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Slug flow from wells or multiphase flow lines from subsea satellite fields is one major disturbance at offshore oil and gas processing facilities. Slugging is an undesirable flow pattern with liquid blockages that can occur in multiphase flow lines transporting both gas and liquid. The original contribution in this paper is the successful application of advanced control for the inlet facilities, where active slug control for two 11.7 km long multiphase flow lines is combined with model predictive control to handle slugs entering the inlet separators.

Systems-Theoretic Process Analysis (STPA) is a recently developed hazard identification technique that is based on control and systems theory. Previous studies on STPA emphasize two major strengths of the method: (1) STPA provides a systematic top-down approach that enables early identification of system flaws, and (2) STPA covers a wider scope of hazards compared to traditional methods. Despite these advantages, there are only a limited number of studies that have applied the method to subsea systems. It is therefore of interest to investigate how STPA can be used to formulate new or verify existing requirements to safety-critical systems for subsea facilities. One example is the isolation of subsea wells initiated by the platform emergency shutdown (ESD) system. The purpose of this paper is to apply STPA to this function, and to discuss opportunities, challenges and possible implications of the results obtained from the analysis. The paper starts with a thorough literature study and includes an analysis of the insights and recommendations made from other industry sectors and application areas. This review is followed by the STPA analysis of the proposed system, with focus on the identification of the unsafe control actions and safety constraints for subsea well isolation. It is investigated how STPA is able to address specific design philosophies and subsea operating conditions, like fail-safe function of subsea ESD valves, long distance between top-side control system and subsea valves, and dynamic behavior of the control structure. The paper concludes with discussions and suggestions on how the STPA procedure may be improved for application to subsea systems.

Compressors are key components in the refrigerant circuits of the Snøhvit LNG plant and contain large amounts of mechanical energy. Thus it is imperative that the control system is able to keep the compressor out of surge in case of driver trip. A dynamic process simulator describing the total LNG plant has been developed by Kongsberg Process Simulation and the simulator has been applied in the engineering phase for the design and process verification. The simulator has also been used to verify the robustness of the closed loop refrigerant circuits and to verify that the refrigerant compressors are sufficiently protected after a driver trip. The presented work demonstrates the value of dynamic simulations for verification of compressor protection systems. In addition it shows the importance of using correct data for polar inertia of the rotating equipment, as well as the opening and dead times of the anti-surge valves. It is recommended to include a sensitivity analysis of these parameters as part of plant verification studies. INTRODUCTIONCompressors are key components in the refrigerant circuits of LNG plants. It is imperative that these compressors are properly protected against surge,, Compressors in surge can result in a severe damage to machinery and disturb plant regularity.One of the most serious incidents that can happen to a compressor system is a driver trip. Gas turbines and electrical motors are the most common types of compressor driver and a trip of the driver here is more serious than a process trip. This is because in a process trip the shutdown of the driver itself can be delayed until the surge protection valves have been opened. Important factors controlling the run down of the compressor after a trip are opening times of the anti-surge valves, piping layout, inlet and discharge volumes as well as the polar inertia of the rotating equipment including the compressor, gear and driver. It is essential to be able to validate that the compressor is properly protected during process upsets including the case of a driver trip.The use of dynamic models for analysing compressor transient behaviour has been addressed by several authors in various scientific journals and conference papers and has also been demonstrated by the supplier industry. Patel [1] discusses dynamic simulators as an important tool in the design and operation of compressor systems. A good overview of the scientific research within this area is given in [2].

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