Subsea Control System 

The purpose of the Subsea Accumulator Module (SAM), is to provide a stored volume of fluid at seabed close to the manifold or Xtree, at the desired pressure, normally system LP pressure. When tree or manifold valves are operated, the SAM provides the immediate fluid volume to avoid pressure drops and system lag. The volumetric requirements of SAM are derived from the Hydraulic analysis.

When a subsea system is required to operate a number of trees located along distance away from the host Platform/FPSO, the hydraulic fluid from the Topsides HPU will take a considerable time to reach the subsea equipment, particularly where small hoses are used in the umbilical. This can result in a drop in pressure at the subsea tree when a valve is opened, as the pressure cannot then be restored immediately via the umbilical.

If the pressure drops, other open tree valves may begin to close, before the pressure can be restored.

If the pressure drops too much, the pilot valves in the Subsea Control Module will 'drop out' i.e. close, causing one or more tree valves to close irrespective of whether pressure is then restored via the umbilical.

In order to maintain an adequate level of pressure at the subsea location, some degree of local accumulation may be required. This can be provided by individual accumulators on the SCM itself, but more usually, a self-contained skid containing several Accumulator bottles is often provided, this being termed a 'Subsea Accumulator Module' or SAM.

The requirement for an increased Nitrogen pre-charge pressure in deep water applications will decrease the efficiency of subsea accumulation, which will increase the number of accumulator bottles required.

A SAM is a simple skid, primarily housing Accumulators. Nevertheless, it must be designed, manufactured and tested as part of the overall system, and designed to be retrievable for maintenance, as the accumulator pre-charges may periodically need replenishing.

The design may also incorporate a filter and block/bleed valves to allow flushing and testing.

Figure 16.1 - Subsea Accumulator Module Block Diagram

The SAM is usually a stand-alone skid, connected via a mounting base, which is connected to the SDU or the hydraulic tubing supplying the subsea control modules. The SAM is run and retrieved in a similar manner to the subsea control module. In order to allow the SAM to be retrieved for maintenance, ROV operable block, vent, and bypass valves are sometimes incorporated into the Manifold/Template tubing (where the hydraulic distribution is hard piped). This valve will allow production to continue whilst the unit is replaced, but tree valves should not be operated whilst the bypass is in operation, due to the risk of pressure-drop as outlined above.

Figure 16.2 - Subsea Accumulator Module

When block and vent valves are not used, the SAM can be retrieved by pulling off its mounting base and relying on the self-sealing quick connect hydraulic couplings sealing to maintain the pressure in the hydraulic system.

Installation of the SAM is more difficult, as there is a large force required to mate the SAM and SAMMB against the closed force exerted behind the closed hydraulic poppets. It is essential that the hydraulic quick-connect couplings used in this application are fully pressure-balanced to counteract the coupling mating forces, particularly in deepwater applications.

A running tool is required to run and set the SAM onto the SAMMB in deepwater applications.