Health-Aware Control of a Subsea Compression System

Abstract

Subsea processing technology is enabling production from oil and gas fields with larger water depths, longer tie-back distances and harsher climate conditions. Although moving production and processing facilities to the seabed are creating new opportunities for the oil and gas industry, it is also arising new challenges. At the seabed the equipment is not easy accessible, and maintenance interventions are both time demanding and expensive. To reassure unanticipated breakdowns stringent requirements on safety and reliability are imposed on subsea processes, often leading to conservative designs and operation strategies. This may in turn lead to that the economic potential of the field is not fully realized.

A way to achieve better economical performance is to employ prognostics and health monitoring (PHM) in the decision making. Meaning that the state of the system is monitored and projected into the future, and considered when calculating the optimal control moves. A control structure that pro-actively adjusts the inputs to prevent a fault from occurring is often described by the term health-aware control. The aim of the current work is to explore the use of health-aware control on a subsea compressor subject to degradation.

A multi-layer control structure is developed and applied to a subsea compressor with the objective of maximizing the gas throughput while ensuring continuous operation until the next planned maintenance stop. To combine the short-term control objectives and the long-term profit and reliability targets the control structure is designed with a three level hierarchical architecture. In the top layer a dynamic real-time optimization (DRTO) problem is solved to find the long-term optimal operation strategy when the compressor is subject to load-induced degradation. For the given operation strategy, the optimal operating point is found in the below short-term optimizing layer. The computed set-points and parameters are taken into the supervisory control layer where self-optimizing control (SOC) is used to reject disturbances. In the regulatory control layer linear feedback control is used to stabilize the compressor.

How do the results answer the two first points above? The results from the current work show that the health-aware control structure are able to ensure operation of the compressor for the given time horizon. The health of the compressor are kept under the threshold value for the planned operating time, while maximizing the throughput.