imcad044.pdf

AB Guidelines for

Isolation and Intervention: Diver Access to Subsea Systems

International Marine Contractors Association

www.imca-int.com

IMCA D 044 October 2009

AB

The International Marine Contractors Association (IMCA) is the international trade association representing offshore, marine and underwater engineering companies. IMCA promotes improvements in quality, health, safety, environmental and technical standards through the publication of information notes, codes of practice and by other appropriate means. Members are self-regulating through the adoption of IMCA guidelines as appropriate. They commit to act as responsible members by following relevant guidelines and being willing to be audited against compliance with them by their clients. There are two core activities that relate to all members:  Competence & Training  Safety, Environment & Legislation The Association is organised through four distinct divisions, each covering a specific area of members’ interests: Diving, Marine, Offshore Survey, Remote Systems & ROV. There are also five regional sections which facilitate work on issues affecting members in their local geographic area – Asia-Pacific, Central & South America, Europe & Africa, Middle East & India and North America.

IMCA D 044 This guidance has been prepared for IMCA under the direction of IMCA’s Diving Division Management Committee based on material provided by Torquil M Crichton and other co-authors of Technip UK Limited.

www.imca-int.com/diving

The information contained herein is given for guidance only and endeavours to reflect best industry practice. For the avoidance of doubt no legal liability shall attach to any guidance and/or recommendation and/or statement herein contained. © 2009 IMCA – International Marine Contractors Association

Guidelines for Isolation and Intervention: Diver Access to Subsea Systems IMCA D 044 – October 2009

1 

Introduction ........................................................................................................... 1 

2 

Glossary .................................................................................................................. 2 

3 

Principles of Isolation ............................................................................................ 5 

4 

5 

3.1 

Principles of Isolation ........................................................................................................................................ 5 

3.2 

System Isolations................................................................................................................................................ 5  3.2.1  Liquid and Gas Equipment ................................................................................................................... 5  3.2.2  Electrical Equipment ............................................................................................................................. 6  3.2.3  Optical Equipment ................................................................................................................................ 6  3.2.4  Hydraulic Equipment ............................................................................................................................ 6 

3.3 

Specific Risk Assessment .................................................................................................................................. 7 

3.4 

Isolation Precedence ......................................................................................................................................... 7 

Flowline/Manifold/Tree and Wellhead Systems ................................................. 8  4.1 

Isolation................................................................................................................................................................ 8  4.1.1  Types of Flowline/Manifold/Tree and Wellhead Isolations .......................................................... 8  4.1.2  Considerations for Flowline/Manifold/Tree and Wellhead Isolations...................................... 14  4.1.3  Testing Flowline/Manifold/Tree and Wellhead Isolations ........................................................... 16  4.1.4  Integrity of Flowline/Manifold/Tree and Wellhead Isolations .................................................... 22 

4.2 

Intervention....................................................................................................................................................... 24  4.2.1  Types of Intervention ......................................................................................................................... 24 

4.3 

Installation of Subsea Equipment .................................................................................................................. 27  4.3.1  General.................................................................................................................................................. 27 

Subsea Control and Umbilical Systems ............................................................ 29  5.1 

Isolation.............................................................................................................................................................. 29  5.1.1  Types of Subsea Control and Umbilical System Isolations......................................................... 29  5.1.2  Electrical/Communication/Signal Isolations ................................................................................... 31  5.1.3  Optical Isolation .................................................................................................................................. 39  5.1.4  Hydraulic and Instrumentation Isolations ...................................................................................... 41  5.1.5  Mechanical Isolations .......................................................................................................................... 59 

5.2 

Intervention....................................................................................................................................................... 60  5.2.1  Types of Subsea Control and Umbilical System Interventions .................................................. 60  5.2.2  Electrical and Communication/Signal System Interventions....................................................... 61  5.2.3  Optical System Interventions ........................................................................................................... 70  5.2.4  Hydraulic and Instrumentation System Interventions ................................................................. 71  5.2.5  Mechanical System Interventions ..................................................................................................... 83 

5.3 

Installation and Retrieval of Subsea Components..................................................................................... 84  5.3.1  General.................................................................................................................................................. 84  5.3.2  Subsea Control and Umbilical System Components – Installation and Retrieval ................. 85 

6 

Isolation Flowchart and Isolations Summary Table ........................................ 90  6.1 

Isolation Flowchart for Subsea System........................................................................................................ 90 

6.2 

Isolations Summary Table – Subsea Control and Umbilical Systems.................................................... 91 

7 

Typical System Drawings.................................................................................... 92 

8 

References ............................................................................................................ 97  8.1 

