Subsea Distribution Assembly (SDA)

The SDA, as shown in Figure 3-5, 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 satellitewells. The SDA connects to the subsea umbilical through the SUTA.

3.3.4.1. Construction

The SDA 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 SDA can be located on a mudmat, simple protective frame, or monopile.

3.3.4.2. Interface with the Umbilical

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

3.3.4.3. Interface with SCM

The jumpers from the SDA to the subsea accumulator module mounting base (SCMMB) are connected using an ROV.

3.3.4.4. Electrical Distribution

The electrical distribution is usually contained in an oil-filled, pressure- balanced, fabricated and coated carbon steel housing called an electrical distribution unit (EDU). (Non–pressure-balanced, resin-filled junction boxes are sometimes used, but these do not allow future maintenance and require the encapsulated components to be suitable for use at depth; designs requiring current-limiting devices may be housed in a one-atmosphere enclosure.) Entry and exit of the EDU is by flange-mounted electrical controlled environment-type connectors. The connectors are configured so that any connections that may be accidentally disconnected live have the live conductors protected from the seawater. Cable tails from the back of the electrical connectors within the oil-filled housing are connected for distribution as required by the control system architecture and the system redundancy capability. The requirement for fault protection is dependent on the system design and the number of wells that could potentially be disabled by a subsea cable fault. Three types of electrical protection are used: fuses, circuit breakers, and thermal resetting trip devices. Fuses are not effective, as slow-blow fuses are necessary in order to cater to the inrushing current while charging up the umbilical. This makes fuses ineffective in isolating a fault in the distribution system without overloading the remainder of distribution outlets and, generally, a fuse would not blow before the line insulation fault trip in the EPU is activated. Circuit breakers have been used subsea in EDUs, but are not commonly used because the circuit breaker reset mechanism has to penetrate through the EDU housing using O-rings, which introduces a potential fault path. The thermal resetting devices are semiconductor devices and due to the technology required, they are not available from all suppliers.

3.3.4.5. Hydraulic and Chemical Distribution

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 made of type 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. Multiple stab plate hydraulic connections must have some movement in order to allow for alignment during makeup. 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 more exotic 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 for proper orientation.

3.3.4.6. ROV Connection

The access of an ROV to the SDU has to be carefully considered. It is not necessary to have a docking station for ROV makeup, but docking may make Subsea Distribution System 73 certain tasks easier. If the field survey shows strong currents at the seabed and changeable directions, then ROV docking is necessary. With multiwell 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 flying leads. This can cause damage to the flying leads or may entangle the ROV where it would need to cut flying leads in order to free itself. Clear marking of the connection point is essential to ensure that the ROV pilot can orient the ROVat the desired location and to ensure that the correct hookup in low-visibility subsea.