Numerical and experimental study on hydrodynamic interaction of side-by-side moored multiple vessels

This paper aims to investigate the basic interaction characteristics of side-by-side moored vessels both numerically and experimentally. A higher-order boundary element method (HOBEM) combined with generalized mode approach is applied to analysis of motion and drift force of side-by-side moored multiple vessels (LNG FPSO, LNGC and shuttle tankers). Model tests were carried out for the same floating bodies investigated in the numerical study in regular and irregular waves. Global and local motion responses and drift forces of three vessels are compared with those of calculations. Discussions is highlighted on applicability of numerical method to prediction of sophisticated multi-body interaction problem of which motion behavior is very important to analysis of mooring dynamics of deep sea floating bodies.     

The Behaviour of LNG Carrier Moored to a Jetty Exposed to Waves,Swell,Unsteady Wind and Current

—Most terminals for tankers are piers and sea islands,while other types include single pointmoorings and multiple-buoy moorings.The LNG and LPG carrier moored to the jetty is a very commonterminal for transfer of gas in open seas.It is important to estimate the motions and line tensions of theLNG carrier when it moors to a jetty in metocean environment.Normally,the motions of the LNG carrierwould be restricted by the loading arm,which is connected to LNG carrier＇s manifold.An example of125,000m~3 LNG carrier moored to a jetty exposed to a set of environment conditions is given.Amathematical model which is based on the equations of motion in the time domain is used to the analysisof LNG moored to an offshore jetty exposed to waves,swell,wind and current.By means of a time do-main computer program TERMSIM computations are carried out to determine and optimize the lay-outand/or orientation of the jetty and mooring gear in terms of forces in mooring lines and fenders and theenvelope of motions of the loadi     

Time domain simulation of side-by-side floating bodies using a 3D numerical wave tank approach

The coupled system of two side-by-side fixed and/or floating bodies interacting with a large amplitude nonlinear wave is studied using a direct time domain solution method. The numerical model is based on a three-dimensional mixed Eulerian–Lagrangian (MEL) method under certain simplifying approximations permitting Rankine panel scheme to be implemented over a time-invariant boundary surface to solve the boundary value problem for the unknown velocity potentials. A 4th order Adams–Bashforth–Moulton scheme is used for time marching of rigid-body motion histories of the individual bodies and evolution of the free-surface including the gap region in which large resonant fluid motions occur. A systematic study has been carried out to evaluate the performance of the developed time domain method in simulating the forces and motions as well as the fluid motion in the gap region for the two body system under various arrangements and in different wave-headings. At first, the computed numerical results have been validated and verified with computational and experimental results available in literature for standard geometries such as vertical truncated cylinders and rectangular boxes. Secondly, effectiveness of the damping lid model which is introduced to suppress wave resonance in the gap region is investigated including its influence on maximum sway forces on fixed and floating rectangular barges in side-by-side configurations. Thirdly, comparative studies on absolute and relative motion response for two cases (two rectangular barges, and a FLNG-FPSO + shuttle tanker) in side-by-side arrangement are detailed to bring out the importance of nonlinearities arising due to steep nonlinear incident waves. Finally, coupled motions of the two-body system of an FPSO and a shuttle tanker floating in side-by-side configuration in a steep nonlinear wave field are studied in which the two bodies are connected through hawsers, and also the FPSO is moored to the ground. Additionally there is a fender between the two bodies.     

Wave driven free surface motion in the gap between a tanker and an FLNG barge

When two vessels are moored side-by-side with a narrow gap between them, intense free surface motions may be excited in the gap as a result of complex hydrodynamic interactions. These influence the motions of the two vessels, and the forces in any moorings. The present paper uses first and second order wave diffraction analysis to investigate this phenomenon. Key theoretical aspects of the numerical analysis are first summarised, including the vital need to suppress “irregular frequency” effects; and results are given to validate the code used. The case of a tanker alongside a large floating FLNG barge is then considered in detail.     

