DMG29 Corrosion inspection of steel piled maritime structures
Short Description
Ministry of Defence Guide (Draft)...
Description
Design and Maintenance Guide 29
Corrosion inspection of steel piled maritime structures
DEFENCE ESTATES MINISTRY OF DEFENCE Draft May 2002 1
DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Foreword
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Acknowledgements
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Abbreviations
ALWC Accelerated Low Water Corrosion CDM Regulations Construction (Design and Management) Regulations 1994 CP Cathodic Protection HSE Health and Safety Executive LAT Lowest Astronomical Tide MHWS Mean High Water Springs MIC Microbiologically Induced Corrosion MLWS Mean Low Water Springs MoD Ministry of Defence NDT Non Destructive Testing SCUBA Self Contained Underwater Breathing Apparatus SRB Sulphate Reducing Bacteria UCVI Underwater Close Visual Inspection UGVI Underwater General Visual Inspection UT meter Ultrasonic Thickness meter
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Contents
FOREWORD ACKNOWLEDGEMENTS ABBREVIATIONS CONTENTS EXECUTIVE SUMMARY Section 1 – Introduction 1.1 Purpose 1.2 Background 1.3 Scope 1.4 Layout of the Document Section 2 – Corrosion of Steel Maritime Structures 2.1 2.2 2.3 2.4
Introduction Normal Corrosion Microbiologically Induced Corrosion Corrosion Design of Steel Maritime Structures
Section 3 – Overall Approach 3.1 3.2 3.3 3.4
Introduction Initial Engineering Review Appraisal of Results of the Survey Summary of Overall Approach
Section 4 – Competence of Inspection Personnel 4.1 4.2 4.3 4.4 4.5 4.6
Introduction Professional Team Leader Senior Inspector Inspector Diving Engineer Diving Inspector
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Section 5 – Diving Operations 5.1
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Diving Operations
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Section 6 – Frequency of Surveys 6.1 6.2 6.3 6.4
Regular Visual Surveys Standard Surveys Detailed Surveys Cathodic Protection
Section 7 – Methods of Visual Inspection 7.1 7.2 7.3 7.4 7.5 7.6
Introduction General Visual Inspection Close Visual Inspection Underwater General Visual Inspection Underwater Close Visual Inspection Photography
Section 8 – Requirements for Non Destructive Testing 8.1 8.2 8.3
Ultrasonic Thickness Measurements Marine Growth Measurements Protective Coating Thickness Measurements
Section 9 – Extent of Inspection and NDT 9.1 9.2 9.3
Regular Visual Surveys Standard Surveys Detailed Surveys
Section 10 – Checks on Cathodic Protection System 10.1 10.2 10.3 10.4
General Checks on Impressed Current Systems Checks on Sacrificial Anode Systems Cathodic Potential Measurements
Section 11 – Reporting Requirements 11.1 11.2 11.3 11.4
General Regular Visual Survey Standard Survey Detailed Survey
Section 12 – Generic Review of Possible Remedial Measures 12.1 12.2 12.3 12.4 12.5
General Measures to Prevent Further Corrosion Measures to Replace Lost Steel Discussions of Options Construction Methods
Section 13 – References Draft May 2002
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Appendix Sample Inspection Forms
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Executive Summary
ES.1 INTRODUCTION ES.1.1
Steel in maritime structures is in a particularly hostile environment. Methodical and regular inspections are essential to the controlled programming, budgeting and economic execution of maintenance work, and to ensure that the structures are safe.
ES.1.2
This Specification is intended to: 1) 2)
ES.1.3
provide a guide to the corrosion of steel maritime structures for facility managers, who may not necessarily be engineers; provide guidance to the engineers and other technical staff who are responsible for carrying out the inspections.
This Specification applies only to steel components of maritime structures. These will generally be steel bearing piles supporting structures or structures having steel sheet piling. It does not apply to dock gates or caissons, link spans or floating structures, though the same considerations will apply.
ES.2 CORROSION OF STEEL MARITIME STRUCTURES ES.2.1
Corrosion of steel in maritime structures can be divided into two types, namely “normal corrosion” and bio-corrosion.
ES.2.2
Normal corrosion refers to the normal rusting of steel as the result of a chemical reaction, which occurs wherever unprotected steel is in the presence of water and oxygen. It is reasonably well understood and predictable.
ES.2.3
Bio-corrosion is a similar process to normal corrosion except that the corrosion of the metal is influenced by micro organisms. Different mechanisms are known, but for maritime structures, that due to sulphate reducing bacteria is important. This is commonly referred to as Accelerated Low Water Corrosion (ALWC) or Microbiologically Influenced Corrosion (MIC). The latter term is used in this Guide.
ES.2.4
MIC causes only localised corrosion, but the rate of corrosion is much higher than for normal corrosion. Steel sections can be perforated in only a few years.
ES.2.5
MIC usually occurs in a zone approximately 1m high, centred between mean low water and Lowest Astronomical Tide (LAT), hence it is often referred to as ALWC. However it can occur down to sea bed level, and has also been found above LAT, where the sea bed rises above LAT.
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ES.2.6
MIC in maritime structures has been known about since the 1980’s but has only been identified as a significant problem since the 1990’s. Latest figures for the UK indicate that as many as 9 out of 10 harbour and port facilities are affected.
ES.2.7
Why and where MIC occurs is not fully understood, so it is not possible to predict accurately whether it is likely to affect a steel structure.
ES.2.8
MIC is caused by two different bacteria. It is recognised as follows. There is a layer of orange-brown poorly adhered paste, comprising aerobic bacteria and corrosion products. When this is removed, a layer of black odorous sludge is revealed, which comprises anaerobic sulphate reducing bacteria. Beneath this sludge, the steel is clearly pitted and shiny, sometimes described as being like gunmetal.
ES.3 OVERALL APPROACH ES.3.1
Three levels of inspection survey are specified. These are: 1)
Regular Visual Survey. This is a relatively quick annual inspection survey, probably by on site staff who should be competent but do not necessarily have to be technically qualified. The purpose of the survey is to provide early warning of specific corrosion problems between the standard inspections. It should also be used to check for damage. As MIC tends to occur around LAT, it is important that this survey is carried out at the lowest state of the tide as possible.
2)
Standard Survey This is a regular inspection of the structure by suitably qualified technical staff. It will usually involve underwater work, but in specific cases this may be omitted. The frequency will depend on the local conditions, but typically would be every four years. As well as being carried out on a regular basis, a Standard Survey should also be undertaken if the Regular Visual Survey indicates that there is an unexpected and significant corrosion problem
3)
Detailed Survey Where the Standard Survey shows that there is corrosion that needs remedial works, then a Detailed Survey may be necessary. This would be tailored to suit the individual circumstances.
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ES.3.2
For the Standard and Detailed Surveys, a suitably qualified Professional Team Leader shall be appointed. He shall be a Chartered Engineer and take responsibility for the technical content of the inspection survey.
ES.3.3
For the Standard Survey, ideally an Initial Engineering Review should be carried out under the supervision of the Professional Team Leader before specifying the extent of inspection. This will enable the inspection to be tailored to suit the structures under examination.
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ES.3.4
For the Initial Engineering Review, the following information on the structures should be obtained where available: a) b) c) d) e) f) g) h)
as built drawings of the structure; the original design criteria, including how the design catered for corrosion; as built drawings of any subsequent modifications or repair works, including the design criteria for these works; any operation and maintenance manuals for the structure, particularly if cathodic protection is provided; details of tidal range and currents. Also details of any water quality tests that have been carried out; records of all previous inspections of the structures concerned; any reports on corrosion of other steel structures in the same port; the Health and Safety File where it exists.
ES.3.5
The Initial Engineering Review should assess the available information as above and also take into account the local factors. It should then adjust the scope of the Survey from that specified in this document, to ensure that the inspection is appropriate. It should also identify criteria against which the inspection can be judged, such as acceptable loss of steel.
ES.3.6
The Initial Engineering Review should be fully documented so that it can be used for future surveys.
ES.4 REQUIREMENTS FOR SURVEY ES.4.1
This Guide includes specific guidance on the following:
competence of inspection personnel; requirements for diving operations; frequency of inspections; methods of visual inspection; requirements for non destructive testing (NDT); extent of inspection and NDT; checks on cathodic protection systems; reporting requirements.
Table ES1 summarises the requirements for the surveys. ES.4.2
Section 12 provides a generic review of possible remedial measures. In practice every structure will need to be assessed individually.
Table ES1: Summary of Recommended Inspection Procedures Clause Initial Engineering Review Frequency Personnel requirements
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Clause 3.2 Section 6 Section 4
Regular Visual Survey Annual Inspector
Standard Survey Every 4 years Professional Team Leader Senior Inspector, Diving Engineer/ Inspector
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General visual inspection, Clause 7.2 above water Underwater general visual Clause 7.4 inspection Close visual inspection, Clause 7.3 above water Underwater close visual Clause 7.5 inspection Photographic record Clause 7.6 Ultrasonic thickness Clause 8.1 measurements Marine growth Clause 8.2 measurements Protective coating Clause 8.3 measurements Cathodic protection survey Section 11 Report Section 12 Notes: 1 The Initial Engineering Review may modify the requirements for the Standard Survey, including the frequency. 2 The recommended spacings of the close visual inspections and measurements are given in Section 9.
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1 - Introduction
1.1
PURPOSE
1.1.1
Steel in maritime structures is in a particularly hostile environment. Methodical and regular inspections are essential to the controlled programming, budgeting and economic execution of maintenance work. Inspections are also necessary to establish that the structures have not deteriorated to such a degree that they can no longer meet their designed purpose.