Reference Documentation ............................................................................................................................. 97  8.1.1  IMCA Guidance ................................................................................................................................... 97  8.1.2  Other Documents .............................................................................................................................. 97 

8.2 

Applicable Standard Graphical Symbols ...................................................................................................... 98 

8.3 

Laser Classifications Summary ...................................................................................................................... 99 

Figures Figure 1 – Double block and bleed arrangements at intended break .............................................................................. 9  Figure 2 – Small bore isolation valve configurations .......................................................................................................... 11  Figure 3 – Typical test downline configuration – DSV to subsea worksite .................................................................. 17  Figure 4 – Positive test method ............................................................................................................................................. 19  Figure 5 – Negative or in-flow leak off test method.......................................................................................................... 20  Figure 6 – Volume calculation test method ......................................................................................................................... 21  Figure 7 – Integrity test graph – acceptable......................................................................................................................... 24  Figure 8 – Integrity test graph – unacceptable .................................................................................................................... 24  Figure 9 – Typical valve arrangement for post-installation flooding ............................................................................... 28  Figure 10 – Minimum valves on typical pig launcher/receiver.......................................................................................... 28  Figure 11 – Typical subsea control and umbilical system isolations ............................................................................... 30  Figure 12 – Double block and bleed (DBB) valve manifold for instrument device ..................................................... 45  Figure 13 – Block-block and bleed (BBB) valve manifold for instrument devices ....................................................... 46  Figure 14 – Block and bleed valves with self-sealing diver coupling ............................................................................... 48  Figure 15 – Isolation testing double block and bleed plus self-sealing coupling ........................................................... 53  Figure 16 – Isolation testing single block and bleed plus self-sealing coupling ............................................................. 55  Figure 17 – Isolation flowchart for subsea system ............................................................................................................. 90  Figure 18 – Fundamental considerations .............................................................................................................................. 92  Figure 19 – Typical manifold and flowline P&ID ................................................................................................................. 93  Figure 20 – Typical subsea tree P&ID ................................................................................................................................... 94  Figure 21 – Typical subsea control and umbilical system schematic .............................................................................. 95  Figure 22 – Typical DSV to subsea worksite test downline ............................................................................................. 96  Figure 23 – Standard graphical symbols................................................................................................................................ 98  Tables Table 1 – Potential energy sources in subsea workscopes ................................................................................................ 5  Table 2 – Isolation and intervention considerations .......................................................................................................... 26  Table 3 – Subsea electrical power categories ..................................................................................................................... 62  Table 4 – Subsea components with optical elements ........................................................................................................ 70  Table 5 – Hydraulic system connection categories for subsea components ................................................................ 72  Table 6 – Subsea instrumentation types and categories ................................................................................................... 79  Table 7 – Subsea control and umbilical system components – installation and retrieval .......................................... 89  Table 8 – Isolations summarised ............................................................................................................................................ 91  Table 9 – Laser classifications ................................................................................................................................................. 99 

1

Introduction

This guidance document is primarily aimed at project managers, project engineers, offshore construction managers, diving supervisors and safety personnel, all of whom have a responsibility for developing safe schemes of isolation and intervention for divers accessing subsea systems. Additionally, engineering personnel involved with the design of such systems should also use this document to ensure that all new (or being modified) subsea systems incorporate adequate isolation facilities. This document sets out what is considered to be good practice for ensuring a safe degree of isolation is established prior to conducting diver intrusive works on any energy-conveying system in which pressure differentials, electrical power or laser power may exist at levels which – on loss of containment – would be harmful to personnel or cause damage to the environment or equipment. The guidelines are applicable for use when preparing workscopes, procedures, reviews and risk assessments for any diver related work. These energy sources (pressurised liquid, pressurised gas, electricity and laser light) may be found as a conveyed product or service utility within either or both of the following two major subsea equipment categories: 

Flowline/manifold/tree and wellhead systems (containing any of – oil, gas, condensate, water injection, chemical injection – either separately or in various combinations);



Subsea control and umbilical systems (containing any of – hydraulic fluid, high and low voltage powered equipment, communication signals, instrumentation signals, optical data signals, power transmission and distribution, chemicals, gas – each within dedicated sub-systems).

