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BACKGROUND
It is recognised that there are increasing demands on the diving industry to perform tasks on or alongside pipelines. Over recent years a number of incidents have occurred from which a great deal of knowledge has been gained.
This guidance has been written to assist both operators and contractors in reducing the risks to divers when executing such tasks. Typically these tasks may involve pressure testing, damage inspection, span rectification and handling of protection mattresses. When planning such tasks, hazard identification and work control are critical, especially in connection with pressure testing and provision of adequate isolation.
SCOPE
The safety of diving personnel should be addressed by the application of the following:
an adequate Safety Management System
hazard identification and risk assessment
provision of working procedures approved by relevant parties as defined within the Safety Management System
adequate briefings prior to commencement of diving operations
SAFETY CONSIDERATIONS TO BE TAKEN INTO ACCOUNT PRIOR TO DIVING IN THE VICINITY OF A PIPELINE UNDER PRESSURE
Consideration should always be given to the consequences of a sudden release of either gas or liquid from the pipeline under pressure. This can be of particular concern when working in the immediate vicinity of a pipeline under test, and should be an important criterion for establishing safe working distances and procedures.
The following safety precautions should be implemented when working on a pipeline under pressure:
Where applicable, warning notices should be erected on the pumping equipment topside.
Boundaries should be marked prohibiting unauthorised persons from approaching the pipeline test equipment/area.
Where pipelines are under test divers’ access should be restricted.
The access restriction must be commensurate with the level of risk for the task in hand.
Compliance with the “Permit to Work” system.
Consideration should be given to the relative positioning of the divers deployment device, i.e. basket or bell, and the support vessel to the pipeline, particularly where there could be a risk of diver or bell contamination and/or vessel stability from a sudden release from the pipeline.
Consideration should be given to the medium used for the test.
Note:
The risk assessment should consider the consequences of a sudden release from a pipeline failure under test, taking into account the test medium. While the hazards associated with a sudden release from a pipeline under a pneumatic test are recognised, it should be noted that the sudden release of hydraulic test media or hydrocarbon inventory from a pipeline under pressure could also be hazardous.
CONTAMINATION
During the risk assessment, careful consideration should be given to the possibility of contact between divers or divers equipment and any chemicals or petroleum products which could lead to the contamination of the diver and / or the bell atmosphere.
PIPELINE TESTING
Pipelines under test can lose pressure for a number of valid reasons without any leak being present. Temperature variations do occur and are not always slow or small. Entrapped gas can cause large fluctuations in test pressure, especially in long pipelines. Pipeline expansion under pressure, notably in flexibles, also causes loss in test pressure. Adequate stabilisation periods should be allowed for, as it is only after a proper stabilisation period has elapsed that leaks, even relatively large ones, can be identified.
In principle, a Remotely Operated Vehicle (ROV) should be the first choice for leak detection. However, if the leak is suspected to be from an area which an ROV cannot access then it may be necessary to use divers.
There should be no requirement for a diver to be in the vicinity of pressure testing unless it has been established that there is a leak. In the case of a leak where diver intervention is necessary, it should be properly planned and the pipeline pressure reduced to a safe level prior to diver intervention. The intervention procedures should detail the predetermined appropriate safe pressure parameters that will have been established and detailed in the pressure testing procedures. These safe pressure parameters should be developed from a full engineering and hazard assessment.
Leak Testing
The preferred test medium for integrity leak testing, depending on operational circumstances, may be either a liquid or a gas. It should be recognised that the use of gases for leak testing creates a greater risk in the event of failure and therefore additional precautions must be predetermined and detailed in the testing procedures.
No attempt should be made to rectify any leak by diver intervention until the pressure is reduced to the predetermined safe pressure, as detailed in the testing procedures, as developed from the engineering and hazard assessment.
Damage Inspection
In planning to undertake damage inspection on pressurised pipelines it is important that the assessment of hazards encompasses not only the possible failure modes but also the associated risks to the diver, diving support vessel (DSV) and environment.
The DSV should be positioned in such a way that both the bell and vessel are protected from the potential hazards associated with the effects of current conditions on any loss of product to the local environment.
ROV inspection should be used to conduct any initial inspection. Diver access should not occur until the pipeline has been depressurised to a level which has been established as safe through the engineering hazard assessment.
Work on Pressurised Pipelines
In all cases where work is to be carried out close to pressurised pipelines, for example the installation of support spans, the placing of protection mattresses, and rock dumping, etc., pressure should be reduced to the level which has been established as safe from the engineering hazard assessment.
All precautionary measures such as not lifting loads directly above a pipeline, except where necessary, e.g. mattress final placement, should be included in procedures. Wherever possible the work should be carried out during planned shutdowns with the pipeline depressurised to ambient. All work should be detailed in written procedures derived from a risk analysis. Changes to approved procedures should be appropriately controlled.
In general, the following list can be used as the base guide words for a hazard identification:
Entrapment, entanglement
Dropped objects
Overpressure/underpressure
Diffusers/exhausts
Flange protectors ventilated
Local environmental controls
Pressure testing medium
Underwater pressure vessels
Flexiumbilicals
Pipeline/bundle breakout configuration
ESV’s
Remotely operated electrohydraulic operations
Elevations
Accidental hydrocarbon releases
Adjacent vessel/platform operations
Simultaneous operations/activities
Loss of containment
Impressed current anodes
Drill muds
Chemicals (including brine), inhibitors, biocides, diver contamination
Permit to work
Isolations / barriers
Use of substances hazardous to health
Hydrate plugs
Isolation
Adequate and appropriate isolations must take place. Where practicable such isolations should be tested prior to the operation taking place.
Prior to the commencement of operations, detailed written procedures should be developed which have been derived using appropriate risk analyses. Any changes to approved procedures should be appropriately controlled and consideration should be given to whether further risk assessment is needed.
Where the inherent safety of the planned operation is dependent on remote isolations not under the direct control of the Diving Supervisor, consideration must be given to the integrity of isolations and their control mechanisms. Inadvertent operation of such systems may lead to diver injury.
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