Traditionally subsea flowlines are rated to the full shut-in tubing pressure (SITP) of the well. Most of the time, the operating pressure is far below the SITP. If the flowlines can be designed for operating pressure rather than for SITP, field development costs can be significantly reduced, especially where long tie-backs are involved in combination with high SITP. This helps make the development of marginal fields a commercial proposition.
With the current trend to increasing water depths and increasing well SITP (15,000 to 20,000 psi in the Gulf of Mexico), the use of (HIPPS) High Integrity Pressure Protection System avoids the need for extremely large flowline and riser wall thicknesses. Excessive wall thicknesses can make installation difficult or impossible, and can reduce weld reliability, increasing overall risks.
A subsea HIPPS is a system of pressure sensors, controls, and barrier valves that provides Traditionally subsea flowlines are rated to the full shut-in tubing pressure (SITP) of the well. Most of the time, the operating pressure is far below the SITP. If a pressure breaks between the upstream subsea system, which is rated for the SITP, and the downstream system, which is rated to a lower pressure. If the production system pressure at the HIPPS location rises to a certain level, the HIPPS valves close to isolate the lower rated downstream equipment from the high-pressure source. High pressure can result from loss of well control, topsides process trip, inadvertent downstream valve closure, or hydrate/wax blockage.
A subsea HIPPS works in conjunction with the subsea process shutdown system (PSD). The latter closes the production master and wing valves on the X-tree. Typically, the HIPPS pressure trip point is set somewhat higher than the trip point for the subsea PSD but below the allowable pressure for the flowline (the flowline allowable pressure is code dependent). The difference between the HIPPS trip setting and the allowable flowline pressure is designed to ensure that the pressure in the protected equipment remains acceptable during the time it takes for the HIPPS barrier valves to close and cut off the pressure source.
A subsea HIPPS is designed to be independent of the PSD and any other control systems, and to shut down in response to detection of high pressure without need for any actions external to the system. Although designed to work in conjunction with, and complement, the PSD, it is in no way interconnected with the PSD but is an entirely autonomous system.
There are at least three types of subsea HIPPS: a well-based HIPPS is located near a well and provides protection against high pressure in that single well; a manifold-based HIPPS provides protection against high pressure arising in any of the several wells that feed the manifold; while a flowline-based HIPPS may provide protection against high pressure in multiple wells/manifolds located upstream. With a well-based HIPPS, the valve size is kept low and the HIPPS need only be installed on those wells that need it, which might be just one well in the field. When multiple wells have high SITP, a manifold-based or flowline-based system can reduce the required number of HIPPS at the expense of increasing the HIPPS valve sizes. When multiple HIPPS are used, the overall reliability requirement must be maintained by increasing the reliability of the individual HIPPS. Reliability requirements may be established and verified following guidelines in API RP 17O, IEC 61511, and IEC 61508.
A subsea HIPPS has three major elements – a pressure sensing system, a logic solver, and the HIPPS barrier valves, as shown in Figure 1. The pressure sensing system comprises a number of independent pressure sensors hardwired to the logic solver which applies a voting logic (such as 2 out of 3, 2 out of 4, etc.) to the sensor outputs. The logic solver initiates closure of the HIPPS valves if flowline pressure is detected to have reached the trip setting. The logic solver typically has a duty unit plus a hot backup that assumes control if the duty unit fails. The HIPPS barrier valves usually comprise two independents, through-conduit, fail safe closed, slab gate valves in series.
A fortified flowline section may be needed immediately downstream of the HIPPS to accommodate transient pressures during the time it takes for the HIPPS to respond to the detected high pressure. The pressure rating of the fortified zone is project specific and may be as high as SITP. In cases where only a slow pressure build up is possible, a fortified zone may not be needed at all.
The only code of practice specifically for HIPPS, API RP 17O (released in 2009), recommends that a fortified zone should also be provided near the host facility. The intent is to ensure that any loss of containment would occur at a safe distance from the platform.
There are several possible design philosophies for the non-fortified flowline section:
Burst critical – the flowline will rupture if there is a pressure excursion to well SITP and the HIPPS fails to close. Reductions in CAPEX are large.
Yield but not burst – the flowline may be damaged if there is a pressure excursion to well SITP and the HIPPS fails to close. Extensive inspection, analysis, revalidation, and possibly repairs would be needed before the flowline could be brought back into operation. Reductions in CAPEX are moderate.
No damage – the flowline is designed such that occasional pressure excursions to SITP can be tolerated without damage. Reductions in CAPEX are slight.
The above are not the only conceivable design philosophies but they are the most common. In regulatory regimes that are performance-based rather than prescriptive, risk assessment may be used to determine the optimum design philosophy for a given development.
The first subsea HIPPS was installed in 1996. At time of writing, there are some 16 operating installations, all but two of them in the North Sea. Given that there have been so few subsea HIPPS developments, few engineers have had the opportunity to work on a subsea HIPPS project. Peritus has a number of engineers with HIPPS design experience, covering the key disciplines of subsea hardware, subsea controls, and risk/reliability. Peritus also has numerous other specialist engineers to provide support.