The general principles of isolation philosophy and isolation practice, as applicable to such systems, are given in section 2, whilst detailed guidelines regarding isolation and intervention are given in sections 4 and 5 respectively. Adequate planning is essential for an effective isolation, not only to ensure awareness of the task requirements and ready availability of all materials, tools, etc., before work begins, but also to identify and assess the isolation options and their associated hazards and effects. Safe standards of isolation are primarily determined by the size and nature of the potential hazards associated with the equipment to be worked on. Other fundamental factors which should be addressed are: i)

an understanding of all the parameters associated with the energy source being isolated;

ii)

status, condition and accessibility of available isolation hardware;

iii) identification of adjacent live systems which may influence or be affected by the isolations; and iv) the anticipated duration of the actual intervention work. Occasionally, the requirement may arise to utilise divers to conduct work on items of hardware which have been specifically designed for ROV installation, operation or recovery. In such instances, the isolation and intervention guidelines set out in this document should still be applicable. Whilst it is not possible for these guidelines to account for the detailed and specific complexities of each and every subsea system encountered, the principles set out in this guidance should be applicable.

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2

Glossary

AAV

Annulus access valve

AC

Alternating current

ACPI

Annulus choke position indicator

ACV

Annulus choke valve

AMV

Annulus master valve

APT

Annulus pressure transducer

AWV

Annulus wing valve

BBB

Block-block and bleed (valve)

BBV

Block-block-and-vent (valve)

Bleed valve

A valve for draining liquids, or venting gas, from a pressurised system

Blind flange

A component for closing an open end of pipework which is suitably rated to maintain the pressure rating of the pipe

Block valve

A valve which provides a tight shut-off isolation purpose

Charged

The item has acquired a charge either because it is live or because it has become charged by other means such as by static or induction charging, or has retained or regained a charge due to capacitance effects even though it may be disconnected from the rest of the system

CIV

Chemical injection valve

DB

Double block (valve)

DBB

Double block and bleed (valve)

DC

Direct current

DCS

Distributed control system

DCV

Directional control valve

Dead

Not electrically ‘live’ or ‘charged’

Design working pressure

Maximum working pressure at which a hose or tube is rated for continuous operation

DHPT

Down-hole pressure and temperature (sensor)

DHSV

Down-hole safety valve

Disconnected

Describes equipment (or part of an electrical system) which is not connected to any source of electrical energy

Double block and bleed

An isolation method consisting of an arrangement of two block valves with a bleed valve located in between

Double seated valve

A valve which has two separate pressure seals within a single valve body. It is designed to hold pressure from either direction as opposed to a single seated valve

DSV

Diving support vessel

DWP

Design working pressure

EDB

Electrical distribution box

ELCB

Earth leakage circuit breaker

Electrical equipment

Includes anything used, intended to be used or installed for use, to generate, provide, transmit, transform, rectify, convert, conduct, distribute, control, store, measure or use electrical energy

EPU

Electrical power unit

ESD

Emergency shutdown

2

IMCA D 044

Final isolation

Subsea isolation, local to the worksite. This isolation should consist of a secure physical separation. It is a readily understood way in which prevention of the uncontrolled release of energy can be confirmed to diving personnel tasked with carrying out the work

FOP

Fibre-optic processor

HAZOP

Hazard and operability (study)

High voltage

Within this document used to refer to any voltage over 1000V and up to 30KV

HIRA

Hazard identification and risk assessment

HPU

Hydraulic power unit

IEC

International Electrotechnical Commission

ISO

International Organization for Standardization

Isolated

Indicates equipment (or part of an electrical system) which is disconnected and separated by a safe distance (the isolating gap) from all sources of electrical energy in such a way that the disconnection is secure, i.e. it cannot be re-energised accidentally or inadvertently

Isolation

The separation of plant and equipment from every source of energy (pressure, electrical, mechanical and optical), in such a manner that the separation is secure

Laser

Light amplification by stimulated emission of radiation

Let go current

The upper limit of current at which the muscles of the forearm can be used

LIM

Line insulation monitor

Live

Equipment in question is at a voltage by being connected to a source of electricity. This implies that, unless otherwise stated, the live parts are exposed so that they can be touched either directly or indirectly by means of some conducting object and that they are live at a possibly hazardous potential

Low voltage

Within this document used to refer to any voltage up to 50V

MAOP

Maximum allowable operating pressure

Master control station (MCS)

Generic name for the topside computer system dedicated to control and monitoring of the entire subsea control and umbilical system

Maximum permissible exposure

Level of laser radiation to which, under normal circumstances, persons may be exposed without suffering adverse effects (see BS EN 60825-1: 1994)

MCS

Master control station

MEG

Monoethylene glycol

Medium voltage

Within this document used to refer to any voltage between 51V and 1000V

MPE

Maximum permissible exposure

Nominal (value)

Minimal value in comparison with the normal expected value

Normally open

A device which, when closed, will perform the function of a closed isolation

Obturator

An internal part of a valve such as a ball, gate, disc, plug or clapper which is positioned in the flow stream such that the flow may be either blocked or permitted to pass

OEM

Original equipment manufacturer

P&ID

Process and instrumentation diagram

PCPI

Production choke position indicator

PCV

Production choke valve

Perception current

The lower limit of current which can be felt

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Pig

A device that can be driven through a pipeline by means of fluid pressure for purposes such as cleaning, dewatering, inspecting, measuring, etc.