Structural Safety Assessment for FLNG-LNGC System During Offloading Operation Scenario

The crashworthiness of the cargo containment systems (CCSs) of a floating liquid natural gas (FLNG) and the side structures in side-by-side offloading operations scenario are studied in this paper. An FLNG vessel is exposed to potential threats from collisions with a liquid natural gas carrier (LNGC) during the offloading operations, which has been confirmed by a model test of FLNG-LNGC side-by-side offloading operations. A nonlinear finite element code LS-DYNA is used to simulate the collision scenarios during the offloading operations. Finite element models of an FLNG vessel and an LNGC are established for the purpose of this study, including a detailed LNG cargo containment system in the FLNG side model. Based on the parameters obtained from the model test and potential dangerous accidents, typical collision scenarios are defined to conduct a comprehensive study. To evaluate the safety of the FLNG vessel, a limit state is proposed based on the structural responses of the LNG CCS. The different characteristics of the structural responses for the primary structural components, energy dissipation and collision forces are obtained for various scenarios. Deformation of the inner hull is found to have a great effect on the responses of the LNG CCS, with approximately 160 mm deformation corresponding to the limit state. Densely arranged web frames can absorb over 35% of the collision energy and be proved to greatly enhance the crashwo-rthiness of the FLNG side structures.     

An experimental study of the wave excitation in the gap between two closely spaced bodies,with implications for LNG offloading

The side-by-side offloading of liquid natural gas (LNG) at offshore terminals involves a fixed and a floating body in close proximity; the offshore terminal being the fixed body and the LNG tanker the floating body. The closeness of the two bodies leads to the formation of a long and relatively narrow gap, within which there is the potential for large amplifications of the water surface elevation. The present paper uses experimental results to characterise both the size and nature of the excitation within the gap. It also illustrates the effect of the vessel motion on this amplification by considering a 1:100 scaled model of an LNG tanker as well as its fixed approximation. It is found that the body's ability to move acts to increase the frequency at which resonant amplification within the gap occurs (the resonance frequency). The incident wave conditions considered include regular and irregular waves in both beam- and head-sea orientations; the latter leading to very different gap end conditions. The nature of the resonant amplification for the floating LNG tanker is shown to be similar for the two orientations, suggesting that the gap end conditions do not drive the resonant amplification. Consideration of the nonlinearity within the gap illustrates that resonant amplification occurs at the resonance frequency, irrespective of whether the fluid motion is first or second harmonic. The present paper provides data relevant to the safe offloading operations of an LNG tanker and demonstrates the importance of incorporating the vessel motion in numerical modelling procedures.     

Fluid impact onto a corrugated panel with trapped gas cavity

Initial stage of incompressible liquid impact onto a corrugated elastic panel with account for compressible gas trapping between the corrugations is studied. The liquid free surface is flat and parallel to the panel before impact. The impact velocity is constant in this study. The corrugations are modelled as identical rigid short structures on the surface of the flat panel. The panel is either of infinite or finite length. There are only two corrugations which are placed symmetrically on the panel. Only a part of the panel between these two corrugations is elastic. The liquid free surface closes the gas cavity between the two corrugations at the initial instant of impact and compresses the gas before the fluid comes in contact with the elastic part of the panel. The elastic deflections of the panel are caused by gas pressure in the cavity. The elastic deflections modify both the pressure in the cavity and the hydrodynamic pressure distribution along the wetted part of the panel. The hydroelastic problem is solved within the Wagner approach. The effect of gas compressibility on the elastic behaviour of the corrugated elastic panel is investigated. It is shown that the pressure in the gas cavity and elastic deflections grow beyond all bounds for the panel of infinite length and are finite if the panel is of finite length. The present model is relevant for the strength assessment of the cargo containment system (CCS) in the tanks of LNG carriers.     

Financing LNG carriers

The author examines the various methods of finance for LNG carriers at each stage of funding. First, he describes the various factors involved in the funding of LNG carriers, and continues by highlighting the different methods of finance. He concludes by summarizing the prospects for shipping finance in a severely depressed ship sale and purchase market.     