1.1.2
This Guide is intended to: 1) 2)
1.1.3
provide a guide to the corrosion of steel maritime structures for facility managers, who may not necessarily be engineers; provide guidance to the engineers and other technical staff who are responsible for carrying out the actual work.
The inspection programme has to be a balance between the following factors:
cost of inspections; risk of significant corrosion not being discovered during an inspection; risk of significant corrosion occurring between inspections; effect of significant corrosion on safety and required maintenance; disruption to operations.
These factors will vary for different structures and locations. This Guide gives specific guidance based on typical conditions, but it may be appropriate to vary this depending on the actual conditions.
1.2
BACKGROUND
1.2.1
Guidance on the inspection of maritime steel structures was issued by the Property Services Agency in the 1980’s. However this advice predated the realisation that Microbiologically Induced Corrosion (MIC) was becoming a significant problem in maritime steel structures. Prior to the preparation of this Guide there was no up to date guidance for property managers of military establishments on what inspection regime they should adopt.
1.2.2
Corrosion of steel is an important consideration in the maritime environment. Many steel maritime structures have been designed using unprotected steel, with additional sacrificial
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metal to allow for this corrosion. Predictions of normal corrosion rates have been formulated for unprotected steel under various exposure conditions. These typically vary from 0.04 mm/year to 0.17 mm/year for each exposed side in temperate climates. 1.2.3
Since the 1980’s however higher corrosion rates have been found in the low water zone in British estuarial waters, associated with microbiological activity. This is called MIC (Microbiologically Induced Corrosion) in this Guide. It is often referred to as Accelerated Low Water Corrosion (ALWC) though there is evidence of it occurring not just at Low Water. Normally its effect is concentrated in discrete areas and the corrosion rates can typically be 0.5 mm/year, though higher rates have been reported.
1.2.4
MIC can result in the perforation of steel structures at a much more rapid rate than would be expected with “normal” corrosion. Depending on the structure involved, this can have serious consequences for safety, though to date no catastrophic failures have been reported in the technical press.
1.2.5
The existence of MIC in maritime structures has only been identified as a significant problem since the 1990’s, though its existence in other areas such as shipping has been known for much longer. There is ongoing research on various fronts, but the reasons why and where it occurs is not fully understood.
1.2.6
This Guide provides guidance on inspecting steelwork, based on the current state of knowledge.
1.3
SCOPE
1.3.1
This Guide applies only to steel components of maritime structures. These will generally be steel bearing piles supporting structures or structures having steel sheet piling.
1.3.2
This Guide does not apply to dock gates, caissons, floating structures such as catamarans and pontoons, or linkspans though many of the considerations will be the same.
1.3.3
Nuclear safety implicated structures have their own prescribed inspection regimes that take into account their Safety Function in relation to the Facility Safety Case. However, the guidance in this Guide can be taken into account in developing inspection regimes for such structures.
1.3.4
The Guide does not apply to steel embedded within concrete components.
1.3.5
The Guide is intended to be used in temperate, tropical and desert climates. It does not cover arctic climates, though the considerations will be similar. Advice on specific cases can be obtained from Defence Estates.
1.3.6
The Specification also considers repairs to corrosion damage in a generic way. In practice the repairs for any structure will be dependent on many site specific factors, including the type of structure, operational requirements and local availability of resources.
1.4
LAYOUT OF THE DOCUMENT
1.4.1
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This Guide is laid out as follows:
DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Section 2 gives technical guidance on the different types of corrosion. This includes how to recognise MIC. Section 3 describes the overall approach to the inspections. Section 4 gives requirements for the competence of inspection personnel. Section 5 gives specific requirements for diving operations, in particular relating to safety aspects. Section 6 gives guidance on how often the inspections should occur. Section 7 specifies the methods of visual inspection. Section 8 specifies methods for non destructive testing. Section 9 specifies the inspection and testing requirements for each inspection. Section 10 covers cathodic protection systems, which are not covered by the previous sections. Section 11 gives the reporting requirements. Section 12 gives a generic review of possible remedial measures. In practice every structure will need to be assessed individually.
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2 –Corrosion of Steel Maritime Structures
2.1
INTRODUCTION
2.1.1
There are several different forms of corrosion. The most important for steel in maritime structures can be divided into two types, namely “normal” corrosion and Microbiologically Induced Corrosion (MIC).
2.1.2
“Normal” corrosion as used here refers to the normal rusting of steel as the result of a chemical reaction. This occurs wherever unprotected steel is in the presence of water and oxygen. It is reasonably well understood and predictable. There are several different forms, and these are described in Section 2.2.
2.1.3
MIC, also called bio corrosion, is a similar process to normal corrosion except that the corrosion of the metal is influenced by micro organisms. Different mechanisms are known, but for maritime structures that commonly described as Accelerated Low Water Corrosion (ALWC) is important. This is described in Section 2.3.
2.1.4
The Institution of Structural Engineers’ “ Guide to Inspection of underwater structures”, October 2001, gives further guidance on the different forms of corrosion in Table 2. This includes the following forms of corrosion that are outside the scope of this Guide and are not discussed further: a) selective leaching. This is particularly significant in cast iron. The more anodic components in the cast iron’s microstructure are preferentially corroded, leaving a porous mass of graphite and iron oxides. b) intergranular corrosion. This occurs in certain grades of stainless steels and other metals that have been subjected to uncontrolled heating. It is therefore often found around welds.
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2.2
“NORMAL” CORROSION
2.2.1
Corrosion of steel is where the steel undergoes oxidation. It is an electro chemical process consisting of two half cell reactions: a reaction at the anode where the steel is oxidised and a cathodic reaction involving the reduction of a chemical species at the cathode. The resultant of these reactions for “normal” corrosion is that iron combines with water and oxygen to form rust. For normal corrosion it is therefore essential to have both water and oxygen.
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2.2.2
The corrosion products (i.e. the rust) have a volume of some ten times the original steel. This means that a steel member can appear to be severely corroded with a substantial thickness of rust, but when this rust is cleaned off the loss of section is found to be relatively small.
2.2.3
The rate of corrosion is affected by contaminants, of which the most important in the marine environment are chlorine and to a less extent sulphur. The resulting iron compounds of these elements stimulate the corrosion process, without being consumed themselves, having an effect disproportionate to their concentration.
2.2.4
The formation of anodes and cathodes requires non uniformities within the material. In steel these are always present as a result of the crystalline structure and also as a result of the fabrication processes such as cold working and welding. As corrosion proceeds, the anodes and cathodes move on the surface and the resulting corrosion is uniform. However in some cases due to specific non uniformities, the anodes and cathodes do not move and pitting occurs.
2.2.5
For some metals such as aluminium, the corrosion results in an oxide that forms a very dense and adherent film, preventing any further corrosion. The rust of steel however provides only very limited protection. It easily flakes off, allowing corrosion to proceed.
2.2.6
The marine environment therefore has all the ingredients for the corrosion of steel. Typical rates of loss of metal thickness have been published for temperate climates and the figures are given in Table 2.1. These vary depending on where the steel is in relation to the tidal levels, which affects the availability of oxygen and water.
Table 2.1 Typical Rates of Corrosion for Structural Steels in Temperate Climates Exposure Zone
Atmospheric Zone -above splash zone and where direct wave or spray impingement is infrequent Splash Zone -above mean high water to a height depending on mean wave height and exposure to wind Tidal Zone -between mean high water and mean low water springs level Inter-tidal Low Water Zone -between low water springs and 0.5m below LAT Continuous Seawater Immersion Zone -from 0.5m below LAT to sea bed Below Seabed Level or in Contact with Soil Note: 2.2.7
Corrosion Rate mm per side/ year Mean Upper Limit 0.04 0.10 0.08
0.17
0.04
0.10
0.08
0.17
0.04
0.13
0.015 max The upper limit figures are the 95% probability values. Reference: BS6349 Part 1:2000 There is little published data on corrosion rates in tropical and desert climates, and what there is is inconclusive. However above the continuous seawater immersion zone, the rates are
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likely to be higher due to the increased temperature. As an order of magnitude, they could be twice the typical values for temperate climates. In the submerged zones, the rates are likely to be similar as oxygen is less soluble in warmer waters. 2.2.8
Other factors affect the rate of corrosion, and can cause significantly higher rates locally. These include: a)
repeated removal of the corrosion product layer, particularly in the low water or immersion zones by the action of fendering systems or other moving parts such as chains (fretting corrosion);
b)
repeated removal of the corrosion product layer by the movement of water, particularly if it contains abrasive particles such as sands and gravels (erosion corrosion). This can be caused by bow thrusters and propellers;
c)
bimetallic corrosion, where steel is electrically connected to other metals. This can also occur when weld metal is significantly different to the parent metal (galvanic corrosion);
d)
stray electric currents. This can be a problem where cathodic protection (CP) is used on steel structures adjacent to steel structures without CP. The stray currents can cause higher corrosion rates in the steel not protected by CP. There can be interaction with a ship, where either the structure or the ship has CP, resulting in localised higher corrosion rates. This is not usually a problem unless the ship is laid up at the berth, and therefore present for a considerable time. Stray currents can also occur as a result of bonding of electrical earths, and lightning protection, though this effect is not usually significant;
2.3
MICROBIOLOGICALY INDUCED CORROSION (MIC)
2.3.1
There are several forms of MIC, but those involving Sulphate Reducing Bacteria (SRB) are important for maritime steel structures. MIC is used in this Guide to describe this particular corrosion, but it should be noted that there are other forms of MIC found in other environments. MIC is commonly referred to as Accelerated Low Water Corrosion, or ALWC. This term has the advantage over MIC in that it is specific to the type of corrosion found in maritime structures. However MIC has been found at locations other than Low Water, and therefore this Guide uses the term MIC in preference. BS6349 Part 1, Maritime Structures uses the term “concentrated corrosion”, but this also covers some other forms of corrosion.
