With a significant increase in the role of subsea production systems the need for reliable subsea control equipment has never been greater. This need is emphasized by the inclusion of advanced modules within the subsea controls package to manage the performance of new mission critical process elements (such as subsea processing packages and compression systems) designed to boost yield and reduce OPEX. Additionally, safety critical components such as pipeline protection systems may significantly reduce CAPEX by de-rating infrastructure pressure containment levels and allowing the tie back of new well sets into aged assets. Central to all such application is the knowledge that the control system itself is of high availability and reliability. In this paper, the acquisition and use of field and accelerated-lifecycle test data is illustrated and a case is built up for the ultimate reliability figures for the subsea control module The development of a HIPPS (High Integrity Pipeline Protection System) is used to exemplify the reliability arguments required to justify the use of a subsea control system in this particular safety critical application. Consideration is given to the mechanical components and failure modes associated with process fluid contamination. Finally the cost benefit argument is demonstrated based on ready deployed equipment

Why Subsea HIPPS?

Historically it has been the case that subsea flowlines and infrastructure have had to be rated such that they are capable of containing the fill shut-in pressure of the wells to which they are connected This is to cater for the eventuality at more produced fluid are admitted to the line than are being removed from it Examples of events that could lead to such as situation are

  • Failure of a choke at the wellhead causing a sudden rise in production rates

  • Hydrate blockage within the flowline

  • Unexpected or sudden closure of a valve within the pipeline or downstream infrastructure

  • Inadequate production control operations and procedures

The occurrence of these events cannot be ruled out with any degree of certainty and the expected frequency is such that, In order to provide a safe system, a high flowline and infrastructure rating has previously been required

The use of HIPPS allows pipeline and associated downstream components to be rated for a lower pressure than the well shut-in pressure Safety Instrumented Systems (SIS) such as HWPS are not new and have been used frequently in topside application to protect a relatively low-pressure system from a high-pressure source. These are already well established for topside applications as a means of safely de-rating pressure systems. This technology has now been developed by KOP to extend to subsea production systems, and in particular to High Pr essure/High Temperature (HPMT) hydrocarbon developments, which are now being actively pursued in many Central North Sea areas These prospects tend not to be large reserves aid often have Closed In Tubing Head Pressures (CITHP) in the region of 10,000 psi This leads to marginal economics of such fields, when the costs of higher specification pipelines are taken into account

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