Shipborne GPS attitude determination during MMST-93

The attitude parameters of a ship underway were measured using a configuration of four 10-channel NovAtel Model 951 narrow-correlator-spacing receivers. These C/A code receivers have output rates of up to 10 Hz and maintain effective carrier phase lock under relatively harsh ship dynamics. The attitude parameters are calculated independently at each epoch using differential carrier phase measurements, carrier phase ambiguities are resolved on-the-fly by constraining the approximately known distances between the antennas that are rigidly mounted on the ship. Carrier phase thermal noise and multipath are minimized by mounting the antennas as far apart as possible. The four-antenna configuration provides redundancy and further improves accuracy. During the Matthew Motion Sensor Trials (MMST-93) conducted off the coast of Halifax, Nova Scotia, in June-July 1993, the GPS-derived attitude parameters were compared with those obtained with a Honeywell HG1050 ring laser gyro inertial navigation system (INS) which provides roll and pitch with an accuracy of 15 arcsecs and heading with an accuracy of 2 arcmins. To simulate rough weather conditions, sharp maneuvers were performed to induce roll angles in excess of 10°. No accuracy degradation nor any loss of GPS measurements occurred. The RMS agreement between GPS and INS derived attitude parameters is better than 2 arcmins in heading, 1 arcmin in pitch and 3 arcmins in roll. This level of accuracy demonstrates the capability of GPS for cost-effective shipborne attitude determination at an accuracy level of 0.05     

Pressure resistance of the corrugated stainless steel membranes of LNG carriers

Corrugated stainless steel membranes have been used as the primary barriers for LNG carriers to reduce thermal stresses at cryogenic temperatures. As the capacity of the LNG cargo is increased, however, the corrugated stainless steel membranes suffer from buckling and collapse due to increased sloshing loads in large-capacity LNG cargo carriers. In this study, the pressure resistance of the corrugated part of the stainless steel membrane was evaluated using a finite element method for better design of the LNG containment system, and an experimental equipment was developed and the pressure resistance of the stainless steel corrugations was measured and compared with the calculated results. In addition, a new membrane with high-pressure resistance and its reinforcing method were developed and evaluated experimentally.     

Time-Domain Simulation for Coupled Motions of Three Barges Moored Side-by-Side in Floatover Operation

Simulating the coupled motions of multiple bodies in the time domain is a complex problem because of the strong hydrodynamic interactions and coupled effect of various mechanical connectors. In this study, we investigate the hydrodynamic responses of three barges moored side-by-side in a floatover operation in the frequency and time domains. In the frequency domain, the damping lid method is adopted to improve the overestimated hydrodynamic coefficients calculated from conventional potential flow theory. A time-domain computing program based on potential flow theory and impulse theory is compiled for analyses that consider multibody hydrodynamic interactions and mechanical effects from lines and fenders. Correspondingly, an experiment is carried out for comparison with the numerical results. All statistics, time series, and power density spectra from decay and irregular wave tests are in a fairly good agreement.     

Use of Helical Strakes for FIV Suppression of Two Inclined Flexible Cylinders in A Side-by-Side Arrangement

The experimental studies on flow-induced vibrations(FIV) reduction of two side-by-side flexible cylinders inclined at 45° by using the helical strakes were carried out in a towing tank. The main aim of the experiment is to check whether the helical strakes with a pitch of 17.5 D and a height of 0.25 D, which is considered as the most effective vibration suppression device for the isolated cylinder undergoing vortex-shedding, still perform very well to reduce FIV of two inclined flexible cylinders in a side-by-side arrangement. The vibration of two identical inclined cylinders with a mass ratio of 1.90 and an aspect ratio of 350 was tested in the experiment. The center-to-center distance between the two cylinders was 3.0 D. The uniform flow was simulated by towing the cylinder models along the tank.The towing velocity varied from 0.05 to 1.0 m/s with an interval of 0.05 m/s. The maximum Reynolds number can be up to 1.6×104. Three cases were experimentally studied in this paper, including two side-by-side inclined smooth cylinders, only one smooth cylinder fitted with helical strakes in the two side-by-side inclined cylinders system and both two cylinders attached with helical strakes. The variations of displacement amplitude, dominant frequency, FIV suppression efficiency and dominant mode for the two side-by-side inclined cylinders with reduced velocity were shown and discussed.     

Stiffness of mooring lines and performance of floating breakwater in three dimensions

In the present paper, the performance of a moored floating breakwater under the action of normal incident waves is investigated in the frequency domain. A three-dimensional hydrodynamic model of the floating body is coupled with a static and dynamic model of the mooring lines, using an iterative procedure. The stiffness coefficients of the mooring lines in six degrees of freedom of the floating breakwater are derived based on the differential changes of mooring lines' tensions caused by the static motions of the floating body. The model of the moored floating system is compared with experimental and numerical results of other investigators. An extensive parametric study is performed to investigate the effect of different configurations (length of mooring lines and draft) on the performance of the moored floating breakwater. The draft of the floating breakwater is changed through the appropriate modification of mooring lines' length. Numerical results demonstrate the effects of the wave characteristics and mooring lines' conditions (slack-taut). The existence of ‘optimum’ configuration of the moored floating breakwater in terms of wave elevation coefficients and mooring lines' forces is clearly demonstrated, through a decision framework.     