Occurrence 2.3.2
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The origins of MIC are uncertain. It has long been established that high rates of non-uniform corrosion occur at sites of bacterial growth, and the shipping and offshore industries have been dealing with the problem for over a century.
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2.3.3
The corrosion colonies, which in the case of MIC act in unison or partnership, produce extreme rates of corrosion. The attacks are localised, but can be so numerous as to affect an almost continuous band.
2.3.4
MIC has been noted at locations along the coastlines of all the northern European countries, the West Indies, Japan, South East Asia and recently in Australia. Latest figures for the UK indicate that as many as 9 out of 10 harbour and port facilities are suffering from MIC. It has also been noted in European fresh water canals.
2.3.5
Surveys at a number of locations originally led to the conclusion that MIC was always isolated to a zone approximately 1m in depth centred between mean low water level and LAT. However, it has also been found down to bed level. In one instance, divers found MIC on a tubular pile at bed level some 7m below CD, where the pile was completely disconnected. It has also been found above low water, but only where the sea bed level rises above low water when it can occur at the bed level.
2.3.6
The reason for bacterial colonisation is not understood and at the moment it is not possible to predict where MIC is likely to occur. It is not clear whether MIC is affected by the particular chemical or physical conditions of seawater. Incidences have been recorded in polluted, clean and brackish water, with or without strong tidal currents. Lighting conditions do not appear to affect the growth either, with divers reporting occurrences in both clear water, with exceptional visibility, and in turbid conditions with poor visibility.
2.3.7
There have been suggestions that the increased occurrence of MIC is linked to a cleaner environment, i.e. oxygen levels in water are improving due to a reduction in toxic effluents. It has been reported that MIC is suppressed at locations near sewage (assumed to be untreated) out-falls, which may support this claim.
2.3.8
It has been suggested that steel produced prior to the 1950’s does not suffer from MIC, but this has not been verified. If pre 50’s steel does have improved durability, this may be due to changes in the production process, which result in modern steels being ‘purer’.
2.3.9
The evidence to date indicates that a properly operating cathodic protection system (sacrificial anode or impressed current) prevents the occurrence of MIC.
2.3.10
Some structural elements appear to suffer in a predictable way, for example: a)
Larssen sheet piles normally corrode on the ‘out-pan’. However in one commercial southern UK port, Larssen piles were visibly corroded and holed at the edges and webs.
b)
Frodingham sheet piles normally corrode on the webs and corners. The corrosion pattern on the Frodingham pile can be quite extensive with elongated holes as much as 1m long developing (see photograph).
c)
Straight web sheet piles normally corrode in the centre of the pans, similar to Larrsen.
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d)
On tubular piles, the welds have been reported as being particularly susceptible. Poor selection of weld consumables can result in the welds being more susceptible to corrosion.
Other standard sections such as columns and beams also suffer from MIC, but records of these are too few to establish a pattern.
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FRODINGHAM PILES SHOWING ELONGATED CORROSION HOLES
Picture courtesy of Posford Haskoning Description of MIC 2.3.11
In general at MIC sites, the seaward side of the pile has a layer of calcareous deposits together with shellfish, algae and weed in direct contact with the seawater. Beneath this is a layer of orange-brown poorly adhered paste. This layer, which is exposed to oxygen, is composed of aerobic bacteria and ferrous and metallic corrosion products and hydroxides. Below this, in direct contact with the steel surface is a layer of black, odorous sludge. The adherent sludge is generally in a liquid or semi-liquid state with black particles held in suspension. It is composed of anaerobic sulphate reducing bacteria (SRB) which produce hydrogen sulphide (H2S). The latter has a characteristic “rotten egg” smell.
2.3.12
Sulphate reducing bacteria (SRB) are anaerobic (they are able to exist in the absence of oxygen). Their metabolism is based on the reduction of sulphate leading to the production of hydrogen sulphide. The sulphate is employed as the electron acceptor in the respiration process, which in addition drives the oxidation of organic carbon. The donors used are low molecular weight carbon compounds such as mono or dicarboxylic acids, alcohols etc. The most well known SRB are medium temperature loving mesophiles of which there are 40 types.
2.3.13
It is recognised that SRB are involved in the corrosion process, but the precise mechanics are unconfirmed. The process is now thought to be more than just a simple cathodic depolarisation.
2.3.14
As their name suggests, the aerobic bacteria thrive in the presence of oxygen. There are three types; two forms of Thiobacilli (sulphur oxidising bacteria and iron bacteria) and Vibrio.
2.3.15
Thiobacilli thrive in the presence of sulphur compounds; these types of bacteria are autotrophic, which means that they obtain their food from inorganic sources, i.e. they do not need to eat other organisms. Despite the ability of the Thiobacilli to exist in conditions that are very different to those required by the SRB, they often coexist with SRB because of the H 2S by-product necessary for their growth. A by-product of the Thiobacillis’ synthesis of pyrites and sulphides is sulphuric acid. It is believed
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that it is this acid which subsequently facilitates the corrosion process.
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Corroded Piles, showing orange patches typical of aerobic bacteria. Picture courtesy of Posford Haskoning
Example of Pitted Surface, biofouling removed to reveal an example of the corrosion surface. Rates of Corrosion 2.3.16
For MIC, typical corrosion rates of 0.5 mm/ year are quoted, but higher values of 1 mm/year or more have been reported. There is some uncertainty in the actual rate of corrosion, because where it has been measured it has not been possible to determine exactly when the attack started.
2.3.17
Such high rates of corrosion can lead to perforation of the steel in a relatively short period. For example a typical sheet pile wall section could have 9 mm wall thickness, which would be perforated in 9 years at 1mm per year if no action is taken.
2.3.18
To date no major failures of marine structures resulting from MIC have been reported. This is in part because the attacks are localised, and for sheet pile walls, the maximum stresses are not usually at low water level, the most likely area of attack. The most common failure mode for sheet pile walls suffering from MIC is loss of material from behind the wall through the perforation. The loss of this material has caused large settlements in the ground behind the wall.
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2.4
CORROSION DESIGN OF STEEL MARITIME STRUCTURES
2.4.1
This describes in outline how steel maritime structures are or were commonly designed to cope with corrosion. It is necessary to understand this in order to assess the significance of any corrosion.
2.4.2
Traditionally corrosion of steel piling in the maritime environment has been addressed in one of the following ways: 1)
the steelwork is not protected, but the members are sized to allow for a certain loss of metal thickness over the design life of the structure. This would be calculated based on forecast corrosion rates, see Section 2.2 and in particular Table 2.1.
2)
the steelwork is fully protected with paint, wrapping or concrete, which is maintained throughout the design life of the structures. There is no allowance for loss of metal as a result of corrosion. A properly specified and applied painting system could have a design life up to about 15 years in the marine environment.
3)
cathodic protection is provided, see Section 10 for further details. It should be noted that cathodic protection only protects the steelwork that is immersed. It therefore provides full protection to the steel below LAT, partial protection for steel between LAT and MHWS and no protection above MHWS. Cathodic protection is often combined with protective coatings. The protective coating can provide full protection above LAT, if it is properly maintained. Below LAT, it is not generally practicable to maintain the coating. Therefore with time the coating in this area is only partially effective, due to damage and deterioration, but the CP ensures full protection. However the protective coating substantially reduces the demand on the CP system.
4)
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A combination of 1) and 2). The steelwork is painted, but it is assumed that this is not maintained and only lasts a certain period (not more than 15 years). The remainder of the design life is then achieved by allowing for the loss of sacrificial metal thickness as a result of corrosion.
2.4.3
The approach adopted will have depended in part on the design life of the structure. For steel maritime structures, the design life is likely to be between 25 and 50 years, or possibly up to 60 years.
2.4.4
The advantage of the first approach, a sacrificial corrosion allowance, is that it does not rely on the structure being maintained. It is often the most cost effective solution for the shorter design lives, say 25 to 30 years, because: a)
for sheet pile walls, the highest stresses are often not at locations where high corrosion is predicted;
b)
for driven bearing piles, the in service stresses are less than the stresses during installation of the pile. The pile has to
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be sized for the installation case and will have spare capacity in the completed structure. 2.4.5
Up to around 1990, many designers of steel sheet piles allowed much higher steel stresses for the corroded section at the end of its life. This was recommended by The Piling Handbook of the time, published by British Steel (now Corus), which gave a design life of unpainted piles in sea water of between 45 and 125 years dependent on the actual section. This design life was based on the loss of steel from corrosion being compensated for by the increase in allowable stresses. This approach is no longer in favour.
2.4.6
In some cases, steel tubular piles were /are driven and filled with reinforced concrete. The design is based solely on the concrete section, and the steel pile is treated as permanent sacrificial formwork. Corrosion of the steel pile in these cases is therefore of no concern. Care should be taken to ensure that this is the case, as steel piles can be filled with concrete for other reasons where the steel pile is still essential to the design. For example, it has been suggested that tubular steel piles should be filled with concrete or sand to prevent MIC becoming established inside the pile.
2.4.7
The use of unprotected steel and a corrosion allowance was probably the most common approach until MIC was found to be a widespread problem in sheet pile walls in the early 1990’s. However the loss of metal thickness as a result of MIC is much higher than for normal corrosion, and it is not economic to provide additional steel to cater for this over the typical life of the structure. If modern structures are designed with an allowance for loss of metal thickness as a result of corrosion, it is therefore assumed that the structures will be inspected regularly and action taken if MIC is found.
2.4.8
Cathodic protection has been found to prevent MIC, and therefore this is becoming more common where it is considered that MIC is likely to occur.
