Subsea high integrity pressure protection system (HIPPS) technology was initial developed some 15 years ago, although today relatively few projects have a subsea track record sufficient to achieve a technical readiness level 7 (TRL 7) designation. Standards defining the rules for instrumented safety systems (International Electrotechnical Commission, 2000, 2003) are well rehearsed, although the standard for subsea, in API 17O (American Petroleum Institute, 2009), has been slow to emerge. This paper discusses to what extent the API standard has settled the debate for subsea and in which other areas, if any, is there an ongoing tension between principle and practical application.
Fundamentally, the function of the HIPPS is to protect downstream equipment from over-pressure by continuously monitoring line pressure and automatically closing an isolation valve(s) if the pressure rises above a preset level.
Traditionally subsea pipelines/flowlines are rated to the full shut-in pressure of the reservoir, which can represent a very significant proportion of the field development cost, especially where long offsets are involved in combination with high pressures.
The cost of the pipeline can be lowered by reducing its pressure rating; a thinner pipe wall results in less weight, which equates to lower capital cost and lower installation cost. Further, the subsea HIPPS also allows for a reduced rating of the downstream equipment (risers, separators, process plant, etc) and can make high pressure field developments commercially viable by allowing them to tie into existing lower pressure rated infrastructure.
In an extreme case and particularly in deep water, it may not even be possible to develop the field without using subsea HIPPS because the pipeline would be too heavy to install using current installation techniques. Interestingly, at least one major oil company will only consider using HIPPS where there is technically no other choice, preferring not to use it as a cost-saving alternative where it is possible to install a pipeline rated to the shut-in pressure.
The pipeline wall thickness can be reduced through three distinct criticality levels: no damage, no burst and burst critical. "No damage" is where the pipeline yield and burst pressure both exceed the shut-in pressure. "No burst" is where the yield pressure is lower than the shut-in pressure, but the burst pressure still exceeds the shut-in pressure. "Burst critical" is where the pipeline yield and burst pressure are both lower than the shut-in pressure. The choice of criticality level must be made cognisant of the fact that increased cost savings will result in increased risk, as illustrated in Figure 1.
Figure 2 illustrates a generic HIPPS arrangement comprising two barrier valves, known as the final element, plus three dual pressure sensors and the so-called logic solver. This decision-making logic solver device is most elegantly installed inside the subsea electronics module (SEM) contained within the subsea control module (SCM).
The logic solver compares output from each pressure sensor against a preset trip threshold.