The last 15 years have seen the exploitation of most of the major reservoirs of the North Sea; the probability of finding large reservoirs in the coming years is obviously reducing all the time. Hence, the emphasis will increasingly be on finding and developing infill deposits that can use the infrastructures already in place. As a result of this, we see a growing use for subsea systems, both in the UK and worldwide.
Figure 1 shows the number of subsea wells installed to the year 1995, by which time some 1200 subsea completions will have been installed worldwide. This compares with the current figure of approximately 450 subsea completions since 1961.
Some of these wells will be located on templates and the Christmas tree arrangement may be significantly affected by manifolding, structural and control system constraints as well as inspection, maintenance and repair (IMR) considerations. All of these factors make standardization difficult. The majority of wells, however, are predicted to be free-standing satellites, either in clusters around manifolds or tied back individually to processing facilities.
Of the 750 wells to be installed between now and 1995, it is estimated that some 500 to 550 will be satellites, continuing the historic trend shown in Fig. 2 Such wells have fewer systems requirements than template wells and are, therefore, suitable candidates for standardization.
In this contribution it is argued that for satellite wells the traditional pattern of designing, specifying, procuring and manufacturing equipment on an individual basis is neither cost-effective nor technically efficient. This is especially true in an expanding market. A standardized satellite well system is proposed that is suitable for most applications while offering considerable cost and program savings to operators(Fig. 1 and Fig are available in full paper)
Typical applications of subsea systems to date have been as follows:
a typing back remote or outlying discoveries to existing facilities-usually within 10 km distance;
a early production systems to accelerate returns on project investments;
a marginal fields, often in conjunction with a floating production facility;
a deep-water developments that cannot be exploited by conventional means.
Within each of these categories there have, of course, been many different variations in terms of water depth, service, completion size, etc. This variety of end-uses has tended to encourage a plethora of designs on a project-by-project basis.
In order to identify areas where cost benefits might be obtained, an analysis was performed of the current philosophy for design, manufacture and installation of subsea systems; a philosophy that traditionally is project-orientated rather than product-orientated.
The starting point for most production systems is flowrate: exploration and appraisal wells have been drilled, recovery rates forecast, and tubing, casing and flowline size estimated. Usually, this fixes the wellhead and tubing hanger design Thereafter the following items are addressed:
a christmas tree;
a flowline system;
a control system;
a workover system;
a intervention (maintenance) system