PIG

Pipeline internal gauge

PLMV

Production lower master valve

PPT

Production pressure transducer

PPTT

Production pressure and temperature transducer

Preliminary isolation

Initial isolation. Set as precursor to facilitate the obtaining of a further final isolation local to the worksite (where by design it is possible to do so). Generally it is a physical separation or (exceptionally) a software inhibit

PSL

Product specification level

PUMV

Production upper master valve

PWV

Production wing valve

Rated working pressure

The maximum internal pressure which the equipment is designed to contain and/or control

RCD

Residual current device

ROT

Remotely operated tool

ROV

Remotely operated vehicle

RWP

Rated working pressure

Safe body current

The maximum current which can be allowed to flow through the diver’s body safely (explained in detail in IMCA D 045/R 015 – see section 8.1). It is not the current flowing in the electrical equipment

SAM

Subsea accumulator module

SBB

Single-block and bleed (valve)

SCADA

Supervisory control and data acquisition

SCM

Subsea control module

SCMMB

Subsea control module mounting base

SCSSSV

Surface controlled sub-surface safety valve

SEM

Subsea electronic module

SIL

Safety integrity levels

Spade

A solid plate for insertion in pipework to secure an isolation

SSIV

Subsea safety isolation valve

SSSV

Sub-surface safety valve

SST

Spheri-seal test

SUDA

Subsea umbilical distribution assembly

SUT

Subsea umbilical termination

SUTA

Subsea umbilical termination assembly

TCT

Tree-cap test

Tested

Integrity has been proven and/or can be monitored

TUTU

Topside umbilical termination unit

Ultra-high voltage

Within this document used to refer to any voltage greater than 30KV

UPS

Uninterruptible power supply

Vent valve

A valve for draining liquids, or venting gas, from a pressurised system

XOV

Cross-over valve

XT

Christmas tree

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IMCA D 044

3 3.1

Principles of Isolation Principles of Isolation The general principle of isolation is, where practicable, the removal of hazards or sources of energy from within the system to be worked upon, through the provision of an appropriate physical separation which can be confirmed to provide adequate disconnection of that system from any potential source of further energy. The hyperbaric nature of subsea work means that divers are regularly exposed to the particular hazard of negative pressure systems during activities associated with system equalisation, as well as the normal potential hazards associated with the positive release of pressure from a system. Even a very small aperture with an associated pressure profile can cause severe injury should a diver come into contact with it. Thus when working on any subsea system containing liquid or gas under positive or negative pressure, there should be no pressure differential, relevant to the seabed ambient, trapped within a space or void. Similarly, divers may become exposed to live electrical or optical connections containing electrical or laser energy at potentially hazardous levels which may also cause injury without warning. Thus, for any subsea system conveying electrical energy, or laser energy, there should be no exposed live electrical connections, or optical contacts located subsea. In many cases, diving operations cannot commence until the topside installation has firstly applied primary isolation(s) to the main energy source(s), following which, manual and tangible final isolations will then need to be applied at the subsea worksite location. All isolations need to be proven, to demonstrate to diving personnel that protection from all potential energy sources has been established. Potential energy sources which may be associated with subsea isolations are: Source

Description

Reservoir

Primary source of high pressure hydrocarbons

Process pipework

Large capacity pipework containing hydrocarbons

Main oil line pumps

High pressure and high volume hydrocarbons

Gas compressors

High pressure and high volume gas compositions

Water injection pumps

High pressure and high volume treated water

Chemical injection

High pressure, low volume chemical solutions

Hydraulic control systems

High pressure, low volume accumulated systems

Electrical power supply systems

High voltage/current electrical energy

Electrical control systems

High voltage/current electrical energy

Fibre-optic data systems

High intensity (laser) light energy

Instrumentation pipework

Small capacity pipework containing fluid/gas

Table 1 – Potential energy sources in subsea workscopes

3.2

System Isolations 3.2.1

Liquid and Gas Equipment For subsea liquid and gas conveying equipment, the general principle is that a minimum of two independent and tested isolations should be established between personnel engaged in any task where the presence of potential hazard from a positive or negative pressure source exists. Where practicable to do so, at least one of the isolation tests should take the form of a positive test, in the direction of flow, or alternatively, a negative test by reducing the pressure

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5

downstream of the isolation. Exceptionally, it may be appropriate to test both isolations against the direction of flow. 3.2.2

