BP Amoco No Corrosion Project - Technical Bulletin
Rev. 1, February 1999
TOPSIDES BOLTING - CORROSION PROTECTION SUMMARY Corrosion of topsides bolting: • Threatens mechanical integrity • Increases maintenance costs (e.g. nut seizures) • Creates safety concerns (e.g. hot bolting operations, loss of integrity) BP Amoco’s North Sea experience over recent years has been of unsatisfactory performance of PTFE coated and electroplated low alloy steel bolting and that hot dipped spun galvanised bolting has provided better corrosion protection at reduced cost. This bulletin outlines the methods of corrosion protection and recommends the use of spun galvanising for low alloy steel bolting in the majority of topsides flanged connections.
IMPACT AND COSTS Safety • In the longer term, the poor corrosion protection offered by PTFE and electroplated bolting will impact plant integrity. • More immediately, personnel are subject to greater risk during normal and hot bolting operations due to the difficulties and additional time required to remove seized bolts.
Cost The cost impact of poor corrosion performance of coated/plated bolting arises in two areas:
Preservation Costs: Assets becoming aware of the impact of bolting corrosion have instigated programmes of retrospective bolt preservation using inhibited wax. (N.B. Wax preservatives cannot be used at temperatures >70°C). Typical costs of preservation programmes are:
Aset Miller Schiehallion Bruce Gyda
Bolting after five years exposure in a marine environment. Top: Electroplated Zinc Middle: Hot Dip Spun Galvanised Bottom: Sheradised
CostfRerpcivPan ~ £250,000 ~ £160,000 ~ £150,000 Cost not available (preservation + replacement with galvanised bolting)
Maintenance Costs: It is difficult to assess the overall cost impact on maintenance and repair times due to seized bolting. All Assets contacted (Gyda, Ula, Harding, Miller, Andrew, Magnus, Thistle) reported that there was a regular need to remove seized bolts, which is expensive in time and causes delay, especially if specialist tools need to be brought in. Repair and maintenance work takes longer, sometimes causing considerable additional downtime and production losses. Assets report repairs which needed only minutes to carry out extending to hours by the need to remove corroded bolting. This Bulletin has been sponsored by BP Amoco's "No Corrosion" Project to assist sharing of operations experience on corrosion and materials issues to New Projects, Partners and Business Units.
BACKGROUND The normal low alloy steel bolting material used on topsides process and pipeline connections is either ASTM A193 Gr B7/B7M & or A320 Gr L7/L7M. They are low alloy, quenched and tempered Cr-Mo steels. The corrosion resistance of these materials is low and they require corrosion protection in an offshore environment. Although corrosion resistant alloys (e.g. stainless steel, duplex stainless steel, precipitation hardened cupronickel) are used in small quantities for topsides plant bolting, cost precludes their widespread use. Low alloy steel bolting with some form of corrosion protection make up the majority of topsides fasteners. Bolts can be supplied with a variety of surface treatments. The common options are:
Type
Standr
MinmuThckes
Zinc and Cadmium electroplate
BS 1706
8µm
PTFE coating + phosphate
–
30µm PTFE
PTFE coating + electroplate (Zn or Cd)
–
30µm PTFE / 8µm Zn/Cd
Sheradising (Barrelled in hot zinc dust)
BS4921 Class 1 / 2
30µm/15µm
Spun galvanised (Dipped in molten zinc)
BS 729
43µm
OPERATIONAL EXPERIENCE Electroplated Zinc and Cadmium Bolting • Electroplated cadmium and zinc only protects bolting during storage prior to installation. In a marine environment the thin sacrificial coating is depleted within weeks. • The Miller platform suffered noticeable rust staining of corrosion resistant alloy process plant from electroplated zinc bolting, leading to concern over long term bolting integrity. • The Ula platform experienced corrosion of cadmium plated bolting within 1-2 years of operation. • Other operators have experienced similar poor corrosion performance with electroplated cadmium bolting. • Concern over the toxicity of cadmium has led some operators to ban its use.
