Jim O’Sullivan

Validation of HVS Semisubmersible VIM Performance by Model Test and CFD

The present paper describes the validation of the HVS (Heave and VIM Suppressed) semisubmersible’s VIM response by model test and Computational Fluid Dynamics (CFD). The HVS semisubmersible is designed to achieve significantly improved motion characteristics for supporting steel catenary risers in hurricane and Loop Current environment of the Gulf of Mexico. In the development of the HVS design, a simplified and efficient CFD analysis in which the rotational motions are ignored was first used as a high-level screening tool. A towing tank model test was then performed to validate the VIM performance of the HVS design. Both the model test and CFD analysis demonstrated that the HVS design has superior VIM response compared to an equivalent conventional semisubmersible design.Copyright

Ringing Analysis of a Vertical Cylinder by Euler Overlay Method

A Computation Fluid Dynamic (CFD) analysis procedure is demonstrated for estimating the ringing loads on an offshore structure modeled as a simple vertical cylinder. The procedure is based on the Euler Overlay Method, where the computational domain of CFD solution is minimized to the immediate vicinity of the structure by overlaying Euler solution defined in the outer domain. The simulated ringing load shows good agreement with experimental data. Dynamic amplification factor due to the simulated ringing load is investigated for different damping ratio and natural frequency of the structure. Ringing response due to breaking wave shows significance of ringing load when evaluating dynamic amplification of wave load.Copyright

Investigation on the VIM Mitigation of the HVS Semisubmersible

The mitigation of Vortex Induced Motion (VIM) of the HVS (Heave and VIM Suppressed) semisubmersible is investigated through extensive comparisons between CFD analysis and VIM model test results. It is shown that the lower VIM response of the HVS semisubmersible results from the break in coherence of vortex shedding along the length of column due to the column step. The present CFD application was carried out on the basis of in-house best practices for VIM analysis of multi-column floaters. The analysis results show excellent comparison with the model test results. The present findings and methodology can be applied to optimize semisubmersible hull designs for suppressed VIM response.Copyright

Benchmark of CFD Modeling of TLP Free Motion in Extreme Wave Event

In this study, a numerical wave tank was set up to simulate the free motion of a Tension Leg Platform (TLP) in extreme wave event. For better computational efficiency, a nonlinear potential flow solver is coupled with a CFD software, with the former to simulate the far-field wave domain and the latter to simulate the near-field wave domain and platform motion. In order to benchmark against model test, a five-minute time window of interest was selected from the extreme sea state in model test. The incoming irregular wave was firstly reconstructed from the measured wave time history using the nonlinear potential flow solver and then applied as input to CFD simulations for two different headings to the platform. Static offset tests and free decay tests were simulated in CFD initially to confirm that the platform and tendon properties were properly modeled. The 6-DOF platform motions were then obtained from the CFD simulations and the time histories of motion, air gap, and tendon tension were compared with model test measurements. Good agreements were achieved except for the initial transient period and low-frequency motions. In particular, the air gap or relative wave elevation compared well for all the locations around the platform. The high frequency response in tendon tension and the different tension characteristics of weather side tendons and leeside tendons were also well captured.Copyright

Validation of the HVS Semisubmersible Global Performance by Model Tests

Validation of the HVS (Heave and VIM Suppressed) semi-submersible’s global performance is carried out through wave basin model tests and correlation with numerical analysis. As an application for a wet-tree floater, Technip designed the HVS semisubmersible to have reduced heave and VIM (Vortex Induced Motion) response. The HVS semisubmersible has blisters attached to the columns. The blister breaks the coherence of the vortex shedding along the length of the column and as a result reduces the VIM response. The redistribution of pontoon volume because of the blister reduces the heave excitation loading. The blister also provides adequate buoyancy for quayside integration.To validate the improved hull global performance, the wave basin model tests were performed. In the tests, collinear hurricane environments of the Gulf of Mexico were considered for two different headings. Truncated mooring was applied to simulate prototype mooring system. Wind loading was applied by means of a dynamic controlled wind winch. Current was generated by a wire attached to a system of pulleys. The measured hull responses were correlated with MLTSIM, a Technip in-house time-domain nonlinear motion analysis program. Through the correlation with model test results, the improved HVS semisubmersible global motion in hurricane environments is validated.Copyright