Electrical Equipment For subsea electrical equipment, the general principle (assuming that the voltage is higher than is safe for the diver to work beside) is that the main power circuit of the electrical equipment, together with any associated auxiliary circuits which constitute a hazard, should be isolated and any stored energy in the electrical circuits should be discharged. Isolation can be achieved by disconnecting and separating the electrical equipment from every source of electrical energy in such a manner that this disconnection and separation is confirmed and secure, i.e. it cannot be re-energised accidentally or inadvertently. A minimum of two independent and certified isolations should be established between personnel engaged in any task where the presence of a potential hazard from electrical energy at potentially hazardous levels exists. Normally at least one of these isolations should be located on the topside host installation. However, it may be possible to set isolations local to the subsea worksite by physical disconnection of an inductive coupler (this does not apply to conductive connectors).

3.2.3

Optical Equipment For subsea optical equipment, the general principle is that the main power circuit of the fibre optic equipment, together with any associated auxiliary circuits which constitute a hazard, should be isolated. Isolation can be achieved by disconnecting and separating the fibre-optic equipment from every source of electrical power (topside) and final optical interface (subsea) in such a manner that this disconnection and separation is confirmed and secure, i.e. it cannot be reenergised accidentally or inadvertently. A minimum of two independent and certified isolations should be established between personnel engaged in any task where the presence of a potential hazard from laser light energy at potentially hazardous levels exists. Normally at least one of these isolations should be located on the topside installation. If, however, the laser sources are Class 1, Class 1M, Class 2 or Class 2M laser sources, then isolation is not a requirement.

3.2.4

Hydraulic Equipment For subsea hydraulic equipment, the general principle is that the main power circuit of the hydraulic equipment, together with any associated auxiliary circuits which constitute a hazard, should be isolated and any stored energy in the hydraulic circuits vented. Isolation can be achieved by disconnecting and separating the hydraulic equipment from every source of hydraulic power in such a manner that the disconnection and separation is confirmed and secure. A minimum of two independent and tested isolations should be established between personnel engaged in any task where the presence of a potential hazard from hydraulic pressure at potentially hazardous levels exists. Normally at least one of these isolations should be located on the topside host installation. However, it may be possible to set isolations local to the subsea worksite by either physical disconnection of stab plate halves or by operating manual isolation and vent valves (the vent port needs to be fitted with a diversafe pressure relief cap) in combination with the physical disconnection of self-sealing hydraulic couplers. Note: Hydraulic systems operating sub-surface or down-hole safety valves may provide a conduit for well bore fluids to return to the surface and these may be present in these systems. This possible hazard should be considered in any assessment of the isolation requirements for such systems.

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IMCA D 044

3.3

Specific Risk Assessment For the isolation of the subsea equipment described above (liquid and gas conveying equipment, electrical equipment and optical equipment), if, due to limitations in actual subsea architecture, two tested and independent isolations cannot be achieved, then it may be possible to identify an alternative method for undertaking the work, without compromising the safety of the operation. Any such alternative method needs to be subjected to a task specific risk assessment by competent personnel with appropriate company review and approval (see Figure 17).

3.4

Isolation Precedence In certain projects it is possible that isolation techniques other than those set out in this document may be suggested. As an example, there may be a client or main-contractor isolation philosophy document containing detailed procedures for isolation. Any such alternative methods should be compared with the techniques contained within this document and the more stringent requirement used. In all cases the need for double isolation remains a fundamental principle.

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4 4.1

Flowline/Manifold/Tree and Wellhead Systems Isolation 4.1.1

Types of Flowline/Manifold/Tree and Wellhead Isolations A minimum of two independent and tested isolations should be established between personnel engaged in any task where the presence of a potential hazard from a pressure source or vacuum exists. The physical isolation of pressurised systems is generally achieved by using various combinations of valves, spades or blank flanges. Isolations for subsea flowline/manifold/tree and wellhead systems are primarily provided in standard form by valves located between the diver intervention workface and the potential energy source. There are also many instances whereby pre-installed and tested blind flanges may provide isolation. The type, configuration, location and testing of such isolations are considered in further detail throughout this section. Consideration is also given to certain alternative and specialised isolation techniques, which may not be appropriate for standard applications but, depending on system architecture, may require to be utilised. The following isolation terminology is applicable both to bulk subsea systems (i.e. flowlines, manifolds, trees and wellheads) and the associated smaller, but more complex, subsea control and umbilical systems (see section 5). The process of achieving an appropriate overall isolation scheme for subsea intervention work invariably has implications for both systems, therefore there needs to be a common understanding of the basic principles involved. Preliminary  Initial isolation. Set as a precursor to facilitate the obtaining of a further final isolation local to the worksite. Generally this is a physical separation or (exceptionally) a software inhibit. Final  Subsea isolation, local to the worksite. This isolation consists of a secure physical separation. It is the tangible mechanism by which prevention of the uncontrolled release of energy is confirmed to those intending to carry out the work. 4.1.1.1 Standard Isolation Methods 4.1.1.1.1

