ABSTRACT

This paper considers the design criteria-for a subsea "High Integrity Pipeline pressure Protection System" (HIPPS), which enables subsea pipelines to be designed for the operating, rather than the shut in wellhead pressure. Such systems will save considerable investment cost in the development of high pressure offshore oil and gas fields, particularly where the distance to the processing infrastructure is long. The conceptual design of a HIPPS which comprises two rapidly closing valves to protect the pipeline from over-pressure, is described. The reliability of the system is assessed and dynamic simulation of the valve and process flow are discussed.

INTRODUCTION
General

A number of subsea field development studies, in particular for marginal fields, have identified that the flow and pipeline cost represents a significant portion of the total field development cost. The pipeline cost is of course also largely dependent on the reservoir pressure. In some cases where the reservoir pressure was high, the studies clearly showed that the field could not be economically developed due to the high pipeline cost.

It was also clear from these studies that if the pipeline could be designed for the maximum operating pressure (MOP) instead of the Shut-in Welihead Pressure (SWHP), many of the developments would be economically feasible.

This background prompted the view that a pipeline safety system would be required for the development of high pressure marginal fields. This would then permit transportation of hydrocarbons at reduced pressures. Such a safety system must then ensure that the pipeline pressure can not exceed the design pressure, under any ircumstances.

The need for such a system has become even more pronounced over the last few years, with the declining oil price.

A safety or protection system should be of high integrity and reliability. Apart from land based application, such a system has already been implemented on an unmanned offshore platform in the North Sea (Sleipner West), to protect the export pipeline (20", 12 km) to the Sleipner B latform. Such systems became known as "High Integrity Pipeline Protection Systems", HIPPS for short.

A HIPPS system for subsea application, although principally identical in function to the land based or platform based system, should have even higher reliability and most certainly higher availability due to the difficulty and cost associated with intervention.

The probability of hydrate formation and subsequent blockage of the pipeline must be considered. In addition, the special problems associated with a subsea environment would have to be solved.subsea HIPPS system should be able to cope with bothgas and oil fields (high GOR) of high pressure.

Design basis

In early 1993, in-house studies were initiated with the objective of identifying the problems associated with subsea HIPPS applications and to establish the scope of work for the future development of such a system.

The studies concluded that a prototype HIPPS system should be designed, built and qualified. The design work started in august 1994 on the following basis:

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Some of the arguments used in arriving at the design basis are discussed below.

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