Naval Architectural Considerations in the Design of the Ninian Field Southern Platform

Abstract

Installation costs and technology are important factors when selecting economically feasible offshore platform design concepts. This paper discusses naval architectural considerations that influenced the self-floating configuration of the Ninian field southern platform. Introduction The installation of the Ninian field southern platform in May 1977 marked the beginning of the platform in May 1977 marked the beginning of the development of the Ninian field in the North Sea. This field was discovered in 1974 and was subsequently delineated as approximately 23 km long and 6 km wide. Reserves were estimated at 1.0 to 1.2 billion bbl. The operators initially decided to develop the field from two platforms, selecting a concrete structure for the central portion of the field and a steel tower-type platform for the southern region.Development plans for the southern location were carried out in 1974 to evaluate two possible platform types: a barge-launched jacket and a self-floating tower. Both types of platform were within the state of the art at that time, and the evaluation process considered many factors, including operator preference. A more complete discussion of the preference. A more complete discussion of the development concept studies is presented in Ref. 1. Selection of Tower The candidate structure types were evaluated according to deck-load capacity, structural weight, fabrication cost, installation cost, and construction capabilities of various fabrication yards. A concrete gravity structure, chosen for the central platform, contained the major processing equipment. A steel tower or jacket would serve adequately if sufficient deck-load capacity could be provided to support the process equipment required for the southern process equipment required for the southern location.Barge-launched jackets and self-floating platforms were under consideration or were being constructed for other fields. Large steel self-floaters had been selected for the neighboring Brent and Thistle fields for water depths up to 162 m. Large steel jackets were also under design at the time, although the weight record for a launched jacket was approximately 12 000 t for the Claymore jacket installed in 110 m of water. The projected 22 000-t weight of the southern platform exceeded the capacity of existing launch barges and raised doubts that a jacket could be launched using existing technology or equipment.Several factors favored the installation of a self-floating platform. It could be designed to carry built-in deck trusses complete with skid beams, thereby eliminating the customary offshore installation of deck trusses, and the proposed concept did not require auxiliary flotation for the installation.Finally, economic considerations made it practical to construct a tower in the U.K., where several fabrication yards had large graving docks and were experienced with other self-floaters. These considerations led to the decision in late 1974 to proceed with the design of a self-floating tower (Fig. 1).The design program resulted in a tower with the principal particulars given in Table 1. principal particulars given in Table 1. JPT P. 1357