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The increased interest in floater designs for ultra-deep water has produced a number of dry tree semisubmersible designs that allow the use of top tensioned risers (TTRs). The primary advantage of the TTRs is that they facilitate direct vertical access to production wells and thereby offer access for well completions and interventions. The principle behind several of the dry tree semisubmersible designs is to reduce the motions of a traditional shaped semisubmersible to a level that can accommodate TTRs. This is accomplished by using heave plates that are positioned beneath the semisubmersible hull and are supported by a lower structure, such as a truss. To a certain extent, the motion responses above and below the response amplitude operator (RAO) cancellation period can be manipulated by designing the relative sizes of the pontoons and columns as well as the size and depth of the heave plates such that the sum of the forces interacts to minimize the heave motion. The paper presents and discusses two semisubmersible designs that assume a common topsides and riser payload. The two designs are sized and analyzed for the new Gulf of Mexico metocean criteria. The comparison is based on hull dimensions, including heave plate and structural support construction. In both cases, results of the hull performance predicted by numerical simulations from fully coupled models are compared and discussed.

This paper presents the design methodology and analysis results for the Baldpate tower for fatigue and wear. It discusses a strategy, that utilizes both spectral (frequency domain) and time domain analysis approaches, for the fatigue design of the connections. The analysis procedures for each of these two approaches (spectral and time domain) are presented and described. The impact on the fatigue life prediction due to the low frequency responses is assessed. The assessment results indicate that for fatigue sensitive joints, the effect of the low frequency responses could be important and need to be properly taken into account in the fatigue predictions. The Baldpate tower has been shown to possess good fatigue characteristics and no significant fatigue problems were encountered in the design phase. This paper also presents an approach for predicting wear volumes at the wear locations (axial tube guides and conductor guides) of the Baldpate tower. A procedure for evaluating wear depths based on the predicted wear volumes and wear ring configurations is described. Two different wear ring configurations are addressed and their pros and cons are discussed. Introduction The Baldpate tower, in 1648 feet water depth, is the first bottom-founded, non-guyed compliant tower installed in Gulf of Mexico (also in the world). Because of compliancy, design and analysis of the Baldpate tower for the in-place conditions was challenging and required thoughtful and rigorous analytical techniques. The in-place design and analysis approaches for conventional fixed platforms are no longer sufficient for the Baldpate tower. More extensive analyses are necessary for the structural components unique to the Baldpate tower concept. The design of the joints and major connections of the Baldpate tower included provisions for long term effects caused by fatigue and wear. The overall fracture control program included a number of measures, such as specification of minimum levels of material toughness at fracture critical locations, weldability verification testing, NDT, and weld profile control. This paper addresses the unique aspects of design required for fatigue and wear effects. Low frequency response is an inherent characteristic of all compliant structures and in general is caused by dynamic wind, wave and current loadings. Fatigue life assessments of the compliant tower connections require treatment of the low frequency responses which can exacerbate the fatigue damage in the connections. The impact on the fatigue damage of the compliant tower due to the low frequency responses, however, can not be fully captured by the traditional spectral fatigue analysis approach. In order to offset and complement the spectral approach, a time domain direct integration approach has to be adopted for fatigue life prediction of the compliant tower. Nonetheless, the time domain approach would result in greatly increased computation times over the traditional spectral approach. Hence, a strategy was adopted for Baldpate which used a spectral approach for screening level assessments to identify fatigue sensitive connections. The time-domain approach was then used to develop the fatigue life predictions for these locations. This paper describes both the spectral and time domain procedures used to predict fatigue lives of the Baldpate joints. The relative importance of the frequency and time-domain approaches is evaluated. A discussion of the important parameters used, including stress concentration factors, S-N data, joint classification, and "Nath Hole" assessment is also included. The fatigue life predictions from both the frequency and time domain approaches are presented and the impact on the

This paper reviews the current practice for the in-place design of Spar hulls. Both the commonly-used approach and the state-of-the-art procedure for the maximum strength and fatigue conditions will be presented. Key assumptions for various design approaches will be discussed along with advantages and disadvantages of each approach. The review will focus on how each approach generates hydrodynamic loadings, performs global motions analysis, and maps design loads from motion analyses to structural finite-element model. Important aspects relating to Spar design will be addressed. In particular, effect of vortex induced hull motions (VIM) will be discussed, and an approach for including the VIM effect in the design of moorings and risers will also be described. Impact on the maximum strength and fatigue capacity of critical structural components due to the assumptions employed in the commonly-used design approach will be evaluated and quantified as compared to the results from the more rigorous state-of-the-art approach.

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.