Valves Valves provide the simplest conventional form of preliminary and/or final in-line isolation device across the dimension range, from large diameter trunk pipelines through to small-bore injection tubing. When utilised in subsea systems they are defined within two specific categories: either manually operated (i.e. by diver or ROV) or remotely actuated (i.e. by subsea control system). Certain designs of remotely actuated valves may also be operated by purposedesigned diver/ROV override mechanisms. The optimised isolation configuration for accessing a subsea bulk system for intervention purposes should consist of two sets of main double block valves, each with a bleed valve located between them. This ‘bleed’ facility itself should consist of an arrangement of small-bore valves in a double block and bleed configuration, as they connect directly into the bulk system. Such valving should be in place on both sides of any intended break (see Figure 1).

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IMCA D 044

BLEED PORT

BLEED PORT TEST DOWNLINE CONNECTION POINT

TEST DOWNLINE CONNECTION POINT

LOCATION OF INTENDED BREAK

PRESSURISED BULK SYSTEM

PRESSURISED BULK SYSTEM VALVE 1A CLOSED

VALVE 2A OPEN

VALVE 2B OPEN

VALVE 1B CLOSED

Figure 1 – Double block and bleed arrangements at intended break Wherever practicable, it is prudent to utilise at least one manually operated valve for one of the isolations. When two remotely actuated valves require to be utilised to establish the bulk system isolation scheme, the supply lines to both should be locally isolated at the worksite. In the absence of any means to implement such isolations then the additional potential hazards arising need to be assessed with a view to either proposing an alternative isolation scheme or identifying an increased isolation envelope. Valves should be capable of providing a reliable and positive shut-off seal for the isolation of a hazardous substance and/or energy source. They should be suitable for the expected service and associated potentially hazardous conditions to be encountered. There are the two fundamental valve properties to consider: i)

type; and

ii)

seat and seal material.

Standard valve types will conventionally be either ‘gate’, ‘plug’, ‘globe’ or ‘ball’. Seat and seal material will be either metal-to-metal or metal-to-elastomeric/ polymeric. Full details of valve specifications and other applicable bulk system parameters should be obtained at an early stage in the onshore phase of the project. This should help avoid unnecessary delays during offshore integrity tests for a given isolation scheme. Small-bore valves which form the directly-connected vent/bleed outlet in any subsea pipework system should always be arranged in a block-vent-block (‘double block and bleed’) valve configuration. The block valves provide two inline isolations, which should be kept closed during the initial diver intervention activities (e.g. when connecting a dive support vessel (DSV) test downline to the main outlet port on the same valve assembly). The bleed valve provides a local safety vent through the bleed port.

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The alternative, dual-in-line block only (‘double block’) valve, i.e. without any incorporated vent facility, should be considered the minimum form of smallbore valve isolation. The protective cap fitted to the outlet port on small-bore double block or double block and bleed valves should be the ‘diver-safe’ integral-vent type (i.e. pressure vents prior to full disengagement). Such devices are designed to ensure any initial differential pressure equalisation occurring within, or through, the valve assembly (when preparing the cap for removal) can be vented in a safe manner, without the potential hazard of gross loss of containment or of the cap coming off in an uncontrolled manner. The use of any other type of cap (or plug) which does not incorporate a secondary pressure-relief mechanism is not considered suitable for diver intervention work. The utilisation of a single-block valve only, in combination with either a nonventing cap/plug or an integral-vent type cap, fitted to the valve outlet port, is not considered appropriate to meet the principles for safe diver intervention given in these guidelines. The suitability, or otherwise, for the various configurations of small-bore isolation valves and their caps/plugs is summarised in Figure 2. The outlet port on small-bore valve assemblies should also be of suitable design to guarantee a fixed pressure-retaining connection when the DSV test downline is attached (and subsequently pressurised) to check for flow, either into or out of the cavity. This ensures a safe and secure facility is maintained for the equalisation of any entrapped pressure throughout the work.