PTFE Coated Bolting • PTFE over phosphate has not been effective in the offshore environment. Damage to the PTFE during make-up exposes the phosphate layer which provides little protection. However, this system has worked well in the Middle East. • It was anticipated that PTFE over zinc or cadmium would offer significantly better performance than electroplated coatings. Experience on Gyda has been that PTFE coated electroplated bolts were corroded by the time the plant was commissioned. In the longer term, nuts have seized and have had to be removed with a nut cracker or cut off. • This poor experience has been confirmed by the Forties, Andrew, ETAP, Bruce and Schiehallion Assets, where PTFE coated bolts started corroding within weeks.
Hot Dip Spun Galvanised Bolting • The corrosion performance of hot dip spun galvanised bolting has been much better than the plain electroplated or PTFE coated equivalents. • BP Amoco Norway has been using galvanised bolts for almost ten years on carbon steel, 316 stainless steel, duplex stainless steel and cupro-nickel topsides systems on Ula. PTFE bolting is replaced with galvanised bolting during any maintenance operations on Gyda. • Gyda has also used galvanised steel bolting on hot produced water valves to replace stainless steel bolts which have failed by stress corrosion cracking, when packing boxes have leaked. • The Forties Unity platform has used hot dip spun galvanised bolting for more than six years. During recent construction work, a tie-in flange was hot bolted with no problems of seizures of nuts, which could have been reused. • Amerada Hess has used galvanised bolting since the mid-eighties with proven success.
QUESTIONS Why use hot dip spun galvanised bolting? Alternatives such as electroplated coatings and electroplated plus PTFE coatings have performed badly with failure occurring after only a few weeks on all of BP Amoco's new projects. The PTFE coating is inevitably damaged during make-up, corrosion prevention then relies on the electroplated coatings which are ~10µm thick. A galvanised coating is much thicker at >43µm. The life of a sacrificial coating is proportional to thickness. Once the PTFE coating is damaged, accelerated corrosion will occur at damaged areas, hence the rapid rates of failure experienced.
Is liquid metal embrittlement (LME) of stainless steel by zinc from galvanised bolting a concern? LME of austenitic and duplex stainless steels can occur in the simultaneous presence of molten zinc, high tensile stresses and a temperature >750°C. The HSE issued a technical note 53/1 after the Flixborough incident alerting industry to the risks. Over the years this problem has assumed a significance out of all proportion to the real risk. In the event of a fire the mechanical properties of stainless steels will have been greatly reduced before the critical temperature for LME, such that in a fire scenario it will not be the primary failure mechanism. As the risk of LME by zinc is considered to be very low, zinc coatings (electroplated and spun galvanised) are acceptable for use on bolting on stainless steel piping systems.
Are thread clearances an issue on galvanised bolting? Extra clearance must be provided on the threads to allow for the zinc coating thickness. For hot dip galvanised bolts and nuts, it is normal for standard bolts from stock to be galvanised; the nuts are galvanised as blanks and then tapped oversize by an amount related to the coating thickness and thread type (pitch). When assembled, the nut thread is protected by contact with the zinc coating on the bolt. Some galvanised bolting supplied to the Miller platform suffered a very slack fit between the nuts and bolts. While not inspiring confidence that the bolting was fit for purpose, detailed measurement showed they were within tolerance and tensile testing demonstrated that the shaft of the bolt failed rather than the nut pulling from the bolt. Testing did not cover the case of galvanised nuts with oversize threading being fitted to PTFE coated or electroplated bolts. Systems and training are necessary so that galvanised nuts are NOT fitted to PTFE coated or electroplated bolts.
Cross section through galvanised nut and bolt.
Have any problems been found when hydratightening galvanised bolting? The recent Forties Unity Riser project used galvanised bolting per the original project. Nut seizure problems were experienced during hydratightening of the large diameter 31/4" flange bolting. This was found to be due to the hydratight company using a hydratight collar that was only suitable for PTFE coated bolting. The use of an undersized collar was compounded by a small ridge of galvanising along the threads of the large diameter bolting due to the cooling of the bolting when the operator has to lift them into the barrel of the centrifuge. This should not be a problem but if the tensioning nut is found to be sticking there exists the real possibility of nut seizure. In this case one end of the studbolt where the tensioning nut is installed should have the thread cleaned by thread chasers.
