Tao Peng

Low-frequency drift forces and horizontal motions of a moored FPSO in bi-directional swell and wind-sea offshore West Africa

Frequencies of floating production, storage and offloadings (FPSOs) horizontal motions are much smaller than the natural frequencies of waves and thus 2nd-order difference-frequency wave drift forces could excite large response. In this paper, the 2nd-order wave drift forces and low-frequency (LF) horizontal motions of a multi-point moored FPSO are computed in bi-directional swell and wind-sea conditions offshore West Africa (WOA). Newmans approximation has been applied as the computation theory. Numerical approximation compares reasonably well with measured results of model tests. Under environmental conditions in WOA, it is concluded that swell induces larger drift loads than wind-sea at the surge/sway natural frequency, and they both are indispensable to the LF motion. However, the coupling interacting drift loads between different incident angles could be neglected due to the large peak period deviation between swell and wind-sea. Simplified approximation will be effective by superposition of drift force contributions from uni-directional long-crested swell and wind-sea. Newmans approximation provides a tool to accurately simulate drift loads on FPSO in the bi-directional waves in WOA. An important reference has been obtained for both LF motion prediction and mooring system design in bi-directional swell and wind-sea WOA.

Model Test Study on Vortex-Induced Motions of a Floating Cylinder

Vortex-Induced Motions (VIM) under current flow is an important issue for surface piercing cylinders, such as Spar platforms and floating buoys, since it affects the motion performance of these structures greatly. In recent years this phenomenon attracts much attention and many researchers have been making efforts to deal with this problem. VIM is such a complicated phenomenon that more fundamental studies are needed to understand the essence behind VIM. This paper mainly concentrates on a circular cylinder, aiming to eliminate outside influences and reveal the inherent characteristic of vortex-induced motion mechanism. A circular cylinder with an aspect ratio of 1:2.4, which could be considered as a scale model for the hard tank of a typical Truss Spar, is studied by experimental method to investigate the surrounding fluid field, the excitation forces and Vortex-Induced Motion characteristics under various governing parameters, such as the current velocity and direction, the mooring stiffness and distribution, the use and efficiency of helical strakes, and so on. By using a simple flow visualization system, the unsteady flow passing the circular cylinder and the vortices in the wake are captured and recorded. The cylinder is tested respectively under fixed, forced-motion and elastically moored conditions. The fluid field, the vortex structures, and the lift and drag forces under fixed and forced-motion conditions are measured, the VIM performance of the cylinder with two different mooring distributions are studied, and strake efficiency is studied considering current directionality and strake height influence.Copyright

Strake Design and VIM-Suppression Study of a Cell-Truss Spar

Along with the development of offshore technology, Spar platforms have been widely used in deep-sea oil and gas exploitation. Due to the deep draft cylinder hull piercing into the water, Spar platforms could be subjected to Vortex-Induced Motions (VIM) in certain current conditions. To mitigate VIM, helical strakes are used on the Spar hulls, and they have been proved to be effective. Cell-Truss Spar is a new concept of Spar platform which has recently been put forward by State Key Laboratory of Ocean Engineering (SKLOE) of Shanghai Jiao Tong University. It combines some good qualities of the Cell Spar and Truss Spar designs, aiming to bring in the lighter truss section and heave plate damping feature of the Truss Spar to obtain satisfactory heave motion performances, while reduce manufacture and installation difficulties by means of cell concept. Since the Cell-Truss Spar is a new design concept that has physical characteristics which are different from the existing Spars, the global motion performance should be carefully studied and verified. Researches about the VIM performance of the Cell-Truss Spar have been carried out recently (see Wang Ying et al, 2008, etc). Since it is still at the concept design stage, the Cell-Truss Spar configuration is considered without detail strake design in these studies. For the Cell-Truss Spar, which is still on concept design stage at the present time, the design and optimization of the helical strakes is very important to control the VIM response and improve the hydrodynamic performance. In this paper, strake design and VIM-Suppression Study of the Cell-Truss Spar is carried out. As a result of the unique characters on the hull, the outer surface of the Cell-Truss Spar does not form a regular cylinder. Hence, the strakes should be designed more carefully. In this study, four different configurations of strake groups are put forward and studied, and the one with the highest efficiency is chosen to be applied on the Cell-Truss Spar. The fluid field around the hard tank of the hull, the vortex disturbance near the strakes, and the forces acting on the hard tank with different strakes are simulated by CFD method, and the strake efficiency is assessed through model test combining with CFD computation. The optimized strake configuration is finally chosen, and the VIM performance of the strake-equipped Spar is studied.© 2010 ASME

A free surface interpolation approach for rapid simulation of short waves in meshless numerical wave tank based on the radial basis function

The meshless Numerical Wave Tank (NWT) has been developed based on the collocation method and the radial basis function. For simulating short waves, a free surface interpolation approach is proposed in this study in order to mitigate numerical dissipation and accelerate the simulation. A number of fundamental free surface nodes are employed in the procedure of solving algebraic equations with a full coefficient matrix, while many more free surface nodes are utilized in the time-stepping and smoothing procedure by applying the interpolation technique between each adjacent fundamental nodes. The NWT with the free surface interpolation approach is applied to simulate regular waves and irregular waves, and is then validated by both analytical solutions and experimental results. The numerical results are significantly improved by using the approach to increase the number of free surface boundary nodes, whilst the time consumption increases proportionally. For shorter waves, more interpolation nodes need be used. The good agreement between the present numerical results and the analytical and experimental results indicates that the free surface interpolation approach succeeds in rapidly and accurately simulating the propagation of short waves and irregular waves, covering a wide range of wave frequencies.

