For offshore oil and gas installations, one of the major hazards identified is the possibility of a hydrocarbon leak on the pipeline side of the riser Emergency shutdown valve (ESDV). The consequences of such an event can be catastrophic as the subsea pipelines are generally several kilometers long and contain large inventories.

In order to prevent the backflow of the hydrocarbon inventory from the subsea pipeline in the event of a leak on the pipeline side of the riser ESDV, the requirements of installing a Subsea Isolation Valve (SSIV) are assessed. The purpose of installing a SSIV in the subsea pipeline is to limit the duration and severity of release which can in turn potentially reduce the risk to personnel, asset and environment. In addition to this function, the subsea isolation system also provides an alternative means of isolation in the event of failure of the topsides isolation system. In some instances, however, the cost and operational impact of installing an SSIV may outweigh these benefits.

The assessment for benefits versus non-benefits of installing a SSIV are carried out using Risk Assessment techniques and the decisions are generally based on Cost Benefit Analysis (CBA). The use of risk assessments in establishing the requirement or non-requirement of SSIV's in the subsea pipelines is currently the best tool available for the oil and gas industry. However, these assessments and the conclusions drawn there from should be based on a holistic approach considering personnel, asset and environmental risk, production availability, operational impacts, SSIV reliability and the cost benefit analysis.

The purpose of this paper is to elaborate on the risk assessment process undertaken for establishing the requirement or non-requirement of a SSIV in a subsea pipeline. It further cautions the assessor about possible pitfalls and identifies possible challenges in such assessment processes.

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