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Introduction

Over the last decade, there has been a huge increase in the application of subsea systems for the production of oil and gas from subsea wellheads. A subsea production system comprises a wellhead, valve tree (‘x-mas tree’) equipment, pipelines, structures and a piping system, etc., and, in many instances, a number of wellheads have to be controlled from a single location. A subsea control system is part of a subsea production system, and proper performance of the control system is the critical factor in ensuring its reliable and safe operation.

The control system provides operation of valves and chokes on subsea completions, templates, manifolds and pipelines. In addition to satisfactory operational characteristics, the design of a control system must also provide the means for a safe shutdown on failure of the equipment or on loss of hydraulic/electrical control from the topsite (a platform or floating facility) and other safety features that automatically prevent dangerous occurrences. One example of such a safety feature is the employment of fail-safe- operated subsea valves that close upon loss of hydraulic pressure.

The control of various production functions, executed at the sea bed, is carried out from a topside production facility (a platform or a floating vessel), and a satisfactory response time for a control system is an important factor that may have a dramatic effect on reliability and safety of environmentally critical operations.

As communication distance between topside production facilities and subsea installations increases, due both to multiple well developments and water depth, early methods of well control using direct hydraulic control of subsea valves have become less feasible due to operational limitations of such controls and due to both the size and cost of the multi-core umbilicals required to provide hydraulic power transmission. This has led to the development of more advanced and complex control methods using piloted hydraulic systems, sequential piloted systems and electrohydraulic systems (hard-wired and multiplexed). The complexity and performance characteristics of subsea control systems depend on the type of control used and are application-specific. The selection of the type of control system is dictated predominantly by technical factors like the distance between control points (offset distance between the platform and the tree), water depth, required speed of response during execution of subsea functions and type of subsea installation (single or multiple wellheads).

To ensure reliable and safe operation of the subsea system, the design, operation and testing, etc., of a subsea control system is regulated by industry, national and international standards, and the systems are subjected to stringent quality review processes like failure modes, effects and criticality analysis, factory acceptance tests and reliability availability and maintainability analysis, etc. (1) 

Control Equipment – Topside

Topside control system equipment comprises a hydraulic power unit (HPU), an electronic power unit (EPU) and a well control panel. The HPU provides high and low-pressure hydraulic supplies and is usually powered by electric motors, although redundancy is sometimes provided by air drives. The HPU includes tanks, pumps, a contamination control system and hydraulic control valves, etc. Emergency shutdown facilities are provided to bleed off hydraulic fluid and thus to close subsea fail-safe valves. The hydraulic components are fairly standard.

Two types of fluid are commonly used for subsea production systems: high water content-based or synthetic hydrocarbon control fluids. The use of synthetic hydrocarbon control fluids has been infrequent in recent years, and their use is usually confined to electrohydraulic control systems. Water- based hydraulic fluids are used most extensively. The characteristics of high water content-based control fluids depend on the ethylene glycol content (typically 10% to 40%), and viscosity varies with temperature (typically 2–10°C). As government regulations do not allow venting mineral-based oil into the sea, if the system uses this type of fluid, it must be a closed-loop system, which adds an extra conduit in the umbilical, making it more complex. Required fluid cleanliness for control systems is class 6 of National Aerospace Standard (NAS) 1638.

A programmable logic controller or PC-based EPU may be integrated with the platform control system or it may be a self-contained unit.

Umbilicals

An umbilical is a conduit between the topside host facility and the subsea control system and is used for chemical and/or hydraulic fluids, electric power and electric control signals. The hydraulic power and control lines are individual hoses or tubes manufactured from steel or thermoplastic materials (most common) and encased in the umbilical. The electrical control cables supplying power and control signals can either be bundled with hydraulic lines or laid separately.

To avoid any potential faults, the umbilicals are fabricated in continuous lengths, i.e. without splices. Major problems encountered with umbilicals are permeability to methanol, fluid incompatibility and mechanical damage during manufacture and installation. Current research and development efforts are directed towards improvement of thermoplastic umbilicals. In some cases, it may be advantageous to use metal umbilicals and it should be noted that, due to problems experienced with thermoplastic conduits in umbilicals, some operators are now using only stainless steel tubing for transporting fluid in umbilicals. Umbilicals employing metal tubing are usually considered for deepwater applications and when longer umbilical lengths are required. Metal umbilicals are also advantageous when higher working pressures, greater electrical power requirements and continuous dynamic service are necessary. However, issues of corrosion, fatigue performance and end terminations still have to be resolved.

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