Haining Lu

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.

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

Study on added mass coefficient and oscillation frequency for a Truss Spar subjected to Vortex-Induced Motions

Vortex-Induced Motion (VIM) is an important cause of fatigue damage of a Truss Spar. Response frequency is one of the critical components in estimating VIM. Response frequency is related to the natural frequency of the structure, and it is also impacted by the effect of the added mass. In this study, Reynolds numbers of the experiment varied from 8E04 to 2E05. The experiment results indicate that the added mass coefficient decreases with the increasing reduced velocity and the oscillation frequency of the low mass ratio cylinder locks in the natural frequency instead of still water natural frequency. The estimated added mass coefficient from one oscillation cycle to the next varied considerably. The oscillation frequency from one oscillation cycle to the next is associated with the natural frequency which is estimated by the time-variable added mass coefficient.

Wave Drift Forces on Three Barges Arranged Side by Side in Floatover Installation

Floatover is a new method for installing integrated topside of a spar platform. It has several obvious advantages such as less time and cost compared with derrick lifting. In general, the floatover installation consists of three procedures: firstly a single barge is used for long-distance transportation of the topside in order to get good stability; secondly two barges take place of the single barge for floatover installation near the operating site; finally the topside is transferred from the two barges to the spar hull and the installation is completed. Between the first and second procedures, the case occurs that the single transportation barge is sided left and right by two floatover barges in the second procedure with close proximity. This case is concerned by many designers and operators for the security problem brought by possible large relative motions and forces of the three barges in side by side configuration.The hydrodynamics of side-by-side barges are much more complex than that of a single barge in waves. In numerical simulation, it is a challenge to consider all effects including the hydrodynamic interactions, the shielding effects, the viscous effects and the wave resonance effect which has been observed in the gaps between the barges and has a significant impact on wave drift forces. In this paper, motion responses and wave drift forces were calculated in frequency domain for both the multi-body system and the single body. Far-field, middle-field and near-field method were all carried out to calculate wave drift forces. Numerical analysis was executed using potential flow code WAMIT. Corresponding model tests were also performed in the Deepwater Offshore Basin in Shanghai Jiao Tong University. Comparison between numerical and experimental results shows that numerical results agree well with the experiment and that middle-field method has better convergence than near-field method. The comparison between the multi-body system and single body shows that the hydrodynamic interactions (including wave shielding effect and Helmholtz resonance of water in the gaps) are remarkable and motion responses in the multi-body system are larger than single barge at some frequencies.Copyright

The Effects of Surface Waves and Submergence on the Performance and Loading of a Tidal Turbine

Tidal energy has the advantages of high predictability, high energy density, and limited environmental impacts. As tidal turbines are expected to be used in the most energetic waters where there might be significant waves, the assessment of unsteady hydrodynamic load due to surface waves is of great concern. The objective of this paper is to assess the effects of surface waves and submergence of the turbine on the power performance and loads ∗Address all correspondence to this author. of a tidal turbine by experimental approach. The experiments on a 1 : 25th model tidal turbine were carried out in a towing tank. A wide range of regular waves with periods from 1.0 s to 3.0 s at model scale were generated. Different submergence conditions were considered to investigate the effects of the presence of free surface. The cases with blade tip partly going out of water were also performed. The regular waves did not have significant influences on average loads and power output in the present experiments, but caused large amplitude of the cyclic variation of the loads. The amplitudes of the cyclic load were proved to be 1 Copyright c

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

Numerical, Experimental and Full-Scaled Investigation on the Current Generation System of the New Deepwater Offshore Basin

The new deepwater offshore basin was completed in 2008 at Shanghai Jiao Tong University. The current generation system is considered to be one of the most important and challenging equipments in the basin. A lot of work has been conducted to investigate the characteristics of current generation system in the new deepwater offshore basin. Main particulars and equipments of the new basin are described in detail. Some ocean deepwater tests conducted in this new basin are also introduced. Numerical simulation, model test and full-scaled measurements in the real basin were conducted to investigate the hydraulic performances of deepwater current generation system in detail. It can be concluded that the current generation system has satisfactory performances by different methods. Uniform and constant current flows with different current velocities can be generated in this new basin. Several typical current velocity profiles can also be modeled in measuring area of the basin. It indicates that the hydraulic performances of the current generation system can satisfy requirement of offshore model tests in deepwater.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

The hydrodynamic performance of a tension leg platform with one-tendon failure

ABSTRACT As one of the most widely used types of deepwater offshore platforms, the Tension Leg Platform (TLP) has shown excellent motion performance in waves; however, a detailed evaluation of their performance under tendon failure condition is vital and necessary. In this study, numerical simulations were conducted in both time and frequency domains using the commercial software HydroD and OrcaFlex, respectively. The numerical results were validated by a series of wave basin model tests. The motions of the TLP, with one tendon failure, and loads on the effective tendons, were studied in dynamic analyses to evaluate the potential adverse impact of tendon failure. Moreover, sensitivity studies of TLP performance under tendon failure were also conducted with various quantities of hull ballast. The relation between the response of the TLP and quantities of ballast were established and some advices were provided about lowering the risks of the loss of TLP stability under one-tendon failure condition.

Experimental investigation on the hydrodynamic performance of a quay moored jackup

Abstract Offshore platforms under construction are normally moored along the quayside during the outfitting stage. The safety of the platforms must be guaranteed during the entire process of outfitting which may last for several months. In this study, the hydrodynamic performance of a quay moored jackup is investigated by using a set of wave basin model tests. In the seakeeping model tests, the dynamic responses of the system, including the six degrees of freedom motions of the jackup and the barge, tensions on the mooring lines and the collision forces on the fenders, were measured in various sea states, and statistical analyses were conducted. Meanwhile, the dynamic responses in current-and-wind-only sea states were analysed for comparison with those in the wave conditions to assess the influence of the wind and the current. It is observed that the offshore wind plays an important role in quay mooring systems and the mooring line tensions are found to exceed the strength of the lines in offshore wind conditions. The possible reasons are discussed in detail and suggestions are proposed for optimising the mooring system performance.