The successful design of a subsea well control system depends chiefly upon the selection of highly reliable subsea components If a system fails regularly, due to unreliable components, then the cost to maintain that system may quickly escalate and surpass capital cost

Poor specification means that vendors are often constrained into tendering their cheapest solution, in order to maintain competitiveness The cheapest solution may appear to be 'fit for purpose', but for how long?

It is not sufficient merely to specify the reliability or availability of a system, there are differing requirements for the topsides and the subsea elements It is well known that reliability data for electronic components exists, but for other components, such as subsea mateable connectors, not enough reliable data exists For these components additional criteria should be specified to ensure that reliability is addressed during selection

It is traditional for a reliability study to be carried out during the detailed design phase A reliability study alone is not sufficient, it may be "closing the gate after the horse has bolted" The quantitative specification of reliability, availability and maintainability, before contract award, is the key to ensuring that the system design will be adequate


This paper sets out the approach to a quantitative specification of reliability It seeks to illustrate that a realistic specification is possible and it introduces a yardstick by which any subsea well control system may be specified For simplicity, a single subsea well control system, controlling an oil producer, is considered The electro/hydraulic umbilical is not included, as it is assumed that it would be specified separately Reliability has different meanings to different individuals Some think of reliability in a non-quantifiable way, much the same as dependability, and some confuse it with availability Reliability can be very specific, yet rarely is it quantitatively specified It is more common to see availability specified, even though, as we shall see later, availability is a rather non-specific term

The reliability of subsea control components, particularly electronic components, has been the subject of many papers and studies but there is little in the way of guidance for the Operators How does the industry want us to specify systems so that vendors can be confident that they are bidding competitively? The Operators, for their part, want a quality product in order to avoid extensive recurs in the future We must state therefore, exactly what we need, particularly in terms of a system's ability to operate successfully throughout the life of a development

The prevailing view amongst many Operators is that the industry has reached a suitable level of maturity and that we need to ‘cut our cloth’ to suit Advances in electronics, distributed sensors and connectors prove that we can further enhance reliability with little increase in cost We should all assist the industry in trying to achieve higher reliability Opting for low cost, low reliability solutions, in critical areas, puts the Operator at risk and is blatant ‘short termism’

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