Installation of Subsea Isolation Valves: A Review of Recent Operations in the North Sea

Abstract

ABSTRACT The installation of subsea isolation valve (SSIV) systems to existing offshore platform facilities is likely to continue in the North Sea as part of an overall review of safety systems following the Cullen Report on the Piper Alpha disaster. Many papers have been presented dealing with specific installations or special techniques to achieve retrofit SSIV installation. It is the intention of this paper to give a general overview of all aspects and disciplines required for SSIV installation with particular reference to recent UK North Sea works. The general principles involved in valve selection, configuration and position are outlined along with aspects of installation that affect overall schedule and cost. Experience gained and problems encountered lead the authors to conclude that if the safety advantages that can be afforded to offshore installations by including retrofit SSIV's are to be fairly assessed along with other options available to operators, then a more integrated approach to contracting philosophy, design and interfacing of the many aspects of valve installation is required in the future. INTRODUCTION When planning the configuration of an offshore production facility certain client requirements and field layouts resulted in specifying a subsea pipeline isolation valve in close proximity to the platform. In the UK sector of the North Sea only 7 valves of this type were installed at platform construction phase prior to July 1988. The-catastrophic result of the Piper Alpha disaster in 1987 highlighted the benefits, with regard to platform safety, that could be realised by addition of this type of isolation system, whereby the inventory of pipeline product that may be released during an emergency situation, could be greatly reduced. Shortly after this incident the UK Department of Energy issued directives to operations requiring the installation of Emergency Shutdown Valves (ESV) on platforms as close to riser top as possible however no guidelines were, issued specifically relating to Subsea Isolation Valves (SSIV). At this point in time most facilities planned for installation were revised to incorporate SSIV's in their construction and several operators decided to commence retrofit of SSIV's to their existing facilities. This paper will review the installation of such retrofit SSIV'S in the UK North Sea, when between 1988-1990 approx 30 retrofit valves were installed. Legislation regarding installation of SSIV'S and other platform safety requirements were deferred until publication of the Cullen Report on the Piper Alpha disaster no doubt to ensure a global safety policy could be adopted rather than issue isolated directives on specific subjects. The recommendations of Cullen, which will undoubtedly be enforced, do not specifically call for installations of SSIV's but requires a full safety assessment of each offshore platform to be conducted and then proposals made for minimizing the effect of pipeline inventory during emergencies. These proposals may include:-Re-siting of RisersRe-siting of Platform Facilities (e.g. Accommodation)Enhanced Projection To StructureInstallation of SSIV's