Morpeth SeaStar: Foundation and Tendon System

Abstract

Abstract This paper describes design and fabrication of the innovative Morpeth SeaStar TLP foundation and tendon system. Advanced analytical techniques in the form of coupled, time-domain simulations were used in specifying tendon equipment. This paper describes the use of those methods in developing the components making up a low-cost, long-life, mooring system. The methods developed are generally applicable to all tension or taut-moored structures. The components developed are particularly well suited to the pre-installation planned and carried out for the Morpeth SeaStar. Introduction SeaStar®1 is a proprietary mono-column TLP concept1. The hull comprises a single, surface-piercing column with three radial pontoons of tapering cross section. The first of its kind, the Morpeth SeaStar, was installed for British-Borneo in the Gulf of Mexico in July-August, 1998. A sister platform will be installed in the second quarter of 1999 in the Allegheny Field. All TLP's, no matter what their hull shape, require a vertical mooring (tendon) system to restrain vertical-plane motions and restrict lateral excursions. This system comprises the mooring lines and their attachments to the hull and to the seafloor. Tendon systems have been undergoing steady evolution since the debut of the Hutton TLP with its forged tendon joints. The basic form of today's tendon was introduced with the next TLP, the Jolliet TLWP. It pioneered the large-diameter, welded tendon based on a commodity steel product, linepipe. The Morpeth hull form is radically different from previous TLP's and presents both challenges and opportunities for tendon design. In designing the Morpeth tendon system, we have tried to take advantage of industry's experience and have, wherever possible, integrated existing components and concepts into a unique tendon system. Elements of the Morpeth tendon design and the design methodology employed are the subjects of this paper. Tendon System Design Concept The Morpeth SeaStar tendon system required (1) six tendons, arranged in groups of two, near the end of each of three pontoons, (2) tendon top attached to the hull at external mooring porches, located about 80-ft below mean waterline, and (3) tendon bottom attached to six foundation piles, one directly under each tendon. System Design Approach. Efficient tendon-system design must be based on consideration of both in-place and installation requirements. Failure to do so results in a system that is either overly-costly or difficult to install, or both. In-place Considerations. The tendon system, in addition to the above configurational constraints, must first of all satisfy mooring requirements in terms of strength, weight in water, and hull/tendon-system natural periods. The tendon system must also be designed for the intended inspection method and planned inspection interval. In the Morpeth case, it was designed for (1) regular in-service visual inspection over the 20-yr service life of the platform and (2) an NDE inspection interval equal to the service life of the platform. This required that the tendon components be both easy to fabricate and easy to inspect during fabrication.