During the last five years there has been considerable interest in the design of remote subsea production facilities as witnessed by the SWAT programme. A key element in any subsea installation is that of the control system. Multi-core umbilicals providing both electrical power and hydraulic control are cost effective solution for installation close to surface facilities. However, the cost of such an approach increases rapidly with step out distance. An attractive alternative approach is to consider the use of seawater as an hydraulic fluid for subsea installations at large step out distances. This approach lead to unique design problems associated with the nature of the seawater itself. Seawater is a living medium which is highly corrosive, with low viscosity and poor lubrication properties. This far no such system has been designed and tested. This is a measure of the engineering problems inherent in using seawater as an hydraulic fluid. However, good progress has been made on critical components such as seawater pumps.
This paper provide a summary of the design issues which must be addressed in developing a seawater hydraulics systems for a 25 year lifetime in a subsea production facility. The nature of seawater and its implications on the design and choice of materials for the hydraulic systems are considered. Two systems concepts are contrasted, an open system where the seawater leaks or is exhausted into the sea and a closed system where the seawater is retained and recycled.
Form considering the constraints imposed by using raw untreated seawater. It is concluded that some treatment of the seawater is essential. Thus a successful design must include filtration of the raw seawater and the use of a biocide to eliminate living material. Due to the complex nature of the engineering involved in a seawater based hydraulic system for prolonged subsea operation, economic justification for its use is strongly dependent upon the exact operational scenario anticipated. One specific application is for subsea installations at large step out distances in the region of 15–20 km. thus it is unlikely that seawater will replace conventional fluids for most applications.
The UK Continental Shelf oil and gas fields are now regarded as being in a mature stage of development. The majority of the large reservoirs that have already been brought into production. In order to preserve economic oil recovery from smaller marginal fields. Larger step out distances from existing surface facilities and complete remote satellite subsea installation are being considered.
In the past, the use of subsea production systems was regarded as very innovative and only a few oil companies were prepared to adopt them to any extent. More recently, the situation has changed with an increasing use of subsea wells linked to floating production systems as in Balmoral and Ivanhoe/Rob Ray and step out developments in Highlander, Scapa and Don On the UKCS of the 150 subsea completions made, 100 remain operational.
There is evidence in the approval of projects such as Ness and Central Brae that the trend towards using subsea completions will increase in the future.
