Model Test of a Mooring System for a Pile-Founded Guyed Tower

ABSTRACT The development of structural concepts for deployment in deep waters has been actively pursued in the 1ast decade by a number of oil operators and design contractors. Among the many concepts investigated, the Guyed Tower was the object of intense study, which eventually led to the first commercial application in 1983, when the Lena Guyed Tower was installed in the Gulf of Mexico by Exxon Co. USA in a water depth of about 1,000 ft. As part of development effort, model tests were conducted to assess the viability of the concept. One was performed by Exxon in the mid-70's, with a 1:5 scale model installed in 300 ft water depth in the Gulf of Mexico. A few years later, as consideration was given to exploiting this .technology in the North Sea, for different combinations of payload and water depth, another test was conducted on a 1:50 scale model in a wave tank. This experimental program, sponsored by thirteen major oil companies, was performed in 1982 using an 8.0 m (26ft) model, one of the largest offshore structure models ever used in a wave tank. Over 200 sea states were run past the model and 64 data channels were continuously monitored. Part of the large amount of data generated in the test program was presented in previous papers. This paper deal s mainly with the experimental results relative to the mooring system characteristics and performance. 1.0 INTRODUCTION In early 1980 Brown &Root completed a preliminary design of a pile founded Guyed Tower Platform, GTP, (See Figure 1) as part of a Joint-Industry Project sponsored by a group of North Sea operators and contractors, usually referred to as The Deepwater Production Platform (DPP) Project. The design specifications were for a 350 m (1,150 ft) water depth in the North Sea and a payload of 19,5000 mt. The preliminary design proved the feasibility of the GTP for this environment. It also evidenced, however, the need for additional engineering studies addressing technical aspects best investigated by means of a series of model tests in a wave tank. Therefore, in late 1980 a Joint Industry Project was proposed to carry out a GTP model test program. 2.0 WAVE TANK CHARACTERISTICS The selected test facility was the Ocean Basin at the Norwegian Hydrodynamic Laboratories (NHL), Trondheim, Norway. The basin has an adjustable water depth with a maximum of 10 m. This 1arge depth allowed a reasonable size model compared to other available facilities, with particular consideration to the spread of the guyline array. The basin is 50 m wide and 80 m long (See Figure 2) with two wave makers, one along each side. The maximum regular wave height for wave maker No. 1 is 0.9 m in the period range 2.4–2.8 s. In the wave period range 0.8–2.4s, the maximum wave height is limited by wave breaking. The maximum wave height for the second wave maker is 0.4 m. Both wave generators are computer controlled.