This paper tells the story (in abbreviated form) of the evolution of the Hutton Tension Leg Platform (TLP) design from the preliminary stage through the preparation of the detailed plans, specifications, and other construction and installation instructions to builders, vendors and operators. An account is given of some of the problems encountered and their solutions. Planning and organization of the management of the project design activity are described. The design is presently being fabricated and will become the first large-scale realization of a development system which can be used in much deeper waters.
When Conoco and its partners decided to develop the Hutton Field with a Tension Leg Platform (TLP) it was with a keen appreciation that the design, construction, and installation of this new system would present problems to challenge the resourcefulness of the project team and their associated contractors. project team and their associated contractors. By developing the Hutton Field, which is located about 90 miles northeast of the Shetland Islands beneath 148 m deep water, with a TLP it was intended to demonstrate the practicality of the concept and thus open opportunities for development of fields in still deeper water.
This paper provides an account of most of the evolutions in the path to final Hutton TLP design, including problems encountered and their solutions. The Hutton design programme allowed extra time for design, recognizing that new system features would take longer than normal to complete. Nonetheless, the schedule could only be adhered to by an early design freeze and an intense design effort with strict design change control.
Preliminary design work for the Hutton TLP attempted to adopt Preliminary design work for the Hutton TLP attempted to adopt design solutions by combining successful prior practices. Prudent system development, even for a new system, involves synthesizing concepts, components and configurations for which there is some related service experience. The size and scope of the new design has, however, led to some key components being altogether novel and others requiring extensive development from prior state-of-the-art. This applies particularly to the mooring and well systems. Design practices for an innovative system tike the Hutton TLP are based practices for an innovative system tike the Hutton TLP are based largely on fast-principles rational methods which require more diligent engineering interpretation than established practices which may be applied to conventional configurations. Although some elements of the design were closely related to prior practice (for instance, for the semi-submersible hull) detailed design methods were developed as a first-time effort of this Hutton design group.
Before describing the evolution of the final design some features of the pre design will be reviewed. Also a brief explanation of the Conoco project management approach will be given, highlighting project design activities.
The TLP is a floating structure connected to anchors fixed in the seabed by vertical mooring lines (tension legs) at each comer of the platform. These vertical mooring lines virtually eliminate the vertical plane motions of heave, pitch and roll while the lateral movements in surge, sway and yaw are compliantly restrained. Buoyancy is provided by the vertical columns and the connecting horizontal pontoons connecting the bottom of these columns. An excess of buoyancy which is greater than the platform weight keeps the mooring lines in tension for all weather and loading conditions.
The TLP concept was introduced in the 1960's and many study and development programs have examined its use in deep water oil developments. An important step in the conceptual development of TLPs was made in 1975 with installation of a small version of a TLP offshore California by Deep Oil Technology. This 635 tonne platform was set in 200 ft deep water and collected data during a six month trial period. This test provided practical information on responses period. This test provided practical information on responses to winds and waves such as mooring loads, platform motions, riser stresses and other data which could be compared with design predictions.
At about the same time, a team of Conoco engineers were analysing subsea production systems for deep water. They recommended that designs should be developed to provide above water platform space to accommodate drilling and production facilities in deep water. The tension leg platform production facilities in deep water. The tension leg platform was recognized as a prospective system with costs that should be relatively insensitive to depth of water. An intensive study concluded that this concept was feasible and could be designed to be reliable.
With this background, Conoco and its partners carried out an in-depth preliminary design of a tension leg platform specifically for the Hutton Field. Many of the key conceptual design premises were established at this time, including that the deck will be built separately as an integrated assembly to be mated to the hull in sheltered waters. All components will be designed for a minimum of 20 years service life. Key components such as tension legs and well riser tensioners will be simple and intrinsically reliable, have backups, and will be replaceable for inspection and maintenance. Individual well risers connect each well to the TLP. The TLP is permanently installed in the sense that it can resist the effects of extreme environmental conditions and continue operations in the same manner as conventional fixed platforms.
This Hutton TLP preliminary design was described to the European Offshore Petroleum Conference in London in October 1980 (Ref. 1). Only a brief description of this design will be given here, together with some background information on TLPs in general to facilitate the account of design evolution and to identify key features of the TLP. Figure 1 illustrates both the preliminary and final designs.