Dry tree floating platforms for deepwater offshore developments are typically Spars and Tension Leg Platforms (TLPs). All production semisubmersibles in operation support only wet tree systems because the hull motions are not sufficiently small to allow the use of riser tensioning systems which connect vertical production risers from the subsea wells directly to the topsides. The wet tree semisubmersible designs can potentially be improved for dry tree applications with reduced heave motions. The advantages of dry tree semisubmersibles for deepwater development are presented in this paper with focus on the hull global performance and the cost benefits. This paper presents a dry tree semisubmersible design with low heave motion and reduced vortex-induced-motion (VIM), hence named Heave and VIM Suppressed (HVS) semisubmersible. A case study of the HVS semisubmersible is performed for the dry tree application to support a relatively small topsides payload of 7,000 MT and five top tensioned risers (TTRs) in benign South East Asian environment. Through numerical simulations using Computational Fluid Dynamics (CFD) and conventional motion analysis based on potential flow theory, the hull global performance and maximum riser tensioner stroke range are provided. The stability analysis results for intact and damaged conditions are presented. The constructability and stability in pre-service conditions are briefly discussed to address the practical application of this hull with respect to cost effectiveness. The results demonstrate technical readiness of this hull design by showing its ability in supporting dry tree systems with field proven riser tensioners and its superior stability for drilling operations. The application of this dry tree semisubmersible is technically feasible today in mild-to-moderate environmental conditions and thus it is an attractive option for new deepwater development with potential cost benefits.

The main challenge in the hydrodynamic design of a dry tree semisubmersible is in limiting its motion responses particularly heave motions to enable the use of riser tensioners. Deep draft semisubmersibles have low heave motions but are more susceptible to vortex induced motions (VIM) due to high slenderness ratios of the columns. A novel in-house developed semisubmersible design named the Heave and VIM Suppressed (HVS) semisubmersible has been designed to possess low VIM and low heave responses required for dry tree applications. A case study of the feasibility of a dry tree HVS semisubmersible in South East Asian environment has been published separately [1]. This paper presents the VIM performance of the same hull, estimated using model testing and Computational Fluid Dynamics (CFD) analysis. From the model tests, VIM suppression is observed in the HVS semisubmersible due to the presence of the column steps. CFD simulations of the model tests show results comparable to the measured data for the HVS semisubmersible. Additional CFD analysis is performed to account for the external damping effect of the mooring lines and risers on the VIM performance of the HVS semisubmersible. This paper together with the earlier publication [1] shows the robustness of the HVS semisubmersible design concept in addressing both the heave and VIM issues in semisubmersibles for dry tree applications.

A dry tree floating platform accommodates Top Tension Risers (TTRs) within the well bay in the center of the platform. The reservoir size dictates the number and size of TTRs required and determines the minimum well bay layout and size. In addition to the platform motions, the TTR and its connecting jumper motions, which occur due to environment loading, influence the minimum riser spacing in the well bay. An optimum well bay layout is a key component in the efficient design of a floating platform. Because the TTRs and jumpers, supported by a floating platform, experience different dynamic responses, a range of dynamic analyses is required to assess the performance of the entire system. This study considers several different environmental conditions in the Asia Pacific region and performs dynamic analyses of riser systems and jumpers to create optimized well bay layouts for several small-scale Tension Leg Platforms (TLPs). Commercial and in-house design tools have been used in this study. The in-house tool is effective for quick sizing of well bays for conceptual studies. It is observed that the TTR and jumper motions vary considerably with environmental conditions. The results from the in-house tool facilitates the dynamic analysis using commercial software. The results of this study demonstrate the influence of riser analysis for well bay layout and sizing; i.e. riser system motions should be estimated to a reasonable level of accuracy at an early design stage to facilitate effective detailed design of the topsides and floating platform.