Does a PTFE coating help friction control during torque or tension tightening of bolting? Problems have occurred because there has been an assumption that an anti-seize compound is not required on PTFE coated bolting. Torque tightening results in the coating being scrubbed off and friction control (essential for achieving correct bolt load) is lost. Corrosion protection is also lost. Tension tightening also damages the coating (but not to the extent of torque tightening) and can mean that removal by ‘de-tensioning’ is difficult or impossible.
Is there accelerated loss of zinc when galvanised bolts are used on CRA plant and could consumption of the zinc on the mating faces of bolting cause bolt relaxation? Operational experience of topsides bolting has not shown accelerated loss of zinc. The zinc that is lost is only from the exposed surfaces, there is no significant loss from the mating faces.
What is the life expectancy of a hot dip zinc coating? BS 14713:1997 gives the average corrosion rate of zinc in a high salinity atmospheric environment as 4-8µm/year. The minimum zinc coating thickness when galvanising to BS 729 is 43µm, but the actual galvanising thickness tends to be greater. BP Amoco Norway experience is that for nearly ten years galvanised bolting has provided satisfactory corrosion protection. Spun hot dip galvanised bolting of all sizes have been used on the Forties Unity platform for at least six years. During recent construction work, an existing tie-in flange on the manifold was hot bolted. There were no nut seizure problems on the galvanised bolting, which could have been re-used. Previous experience with electroplated and PTFE coated bolting required the use of a nut cracker to remove seized nuts.
How do the costs compare? The relative costs (August 1998) of a common size of studbolt are shown below. The hot dipped spun galvanised finish was two-thirds the cost of the PTFE coated equivalent.
Relative Costs of 5/8” x 4” ASTM A193 B7 Studbolts and Nuts
Type
Relative Cost
Plain Steel
1
Cadmium Electroplated / Zinc Electroplated
1.6
Phosphate and Oil
1.6
Hot Dip Spun Galvanised
2.3
Cadmium Electroplate + PTFE Coating
3.7
Phosphate + PTFE Coating
3.7
Does it take longer to supply galvanised bolting than PTFE coated bolting? BP Amoco’s current bolting supplier have in-house electroplating and PTFE coating facilities which enable them to provide a turnaround of 1-2 days for urgent deliveries. They supply spun galvanised bolting using galvanisers in the Midlands. Delivery time for urgent orders is extended to 5-6 days.
HOT DIP GALVANIZING The Process: The terms “spin galvanising” or “centrifuge galvanising” are used to describe the process for hot dipping threaded components and other small parts. The procedure is, after chemical cleaning, to immerse them into molten zinc (normally @ ~450°C) in a perforated basket. After the coating has formed, the basket is centrifuged at high speed so that the spinning action throws off the surplus zinc and ensures a clean profile. The Coating: When nuts and bolts are immersed in the galvanising bath, a series of zinc rich alloy layers are formed with a metallurgical bond to the steel. As the adjacent microsection shows, there is no transition between the steel and the zinc, but a gradual transition through a series of alloys which provide the metallurgical bond. These alloys are harder than a mild steel and are normally covered by an outer layer of comparatively soft zinc. This structure gives the coating good resistance to rough treatment. How does it protect steel? Zinc protects steel by a process known as “sacrificial protection”, whereby the zinc corrodes preferentially protecting the steel. If the coating is damaged exposing a small area of steel, the zinc and the steel together with the moisture in the atmosphere form an electrolytic cell. The zinc (anode) corrodes and the cathode (steel) is protected. The corrosion products from the zinc are deposited on the steel re-sealing it from the atmosphere and stifling the corrosion reaction. Scars up to 5mm wide will be protected.
Zinc Coating. Galvanic cell protects steel and corrosion products precipitate on steel coating to protect it.
Paint Coating. Rust creeps under paint film, which is lifted. Corrosion continues.
For more information contact: Paul Badelek (Corrosion Consultant) BP Amoco Exploration, Farburn Industrial Estate, Dyce, Aberdeen AB21 7PB Tel: 01224 834071
E-mail:
[email protected]
01129468, February 1999
Return to document
☞
