When planning the configuration of an offshore production facility certain client requirements and field layouts resulted in specifying a subsea pipeline isolation valves 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 i988 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 operators 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 faculties planned for installation were revised to incorporates SSIV's in their construction and several operators decided to commence retrofit of SSIV's to their existing facilities. 'These actions were taken in anticipation of prescriptive regulations being introduced, insisting on SSIV installation. On publication of the Cullen Report into the Piper Alpha disaster it became apparent that prescriptive regulations installation of SSIVs would not be enforced and that all operators would be required to submit a Safety case to the regulatory body which would demonstrate, among other aspects, that "hazards arising from the inventory of hydrocarbons in risers and pipelines connected to the installation have been minimized." Being more specific the report states that "the operator should demonstrate m the Safety case that adequate provision has been made, including if necessary the use of SSIV's, against hazards from risers and pipelines."
It therefore follows that SSIV installation is not necessarily a requirement but the cost/benefit of this type of system will be assessed along With other methods of protection such as riser relocation, enhanced accommodation protection, resisting of accommodation etc. It may be a natural course of action for operators to look at alternative methods of complying with the Cullen requirements m the perception that retrofit SSIV's are likely to be the most expensive option for existing facilities. However we believe that the industry has been offered a rather unique opportunity to ensure that economic proposals for essentially repetitive installation of retrofit SSIV assembles to be developed by capitalizing on previous experience. Only by this approach will the undoubted safety advantages of SSIV's be fairly assessed.
This paper will review recent retrofit SSIV installations in the UK North Sea and offer proposals to allC7il the experience gamed to date to be utilised to ensure practical and economical proposals are evaluated for consideration for enhancement of platform safety.
Figure 1 illustrates the component parts of a typical retrofit SSIV assembly and can be summarized as below:-
Subsea Isolation Valve - The specification and selection of the correct valve is obviously of great importance to ensure long term suitability for intended purpose and this selection process must review the pipeline product, maintenance requirements, reaction time, compatibility with actuator and control systems.