First Application of the All-Electric Subsea Production System-Implementation of a New Technology
Authors: Bouquier Laurent , Signoret Jean Pierre , Lopez Robert 
Published: October 2008

Operators are continually seeking improved reliability, availability and performance in subsea control systems due to the ever increasing cost of failure from downtime and intervention. Enhancements in functionality including more rapid response and improved condition monitoring of equipment are greatly desired. And, the increased focus on environmental concerns due to venting and leakage of control fluids to sea, coupled with the high carrying costs of control fluids in general, are forcing operators to look for cleaner, more economical alternatives to meet ever restricting standards. Resulting from these and other deficiencies, hydraulic control systems for subsea production have come under increased scrutiny.

The advantages and benefits provided by all-electric systems over conventional, hydraulic based systems for subsea application have been rigorously evaluated by many operators and have demonstrated dramatically positive results. Due to the rapid development and advances in electric controls technologies, operators now have a viable alternative.

The desire to realize improvements in equipment reliability, system availability, operational functionality and environmental performance propelled a major operator to select an all-electric subsea production system for a multiwell development in the North Sea. This paper describes the process, decision criteria and strategic impetus that led to the selection of the all-electric subsea production system for this application and outlines the desired objectives for the implementation of the all-electric technology.

Figure 1 - All-Electric Deepwater Subsea SpoolTree System (available in full paper)

Introduction

Over the past decade, substantial gains have been realized in reliability and functionality of electro-hydraulic multiplexed (EHMUX) control systems for subsea production. These systems are the culmination of advanced hydraulic controls technology dating back to the early days of subsea systems. Since the 1960s, the evolution of control systems technology has proceeded from direct hydraulic to piloted and sequenced systems to provide improved response time and allow for long distance tiebacks. Today, most subsea developments make use of EHMUX control. This is essentially a subsea computer/communication system of hydraulic directional control valves (DCVs). These electrically actuated valves allow stored pressure within subsea accumulators to be routed to individual hydraulic lines and onward to actuated gate valves and chokes on subsea production equipment.

Despite the many advantages provided by EHMUX systems, there is a general industry recognition of persistent weaknesses related to susceptibility of fluid cleanliness, materials compatibility, hydrostatic effects in deeper water and limitations for long distance tieback. The higher cost of deepwater and remote, long offset, subsea developments has focused much attention by operators on improving the reliability of subsea systems in general, and control systems in particular.

Many subsea reliability assessments conducted by operators, in association with specialist academic and reliability consultants identified that a significant portion of reliability problems are attributable to the failure of hydraulic components. Reliability issues persistently occur during installation and in operations associated with high pressure/high fluid volume hydraulic systems. The positive results from reliability studies initiated significant technology development investigations to explore potential advantages and benefits of all-electric subsea controls capability.