2.4.9
The design criteria for the original structure should describe how the effects of corrosion were designed to be catered for. If this is available, this is a very useful document to assist in assessing the structure.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
3 – Overall Approach
3.1
INTRODUCTION
3.1.1
This section describes the overall approach to corrosion inspection of steel maritime structures. It should be noted that the inspection is intended to identify all forms of corrosion, not just MIC.
3.1.2
There are three levels of inspection survey. These are: 1)
Regular Visual Survey This is a relatively quick annual inspection, probably by on site staff, who should be competent but do not necessarily have to be technically qualified. The purpose of the inspection is to provide early warning of specific corrosion problems between the standard inspections. It should also be used to check for damage. As MIC usually occurs around LAT, it is important that this survey is carried out at the lowest state of the tide as possible, compatible with the requirement for an annual survey. Spring tides are generally particularly low and therefore suitable around the equinox, i.e. in March and September.
2)
Standard Survey This is a regular inspection of the structure by suitably qualified technical staff. It will usually involve underwater work, but in specific cases this may be omitted. As well as being carried out on a regular basis, a Standard Survey should also be undertaken if the Regular Visual Survey indicates that there is an unexpected and significant corrosion problem. In such cases, the Standard Survey should be carried out as soon as practicable following the Regular Visual Survey.
3)
Detailed Survey Where the Standard Survey shows that there is corrosion that needs remedial works, then a Detailed Survey may be necessary. This would be tailored to suit the individual circumstances.
3.2
26
INITIAL ENGINEERING REVIEW
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
3.2.1
For the Standard Survey, ideally an Initial Engineering Review should be carried out before specifying the extent of the survey. This will enable the inspection to be tailored to suit the structures under examination.
3.2.2
The following information on the structures should be obtained where available: a)
as built drawings of the structure;
b)
the original design criteria, particularly information on how the design catered for corrosion;
c)
as built drawings of any subsequent modifications or repair works, including the design criteria for these works;
d)
the Operation and Maintenance manuals for the structure, particularly for any cathodic protection (CP) system where this is provided. Where CP is not provided, it is unlikely that any manuals exist.
e)
details of tidal range and currents. Also details of any water quality tests that have been carried out;
f)
records of all previous surveys of the structures concerned, including those for the Regular Visual Surveys undertaken since the last Standard Survey;
g)
any reports on corrosion of other steel structures in the same port.
h)
the Health and Safety File where it exists. This is required by the CDM Regulations in the UK, and applies to all construction work since 1995 (subject to minor exceptions).
In addition, the following may be useful: i)
the original specification for the works;
j)
design calculations.
3.2.3
If there is an existing Initial Engineering Review from a previous Standard Survey, this should form the basis for the new Review (see 3.2.8).
3.2.4
The Initial Engineering Review should assess the available information to ensure that the inspection is directed to the critical areas. It should be carried out under the supervision of the Professional Team Leader (see Section 4), who shall take full responsibility for it.
3.2.5
The Initial Engineering Review should identify criteria against which the inspection can be judged. This would include: 1)
the anticipated corrosion of any unpainted steel based on typical corrosion rates or previous survey information if available. This will enable the inspectors to identify any rogue readings during their survey.
2)
Draft May 2002
where the steelwork is painted, the anticipated life of the paint system, and therefore what condition it should be in.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
3)
for structures fitted with cathodic protection using sacrificial anodes, the anticipated condition of the anodes.
4) the acceptable corrosion with respect to the safety of the structure. This will enable the inspectors to assess how critical their results are during the survey. If there is only limited data available on the original design, it may not be possible to determine the acceptable corrosion without carrying out new design calculations. If this is the case, the new design calculations should not be carried out prior to the inspection, due to the cost and effort required. 3.2.6
The Initial Engineering Review should also consider local factors, such as the following:
Factor Operational requirements Access to the structure Tides and currents
Underwater visibility Local availability of resources 3.2.7
In most cases, the inspection will have to take place when the facility is not being used. Operational requirements may restrict the time available. It will not be economic to provide extensive access scaffolding. Access above water will therefore usually be from the existing structure and from a boat. If there are strong currents, some parts of the inspection may only be possible at slack water. This will limit the time available and with a large tidal range, this could make it difficult to inspect the structure at certain levels The underwater visibility will affect whether underwater photographs and or video is appropriate. At remote sites, it may be difficult and expensive to mobilise the full resources, such as diving inspectors.
The Initial Engineering Review should then adjust the scope of the inspection work, to ensure that it is appropriate for the structure, local factors and the importance of the structure. In particular it should ensure that resources are not wasted attempting to carry out excessive close inspections of parts of structures that are difficult to access but not critical. Subject to contractual requirements, this may include adjusting the frequency of the inspections. In certain situations, it may be concluded that a Standard Survey is not necessary, see Section 6.2.
3.2.8
The Initial Engineering Review should be fully documented so that it can be used for future surveys. The Review for the next Standard Survey will then just be a review of the previous Initial Engineering Review and the results of the subsequent Regular Visual Surveys.
3.3
APPRAISAL OF RESULTS OF THE SURVEY Regular Visual Surveys
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
3.3.1
The appraisal of the Regular Visual Survey will normally be undertaken by the Inspector who carried out the work. Depending on the technical competence of the Inspector and what has been noted, professional and technical assistance may be required with the appraisal. If so, this could be from the Professional Team Leader who was responsible for the most recent Standard Survey.
3.3.2
The appraisal shall consider the following: 1)
has any MIC been noted?
2)
has the structure suffered significant and unexpected corrosion since the last Standard Survey, or indeed the last Regular Visual Survey?
If the answer to either of the above questions is yes, then an Initial Engineering Review and Standard Survey should be undertaken.
Standard Surveys 3.3.3
The appraisal of the results of the Standard Survey should be carried out by the Professional Team Leader. This appraisal should cover the following: 1)
whether the condition of the structure is in line with expectations;
2)
whether MIC is present;
3)
the anticipated remaining design life of the structure, assuming no works are undertaken;
4)
whether any repairs or other works are necessary to ensure the safety of the structure until the next Standard Survey;
5)
whether any repairs or other works are recommended on economic grounds to ensure the structure is likely to safely achieve the required life. The required life will depend on operational considerations, and may not be the same as the original life that the structure was designed for.
6)
when the next Standard Survey should take place.
If significant corrosion and/or MIC is found, then a further, Detailed Survey will normally be required.
3.4
SUMMARY OF OVERALL APPROACH
3.4.1
Figure 3.1 gives a flow chart summarising the steps for the Standard Inspection.
Figure 3.1 Methodology for Standard Survey
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Identify Steel Structures
Collect Drawings, Design Information, Results of Previous Surveys
Appoint Professional Team Leader
Carry Out Initial Engineering Review Determine Scope of Work for Inspection
Appoint Survey Team
Carry out Standard Survey
Assess the Results of the Survey Prepare Survey Report
Is Action Required ?
NO
Regular Visual Surveys at 1 Year Intervals
Next Standard Survey as Required
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YES
To be determined Detailed Survey?
DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Section 4 – Competence of Inspection Personnel
4.1
INTRODUCTION
4.1.1
For inspection at and above water level, three grades of inspection personnel are specified as follows: Professional Team Leader Senior Inspector Inspector. The requirements for these are given in Sections 4.2 to 4.4 below.
4.1.2
For underwater inspections, a diving team will be required. The diving team shall comply with the current UK Regulations and other relevant regulations, see Section 5. This Guide specifies requirements for a Diving Engineer and a Diving Inspector. To satisfy the Health and Safety regulations, a Diving Supervisor with particular qualifications is required, however this is not specified here as this role relates to safety rather than the technical content of the survey.
4.1.3
Notwithstanding the specific requirements, all staff shall be suitably qualified and experienced to carry out their tasks. Where the survey is carried out by a third party, the curricula vitae of the Professional Team Leader, the Senior Inspectors, the Diving Engineers and the Diving Inspectors shall be provided for the Client’s approval.
4.1.4
Advice on the acceptability of qualifications and experience in specific circumstances can be obtained from the contact point identified at the front of this Guide.
4.2
PROFESSIONAL TEAM LEADER
4.2.1
For all surveys except the Regular Visual Surveys, there shall be a Professional Team Leader.
4.2.2
The Professional Team Leader shall be responsible for ensuring that the survey addresses the critical areas appropriate for the structures being inspected. He shall be responsible for the technical content of the survey report, and shall approve any such report.
4.2.3
The Professional Team Leader shall be a chartered civil engineer, with a minimum of 10 years professional experience.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
This shall include 5 years experience of steel maritime structures, including their design and specification. He shall have an in-depth understanding of steel structures in the maritime environment.
4.3
SENIOR INSPECTOR
4.3.1
Senior Inspectors will be responsible for the Standard Surveys.
4.3.2
Senior Inspectors could be either : a)
professional civil engineers, educated to degree level. Ideally they should be chartered, but this is not essential if they have suitable experience. The Professional Team Leader could also be the Senior Inspector;
b)
incorporated engineers;
c)
or full time inspectors.
Senior Inspectors shall have a minimum of 5 years relevant experience, including previous experience of inspections of maritime structures.
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4.4
INSPECTOR
4.4.1
Inspectors may undertake the Regular Visual Surveys.
4.4.2
Inspectors can also assist with the Standard Survey provided it is under the direct supervision of a Senior Inspector. A Senior Inspector shall not supervise more than two Inspectors at any one time.
4.4.3
The Inspector is not required to have specific levels of experience, but shall be competent for the tasks he has to undertake.
