The objective of this paper is to present the new Offset-Pontoon Semi-submersible (OPS) for use as a drilling and production platform. The new semi-submersible features pontoons that are offset to the outside of the columns which lends the vessel a low heave response and enables the use of top tension risers with dry trees. Other advantages of the OPS are a shallow quayside draft, a large payload capacity and the ability to operate in ultra-deepwater.The primary design challenge for dry-tree semi-submersibles is to make their heave motions compatible with the stroke range of existing riser tensioners. Increasing the vessel draft to reduce the heave motions is constrained by installation requirements and stability limits. However, vessel heave motions can be reduced by modifying the hull shape.The OPS has a longer heave natural period and a smaller heave response than a conventional deep-draft semi-submersible. The heave performance of the OPS was compared with a conventional semisubmersible using frequency-and time-domain analyses.In this paper we also present a design study of the OPS for dry-tree field development offshore Brazil. The vessel was sized to support thirteen top-tension risers and a 30,000 ton topside. The paper describes design of the vessel and its global performance. Focus of the study was the wellbay loayout and the tensioner stroke requirements. For the selected design case we found ram-style riser tensioners with a stroke range of 30 ft sufficient. Integration of the hull and topside by float-over is also described.

The Spar continues to be a popular drilling and production platform design for ultra-deep water. In recent years, developers have introduced a number of design variations such as the Arctic Spar, closed centerwell Spar, and long Spar. As the industry moves production into ultra-deep water, the escalation in drilling costs, particularly for deeper more complicated wells, prompts the need to look for new deepwater floater designs, including Spars. This paper introduces some new features to the Truss Spar, including a radial wellbay layout and an adjustable buoyancy centerwell device. This new Radial Wellbay Spar design is investigated and compared to the traditional Truss Spar for the same topside and riser weights and subjected to the same environments. The base case assumes a drilling and production platform with the performance comparison made in terms of hull weights and dimensions and hull motions for post-Katrina Gulf of Mexico conditions. In general, the Radial Wellbay Spar offers a smaller hull with fewer mooring lines for the same payload while maintaining the Spar’s low motion performance.

Tension leg platforms (TLPs) are floating structures moored to the seabed by multiple vertically arranged tension members called tendons . TLPs are mainly used as production and drilling platforms for offshore oil and gas field developments. Since the first TLP was put into service in the North Sea in 1984, a total of 26 TLPs have been installed in water depths ranging from 147 to 1425 m. TLPs are currently not considered viable beyond about 1800 m water depth due to the size and cost of the tendon system required for deeper water. Tendons are kept under tension by excess buoyancy of the hull. Their stiffness restrains the heave and pitch motions of the platform to a large extent. Due to the restrained heave and pitch motions, the main well control valves (i.e., the production trees) can be located on the platform. For most other floating platforms, the production trees have to be located on the seabed.

High-frequency vibrations of Tension Leg Platforms (TLPs), commonly known as ringing and springing, have challenged TLP designers since the first full-scale TLP was installed in the North Sea in 1984. Although current design codes recognize the significance of the ringing and springing response for tendon design, no widely accepted modeling approach for their calculation has yet emerged. This paper presents a nonlinear time-domain model of a TLP that exhibits the ringing and springing response of the vessel. The analysis model uses large displacement theory for the vessel and tendons and a semi-empirical wave model based on a modified linear wave theory. Predictions of vessel motions and tendon loads made with the analysis model were compared to model tests and were found in good agreement with the measurements. The analysis model was also was used to investigate the fatigue damage in the tendons caused by the vessel’s high-frequency response. Tendon stress time histories were computed for nine different unidirectional sea-states. These sea-states represent a condensed wave scatter diagram for the Gulf of Mexico (GoM). The tendon fatigue was calculated from the stress histories by rainflow counting. Fatigue contributions from different frequency ranges were identified by Fourier analysis. The analysis showed that high-frequency response was present in all sea-states even though ringing occurred only in sea-states with significant wave heights above 10 ft. Tendon fatigue damage contribution from high-frequency loads were found to be significant in every sea-state. For all sea-states combined 73% of the up-wave tendon fatigue damage was due to high-frequency response. For the down-wave and the cross-wave tendons, the high-frequency contributions were 57% and 34%, respectively. This paper demonstrates the importance of considering high-frequency response for the fatigue design of TLP tendons. Another finding of the study is that the analysis model using a modified linear wave theory can describe the ringing and springing behavior of a TLP provided other significant nonlinearities of the system are considered.

For the global performance analysis of a floater, the traditional semi-coupled method models mooring lines/risers as nonlinear massless springs and ignores 1) the inertial effects from mooring lines/risers, 2) the current and wave load effects on mooring lines/risers, and 3) the dynamic interaction between mooring lines/risers and the floater. However, these effects are deemed critical for deepwater and ultra deepwater floating structures as they may have a significant impact on the floaters’ motions and mooring line/riser tensions. This paper presents the development and verification of a time-domain nonlinear coupled analysis tool, MLTSIM-ROD, which is an integration of a recently developed 3D rod dynamic program, ROD3D, with the well-calibrated floater global performance analysis program, MULTISIM (Ref [9]). The ROD3D was developed based on a nonlinear finite element method and merged with MULTISIM by matching the forces and displacements of mooring lines/risers with the floater at their connections. MLTSIM-ROD can thus predict the floater’s large displacement/rotation motions and mooring line/riser tensions including all the coupled effects between the floater and mooring lines/risers. In this paper, global performance predictions for a SPAR in the Gulf of Mexico in deepwater were carried out using MLTSIM-ROD. The results were then verified with those from other coupled analysis programs. The paper also presents the results of motions and mooring line/riser tensions of the SPAR using both the coupled and semi-coupled methods. The results from the coupled and semi-coupled analyses indicate that the floater’s motions and mooring line/riser tensions could be significantly influenced by the dynamic interactions between the floater and mooring lines/risers. Hence, the coupled method needs to be considered for deepwater floating structures.