Concrete TLP's in Water Depths Beyond 3,000 ft

Abstract Concrete hull TLP's are established as a viable concept for field development in water depths up to 3,000 ft. The first platform is now under construction for the norwegian Heidrun field. However, as the water depth increases, the platform natural periods tend to increase, which also will increase the tether fatigue. Is there a limit for the concrete hull? This paper describes a large size platform for Gulf of Mexico conditions at different water depths. A comparison is made with a steel hull alternative, under identical functional requirements. In particular, the tether extreme and fatigue responses are analyzed. The results demonstrate that concrete hull platforms are competitive also beyond 3,000 ft. High natural periods can be acceptable for the tether system provided that the platform geometry gives low excitation at roll/pitch resonance as well as low responses at higher periods. Introduction In the last few years, concrete has been introduced as a construction material also for floating production platforms. The concrete hull TLP has been found feasible and economically attractive in water depths up to 3,000 ft, as demonstrated by Wilson /1/ and Lokken /2/. The tether system, that moors the platform, is typically designed for extreme storm condition, which also provides adequate fatigue life (assuming appropriate component design). As the water depth increases, the fatigue life becomes more critical, as the tether stiffness decreases and the natural periods increase. In order to reduce the response at resonance, the natural periods can be reduced by increasing the tether stiffness. This means that the tether system is no longer designed for stress, but for stiffness (steel cross sectional area) " For Gulf of Mexico conditions, a practice is about to emerge, which claims that the natural periods in heave, roll and pitch should not exceed four (4) seconds. This paper sets about to investigate the relevance of such a limit, which may be questioned on the basis that the tether responses are a function of the platform geometry. For comparison, a concrete and a steel hull are analyzed with identical tether system. Design Basis The environmental conditions for this study are based on published information, the extreme conditions presented in Table 1 are taken from Brewer /3/, while the fatigue conditions, the wave scatter diagram, is taken from Kinra and Marshall /4/. Three waterdepths are considered; 3,000 ft, 4,500 ft and 6,000 ft. The platform requirements represent a large size Gulf of Mexico field. The topside mass, including payload and deck structure, is 30,000 tons (27,216 metric tonnes). This carrying capacity is defined for 3,000 ft (914 m), as the hull geometry is kept identical for larger water depths, the deck weight has to be reduced to allow for the increased tether weight. The total riser tension is 14,000 kips (62.3 MN). The riser tension is taken as independent of water depth (assumes that buoyancy is attached in very deep waters).