With oil and gas operators now focusing on smaller assets to develop, every opportunity to reduce costs, with due consideration to safety is being carefully considered.
By derating the transportation flowline substantial cost savings are possible. In the past this would not have been allowed for safety reasons, as the flowline was required to be rated to the maximum pressure from the reservoir.
HIPPS is a new subsea technology which allows the high pressure specification flowlines to be safely derated between the manifold and host platform. This is achieved by reliably isolating the high pressure product before the specification break of the lower pressure rated flowline.
The system is designed to failsafe under all operating conditions. Commercial evaluations are given to justifi the technology. The HIPPS control system is described in detail together with summarized reliability data.
High Pressure/High Temperature (HP/HT) subsea hydrocarbon developments are now being actively pursued in many Central North Sea areas. These prospects tend not to be large reserves but may have Closed In Tubing Head Pressures (CITHP) in the region of 10,000 psi. This leads to marginal economics of such fields, when the cost of higher specification flowlines are taken into account.
Subsea HIPPS presents an option for justifying HP/HT developments, by allowing high pressure flowlines to be derated to standard pressure flowlines, at considerable overall cost savings.
The HIPPS system provides a reliable failsafe means to protect and isolate the derated flowline at the subsea installation from the high pressure at the wellhead. HIPPS works in parallel with the standard control and safety systems, which would all need to I before HIPPS comes into operation. HIPPS also allows potential cost savings in the topside piping system and a reduction in the platform risk levels by a reduction in system pressure.
The control system for subsea HIPPS is separate from the well control system for the subsea equipment having dedicated power, communication and hydraulic lines, These may be shared in a common umbilical with the production control system, and are integrated into the surface control computer and hydraulic/electrical supply units.
Typical costs savings for a step-out distance of 9 km can be in the region of UK E13 million compared to a fully rated flowline system. This saving may be reduced by selecting a ?no burst' option in order to maximise safety considerations. Based on thorough reliability and risk assessment models Ref. 1 it has been shown the subsea system is extremely safe with the risk of a flowline failure in the order of 1.7E-04 per annum or once per 5900 years.
The control hardware used for subsea HIPPS is based on a thoroughly proven topside, certified fixed logic HIPPS system, which has then been marinised for subsea use. The principle feature for the safety critical controller is its ability for failsafe operation under all modes of failure.