Subsea Control Modules

How an SCM works

The umbilical delivers low-pressure (LP) and high-pressure (HP) hydraulic supply, electrical power and communications to the SCM. One or two SEMs decode commands from the topside master control station. They energise solenoid-operated directional control valves, which route hydraulic supply to each valve actuator on the tree or manifold. The HP supply usually serves the surface-controlled subsurface safety valve. Local accumulators hold pressure for fast valve strokes. Water-based fluid usually vents to sea on return, while oil-based fluid returns through the umbilical.

The SEMs also read pressure and temperature transmitters, valve position sensors and, on many trees, downhole gauges. The SCM lands on a mounting base. Its hydraulic couplers and electrical connectors make up as it lands, so an ROV can recover and replace it with a running tool.

Configurations

• Multiplexed electro-hydraulic SCMs, the standard on most fields built in recent decades.
• Single or dual SEM layouts. Dual SEMs give redundancy and a longer mean time to intervention.
• Tree-mounted and manifold-mounted units, which differ in function count and mounting base.
• All-electric control systems, which replace the hydraulic valve pack with electric actuator drives.

Key specifications

Standards and certification

• API 17F, aligned with ISO 13628-6, covers design, testing and qualification of subsea control system components, including SCMs, SEMs and control fluids.
• ISO 13628-6:2006 states that rework and repair of used equipment are outside its scope. The standard alone does not tell you how to requalify a recovered SCM. Agree that scope with the field owner and the OEM.
• Fluid cleanliness is specified to SAE AS4059 or ISO 4406, at the class the OEM sets for the module.

Failure modes and inspection

• SEM faults and communication loss, often in electronics the OEM no longer supports.
• Directional control valves that stick or leak because of contaminated fluid.
• Aged elastomer seals and leaking coupler poppets.
• Water ingress into the SEM housing or connectors, shown as falling insulation resistance.
• Lost accumulator precharge.
• Corrosion, depleted anodes and marine growth on recovered units.

An inspection usually includes a fluid sample checked against the cleanliness class, and external and internal leak tests at LP and HP supply pressure. It adds a function test of every output on a test stand running the OEM's software, and insulation and continuity tests on each SEM. These tests need the OEM's test equipment, so they normally happen at the OEM or an approved shop.

Documentation

• FAT, extended FAT and system integration test reports.
• Qualification test reports for the design.
• As-built data book and material certificates.
• SEM software version and configuration file.
• For recovered units, the recovery report and the fault history that led to recovery.
• For unused units, preservation and periodic check records.

Integration

The SCM must match the mounting base already on your structure, its couplers and connectors, and the running tool your ROV carries. The SEM must run the protocol and software version of your topside master control station. The control fluid must match the umbilical's. Electrical and hydraulic links run through subsea connectors, and installation uses tooling from subsea tools and intervention.

Refurbishment, used or unused surplus

A refurbishment strips the module, replaces seals and worn control valves, and tests or upgrades the SEMs. The shop then flushes the circuits to the cleanliness class, repeats the FAT procedure and preserves the unit again. An unused surplus SCM left over from a field development is the best spare you can find, as long as its preservation records are complete. A recovered unit refurbished and retested by the OEM is the next best option. Check SEM obsolescence on both.

Well-known SCM makers include OneSubsea, TechnipFMC and Baker Hughes. OneSubsea is an SLB-led joint venture that took over Aker Solutions' subsea business in 2023, so older Aker SCMs are now supported through it.