Subsea Control System 

The SDU connects to the subsea umbilical through the Subsea Umbilical Termination Unit (SUTU), and distributes hydraulic supplies, electrical power supplies, signals, and injection chemicals to the subsea facilities. The facilities can be a subsea template, a satellite well cluster, or a distribution to satellite wells. Some projects do not utilise an SDU, instead the distribution takes place directly on the manifold and then to the manifold wells or to the satellite wells via distribution harness or cables, ROV connectors and stab plates.

Figure 15.1 - A typical SDU (OneSubsea design)

The SDU frame is fabricated from carbon steel coated in accordance with a subsea paint specification. The frame is designed for lifting and lowering onto a location on a subsea production structure. Alternatively, the SDU can be located on a mud mat, simple protective frame, or a mono-pile.

The SUTU can connect to the SDU with a vertical stab and hinge-over connection, or alternatively connect via electrical and hydraulic jumpers at seabed level pull-in location, or manifold structure pull-in location using ROV connectors. If the field layout demands, the jumpers can route through a weak link breakaway connector.

The jumpers from the SDU to the SCMMB connect using ROV connectors.

Electrical distribution can be performed in different methods dependent on system design, layout, water depth etc. Some systems utilise an Electrical Distribution Box (EDB) which can be of whereby the distribution is contained in an oil-filled pressure-balanced fabricated and coated carbon steel housing sometimes called an Electrical Distribution Unit (EDU).

Most deepwater projects utilise electrical distribution from the SUTU to the SDU or directly on to the manifold by way of electrical flying leads and harnesses fitted with ROV connectors, e.g. DigiTRON or ODI design. This allows changeabilility subsea in the event of failure and the ability to connect or disconnect whilst live.

The hydraulic distribution is by tubing from the incoming interface connection routed around the structure to the distribution outlets. The stab connections and the tubing are generally of 316 stainless steel. The tubing terminations are all welded for integrity. The tubing, which is usually installed at a fabrication site, has to be flushed and cleaned to the integrity required by the subsea control system.

Chemical injection systems generally require larger volume flows during normal operation, and are also subject to increased viscosity at lower seabed temperatures. Therefore, larger bore tubing or piping is generally used, again welded to maintain integrity.

Multi stab plate hydraulic connections must have some movement in order to allow for alignment during make-up. Also tubing is often installed on structures using clamps with plastic inserts. This can leave the tubing and end connections floating without cathodic protection. It is essential that these items be electrically bonded to the main structure cathodic protection system to avoid rapid corrosion of the system.

Other material that may be considered for the distribution piping or tubing is carbon steel for the chemical injection system, or other materials such as Duplex or Super Duplex stainless steel.

To ensure correct mating of the respective parts, guide pins are used on stab plates, and single connections may have different size quick-connect couplings or may be keyed.

The size of an ROV means that design consideration regarding access to the SDU has to be carefully considered. It is not always necessary to have a docking station for ROV make up, but docking may make certain tasks easier. If the field survey shows strong currents at the seabed and changeable directions, then ROV docking is necessary. ROV grab rails are fitted to the SDU for this purpose.

With multi-well applications where the ROV must remove connectors from parking positions and hook up at positions on the SDU, it is essential that the ROV does not get entangled in any of the other jumpers. This can cause damage to the jumpers or may entangle the ROV where it would need to cut jumpers in order to free itself.

Clear marking of the connection point is essential in order that the ROV pilot may orientate the ROV at the desired location and to ensure the correct hook up in low visibility subsea.