The installation of the Hutton T.L.P. foundations in the summer of 1983 posed a number of new problems with regard to the deployment of large subsea structures. This paper outlines the main installation requirements and discusses those considerations which lead to the development of the final installation procedures. The operational parameters which affected the installation are also reviewed and an historic account of the installation is presented.
The Hutton Field Development is located in Block 211 /27–28 of the U.K. Sector of the North Sea where for the first time a Tension Leg Platform (TLP) is being utilized. The TLP is positioned over a pre-installed well template and is connected by sixteen tension legs, four at each corner, to four separate foundation templates on the seabed 148 meters below the sea surface (Fig. 1). The function of the foundations is to provide an anchorage for the tension legs and to transmit the tension leg loads to the subsoil formation.
The foundations design Ref. 1 consists of four steel templates, each approximately 20.0 meters in diameter, 9.2m high and weighing 985 tones in air (Fig. 2). Each structure is pinned to the seabed by eight piles, 1.83m in diameter, penetrating the seabed to a depth of approximately 59m. Cement grout connects the piles to the template.
Accurate plan location, orientation and "levelness" of each template on the seabed were the primary requirements of this TLP foundation installation.
The degree of accuracy to which the foundation templates are positioned is reflected in the pretension requirements in the tensions. A low degree of plan positional acc4racy, resulting in inclined tension legs, requires a higher pretension and a consequent reduction in deck payload. A high degree of positional accuracy, however, permits relatively lower pretensions and a greater deck payload. The "levelness" requirement, 0.5°, was considered necessary in order to minimise the design angle of the anchor connector fl ex joint and to keep to a minimum the "lean" on the free standing piles prior to and during driving, thus reducing the moment applied to the hammer/pile/template system. The installation tolerances specified for the Hutton templates are presented in Fig. 3.
A review of the foundation design concept was undertaken in early 1980 and the feasibility of driving the foundation piles, using the new underwater hammer, was confirmed. Consideration of the template and pile/hammer weights, combined with the requirements of the installation schedule, lead to a commitment to a semi-submersible crane vessel (SSCV) to carry out the installation. The strict positional tolerances also resulted in the adoption of a one piece structural guide frame to physically locate the foundation templates on the seabed.