4.4.4
For the Regular Visual Surveys, it is envisaged that the Inspector could be an appropriate individual already working at the facility. He shall be conversant with this Guide, and in particular shall have practical experience of recognising all forms of corrosion, including MIC.
4.5
DIVING ENGINEER
4.5.1
Ideally the diving team shall include a Diving Engineer. It is however recognised that suitably qualified Diving Engineers are not common, and in certain circumstances a Diving Inspector may take this role. An example may possibly be if the survey is expected to be particularly routine, or if a Diving Engineer is not available at a reasonable cost. Where it is proposed not to use a Diving Engineer, the Professional Team Leader shall assess and approve this decision, taking into account the qualifications of the Diving Inspector.
4.5.2
The Diving Engineer shall be responsible for all technical aspects of the underwater survey, and shall ensure that the underwater survey addresses the critical areas appropriate for the structures being inspected. He shall be responsible for the
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
technical content of the underwater survey report, and shall approve any such report together with the Professional Team Leader. 4.5.3
The Diving Engineer shall be at least a degree qualified civil or structural engineer, but preferably a chartered civil or structural engineer, with river or marine works experience , a minimum of five years professional experience and hold an HSE commercial diver training qualification with a minimum Inshore Air Diver standard or equivalent approved by HSE. The five years professional experience shall include two years experience of steel maritime structures.
4.5.4
The Diving Engineer shall have a clear understanding of steel structures in the maritime environment, and be thoroughly conversant with, and have practical experience in recognising, all forms of corrosion including MIC.
4.6
DIVING INSPECTOR
4.6.1
Diving Inspectors shall undertake the underwater visual inspections.
4.6.2
The Diving Inspector shall work under the technical direction of the Diving Engineer.
4.6.3
The Diving Inspector shall have at least a basic education to GCSE standard, hold an HSE commercial diver training certificate to at least Inshore Air Diver standard, preferably hold CSWIP 3.1U or 3.2U certification, and have at least two years experience with diving inspections of river or marine works.
4.6.4
The Diving Inspector shall be thoroughly conversant with, and have practical experience in recognising, all forms of corrosion including MIC.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Section 5 – Diving Operations
34
5.1
DIVING OPERATIONS
5.1.1
The Contractor shall provide a competently manned commercial diving team using surface demand diving equipment for all the underwater works. The minimum number of personnel in a diving team shall be four.
5.1.2
All diving operations shall be performed in accordance with the relevant UK Regulations, any local Regulations where outside the UK, and any local MOD procedures. In particular, the operations shall comply with “The Diving At Work Regulations”, UK, S.I. 1997/2776 and the relevant “Approved Code of Practice for Commercial Diving Projects Inland/Inshore”, ref. L104, 1998, or any subsequent revisions to these.
5.1.3
SCUBA diving should not generally be used in support of inspection, construction, maintenance and salvage work except when absolutely necessary and only then permitted when used with hard wire communications and a safety line.
5.1.4
All divers shall be in possession of bona fide commercial diver training certificates, current certificates of medical fitness and diving logbook. In addition, sufficient divers in the team shall have current certificates of diving or general first aid in order to comply with the current regulations. Non-availability of and non-compliance with any of the above shall immediately render the diver to a non-diving role.
5.1.5
The Contractor shall preferably be a member of a recognised diving contractors’ trade association such as the Association of Diving Contractors (known as the ADC).
5.1.6
Information on diving techniques and equipment is given in “Guide to Inspection of underwater structures”, published by the Institution of Structural Engineers.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Section 6 – Frequency of Surveys
6.1
REGULAR VISUAL SURVEYS
6.1.1
The objective of the Regular Visual Survey is to provide early warning that there may be problems with MIC, or other forms of corrosion.
6.1.2
Regular Visual Surveys shall take place annually. As MIC usually occurs around LAT, it is important that this survey is carried out at the lowest spring tides of the year. These usually occur around the equinox, i.e. in late March and late September.
6.2
STANDARD SURVEYS
6.2.1
The objective of the Standard Survey is for technically qualified inspectors to assess the corrosion of the structures.
6.2.2
The frequency of inspection will depend on a variety of factors, including:
the age of the structure; the type of structure; the importance of the structure and the effect of any failure of the structure resulting from corrosion; the protection system of the steelwork; ease and cost of inspection.
For military structures, Standard Surveys should typically take place every four years. The Initial Engineering Review should consider if four years is appropriate, taking into account the above factors, and if not determine a more appropriate period. For new structures where the steel is protected by a protective coating or cathodic protection, a Standard Survey would not normally be required after the first four years. The first Standard Survey would typically be after eight years, and thereafter every four years. Notwithstanding this, an Initial Engineering Review should be undertaken after four years to confirm that the Standard Survey is unnecessary.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
36
6.3
DETAILED SURVEYS
6.3.1
Where the Standard Survey has identified an area of concern, further investigation may be required. The need for a Detailed Survey will be established during the appraisal of the results.
6.4
CATHODIC PROTECTION
6.4.1
Where cathodic protection is provided, this needs regular checks outside the Surveys. These are discussed in Section 10.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Section 7 – Methods of Visual Inspection
7.1
INTRODUCTION
7.1.1
The following types of visual inspections are specified:
7.1.2
General Visual Inspection (above water); Close Visual Inspection (above water); Underwater General Visual Inspection; Underwater Close Visual Inspection.
For all inspections, it is essential to set up a proper referencing system to establish the locations of all defects, close visual inspections and tests.
For sheet pile walls, a chainage regime shall be set up for each structure. For bearing piles each pile shall be given a unique reference. A tape measure shall be used to measure the vertical location. In the case of raking piles, it shall be made clear whether distances are measured vertically or parallel to the pile axis.
7.2
GENERAL VISUAL INSPECTION
7.2.1
General visual inspections are required for all elements of the structure at and above Mean Low Water Springs, under all inspection regimes.
7.2.2
General visual inspections shall be a brief qualitative visual assessment of the condition of the steel, without removal of marine growth. The method shall be suitable for the conditions. As a minimum, it shall be undertaken at a distance of not more than 5 metres from the steel, or not more than 15 m with the aid of binoculars.
7.2.3
For individual piles, the inspection shall cover the element from Mean Low Water Springs to the visible top of the pile. Ideally it should cover all sides of the pile, but where access is difficult and for the Regular Visual Survey, one side is sufficient.
7.2.4
For sheet pile walls, the inspection shall cover the exposed face only, from Mean Low Water Springs to the top of the wall.
7.3
CLOSE VISUAL INSPECTION
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
7.3.1
A Close Visual Inspection involves careful inspection of the element at specific points.
7.3.2
All marine growth shall be removed at each point to expose the protective coating if present or, if there is no protective coating, the steel or rust products. It should be noted that marine growth provides some protection to the steelwork, and therefore no more than is necessary for the purposes of the inspection should be removed. Care shall be taken to ensure that the removal of the marine growth does not damage any protective coating. Where there is a protective coating, the steelwork should preferably be cleaned using a low pressure water jet, at a pressure of around 1,000 to 1,200 psi. This will remove loose products but is less likely to damage the coating compared to mechanical methods. If any protective coating is damaged, this shall be made good at the time of the inspection.
7.3.3
Any rust products shall be investigated during the inspection to establish whether they are as a result of MIC or normal corrosion.
7.4
UNDERWATER GENERAL VISUAL INSPECTION
7.4.1
Underwater General Visual Inspections (UGVI) are required for all elements of the structure below Mean Low Water Springs (or from where the above water inspection could not be done) for Standard Surveys. UGVI’s shall be a brief qualitative visual assessment of the condition of the steel.
7.4.2
The method of UGVI shall be suitable for the conditions, and agreed by the Professional Team Leader. Where visibility is very poor, a UGVI may not be possible or be too time consuming. In these cases, it may be necessary to restrict the extent of the UGVI, or rely on UCVI’s at discrete points, subject to the approval of the Professional Team Leader. Where turbidity varies significantly over time, the UGVI should be scheduled to make use of the best conditions. For example the diving may have to be delayed during heavy rains, if the latter increases the amount of sediment in suspension in the water.
7.4.3
For individual piles, the UGVI shall cover the element from Mean Low Water Springs to the seabed. Ideally it should cover all sides of the pile, but where access is difficult, one side is sufficient.
7.4.4
For sheet pile walls, the UGVI shall cover the exposed face only, from Mean Low Water Springs down to the seabed or estuary bed.
Particular attention should be paid to the areas at LAT and also the bed level. 7.4.5
38
The UGVI shall identify the general condition and features on every structure sufficiently closely to detect major and minor defects or abnormalities. No removal of marine growth or deposits is required.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
7.5
UNDERWATER CLOSE VISUAL INSPECTION
7.5.1
The Underwater Close Visual Inspection (UCVI) shall entail close inspection of specific areas.
7.5.2
All marine growth shall be removed at each point to expose the protective coating if present or, if there is no protective coating, the steel or rust products. The area shall be a minimum of 150mm by 150mm. It should be noted that marine growth provides some protection to the steelwork, and therefore no more than is necessary for the purposes of the inspection should be removed. Care shall be taken to ensure that the removal of the marine growth does not damage any protective coating, see clause 7.3.2.
7.5.3
Any rust products shall be inspected to establish whether it is MIC or normal corrosion.
7.6
PHOTOGRAPHY
7.6.1
A comprehensive photographic record should normally be taken for all Surveys, subject to MOD Regulations. Above water, this shall include colour still photographs of major defects and general photographs of the structure to give an overall impression of the typical condition of the structure. The photographs shall generally be taken in the same locations and of the same areas at each survey, to give a regular record of the condition of the structure.
7.6.2
All shots shall be recorded on suitable log sheets, with their locations clearly identified.