Numerical Wave Tank Analysis of Wave Run-Up on a Truncated Vertical Cylinder

A new far-field closure condition for a CFD-based numerical wave tank that uses a potential wave solution to overlay the outer computational domain of a CFD solution is described. A prescribed potential wave solution covers the region beyond a diameter more than 10 times of floater footprints. The diffracted waves from the body are absorbed by the ‘potential-attractor’ terms applied in the intermediate CFD domain where the CFD solution for Navier-Stokes equation is gradually blended into far-field potential solution. The proposed model provides an efficient numerical wave tank for the case when incoming wave length is much longer than floater. In this case, the required mesh and domain size for numerical accuracy is mainly affected by the floater geometry and local wave kinematics near the floater and less dependent on the length scale of the incoming waves. The new numerical wave tank is first tested for a diffraction of a truncated cylinder exposed to long regular waves. Comparison with theoretical and experimental results demonstrates accuracy and efficiency of the new method.

Model Tests With the HVS Semisubmersible for Dry Tree Application

The global hull motion performance of the HVS semisubmersible for dry tree application is investigated with model tests. The HVS semisubmersible, which has been validated for low heave motion and VIM (Vortex Induced Motion) response, was modified for dry tree application. As a base case, the modification includes a keel plate with riser keel guides at the level of the pontoons. The keel plate is optimally designed to increase the hull heave period to compensate for the heave period reduction in the HVS semisubmersible due to the riser tensioners for the dry tree application. The plate also provides additional viscous damping that decreases the heave response at the heave natural period.The model tests were performed to investigate the in-place hull motion performance for the Gulf of Mexico environmental conditions. The pneumatic riser tensioners were modeled using a spring with dual stiffness. Because of the water depth limit in the wave basin, a truncated mooring was used to simulate the full scale prototype mooring system.An alternate modification to the HVS semisubmersible that includes pontoon plates was also tested and the measured response was compared to the response of the base case. The measured hull responses were correlated with MLTSIM, a Technip in-house nonlinear time-domain 6-DOF motion analysis program.Copyright

Global Performance and Sloshing Analysis of a New Deep-Draft Semi-Submersible LNG FPSO

A new concept of LNG FPSO based on a deep-draft semi-submersible hull is introduced. With the deep draft, small water plane area, low center of gravity and large radius of gyration, the new LNG FPSO offers very low motions. This low-motion LNG FPSO platform provides more options and flexibilities in the selection of LNG liquefaction units, LNG containment systems, construction sites, installation methods, mooring systems (i.e. no requirements for weather-vaning), riser system and less down time compared with a conventional FPSO hull. Global performance and sloshing analyses for the new LNG FPSO hull and the conventional FPSO hull are performed to compare their operating performance for West Africa and the Northwest Australia environments.Copyright

A Comparison Study of Improved SCR Response for HVS Semisubmersible

A new deep draft semisubmersible HVS (Heave and VIM Suppressed) has been developed at Technip as another wet-tree floating platform option, which achieves significantly improved heave motion and Vortex-Induced-Motion (VIM) performance as compared to conventional semisubmersible concepts. This paper presents results from a study that compares the strength and fatigue performance of a gas export (20 inch) SCR that is supported by either the HVS semi or a comparable conventional semi in 4000 ft or 6000 ft of water depth. Both floaters are designed for the central Gulf of Mexico, and have comparable hull principal dimensions and mooring configurations. For the HVS semisubmersible in an extreme sea state (100-yr hurricane), the reduced heave motion significantly improves SCR strength performance at the Touch Down Zone (TDZ) and effectively eliminates the compression load at the sag bend region. The assessment of SCR fatigue responses indicates that most damage near the TDZ results from wave induced motion fatigue and hull VIM fatigue. The suppressed VIM response of the HVS semisubmersible due to the unique blisters attached to the four hull columns significantly enhances the SCR fatigue life near the critical TDZ. The results from this study confirm HVS as a SCR-friendly wet tree host concept for the deep waters in GoM.Copyright