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IMCA D 044

P R E S S U R E

P R E S S U R E

UNACCEPTABLE PRODUCT FLOW

UNACCEPTABLE PRODUCT FLOW

P R E S S U R E

P R E S S U R E

ACCEPTABLE (but not recommended)

PRODUCT FLOW

ACCEPTABLE - BASIC PRODUCT FLOW

P R E S S U R E

P R E S S U R E

PRODUCT FLOW

ACCEPTABLE (but not recommended)

PRODUCT FLOW

ACCEPTABLE - OPTIMISED

LEGEND

Figure 2 – Small bore isolation valve configurations 4.1.1.1.2

Blind Flanges The ends of pipelines, headers and spools are prepared with precision-machined flange faces such that they can be inter-connected to form a pressure-containing liquid/gas transportation system. These flanges are specified to at least the same design and test standards as the item to which they are attached. The flange faces require to be maintained in their factory-finished condition throughout the load-out and offshore installation activities and for the duration of the field life. Protection for the sealing surfaces is therefore provided in the form of a matching circular blanking cover/plate or blind flange. These provide physical protection and, where required, comply with the system installation and commissioning specifications (i.e. free-flooding or pressure-tight), in combination with the intended field development programme (i.e. immediate hook-up, or future tie-in).

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Blind flanges are therefore specified and prepared with either a single, or a dualpurpose role, as follows: Single Duty – To provide physical protection only, for the sealing surfaces of the flange face. This is usually associated with a short-term requirement, the hookup of adjacent items following soon after deployment. The flange face may be protected with some simple covering arrangement or a proprietary blind flange which should not be fully tightened into place (e.g. by inserting spacer washers). With this type of protection arrangement, the flange interface is designed to free-flood and should therefore present no differentialpressure equalisation hazards for diver intervention. In certain circumstances it may be a requirement to tighten the blind flange in place on surface, prior to deployment to the seabed (as it may be intended to allow the system to free flood in some other manner). Therefore the blind flange should be prepared with a welded outlet port to which is fitted, as a minimum, a small-bore double block valve, complete with either a ‘T’-piece or a diffuser. This is to ensure that there is no possibility of diver finger/hand entrapment during differential-pressure equalisation at depth. Dual Purpose – To provide physical protection for the sealing surfaces of the flange face, plus the capability to maintain a pressure-containing isolation equal to the system design. The flange face will normally be fitted with a proprietary blind flange and ringgasket, and set in place with the full complement of tensioned studs. This level of preparation enables full pressure-testing against the blind-flange during pipeline commissioning. It also provides the capability, if required, of leaving the blind flange secured in place as a proven isolation, for some future tie-in. With this type of flange protection, there exists the potential hazard of a trapped inventory of positive or negative pressure remaining in the cavity between the blind flange and the next (closed) valve in the bulk system. Therefore the blind flange should be prepared with a welded outlet port to which is preinstalled, as a minimum, a small-bore double block valve, complete with either a ‘T’-piece, or a diffuser. As an alternative, a small-bore double block and bleed valve arrangement could be preinstalled. In the absence of any means to safely depressurise the bulk system prior to removal of the blind flange, then the additional potential hazards arising need to be assessed with a view to identifying an increased isolation envelope. 4.1.1.2 Alternative Isolation Methods Certain other types of special or novel isolation techniques are available. Depending on specific design, these may or may not align with the recommended ‘double block and bleed’ isolation principle. Their utilisation should therefore be considered through detail engineering review processes. The various techniques available are outlined below: 4.1.1.2.1

Double-Seal within Valve Body Double-wedge gate, parallel-expanding gate or double-seal (double-piston effect) ball valves, which provide a double-seal in a single valve body with a bleed in between, may be utilised if necessary. There are, however, certain limitations and restrictions to this type of valve which should be considered: i)

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In some applications both isolations cannot be easily tested;

IMCA D 044

ii)

The status of the double isolation depends upon the immobilisation of a single valve operating stem therefore there should be no possibility of it being operated during the intervention work;

iii) The valve body outlet bleed port directly accesses the inventory of the valve cavity. Also, this outlet is only protected from the potential energy in the bulk inventories (i.e. on either side of the valve unit) by the single isolations provided by each of the obturators within the valve assembly. The outlet should be fitted with a permanently attached double block access/vent valve arrangement as a minimum (a double block and bleed is recommended). The ‘double seal’ valve design should only be used in preference to the conventional bulk system isolation arrangement (i.e. double block and bleed) after the increased hazards have been reviewed through the appropriate risk assessment process. 4.1.1.2.2

Pipeline Plugs The utilisation of a bespoke in-pipe plug (or combination of plugs) to form a proven subsea isolation scheme is considered to be an appropriate form of novel isolation technique, subject to the following considerations. Redundancy and independence should exist within, or between, the plugs such that failure of a part of the sealing system does not cause total loss of sealing capability. Similarly, power, control and monitoring systems for the isolation plug should be suitably robust and/or dual-redundant to account for any possible in-situ damage or failure. Certain designs of in-pipe plugs may only be capable of providing an appropriate form of single isolation, whilst others may claim to provide full double block and bleed isolation. Due to variations in manufacturers’ design and the techniques by which these isolations are achieved, tested and maintained, the proposed device should be subject to thorough engineering evaluation and review at an early stage in the project. The potential hazards associated with the utilisation of in-pipe isolation plug devices should be considered on a case by case basis under the appropriate risk assessment process.