NUMERICAL AND EXPERIMENTAL RESEARCH ON CURRENT GENERATION IN DEEPWATER OFFSHORE BASIN

During the design and construction of the deepwater offshore basin, its current generation system is considered to be one of the key technologies. In this article, the state-of-the-art deepwater offshore basin at Shanghai Jiaotong University was examined as a case study. Different 3-D numerical models of the current generation system were developed and calculated. The uniformity of flow field in the basin, such as horizontal and vertical current velocity profiles, were calculated in detail. Besides, a 1:10 scaled model test was also carried out. The current velocities at different locations in the basin were measured and compared with the calculated results. It is concluded that satisfactory agreement can be resulted between the numerical simulation and model test. In addition, it is suggested that transition sections and turning vanes need to be set in inflow and outflow culverts to improve the uniformity of flow field in the basin. And the hydraulic performance of the deepwater current generation system can meet the requirements of model tests in deep water.

Experimental Investigation on Added Mass Coefficient of a Truss Spar Subjected to Vortex-Induced Motions

The prediction of response frequency of Truss Spars is important in the estimation of fatigue damage rates. Response frequency depends on the natural frequency of the structure which is influenced by the added mass.The 1:60 scale Truss Spar model tests have been conducted in towing tank at Zhejiang Ocean University (ZOU). The Truss Spar model consists of a hard tank with removable helical strakes, a truss section and a square soft tank. Model tests were carried out with simplified mooring system.In this study, results from the experiment subjected to uniform current were reported. The Reynolds numbers varied from 8E04 to 2E05. The mean added mass coefficient and the time-variable added mass coefficient were calculated. The results shown that the predicted natural frequency based on the measured added mass coefficient was approximately equal to the measured mean oscillation frequency. The added mass coefficient calculated from one oscillation cycle to the next varied considerably. The oscillation frequency from one oscillation to the next corresponded to the natural frequency including the added mass coefficient for the same cycle.Copyright

Hydraulic Performances of Current Generation System in the New Deepwater Offshore Basin

The state-of-the-art deepwater offshore basin is presently under construction at Shanghai Jiao Tong University. Its current generation system is considered as one of the most challenging and crucial equipments in the basin. Hydraulic performances of the current generation system in the new deepwater offshore basin are investigated using both CFD and experimental methods. 3D numerical models are applied and calculated. Besides a 1:10 scaled model test is also carried out to validate the numerical simulation. Uniformities of the flow field in the basin, including horizontal and vertical current profiles with various mean values are studied in detail. In addition turbulence levels are also analyzed. Satisfactory agreements can be achieved between numerical calculation and model test. It is concluded that hydraulic performances of current generation system in the new basin can satisfy the requirements of model tests in deepwater. Uniform and constant current field can be obtained in measuring area. Besides various typical vertical current profiles in deep water, including uniform profiles and sheared profiles etc. can be modeled. And suggestions are also presented which will be helpful instructions for the design and operation of the new deepwater offshore basin.Copyright

EXPERIMENTAL INVESTIGATIONS ON THE HYDRODYNAMIC CHARACTERISTICS OF HEAVE PLATE

Heave plates are installed in Spar platforms to improve the heave motion performance of the platforms. The heave plates not only increase the equivalent added mass of the platform but also provide extra damping to the system. This article presents an experimental study on the hydrodynamic characteristics of an isolate heave plate. The plate is forced to oscillate in the water at rest. The added mass and damping of the plate are used to represent its hydrodynamic characteristics. Influences of the thickness ratio, the plate shape, the corner radius and the porosity on the added mass and the damping coefficients of the heave plate are evaluated at various KC and β numbers.© 2013 ASME

Numerical and Experimental Investigation on Hydrodynamic Characteristics of FLNG With an Account of the Inner-Tank Sloshing

FLNG is a new type of floating LNG platform which is ship-shaped FPSO hull equipped with LNG storage tanks and liquefaction plants and has been developed as an alternative to long pipelines to LNG liquefaction plant on land for stranded offshore fields. Different to the conventional FPSO, the hydrodynamic performance of the being-proposed FLNG can be significantly affected by the inner-tank liquid sloshing, due to the fact that LNG performs better liquidity than crude oil. In order to investigate the hydrodynamic characteristics of FLNG with large hull and high center of gravity, a series of numerical simulations on an FLNG designed by CNOOC and MARIC were conducted with the help of the well-known code WADAM. In addition, corresponding model tests on the same vessel with solid ballast were completed, serving as a verification of the numerical simulations. In order to inspect the sloshing effects of inner tanks on the global motion responses of the FLNG vessel, model tests with liquid ballast were also conducted. Comparisons were made between numerical and experimental results with methods of Fast Fourier Transformation technology and spectral analysis. The numerical simulation results were in good accordance with those of the model tests, which proves the feasibility of the numerical simulations. Furthermore, the effects of the inner tank sloshing on the FLNG hydrodynamic characteristics were summed up. Conclusions obtained from the comparison results would be of significant importance for the design of the FLNG system.Copyright