Subsea control systems are becoming more complex. As they are moving into even deeper water, using more complex equipment and collecting more data, the demands on their performance are increasing. This has resulted in the need for faster data retrieval, more complex programmable processors, bigger power-hungry devices and continual technological progression.
Thus whilst there is a desire in each project to keep innovation to a minimum and use only what is tried and tested, in reality the industry is seeing a steady product development and evolution. Nowhere is this more apparent than in the heart of the control system's subsea control module (SCM). It is no secret in industry that the SCM is one of the systems with the biggest reliability challenges. Chevron collects all the subsea reliability data in a database called Subsea Master. On reviewing the data of the SCM reliability statistics, not surprisingly, Chevron has found them to be one of its principal bad performers.
Further analysis revealed that SCM reliability was not as high as desired or expected, and attempts to correlate against the various fields and environmental conditions revealed little. It appeared that there was no correlation between environmental factors and failure rate, or even the age of the systems and failure rate. However one thing clearly stood out, early life failure was a major problem.
Chevron reviewed all of its conventional electrohydraulic (EH) systems with the SCM installed on or near the tree or manifold (see Figure 1). In most cases the control system is by a water based control fluid.
An SCM typically controls the functions on a subsea tree or manifold and is designed to be installed and retrieved without disturbing the major pieces of hardware. Depending on water depth, this operation may be performed by a diver or a remotely operated vehicle (ROV). There are three main types of SCM currently in existence: all-hydraulic, which was historically used in the late 1970s and early 1980s; EH, which is the most common technique used since early 1980s; and all-electric, a primarily new technology that is likely to be the way in which controls will evolve.
The SCM is the brain of subsea controls equipment. Made of computer-like electronics called the subsea electronics module (SEM), it normally receives power and communications through the subsea umbilical. The SEM, in turn, controls a hydraulic manifold system using a selection of solenoid driven control valves, which then directly drive the subsea hydraulic functions.
Additionally, the SEM is connected to external sensor systems providing production condition information, and a selection of internal sensors condition monitors the overall system. The SCM's components and electrical connections are encased inside an oil-compensated outer housing that protects them from the elements. The hydraulic, electronic and electrical interfaces feed and distribute electrical and hydraulic power to the external sensors and valves that are usually installed into the SCM's base plate.