4.1.1.2.3

Hot Tapping Occasionally, for various reasons of operational or delivery constraints, it may be necessary to perform a live intrusive intervention on a pipeline whilst it remains at a percentage of (or even full) service pressure throughout the work. Accessing a bulk system in this manner is termed ‘hot tapping’. This method of intervention has been successfully utilised onshore and subsequently adapted for subsea applications. The section of pipe requiring intervention (e.g. to fit a valve assembly for some future tie-in) should be accessed by means of a hot tap clamp and drilling assembly, which should be suitably designed and tested for containing full pipeline pressure. This stack-up should also incorporate a suite of valves to facilitate the future tie-in. These should be arranged in a double block and bleed configuration which, on completion of the hot-tapping operation, should be subjected to full test pressure to confirm suitability as a permanent isolation.

4.1.1.2.4

Pigs Pigs are not considered an appropriate form of subsea isolation. The utilisation of a pig or a series of pigs (separated by slugs of nitrogen, diesel, glycol, water, etc.) does not provide a reliable form of static isolation which can be fully tested

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and accepted in the terms and recommendations of these guidelines. The ‘isolation’ properties previously offered by pigs have now been superseded by those of pipeline isolation plugs (see ii), above). 4.1.1.3 Specialised Isolation Methods Techniques for directly isolating the pipeline or hydrocarbon reservoir from the subsea equipment, such as pipe freezing, hydrostatic column1, bridge plug, cement plug or other such specialised methods, are considered to be outwith the scope of these guidelines and are therefore excluded. Should it be necessary to utilise any of these as effective isolations (through the services and supporting expertise of a specialised vendor) then their incorporation into the isolation scheme would need to be subject to detailed review through the appropriate risk assessment processes of both the client and the diving contractor. The following valve-type is not considered suitable for intervention isolations: 

Choke valves – the seats on flow-control elements of these valves are not designed to be pressure-retaining when fully closed. During interventions, the choke should be previously set to at least 25% open to reduce possibility of a potential pressure differential (e.g. due to restriction within choke).

The following valves are not normally considered suitable as intervention isolations:

4.1.2

i)

Down-hole safety valves – these valves have the potential to self-equalise/ open if pressure develops in well-bore column above valve obturator. However, exceptionally, it may be permissible to accept this type of valve as a suitable isolation, but only if it has been possible to prove the sealing properties of the valve to at least the maximum anticipated pressure differential.

ii)

Check valves – the condition and status of a ‘check’ valve cannot be guaranteed. However, exceptionally, it may be permissible to accept this type of valve as a suitable isolation, provided: a) the valve can be positively locked closed throughout the workscope; b) the valve is only utilised in conjunction with other proven valves in the bulk system isolation scheme; and c) it has been possible to prove the sealing properties of the valve to at least the maximum anticipated pressure differential. Caution is required for smaller size check valves (e.g. in chemical injection lines) as a blockage may mask the test.

Considerations for Flowline/Manifold/Tree and Wellhead Isolations 4.1.2.1 Requirement to Flush Before any intervention operations are conducted, consideration should be given at the planning stage to the contents of the relevant subsea pipework/tree-cavity. Applicable details regarding the bulk systems pressure, volume quantity, temperature, flowrates and chemical composition should be obtained for the risk assessment. To provide a safe worksite for the diver and to minimise damage to the environment, it may be necessary to flush the subsea pipework/tree-cavity to remove harmful contents prior to placing isolations. These operations are usually required when hydrocarbon inventories are involved; it is recognised industry

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The utilisation of a column of fluid in the well-bore of sufficient specific gravity such that its weight exceeds the up-thrust due to the formation pressure below – thus having the effect of forming an ‘isolation’. Key aspects to the reliability of this technique are: a) typically the overbalance pressure margin should be greater than 14 bar (200 psi); b) fluid level in the isolation column should be capable of being monitored continuously; and c) gas migration through the isolation column (from the reservoir) may occur, therefore any such inventory should be safely managed.

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IMCA D 044

practice to reduce the hydrocarbon content to less than 40 parts per million (