Subsea Control System 

Early subsea control systems were born with the concept that Aerospace Quality was essential for design and build of equipment for subsea applications. Aerospace hydraulic valves were used for control functions, hydraulic systems were flushed to Aerospace Standards, and electronic components used were Military (MIL) Specification. Dual Systems were used to eliminate single point failure.

Components were, and still are, subjected to Qualification Testing. This requirement was for life cycle testing of valves for wear, leakage, and failure. Pressure Testing of components and housings at pressures usually double working depth, burn in and cycling electronics for temperature, shock and vibration. Testing hydraulic connectors for make-up and leakage, testing of electrical connectors for make-up, continuity and Insulation Resistance.

Material compatibility was (and still is) a major issue. This can be due to electrical cells between different metals in seawater, compatibility of seals in valves or hydraulic connectors, or compensation bladders with hydraulic oils, dielectric oils, or chemicals.

Hydraulic tube connections were welded for Integrity. Weld Procedures and Welder Qualifications are necessary for maintaining integrity in systems. Where welds are in seawater the welds are over alloyed to provide a more noble material in the weld and heat affected zone.

There was a great step change in policy and thinking from the requirements for the Oil Patch to the requirements of subsea technology. The change was brought about due to the high cost of failure for intervention, paralleled to the disastrous results of similar failures in the aerospace industry. Modern Quality Assurance techniques and procedures have been refined from the original aerospace applications and are now accepted as standard in the subsea industry.

Planning quality is paramount in aiding the purchaser to achieve the criteria that has been prescribed within the contract and associated documentation with the ISO 9001 Standard being the foundation, covering all areas from contract review to delivery of the completed product or service.

Experience over the years has revealed that quality has to be built into the contract at the onset, and typically, mandatory documentation deliverables for purchaser approval would be:

Following on from qualification testing, and demonstrating the reliability of system components (some of them prototypes), there is the requirement to manufacture in production quantities. Hydraulic components are commonly manufactured in materials that are usually difficult to machine, to high tolerances. The requirement to miniaturise components in order to make control modules smaller and easier to handle and install in deep water has necessitated tighter tolerances and less room for error. Gun Drilling is necessary for long hydraulic galleries, Cross drilling often leaves burrs at the point of intersection, which have to be removed by laser de-burring and ultrasonic techniques.

Production testing of components, sub-assemblies, assemblies, and the system is necessary to maintain quality and to ensure that the system will perform after installation to the design criteria.

Factory Acceptance Testing (FAT) is fully covered in Section Chapter 26, Factory Acceptance Testing (FAT).

A Supplier's Quality Assurance system (in conjunction with the allocated Project Manager) will normally provide the control and documentation of all of the above aspects.

However, the Customer is recommended to mobilise its own inspectors to follow all stages of a subsea control system contract in order to fully assure itself that each component is correctly manufactured and tested. This also ensures the eventual data books delivered with the system have already been witnessed by the Customer's representatives and avoids last-minute delays due to uncertainty about any individual item.

The manufacture, testing, and long-term reliability of components and subsea systems is facilitated by the application of the Quality Management System requirements of ISO 9001. However, these systems are very complex with many components, sub-assemblies and interfaces. Quality Objectives can only be achieved in practice by effective Project Management and Control, and by ensuring that the supplier is completely aware of his responsibilities at the onset of the project. It is too late to try to modify quality requirements once the ITP’s and quality plans have been drafted and agreed. In essence, the only real way of ensuring the COMPANY and project quality requirements will be achieved, is by ensuring that each and every FAT, SIT and inspection of every deliverable component of the Subsea Control System is attended by a knowledgeable and competent COMPANY representative.

From a CAPEX point of view this may seem to be an expensive initial cost, however in reality, if there are any inherent faults which have not been identified before installation, the cost to recover, transport and repair the faulty equipment will undoubtedly be far more expensive in terms of capital cost and project delay.