Vortex Induced Motion (VIM) of a Deep Draft Semi® with four square columns has been observed in model tests and in a prototype configuration. The flow field around the columns causes the vessel to oscillate along one of the vessel’s main diagonals under particular conditions. The observations and conclusions of three series of VIM model programs are combined in this report. Each test program was executed with a unique objective in mind and each setup was unique. Many similarities in response characteristics have been identified. Some of the differences in observed response are identified and discussed. A discussion is provided regarding the scaling of VIM problems, and the importance of the Reynolds number under model test conditions is discussed. The VIM of a typical Gulf of Mexico Deep Draft Semi® may affect the fatigue life of the mooring system and risers. A formulation is developed that predicts the distribution of VIM amplitudes which then can be used to estimate VIM induced fatigue damage. Observations and conclusions are provided of several experiments where VIM response was evaluated in the presence of a fatigue sea state and for a system with increased sway damping.

The Morpeth project is the world's first application of a SeaStar®1 Mini-TLP and establishes that mono-column TLP's can be used to economically develop deepwater fields. This paper discusses the evolution and development of SeaStar technology and its niche in the deepwater platform market. Specifics of the Morpeth SeaStar platform are presented along with a discussion of some execution challenges that arose during the execution of this prototype project. Introduction The inventory of undeveloped deepwater discoveries is evidence that the offshore industry's ability to discover deepwater hydrocarbons exceeds its ability to economically exploit them. This inventory, which grew as deepwater wells were drilled, pointed out the need to develop more economic field development solutions for large and small deepwater fields. The Morpeth Field provides a specific example of discovery, deferral, and development of a deepwater discovery. Although discovered reserves were potentially substantial, proven reserves were not sufficient, or near enough to existing platform and pipeline infrastructure, to become a development priority in comparison to other opportunities in the operator's portfolio. Consequently, after the discovery was made, development was deferred. After a long offset subsea tie-back solution was deemed sub-optimal for this field, British-Borneo proposed a standalone SeaStar oil and gas production facility in the Morpeth Field to process oil and gas to pipeline specifications. This strategically located platform simplified the subsea system, improved flow assurance, increased recovery, and improved the economic attractiveness of future subsea developments in the area. The Morpeth Field development project was sanctioned in February 1997 and the SeaStar platform was fully installed by August 10, 1998. First production was achieved on October 11, 1998 despite interruptions in offshore hook-up operations by three named tropical storms, including a direct hit by Hurricane Earl in September 1998. Successful production operations at Morpeth prove that mono-column TLP's can be used to successfully develop fields in deepwater areas. Until Morpeth cost and schedule performance demonstrated otherwise, tension-leg mooring technology was widely believed to only be applicable to platforms that supported large numbers of dry trees. The Morpeth project demonstrates that tension-leg moorings provide reliable, cost-effective, stable, and expeditious means for mooring small deepwater platforms. The Morpeth project also achieved several other industry firsts including the first TLP hull fabricated in the USA, and the first TLP classed by a Classification Society.

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A Tension Leg Platform (TLP) solution is proposed for dry tree applications in ultra-deep water depths (∼8000 ft). Major challenges for a TLP associated with the ultra deep environment are addressed with an innovative air spring – mass vibration absorber (VAB) system introduced by SBM Atlantia. The tendon system for the TLP therefore is able to scale essentially linearly with the water depth, instead of quadratically for a conventional TLP. This paper details the integrated platform design issues, including the hull, tendon design and the coupled motion analysis approach. It also discusses the system weight and dimension scales for typical applications and the constructability in terms of their upper bound limits compared with existing platforms. A design example is presented for the offshore Brazil Santos Basin environment at 8000 ft water depth with consistent current and relatively large fatigue sea states.