INTRODUCTION

For a conventional multiwell offshore field development, traditionally drilling could only commence after a fixed platform had been installed and commissioned. This meant that the gas or oil was not recovered until well into the development programme, and that the cost of developing fields m deepwater or marginal fields became, if not unpractical, then certainly less financially attractive

It is usually cost-effective for an operator to begin production at the earliest opportunity, because recovery of capital outlay cannot commence until the first gas or oil flows. Earlier production can be achieved by pre-drilling the wells through a drilling template previously installed on the sea bed. The wells can then be tied back to surface production facilities typically installed on a fixed platform or a tension-leg platform. The early production of gas or oil must, of course, be offset against the costs of the template, pre-drilling and tie-back operations

Where the cost of providing a fixed structure proves to be uneconomic, then the use of a template containing the production facilities should be considered. It can provide the drilling facilities in addition to accommodating Christmas trees, production piping, well killing and pigging facilities. The template can act as a gathering point for satellite wells, and export by pipeline to either a jacket located in shallower water or via a riser to a floating structure within the development Its use m this fashion allows the economic development of a marginal field and even small pockets of a large field not accessible by deviation drilling from a central platform The technology used and the expertise gamed from such operations will also prove useful in exploiting the reserves in the deepwater fields which are currently being investigated.

The major considerations governing the design and installation of these structures form the basis of this chapter.

DESCRIPTION
General

When a fixed or tension-leg platform is to be used m conjunction with pre-drilled wells it is first necessary to drill some or all of the wells through a previously installed template. The platform is subsequently installed, the subsea wellheads are tied back to the production trees on the deck, and any further wells can be drilled from the platform Such a template structure, which is only required during the drilling operation, is generally referred to as a drilling template, a typical example of which is shown in Fig 1.

As an alternative, it is possible to locate the production trees on a subsea template and then tie-in to a floating production facility (e g a modified tanker or drilling vessel) or a remote platform The oil or gas is exported to the floater via a riser or to the platform by flowline Such a structure is commonly referred to as a production template (a typical example is given in Fig 2), and is expected to act as a guide and support for the drilling operations and to house the Christmas trees throughout the life of the installation

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