RIASSUNTO
Abstract
Well completions for tension leg platforms require special consideration in key areas. Standard downhole equipment including subsurface safety valves can be used in a conventional manner from TLP's. Special designs, however, may be required for annular safety systems or sales risers.
Introduction
Future development of deep water reserves will be dependent upon the successful application of new production structures. Perhaps foremost among these is the tension leg platform (TLP). The installation of the Hutton TLP by Conoco in the North Sea will further our knowledge of these deep water designs and their various sub systems. Figure 1 gives an overall view of a TLP.
Although the detailed design of downhole production components need not change simply because production components need not change simply because the production structure is a TLP, the type of completion and functions required are heavily influenced. For example, the steps required during production riser replacement or workover will production riser replacement or workover will be quite different from a conventional platform. Much of the equipment and techniques developed for bottom-based structures can be used unmodified so that a quantum leap in technology is not required for successful TLP completions. However, certain new applications will no doubt also come forward because of the TLP.
SAFETY CONSIDERATIONS UNIQUE TO TLPS
Constraints placed by concurrent drilling and production from adjacent wells on a TLP can affect the selection of setting depth for Surface- Controlled Subsurface Safety Valves (SCSSV's). When well spacing is sufficiently close, i.e. less than 3m (10 ft), at the drilling template, the kick-off point should be above the setting depth of the SCSSV in the producing well. Some operators may even shut in near-by producing wells until the bit is a satisfactory distance from the producing well bore. Sea floor lithology often dictates the difficulty of this phase of the drilling program so that actual template location is important.
One of the key considerations in TLP design is limiting the forces in the tension legs and production risers. These act as parallel motion production risers. These act as parallel motion linkages, such that horizontal movement due to wind and wave action, produce a vertical displacement. Current TLP design provides for production riser tensioning devices to compensate production riser tensioning devices to compensate for the relative motion between the riser (or wellhead) and the TLP deck. The anticipated vertical displacements for the Hutton TLP, for example, range from 2.5m (i ft) to about 0.5m (1.6 ft) depending upon the season. The production riser terminates through a gimballed support device at the TLP and through a transition joint at the subsea wellhead.
The riser may also experience some bowing and this combined with possible vertical displacement should be given special consideration in the production tubing design. production tubing design. APPLICATION OF CONVENTIONAL COMPLETIONS
Many standard types of completion equipment may be utilized in conventional fashion for TLP completions. Standard subsurface safety valves, circulation devices, tubing hangers and packers can be run, set and operated in a similar manner to completions from bottom- based structures. Completion philosophy may thus, to a large measure, be guided by the needs of the well itself rather than the fact that it is completed from a TLP.
One of the key considerations is the treatment of the annulus. The lateral motion of the TLP results in a bending moment on the tensioned production riser. To accommodate this moment production riser. To accommodate this moment a flexible joint is incorporated in the production riser at the mudline. The presence of these items plus the actual motion of the riser make it important to monitor annulus pressure and be able to condition annulus fluid.
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