7.6.3
For underwater work, stills photographs or video records should be made, subject to available underwater visibility. For visibility less than 1.0 – 1.5m, stills photos are not normally useful. For underwater visibility less than 300mm video recordings are also not practical. Low light subsea cameras can focus to 100mm but the diver in the water cannot. There should not be a reliance on still and video cameras to carry out or assist inspection work of this nature. However if it is possible to provide stills or video, all shots and footage must be recorded on log sheets with their locations, to complement the Ultrasonic Test and Cathodic Protection logs at the same location.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Section 8 – Requirements for NonDestructive Testing
8.1
ULTRASONIC THICKNESS MEASUREMENTS
8.1.1
Residual steel thickness measurements shall be made as specified using an ultrasonic thickness (UT) meter.
8.1.2
The diver held underwater UT meter shall be the multiple echo single probe type with remote read out between 1-99mm range to 0.1mm accuracy.
8.1.3
The UT meter shall be used in accordance with the manufacturer’s recommendations and the following practical procedures.
Procedure 8.1.4
The following procedure should be used. Where appropriate, it may be adjusted slightly to suit site conditions. (a) Calibration – Pre-calibrate the meter with the blocks supplied with the UT Meter. The UT meter shall be calibrated on
the surface before and after the main measurements with 10, 15, 20 or 25 mm calibration blocks. Record the results in the correct place on the UT Log sheets (see sample in Appendix). (b) Readings - At the specified position, clean off the marine growth using a wire brush and/or scraper and remove any loose flaking steel. The cleaned area shall be a minimum of 150mm by 150mm. Squeeze some ultrasonic couplant (not too much) on to the area (note: couplant is not required underwater). Attempt to obtain a reading without any grinding. This is preferable, but if it is not possible grinding will be required, see 8.1.5 below. Take 4 no. readings at each localised spot within a 50-60mm square. Write the four readings in the UT Log Sheet. Check if there are any spurious readings (i.e. any wildly out) and re-do again to end up with 4 no. readings. Check the results against the original steel thickness. Take the average of the four readings to give the result. (c)
40
Draft May 2002
Recording - Use appropriate UT Log sheets (see Appendix for example) to record the results. Draw a sketch of the steel sheet piling/ section to illustrate where the measurements were taken. Measure up the overall dimensions of the section to check that it is as recorded on the drawings. Record the original steel thickness.
DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
8.1.5
Usually the use of a Multiple Echo Measurement Technique (such as a Cygnus 1 underwater UT meter) will negate the requirement of grinding. However when using other systems, if corrosion has caused pitting of the surface, it may not be possible to obtain a UT reading. If this is the case grinding will be required to obtain reading from the UT meter. Grinding should be done very carefully and slowly in 0.5mm stages to minimise the removal of structurally intact steel prior to a successful UT reading being completed. Grinding is stopped either when the bottom of any corrosion pit is reached and the probe can take a reading or when the entire steel surface is visibly bright steel. Above water an electric grinder can be used. Below water either an air grinder or a hydraulically powered grinder will be required.
8.1.6
Assuming that the age of the structure is known, the readings shall be converted to an estimated corrosion rate. Take the original wall thickness (WT) minus the remaining WT and divide it by the age of the structure to give the corrosion rate in mm/year. This can be compared with typical values. Where there are previous readings, the corrosion rate between each reading shall also be calculated, to establish whether this is consistent.
8.2
MARINE GROWTH MEASUREMENTS
8.2.1
The thickness, percentage cover and density of the soft and hard marine growth (MG) shall be recorded on MG Log sheets (see Appendix for an example). The equivalent marine growth thickness shall be calculated from the equation ET =[ (A * B) + 0.5 (C * D)]/100 where ET = equivalent marine growth thickness in mm A = %cover of hard MG B = thickness of hard MG in mm C = % cover of soft MG D = thickness of soft MG in mm
Ultrasonic Testing Meter being used on steel beam to measure wall thickness Photograph courtesy of Posford Haskoning Marine growth measurements will act as a record for future inspections determining the level of cleaning that may be
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
required and therefore, providing guidance for programming on how labour intensive future surveys will be.
8.3
PROTECTIVE COATING THICKNESS MEASUREMENTS
8.3.1
Where the steel has a protective coating, the thickness of this coating shall be measured as specified using a proprietary measuring gauge. Such instruments as the submersible Cygnus Instruments QuaNix 7500 can be used to measure coating thicknesses over ferrous and non-ferrous substrates underwater. Measurement range varies from 0.0 - 2000m.
8.3.2
42
The measuring gauge shall be used in accordance with the manufacturer’s recommendations and the following procedures. -
to obtain an accurate estimate of the thickness of the coating, an average of five readings per inspection area should be undertaken.
-
should a thickness profile be required, readings should be taken from bed level to above the tidal zone, nominally at 1.0 metre intervals.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
Section 9 – Extent of Inspection and NDT
9.1
REGULAR VISUAL SURVEYS
9.1.1
Regular Visual Surveys shall comprise a General Visual Inspection of all exposed steel pile elements between cope level and Mean Low Water Springs (MLWS).
9.1.2
Ideally it should also cover the area from LAT to MLWS. This will depend on the state of the tides at the time of inspection, but the inspections should be timed to provide the best possible coverage.
9.1.3
A photographic record shall be taken where MoD regulations permit. This shall comprise still colour photographs of major defects and general photographs of the structure to give an overall impression of the typical condition of the structure. See Section 7.6.
9.2
STANDARD SURVEYS
9.2.1
The Standard Survey shall comprise the Regular Visual Survey and the additional requirements as given below.
9.2.2
An Underwater General Visual Inspection shall be carried out on all steelwork between sea bed level and the lowest level inspected during the above water survey. The underwater inspections do not necessarily have to take place at the same time as the above water inspections.
9.2.3
Close Visual Inspections, including Underwater Close Visual Inspections, shall be undertaken as follows: Sheet Pile Walls Horizontally at 10 m spacing Vertically
Draft May 2002
at bed level one location half way between bed level and LAT two locations 300-500 mm apart between LAT and MLWS one location at approximately mean tide one location in the splash zone, above MHWS at tie rod level where installed, if this is not covered by one of the above.
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DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
At each location, the inspection shall cover the inpan, side wall and outpan. At tie rod level, it shall also cover the tie rod if exposed. Where the tidal range is less than 1m or there is no tide, the mean tide of the vertical readings may be omitted. Bearing Piles 15% of all piles shall be inspected, with a minimum of at least two per individual structure (dolphin, jetty etc) The piles shall be inspected at the levels specified for sheet pile walls (except that the tie rod level is not applicable), on two sides at each location. 9.2.4
In addition, a Close Visual Inspection shall be carried out of any areas that appear significantly more corroded than the specified areas, along with those areas showing signs of MIC. This may include underwater areas.
9.2.5
Where the structure is protected by a cathodic protection system, the number of Underwater Close Visual Inspections may be reduced, provided the Underwater General Visual Inspection shows that the CP appears to be providing satisfactory protection. The Professional Team Leader should consider this during the Initial Engineering Review and during the survey, and must approve any such reduction.
9.2.6
At all locations of Close Visual Inspections, the thickness of the steel shall be measured using an ultrasonic thickness meter. Where the steel is painted, the thickness of the paintwork shall be measured.
9.2.7
At all locations of Close Visual Inspections, the thickness, percentage cover and density of marine growth shall be recorded (see Section 8.2).
9.2.8
A full photographic record shall be taken of the inspection, see Section 7.6.
9.2.9
Where cathodic protection is provided, tests appropriate to the system being used shall be carried out, see Section 10.
9.2.10
The Professional Team Leader should carry out a preliminary appraisal of the results of the Standard Survey before the diving team leaves site. The objective of this appraisal is to check that the results appear reasonable and consider whether any inspections should be repeated or further inspections are required. This preliminary appraisal should be carried out expeditiously so as to avoid unnecessary delays to the diving team. Ideally it should be carried out in discrete packages during the Survey, and not just at the end. The thoroughness of the preliminary appraisal shall take into account the costs of any delays to the diving team with the cost of remobilising the diving team if additional inspections are required. If the preliminary appraisal concludes that substantially more inspection work is required, it may be preferable to demobilise the diving team, carry out a full appraisal and specify an additional Detailed Survey, rather than trying to complete the diving in one visit.
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9.3
DETAILED SURVEYS
9.3.1
Detailed Surveys are required where the Standard Survey has revealed a cause for concern, and the Professional Team Leader requires additional information. The extent of the Survey will be specified by the Professional Team Leader to suit each case. A Detailed Survey may involve using a suitable method of dewatering, such as a limpet dam, in order to improve the quality of the inspection.
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Section 10 – Checks on Cathodic Protection System
10.1 GENERAL 10.1.1
Cathodic protection provides effective corrosion control on steel structures by creating a potential (voltage) gradient opposing the flow of ions away from the surface and preventing the steel forming anodes. It requires that the steel is immersed in a suitable electrolyte, and therefore only protects the steel that is below the water level. It provides full protection to the steel below LAT, partial protection for steel between LAT and MHWS and no protection above MHWS. Cathodic protection(CP) has been found to be effective in preventing MIC.
10.1.2
There are two basic types of cathodic protection. In an impressed current system, an electric current is supplied by a rectifier, or other direct current source, to a protected structure in order to attain the necessary protection potential.
10.1.3
In a sacrificial anode system, an anode is used to protect a structure by galvanic action. The anode will be a metal that has a more negative potential than steel, and hence the sacrificial anode will corrode instead of the steel. The anode has to be immersed in the electrolyte, i.e. the water, and therefore will be situated below LAT. Specially formulated alloys of aluminium, zinc or magnesium are used for the anodes. The anode will be consumed at a rate dependent on various factors including the area of steel being protected. The weight required is calculated based on the consumption rate and the required design life of the anode. The design life of the anode may be less than the design life of the structure, where it is intended that the anode is replaced during the life of the structure. The driving voltage in the sacrificial anode system tends to be lower than in the impressed current system.
