The Subsea Production Control System (SPCS) which looks after all the above instrument monitoring and control is vital to subsea oil and gas production. The control system is split between a number of platform housed modules (Electrical Power Unit (EPU), Hydraulic Power Unit (HPU) and Master Control Station (MCS) and subsea modules connected together with an Umbilical. The umbilical is just a bunch of hydraulic hoses/tubes, electrical cables and sometimes fibre optic cables. The subsea portion of the controls system relies heavily on a unit called a Subsea Control Module (SCM) which is usually mounted on the Xmas tree structure. This is the heart and brain of the system.
Each SCM contains a hydraulic and electronic portion. The electronic one is known as the Subsea Electronic Module (SEM). This is basically an industrial computer housed in a single atmosphere nitrogen filled chamber which receives power from the platform and likewise communicates back and forth with the platform Master Control Station. Command are sent down to request Xmas tree, manifold and down hole valve movements and valve position information along with all the other process instrumentation is returned. The Hydraulic portion contains fluid accumulators to store hydraulic energy, fluid filters, manifolds and Directional Control Valves (DCV) which direct pressurised fluid from the SCM out through tubes and pipes to the large production and injection valves on the Xmas tree, manifolds and down hole.
The conduit for these signals, the hydraulic fluid and well injection chemicals is the umbilical, comprised of hoses or tubes and cables and fitted with special connectors at each end. The need for underwater mating of connectors drove early designers down the sealed inductive connector path but these proved unreliable over time resulting in the development of conductive pin connector as used in nearly 100% of current systems. Another more recent change was driven by the ever-increasing amount of data being transmitted to and from the platform. This led to a move away from Copper Cables to Fibre Optic Cables. On the hydraulic front, early subsea systems were for relatively shallow water so Thermoplastic hoses were used but as water depths increased, these hoses began to collapse due to the increased hydrostatic pressure. The solution was to move to Steel Tube Umbilicals which also had the benefit of being more suited to harsh chemical service used in downhole injection. One of the drivers for a move to all-electric systems is to simplify the umbilical.
The final piece of the subsea puzzle is Transportation of produced fluids and gasses i.e., the pipeline from the subsea field back to the platform. Subsea control systems also control off-tree valves such as manifold pipeline isolations and manifold instrumentation. Another type of safety system, High Integrity Pipeline Protection Systems (HIPPS), were introduced towards the subsea end of a pipeline to protect pipelines in the event of pressure surges beyond their design limit. Nearer the platform end of the pipeline is a Subsea Isolation Valve (SSIV). These first came to play in the aftermath of the Piper Alpha offshore accident in 1988. The post-accident enquiry recommended the installation of a fail-safe means of isolating the incoming pipeline to protect the production facilities by preventing the unrestricted flow of natural gas in the event of an offshore incident.
Read our extensive Subsea Equipment List PDF here.