In the aftermath of the Piper Alpha disaster in 1988, considerable attention was given by many companies to the risks posed by riser and pipeline failures to their installations. Particular attention was focused on ways of reducing the impact of failures, including specifically sub-sea isolation valves (SSIV). The decision process for deciding whether or not to install a SSIV is complex, and the Exploration and Production function of Shell International Petroleum Mij B.V. (SIPM-EP) felt that a consistent approach for assessing the safety benefit of SSIVs should be adopted by Shell companies. As a result, SIPM commissioned a study to develop a method which would enable their own staff to carry out risk assessments in a standardized repeatable manner. In parallel with the development of the method, a series of six risk assessment studies were carried out, followed by a further two once the method development had been completed.

This paper will describe the key aspects of the methodology and how it is applied, and will present a summary of the results of the applications to the eight installations. The installations studied included a widerange of platform types and environments from shallow water platform complexes in tropical areas to large integrated north sea platforms. Gas, oil and two-phase risers were included in the work. Each study considered a number of options including the installation of top of riser valves and sub-sea valves close to and remote from the facility. These were compared with the base case risk. The results will show that the risk reduction potential of an SSIV varies significantly from installation to installation depending on specific circumstances, and demonstrates the need to consider each case individually in order to ensure that cost effective risk reducing measures are adopted.

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