Subsea Control System 

The materials used in subsea control systems can vary from basic materials such as carbon steel, to more exotic materials such as 'super duplex' or Titanium, for various reasons such as corrosion resistance, strength, or compatibility.

Carbon steel is well understood in terms of material properties, corrosion in seawater, weld ability, and cost. Carbon steel is used for weldments and housings for all major subsea components:

The material sections are fabricated into frames and housings for containment installation and pulling in. Where materials are designed for containment such as electrical distribution units, they are oil filled and pressure balanced. Therefore, deepwater service does not cause a problem.

To slow the rate of corrosion, carbon steel is coated. There are some exceptions however, where areas are not coated in small areas where electrical earth bonding is required for electrical connectors, or where a face seal makes up.

Hydraulic systems and connectors generally use 316L stainless steel as the main component for its external corrosion resistance, and internal suitability with clean hydraulic systems. Quick-connect / disconnect couplings are assembled from many components requiring different materials. Monel is used for screw nuts to prevent galling, and for coupling self-sealing poppets where a higher strength material is required. Material control is required to eliminate problems with Hydrogen embrittlement.

Sometimes Nitronic is used for Quick connect couplings in deepwater applications. Nitronic 60 is used for the female couplings and Nitronic 50 for the male couplings. The Nitronic 50 provides a high strength material and the difference in hardness between the 60 and 50 grades provides anti-galling properties.

Springs have to be from a material with permanent spring properties that does not corrode. Hastelloy is often used for this service, and Elgiloy is now being used.

The O-Ring seals for the female halves of the hydraulic quick connect / disconnect coupling need to be compatible with the hydraulic or chemical fluid used and with seawater. Seals are made from materials such as Nitrile, or Viton for hydraulic service, and Chemraz, Fluortrell or Isolast for severe service such as chemicals. Backup seals to prevent O-Ring extrusion can be of Teflon or Peek. For metal seal couplings, seal material such as Inconel 750 gold plated is used.

Piece parts such as the SCMMB interface, the SCM base and mating stab plate assemblies are all machined from 316 stainless steel.

Electrical Connectors are usually made using 316 stainless-steel bodies (Total standard is super-duplex e.g. Ferralium SD40 or Zeron 100) with water blocks of Ultem and the conductive pins of gold plated copper. The pressure compensation diaphragms and the boot seals for the cables are made from neoprene or synthetic rubber. The pressure containment fluid is silicon based dielectric oil.

The directional control valves (DCVs) within the control modules are made from 316 stainless steel bodies, with internals to seal with metal to metal shear seal action, and also to provide sufficient wear capabilities. In past deepwater applications, seawater has migrated into valves causing corrosion problems and a new generation of valves with seawater tolerant internals such as Inconel, MP35N, and ceramics are being used.

The electronics assembly within the subsea control module is contained in a one-atmosphere housing called a SEM housing. The SEM housings were traditionally made from Aluminium for lightness. For deepwater applications where greater strength is required to prevent the housing collapsing, high strength steel or aviation strength aluminium is used.

Transducers are made from materials to suit interfaces. Choke position indicators housings are made from 316 stainless steel, pressure transducers which interface with Xmas tree flanges, ring grooves and gaskets are generally made from duplex stainless steel.

The noble materials such as stainless steel and Monel do not need coating. Neither does Duplex stainless steel, although it often is coated to reduce the un-coated surface area used in calculations of the amount of anodes for surface protection.

Carbon Steel is coated with a multi-coat epoxy designed for permanent use underwater. The coating of carbon steel is time-consuming, taking several days to apply the coatings, but has the advantage that high-visibility colours, such as white and yellow, can be used.

Aluminium based anodes (Al-Zn-In) anodes are used in a sacrificial mode to protect the subsea structures. The number of anodes required is based on the initial un-coated surface area plus the estimated area that becomes un-coated during the field life.

- Electrical Connectors Earth Bonded to SCMMB

Suppliers providing Cathodic protection for their own equipment in isolation, and lack of system engineering has led to an over-protection of subsea facilities with its own problems. This has led to a change in philosophy so that subsea control equipment is protected under the main structure e.g. an Xmas tree by earth bonding to the main structure Cathodic protection system.

This provides the benefit during installation in deepwater of equipment such as subsea control modules. SCMs without sacrificial anodes attached are lighter and do not have additional protrusions that could prevent easy installation.

Galvanic Cells between different materials in seawater can cause corrosion and pitting between materials. In a similar manner to the electrical cell set up allowing the Zinc Alloy anode to sacrifice by design to protect the more noble Cathode, dissimilar materials in contact or close proximity can also set up a cell and one material will corrode. It is a very important design consideration that should not be over-looked, and those materials in close proximity have similar potential differences in seawater.