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.

Truss Spars that are designed to accommodate dry transportation encounter certain constraints because the overall length and hard tank diameter of the Spar cannot exceed the limits of the transport vessel. When these constraints are removed, hulls can be longer. Increased length allows the fixed ballast and hard tank steel weights to be reduced. Although the truss steel weight is increased for deeper drafts, there is an overall savings in steel when the hard tank and soft tank are taken into account. The heave response of a Spar can be adversely affected by tensioners used to support TTRs. Deeper drafts can improve heave response and an additional heave plate can be introduced if necessary.

A model test campaign on a 1:50 scale model of a Radial Wellbay Spar (RAW) Spar was carried out at the OTRC facility in College Station, Texas. The campaign subjected the model to wind, wave, and current environments from the Central Gulf of Mexico, the Norwegian Sea, and offshore Brazil. Time traces of dynamic wind loads were predetermined from computations using drag coefficients and estimated wind load areas of the topsides. A servo controlled line mechanism was used to apply the wind load to the model topsides. Current forces were modeled using static weights connected at the appropriate elevation on the model. Ten (10) top-tensioned risers (TTRs) were modeled in terms of stiffness and top tension using four equivalent model TTRs. Horizontal restoring forces of the prototype mooring were modeled using a four-model line arrangement. The model was instrumented to measure six-degree-of-freedom rigid body motions, air gap around the deck, wave run-up, water elevation in the riser gap in the hard tank, and mooring and TTR tensions. Global loads on an internal structural component between the centerwell device and hard tank were measured in all environments. Data comparisons were based on selected time traces of various responses and Weibull distributions to predict extreme values. In general, good agreement was found between the measured and predicted values.

Fixed structures operating in extreme offshore environments (eg: arctic) have to resist and survive the challenging conditions like large ice forces. On the other hand, the floating systems in such environments benefit from their ability to be evacuated and transported in the event of severe condition. However, such an ice-management scheme requires the moorings and risers to be disconnected in the severe conditions and reconnected during more clement conditions. Among the floater designs typically considered for extreme offshore applications is the Spar. The Spar offers low extreme motion responses than other shallower draft floaters and as a result, low fatigue damage on the risers. However, this design has versions with open centerwells that can contain leaked product and cause a potential hazardous condition. Furthermore, most designs that require the risers to be disconnected below the mean water line (wet-disconnection mechanism) not only carry the risk of leakage but have poor access for visual inspection. One solution is to use a continuous flexible riser without in-line connections or terminations in the flexible riser between the seafloor pipe line end manifolds (PLEM) and the production deck manifolds (dry-disconnection mechanism). The risers disconnect at an elevation above the water line and the termination point is lowered to a disconnect buoy supported at the keel. Subsequently the floater is moved away. The main difficulty is that the lowered flexible riser has to be suspended from the disconnect buoy and at the same time avoid contact with the seafloor. This paper describes and discusses a design of a dry-disconnectable flexible riser system comprised of a buoy supporting arches to control the bending in the risers during operation and disconnect. The system is particularly effective when the clearance between the keel and the seafloor is restricted. The rationale is based on strength, control of the minimum allowable bend radius and interference among the risers.

A new alternative for large deepwater field development is described. This "Oil Box" (aka "Box Spar") is a multifunction floater capable of drilling, production and storage. It is distinguished from other Floating Drilling, Production, Storage and Offloading vessels by its unique hull form and oil storage system. It's main advantages are flexibility derived from the floatover deck option, use of proven top tensioned riser technology, and motion characteristics which make it operable in a wide range of environmental conditions. Introduction Large oil and gas discoveries have been made in the ultra-deep waters of the Gulf of Mexico. These discoveries are on a par with those found in the other "hot" frontier areas of West Africa and Brazil. The high costs of drilling and producing these ultra deep water fields using conventional approaches naturally yields an incentive to consider new methods and paradigms for field development. One approach being promoted by several contractors for West Africa is the "multifunction" vessel, which can do drilling, production, storage and offloading. These Floating Drilling, Production, Storage and Offloading vessels (FDPSO) have the advantage of reducing the need for costly MODU operations for development drilling and completions, and they allow dry tree operations and workovers. The FDPSO combines the functions of a wellhead platform and an FPSO, reducing thetotal costs for facilities. A disadvantage of the FDPSO approach is that the vessel's schedule is driven by the longest lead time component, usually the process facilities. While combining all functions on one vessel might save some money, project economics might be improved if, for example, the drilling function could be fast tracked with a smaller wellhead platform. Some operators also perceive added risk in "placing all the eggs in one basket". Most FDPSO designs are based on conventional mono-hull construction. These are suitable for subsea wells and flexible risers in most environments, but these hulls are not ideal for rigid risers and steel catenary risers in harsh environments or those subject to cyclonic events. A new FDPSO design has been developed to address these issues. This "Oil Box" (formerly known as the "Box Spar") was originally intended for West Africa (Ref. 1). Recently, the configuration has been optimized for improved motions and is capable of operations in Brazil and the Gulf of Mexico as well. This paper will present a description of the Oil Box as configured for West Africa and as modified for Gulf of Mexico applications. We will also present the current status of development and an economic assessment of the Oil Box for a typical field development scenario. Design Criteria Table 1 lists the design criteria used for designs described here. Table 1 Design Criteria (Available in full paper) Survival environments used for this paper are summarized in Table 2. Two 100-year environments are listed for each location. The environment labeled "current" corresponds to the 100-year current with a 10-year wind/wave environment, and similarly the 100-year "storm" corresponds to a 100-year wind/wave together with a 10-year current.