This paper summarizes VIM towing test results of Houston Offshore Engineering’s Paired-Column Semisubmersible (PC Semi) platform that was performed at the UC Berkeley towing tank. The PC semi configuration is different from a conventional Deep Draft Semi (DD Semi) in three aspects, 1) 8 columns vs. 4 columns; 2) rectangle column vs. square column; 3) larger column slenderness ratio. Typically, a larger column slenderness ratio may result in more pronounced VIM. Since VIM significantly affects mooring and riser strength and fatigue, it is crucial to explore the VIM response characteristics of the PC Semi configuration. VIM has been characterized as a difficult subject with a complicated structural-hydrodynamic interaction. The physical mechanics is generally less well understood and numerical prediction is generally less reliable. Model testing has become an important and widely accepted design tool to derive reliable A/D envelops and drag coefficients. This project builds on the VIM testing experiences obtained in Finnigan and Roddier (2007), which showed that VIM testing at small scale is only slightly conservative. A scale factor of 1:160 was used in the present tests. The Reynolds Number for the tests varied from 15,000 to 30,000 depending on the towing speeds. This manuscript provides some details on the test setup and overall program, and highlights the key results of the tests.

The Paired-Column Semisubmersible (PC Semi) concept was selected as a dry tree alternative for ultra-deep water development by RPSEA in 2009. Since then, RPSEA continuously supported maturation of PC Semi. The PC Semi platform has relatively deeper draft and smaller columns compared to those of a generic Deep Draft Semisubmersible (DD Semi) platform. Due to its larger column slenderness ratio, it might be suspected to have a pronounced VIM response. VIM response characteristics of a floater are the key measures of its performance. It is an essential step to explore VIM response characteristics during concept maturation. An extensive VIM model test program funded by RPSEA was performed at MARIN in June, 2013. Key results are summarized and presented in this paper.Since the VIM response of a floater (Spar or Semi) is a controlling factor for mooring fatigue, VIM induced mooring fatigue analysis was followed by employing the measured test results. A typical loop current scatter diagram for the GoM was considered and the resultant fatigue damage was analyzed and highlighted. A parallel study for VIM induced mooring fatigue of a generic dry tree DD Semi was also carried out to establish a baseline for comparison of the PC Semi response features and mooring fatigue.

The use of semi-submersible platforms has become increasingly popular due to its ability to carry large topsides and the possibility for quayside integration. With recent exploration successes in ultra-deepwater fields of the Gulf of Mexico, major oil and engineering companies are keen to look for a safe, reliable and cost-effective dry-tree option to maximize the value of deepwater field developments. Dry-tree semi-submersible (DTS) emerges as such an option to overcome the water depth and size limits imposed by TLP and Spar, respectively, and enables the platform to carry a large well array and payloads in ultra-deep water. This paper presents the offshore industry’s multi-year efforts to mature two promising semi-submersible platform concepts that can accommodate long-stroke dry-tree risers and have large drilling and production capabilities. Results of technology development and qualification will be highlighted with details on hull performance and hull/riser interfaces. Key structural, mooring and riser analyses and scaled model test results including the long-stroke riser tensioning system will be presented. Remaining challenges that need to be overcome to advance the DTS concepts from “technology acceptance” to “project readiness” will also be discussed.

One key decision operators make when planning field developments is whether the production system will use only subsea equipment or will also include dry trees. Although dry tree floating platforms have primarily utilized SPAR and TLP hull forms, semisubmersible options are being developed and qualified for the Gulf of Mexico. A key component of the dry tree semisubmersible is the riser tensioning system, which must be carefully matched to hull motion characteristics and arrangements. The SPAR platform utilizes self-supporting buoyancy cans or long stroke riser tensioners in a protected location within the SPAR centerwell and in combination with a keel guide. The TLP utilizes short stroke riser tensioners with or without a keel guide and in a location open to interaction with waves and currents. A dry tree semisubmersible is likely to use long stroke riser tensioners in an open wellbay configuration either with or without a keel guide. This paper uses computer analysis and motions calibrated to model test data to evaluate the strength and operational performance of a riser tensioning system for a specific dry tree semisubmersible configuration. The system uses existing riser tensioning equipment in an arrangement that is open to wave and current interaction. The paper compares arrangements with and without a keel guide and presents differences in strength, stroke range and cost. The semisubmersible hull form includes columns arranged in pairs at each corner of the platform and the paper investigates the influence of the paired-column arrangement on wave loading on the risers and supporting structures. The results of the paper indicate what modifications are necessary, if any, for riser tensioning equipment integration. The information and results presented in this paper are applicable to operators evaluating dry tree interfaces on proposed new floater concepts in addition to equipment suppliers planning to provide or qualify riser tensioning equipment for semisubmersibles. The results will also contribute to further development of a dry tree semisubmersible option for Gulf of Mexico projects, which will provide cost and execution plan improvements compared to existing options for deepwater.

Deepwater field developments in the Gulf of Mexico typically consider Spar, Tension Leg Platform and Semisubmersible hull forms as potential candidates for floating facilities. Since 2005, field development studies for floating systems have had to consider more severe environmental conditions, including increased wind and wave criteria released in API Bulletin 2INT-MET in 2007, increased or more prevalent loop/eddy current events and longer wave periods. These changes have quantifiable impacts to Spar hull and mooring design that are evaluated in this paper. In addition to the design challenges presented by the environment, operator functional requirements (e.g. hull-supported top-tensioned risers), robustness requirements (e.g. minimum air gap in survival conditions) and execution plan considerations (e.g. hull dry transport constraints) also have a quantitative impact on the Spar configuration. This paper presents a summary of the recent design challenges affecting Gulf of Mexico Spar design and uses global performance analysis to evaluate different options to update the Spar configuration to effectively satisfy the design challenges. Design solutions that produce acceptable global performance results are further evaluated to quantify the potential benefit to delivery cost and schedule based on overall hull weight. Based on the analysis results, recommendations are made regarding the best solution to meet the identified post-Katrina design challenges. Results indicate that the optimum number of heave plates depends on the top-tensioned riser support system. The effect of overall hull length (a typical execution plan constraint) on overall weight (and therefore cost) for a given payload is identified. The technical solutions and recommendations are applicable to all future field developments that are considering a Spar hull concept to support floating facilities.The offshore oil industry is characterized by more challenging developments, increasing costs, and ever-increasing focus on safety and survivability. The technical conclusions and recommendations from the work discussed in this paper will assist operators that choose the Spar concept in developing more cost effective designs that retain the required robustness and survivability for safe, reliable operations. IntroductionDeepwater production of oil and gas in the Gulf of Mexico utilizes floating and subsea systems. Spar, Tension Leg Platform and Semisubmersible hull forms have been used for existing floating production systems and are typically considered for new developments. All of these hull concepts have unique features to meet the demanding functional and environmental requirements of offshore oil and gas production. As experience is gained in deepwater production, functional and environmental requirements are updated and the hull concepts must be evaluated in consideration of the updated requirements to ensure that each implementation provides a good balance of cost, function, reliability and safety.This paper specifically considers the Truss Spar c...