10.1.4
Which cathodic protection system is used will depend upon physical and financial factors, and in particular the ongoing operational and maintenance requirements. Sacrificial anode systems have the advantages of being simple and robust. As individual anodes provide relatively low currents, galvanic systems are easy to design in terms of the current provision and distribution. Adjacent structures are less likely to be affected by stray currents.
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Sacrificial anode systems are more expensive to install, but require little maintenance. 10.1.5
Impressed current systems are cheaper to install, and usually have a lower whole life cost when compared to sacrificial anode systems. They are intrinsically more complex, more vulnerable to mechanical damage and require regular checks on the electrical system. They require a reliable electrical power supply, though their operating costs are relatively low.
10.1.6
In any cathodic protection system, it is essential that the current provision to achieve the correct protection criteria under the full range of environmental conditions is available at all locations over the structure. It is not sufficient for the cathodic protection system design to just provide the correct total current overall. The position and even distribution of anodes is critical to avoid uneven distribution, providing over protection to some areas and insufficient in others. The designer must consider the potential for salinity changes due to fresh water run off or estuarine conditions over the full design life of the structure. It is important to ensure that the potential is not too high, causing over polarization. This can affect the adhesion of some paints and there is an increasing risk of hydrogen embrittlement of susceptible steels, with a consequential adverse effect on fatigue life.
10.2 CHECKS ON IMPRESSED CURRENT SYSTEMS 10.2.1
An impressed current system will require regular monitoring. Details of this should be set out in the maintenance manuals for the system. This monitoring will be ongoing, and is not part of the Surveys specified in this Guide. For information only, typical requirements are given in sub clauses 10.2.2 to 10.2.4 below. However, for the Standard Survey, where divers are being used, additional checks should be carried out to take advantage of their availability. These are described in sub clause 10.2.5 to 10.2.8.
10.2.2
The maintenance of the power supply equipment, such as transformers and switchgear, will follow normal procedures and should be set out in the Operating and Maintenance manual. Particular attention should be paid to safety and earthing measures and equipment in hazardous areas.
10.2.3
The current output of the transformer rectifier may need to be adjusted from time to time to maintain a satisfactory level of protection. In the first year of operation, it is usually possible to decrease the current as polarization increases. Later it may be necessary to increase the current to compensate for deterioration and damage to any protective coating system.
10.2.4
The transformer- rectifier voltage and current output should be measured regularly, along with structure/electrolyte potential readings at a limited number of representative points. BS 7361 recommends:
Draft May 2002
at least monthly measurements for transformer-rectifier voltage and current output;
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structure/electrolyte potential readings monthly for the first year, thereafter if stable conditions have been reached every 2 to 3 months, depending on local conditions.
Provision should be included for carrying out the structure/electrolyte readings at the selected points from the deck. Fixed reference electrodes are not generally used owing to the difficulty of maintenance. Portable reference electrodes are extremely difficult to position against the structure with any consistency; in order to overcome this, perforated plastic location tubes can be fixed on the structure at the required locations. Systems are available to provide automatic measurements, including remote monitoring.
Standard Survey Requirements 10.2.5
The regular monitoring will normally be done without divers. During the Standard Survey, the presence of the divers should be taken advantage of to provide additional checks.
10.2.6
A visual assessment shall be made of the condition of the cable from the surface, the cable fixings, the anodes and the anode fixings. Damage can occur to any one of those elements. If so, then the system cannot operate as designed.
10.2.7
Structure electrolyte readings shall be taken at selected test positions. These positions shall be selected to ensure that these provided a comprehensive survey of the structure. In particular both areas furthest from the anodes and those close to the anodes shall be included. Procedures shall be in accordance with BS 7361. Measurements shall be taken in mV and logged with their location on the CP log sheet (see Appendix).
10.2.8
Generally when the diver is in the water, the impressed current system shall be switched off. Only the diving supervisor can authorise the switching back on.
10.3 CHECKS ON SACRIFICIAL ANODE SYSTEMS 10.3.1
The sacrificial anode cathodic protection system should have a maintenance manual, which sets out the monitoring required. This monitoring will be ongoing, and is not part of the Surveys specified in this Guide. For information only, typical requirements are given in sub clauses10.3.2 and 10.3.3. However certain checks that require divers could be carried out as part of the Standard Survey. These are described in sub clause 10.3.4 to 10.3.6.
10.3.2
Routine readings of structure/electrolyte potential at a limited number of representative points should be made at suitable intervals. BS 7361 suggests that 3 monthly intervals are usual. Provision should be included for carrying out the structure/electrolyte readings at the selected points from the deck. Where facilities are provided, the current output of each anode should also be measured at 3 monthly intervals. However this is
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only possible where link boxes or similar are provided, and is not possible where the anodes are bolted or welded directly to the structure. 10.3.3
In addition regular checks of anode wastage should be undertaken. Depending on the details of the installation, it may be possible to carry this out without divers; for example at very low tides. Where this is not the case, the Standard Survey should include these checks.
Standard Survey Requirements 10.3.4
A visual assessment shall be made of the condition of the anode (or anode sled), and its fixings. Damage can occur to those elements. If so, then the system cannot operate as designed. Each anode shall be surveyed as follows: -
10.3.5
locate and note position of anode clean off anodes to solid material measure length, width and depth of at least three anodes in different places. calculate their residual volume estimate the percentage depletion or wastage from the original shape. calculate the average rate of loss of the anode per year calculate the number of years of useful anode life remaining, based on the loss of material since new and how long the anode has been the . record all the above on log sheets with sketches.
Structure electrolyte readings shall be taken at selected test positions. These positions shall be selected to ensure that these provided a comprehensive survey of the structure. In particular both areas furthest from the anodes and those close to the anodes shall be included. Procedures shall be in accordance with BS 7361. Measurements shall be taken in mV and logged with their location on the CP log sheet (see Appendix).
10.3.6
There is no danger to the diver from the electrical currents in a sacrificial anode system.
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Section 11 – Reporting Requirements
11.1 GENERAL 11.1.1
It is important that accurate records of each survey are made and kept. This will enable each survey to be compared to the previous surveys to give a more accurate picture of the deterioration of the asset due to corrosion over time. This information will be very useful in deciding what action if any should be taken and when.
11.1.2
Copies of all survey reports shall be retained locally by the MoD Manager who is responsible for maintaining the asset until the asset is decommissioned.
11.1.3
Where quantitative results on corrosion are obtained, consideration should be given to pooling these results, in order to provide a database to enhance the state of knowledge of corrosion in maritime steel structures.
11.2 REGULAR VISUAL SURVEY 11.2.1
Only a brief report is required for the Regular Visual Survey. This shall detail the following: 1) 2) 3)
4) 5)
6) 7)
Structures inspected; Dates and time of inspection, for each structure; Water levels at time of inspections (ideally these should be as recorded at the time of the inspection, but where there is no tidal recording device predicted tidal levels may be used); Brief description of how inspection was carried out (e.g. by boat); Observations made: corrosion noted whether MIC noted condition of any protective coatings any significant damage If unexpected corrosion or MIC noted, what further action was taken; Name and signature of inspector
The photographs shall be appended to the report, with their locations fully referenced. A typical proforma is included in Appendix 1.
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11.2.2
The results of the Regular Visual Survey should be reviewed by a technically competent engineer, if the Inspector who carried out the survey is not so qualified.
11.3 STANDARD SURVEY 11.3.1
A full report is required for Standard Surveys. It shall comprise the following sections: 1) 2 3) 4) 5) 6) 7)
Summary Introduction Description of the Structures Inspection Findings Discussion Conclusions Recommendations Appendices
Each section is described below. 11.3.2
The Summary shall be a concise summary of the inspections undertaken, the main findings, conclusions and recommendations.
11.3.3
Section 1, the Introduction, shall give the background to the inspection and shall include the following:
11.3.4
the Client for the works; the firms undertaking the inspections; the names of the Professional Team Leader and the Inspectors; the structures inspected; the type of inspection; the dates and times of the inspection; any limitations and exclusions.
Section 2, the Description of the Structures, shall give a general description of the structures inspected. This shall include:
a brief history of each structure, including when it was constructed; the structural concept, including the original design approach to corrosion where known; references to previous surveys.
General arrangement drawings shall be included in the Appendices (see below). These shall be in sufficient detail so that the structural concept of each element can be fully understood and to ensure that the system used to locate the measurement sites is clear. For a sheet pile wall this would include a plan, a typical cross section through the wall and an elevation on the wall. Assuming a chainage system was used, this would be indicated. For bearing piles, this would include an overall plan and a detailed piling plan indicating the location of each pile, and its rake. The latter would include either a unique number for each pile, or a system of grid references depending on the reference system used.
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Where Reports on previous Standard Surveys are available, it may be appropriate for this Section to refer to the previous Survey Report. 11.3.5 Section 3, Inspection, shall give general information on the Inspection, including: 11.3.6
methods of inspection and testing; methods of identifying inspection locations and levels equipment used; tidal and weather details.
Section 4, Findings, shall describe the findings of the inspection. The main text shall summarise the findings for each element of the structure, with the detailed measurements and reports included in the Appendices. The location of all defects shall be clearly identified. Colour photographs illustrating the condition of the structures shall be included in the Appendices. These shall cover all significant defects and all elements. Sketches illustrating significant points shall be included in the Appendices where appropriate.
