ABSTRACT

This paper examines the criteria which must be assessed when developing the philosophy for designing a Subsea Isolation Valve (SSIV) Installation By reference to actual designs, it is shown that satisfying these criteria for different applications can produce different solutions to what is essentially a similar problem Consequently it is deduced that the design of such installations should be undertaken on a project by project basis

INTRODUCTION

In the wake of the Piper Alpha incident, and in the light of Lord Cullen's enquiry report [I], it is recognised that an emergency shutdown valve located anywhere above sea level may not always provide sufficient protection against un-isolateable hydrocarbon leaks in a pipeline system As a result, operators are now considering the installation of seabed emergency shutdown/isolation valves, in close proximity to the platform, thereby isolating the topsides from the main pipeline inventory.

Requirement for Subsea ESDVs

Before addressing the design of SSIV installations some SSIV requirement criteria will be briefly discussed The decision as to what system to adopt and where, will be based primarily upon the nature of hydrocarbon product in the pipeline, the requirement being more critical for gas lines.

The occurrences on Piper Alpha and subsequent studies have demonstrated that ESDVs located topsides cannot be fully relied upon to contain a major explosion since they themselves may be disabled Given this scenario, an effective way of limiting the supply of hydrocarbons through a major pipeline to a disabled platform is by a "subsea" isolation valve.

Equally, should a dropped object or an anchor, rupture a pipeline near a platform a fundamental method of limiting the remaining pipeline inventory of hydrocarbons from leaking to the oil slick or gas cloud is by a "subsea" isolation valve. Since the dropped object and anchor hazards decrease with distance from the platform the position of such an SSIV should ideally be at an appropriate distance from platform, but within the 500m exclusion zone (see Figure 1)

At this time the actual requirements for mandatory SSIVs are determined as an integral part of the Full Safety Assessment in the UK sector of the North Sea, but the discussions above clearly demonstrate their necessity where large pipelines serve a platform. Consequently operators are now installing them in both new and existing facilities.

Design Criteria

In developing a shutdown system design, a number of aspects must be assessed in order to arrive at an optimum solution for each application. This paper discusses the principal criteria presenting some associated advantages and disadvantages of each aspect Depending upon the application, a number of varying solutions may be developed for what is, essentially a similar problem

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