11.3.7
Section 5, Discussion, shall discuss the significance of the findings. The depth of the discussions will depend on the Scope of Work agreed.
11.3.8
Section 6, Conclusion, shall give a brief overview of the report, including the areas of concern.
11.3.9
Section 7, Recommendations, shall give recommendations for future action. Where significant corrosion of concern is noted, recommendations for further action shall be made. Depending on the agreed Scope of Work, the recommendations may only be for further work to identify suitable repairs. Depending on the nature of the corrosion, the assessment of the repairs can be time consuming. It is not therefore recommended that the Inspection team should be required to propose repair solutions as part of a fixed price contract for the inspection, as it is difficult to quantify the effort required prior to the inspection.
11.3.10 The Appendices shall include the following: A B C D E F G
Drawings and sketches of the structures. Colour photographs Details of visual inspections Details of marine growth surveys Details of all ultrasound thickness measurements Details of paint thickness measurements Details of cathodic potential measurements
Further information may be added as required.
11.4 DETAILED SURVEY 11.4.1
52
Normally the results of Detailed Surveys should be included in the Standard Survey Reports to which they relate. The
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Professional Team Leader should specify any specific requirements.
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Section 12 – Generic Review of Possible Remedial Measures
12.1 GENERAL 12.1.1
This Section comprises a general discussion on possible remedial measures and is intended to give the reader background information. As such it can not cover every aspect that may be relevant. In practice, every situation will be different and specialist advice will be required in each and every case where remedial measures are required.
12.1.2
Remedial action and repairs to existing structures are ongoing concerns and will always be expensive. Difficult working conditions and operational requirements will normally dictate the optimum solution, but the following factors will also need to be considered:
12.1.3
the type of corrosion; (normal or MIC); the required remaining life of the structure; the type of structure; the original design approach to corrosion; ease of access; cost; operational restrictions.
It is important to establish the objective of the repairs, which will be one or possibly both of the following: A B
To prevent further corrosion; To replace lost steel.
For example, a steel element may have been designed as unprotected but with additional sacrificial steel as a corrosion allowance. If the inspection shows that the loss of steel has used up the corrosion allowance but the facility is still required, then the objective of the repairs could be A, to prevent further corrosion, or B to replace the lost metal. In this case, one would probably investigate both options before deciding which is most appropriate. If MIC or other severe forms of corrosion are present, then it is recommended that measures are taken to prevent further corrosion, unless the required remaining design life is relatively short, say 5 years. It is not recommended that unprotected sacrificial steel plates are adopted for the long term because of the uncertainty of the design life and actual rate of corrosion once MIC is present.
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12.1.4
Any solution should be carefully assessed by a suitable corrosion expert to ensure that it does not produce an extremely aggressive corrosion cell. For example, this can occur with combinations of additional steel and concrete infill.
12.2 MEASURES TO PREVENT FURTHER CORROSION 12.2.1
There are two principal options, protective coatings and cathodic protection.
12.2.2
Protective coatings include the following:
12.2.3
painting; concrete; wrapping.
For painting, it is essential that the paint is suitable for marine conditions and on site application. Several paint manufacturers offer suitable products, with typical design lives up to a maximum of 10 to 15 years. The success of any painting system depends on good workmanship, particularly the preparation of the metal prior to applying the paint. These are difficult to achieve in the marine environment, particularly at low water level (often the most critical area), but the paints are designed to be tolerant of the conditions. Generally painting can only be applied from low water and above, unless the water level can be lowered locally. Specialist paint products are available for application underwater but they are very expensive and their use has not always been successful.
12.2.4
Concrete protective coatings can be used. These would usually be reinforced with a steel mesh and be between 100 mm and 150 mm thick. Any reinforcement steel provided should have adequate concrete cover. Typically this would involve welding shear connectors to the existing steel, fixing steel mesh, placing shutters and pouring concrete into the shutter. There are proprietary systems available for tubular piles that use permanent formwork. The details of the protective coating should be checked to ensure that an aggressive corrosion cell is not set up.
12.2.5
Wrapping can be used on tubular and other bearing piles. It is not however appropriate for sheet pile walls. There are proprietary systems available such as that developed by Denso, which provide suitable protection. In the Denso system, the piles are wrapped in a grease impregnated tape with a protective abrasion resistant sheath.
12.2.6
Cathodic protection can be applied to existing structures. Specialist advice on its design should be taken. Particular care needs to be taken to ensure that stray currents do not affect adjacent structures, which could result in excessive localised corrosion in the adjacent structures.
12.3 MEASURES TO REPLACE LOST STEEL Draft May 2002
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12.3.1
The principal method of replacing lost steel is the welding of additional steel plates to the structural elements. This directly replaces the lost material.
12.3.2
Another option is to replace the lost metal with concrete (but see Clause 12.1.4). This is similar to the concrete protection, except that the concrete may have heavier reinforcement. This option may be appropriate where:
12.3.3
it is difficult to weld additional plates the concrete satisfies other criteria. For example it will provide protection to the steel.
Where the corrosion is severe and extensive, more extensive methods may be required, for example driving an additional sheet pile wall in front of an existing wall, or sleeving tubular piles.
12.4 DISCUSSION OF OPTIONS Sheet Pile Walls 12.4.1
As discussed in Section 2.4, it may not be necessary to replace steel lost from corrosion. This is because: a) b)
the design may have allowed for a certain amount of corrosion; the corrosion may occur where the steel is not highly stressed.
It is however necessary to ensure that there are no holes in the wall, which would allow material to be washed out from behind the wall. 12.4.2
Repairs to sheet pile walls usually comprise the welding of additional steel plates over the affected areas, to increase the structural capacity and plug any holes.
12.4.3
If the objective is to halt further corrosion, then the sheet pile wall will need to be protected. The most usual method is to paint the piles from Low Water to the top, which if properly specified and carried out should provide about 10 years protection. This will not however protect those parts of the wall below Low Water. There may be spare structural capacity in this area because of the lower rate of corrosion, and therefore further corrosion may be acceptable below Low Water. In this case, it may not be necessary to protect the latter, but if it is then a cathodic protection system should be considered. It should be noted that cathodic protection does not protect the steel above the water level.
12.4.4
Where MIC is found, one of the following approaches is usually appropriate: 1)
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If the attack is limited or the required remaining design life is short, say 5 to 10 years, then the affected areas could be plated up only. This will require ongoing monitoring and maintenance but overall may give the lowest whole life cost.
DMG 29 Corrosion Inspection of Steel Piled Maritime Structures
2)
Where the attack is severe and/or the structure is required to be serviceable for the medium term, then after plating up, a cathodic protection (CP) system could be installed, possibly combined with a protective coating above Low Water. A properly designed and maintained cathodic protection is considered the most reliable form of defence against MIC.
3)
A protective coating could be applied. For a paint system, this will require that the water level is lowered locally, see Section 12.5.
The actual repairs will need careful consideration by a suitably qualified and experienced engineer.
Bearing Piles 12.4.5
The same considerations apply for bearing piles as for sheet pile walls, except as noted below.
12.4.6
As discussed in Section 2.4, it may not be necessary to replace steel lost from corrosion. This is because: a) b)
12.4.7
the design may have allowed for a certain amount of corrosion; the critical design consideration for a driven pile may be the stresses during installation. Once installed a certain loss of section may be acceptable.
As an alternative to painting, the piles can be protected by wrapping.
12.5 CONSTRUCTION METHODS 12.5.1
Access to the steel is an important consideration in any remedial works, particularly to those areas below water. In some cases, e.g. dry docks and locks, it is possible to lower the water level to allow access. Design checks should be carried out to ensure that the structures have been designed to allow for this.
12.5.2
One technique for sheet pile walls that has been developed is the use of a limpet dam. This is a box open on one side and the top. The open side is placed against the sheet pile wall, and a seal is formed between the box and the wall. The water can then be pumped out from inside the box, allowing access in the dry to the local section of wall that would otherwise be underwater.
12.5.3
Where limpet dams are not appropriate and underwater work is necessary, then divers will need to be used.
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Section 13- References
1. BS 6349-1:2000, Maritime structures – Part 1: Code of practice for general criteria 2. SI 1997 No. 2776 - The Diving at Work Regulations 1997. 3. HSC “Approved Code of Practice – Commercial Diving Projects Inland/Inshore L104”, 1998. 4. US Navy Diving Manual (Air Diving),1996, Best Publishing Co., USA 5. British Steel Piling Handbook, 1997. 6. CIRIA Report 158, Sea Outfalls – Inspection and Diver Safety, 1996. 7. BS 7361:Pt1:1991, “Cathodic Protection Part 1. Code of Practice for Land and Marine Applications”. 8. BS 5493:1977 Code of Practice for Protective Coating of Iron and Steel Structures against Corrosion (obsolescent, partly replaced by next reference). 9. BS EN ISO 12944-2:1998, Paints and varnishes – Corrosion protection of steel structures by protective paint systems 10. PIANC “Inspection, Maintenance and Repair of Maritime Structures exposed to Material Degradation caused by Salt Water Environment”, Supplement to Bulletin No. 71 (1990). 11. The Association of Diving Contractors, “The Inshore Diving Supervisors Manual”, 2000. 12. The Association of Diving Contractors, “Code of Practice for the Safe Use of Electricity Underwater”, AODC035, Sept. 1985. 13. Svensk Standard, SS 055900:3rd edition:1988, (ISO 850-1:1988), “Preparation of Steel Substrates before Application of Paints and Related Products”. 14. “Professional Divers Handbook”, Submex, London, 1984. 15. The Institution of Structural Engineers, “Guide to Inspection of Underwater Structures”, October 2001.
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Appendix – Sample Inspection Reports
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