液舱晃荡载荷计算与强度校核系统研究

随着薄膜型LNG运输船的需求量不断增加,晃荡载荷已成为船舶安全性研究的重要内容之一。本文结合中国船级社规范所推荐的公式,对薄膜型LNG船晃荡水平的载荷进行研究,提出载荷计算方法和流程,在MSC.patran的基础上结合二次开发语言PCL,设计了一套晃荡载荷计算与校核系统。该系统对有限元模型进行前后处理,设计了舱室识别算法来搜索晃荡载荷的作用域,实现薄膜型LNG船晃荡载荷的自动计算与施加,完成屈服强度评估。通过算例测试证明本系统自动计算结果的有效性和准确性,可以为工程设计人员大大节约工作量,大幅度提高工作效率。     

Application of Helical Strakes for Suppressing the Flow-Induced Vibration of Two Side-by-Side Long Flexible Cylinders

Helical strakes have been widely applied for suppressing the vibration of flexible cylinders undergoing vortexshedding in offshore engineering. However, most research works have concerned on the application of helical strakes for the isolated flexible cylinder subjected to vortex-induced vibration(VIV). The effectiveness of helical strakes attached to side-by-side flexible cylinders in vibration reduction is still unclear. In this paper, the response characteristics of two side-by-side flexible cylinders with and without helical strakes were experimentally investigated in a towing tank. The configuration of the helical strakes used in the experiment had a pitch of 17.5D and a height of 0.25D(where D is the cylinder diameter), which is usually considered the most effective for VIV suppression of isolated marine risers and tendons. The center-to-center distance of the two cylinders was 3.0D. The uniform flow with a velocity ranging from 0.05 m/s to 1.0 m/s was generated by towing the cylinder models along the tank. Experimental results, including the displacement amplitude, the dominant frequency, the dominant mode,and the mean drag force coefficient, were summarized and discussed. For the case where only one cylinder in the two-cylinder system had helical strakes, the experimental results indicated that helical strakes can remarkably reduce the flow-induced vibration(FIV) of the staked cylinder. For the case of two straked cylinders in a side-by-side arrangement, it was found that the performance of helical strakes in suppressing the FIV is as good as that for the isolated cylinder.     

Statics of a three component mooring line

Catenary equations are solved for a three component mooring made up of two lines, connected at a point buoy or sinker where water depth and fairlead tension are given. This configuration is typical of deep water moorings and solutions are required for preliminary design of mooring systems and measurement of stability in service of moored floating platforms. The problem is transformed to a single polynomial equation of degree eight and is solved by Laguerre's iteration. Where there is no buoy or sinker the equation reduces to a quartic and can be solved in closed form. Elongation of the lines is shown to be equivalent to small uncertainties in the weights per unit length. The method is computationally efficient.     

多船旁靠系泊作业关键特性试验研究

旁靠带缆作业是实现海上补给、模块吊运、原油外输等运转的常用方式,针对浮式平台与两艘供应船以旁靠方式作业涉及的关键特征进行水池模型试验研究,阐述了旁靠带缆系统和护舷设备的属性和需遵循的规范衡准,深入分析旁靠系缆的张力、护舷的挤压力、船体之间的相对运动特征随有效波高和波浪入射角度等环境参数的变化规律,确定了适用于多船旁靠作业的海况条件。研究表明:波浪能量随有效波高的增大呈现非线性增大趋势,是影响旁靠作业窗口期的主要外部因素,而波浪入射角度对旁靠作业的影响程度与海况有较大关系;三船在纵荡、横荡以及艏摇方向具有明显的跟随性特点,运动响应的幅值和相位比较接近,浮式平台与供应船之间的相对运动较小,完全可满足作业要求。垂荡、横摇以及纵摇这三个波频方向,各船的运动并不同步,相对运动大,是影响多船旁靠作业窗口期的主要因素;有效波高为2.5 m的海况条件可满足多船旁靠要求。相关结论为实际的工程操作提供参考。     

Seasonal variations of wind and waves over Taiwan waters

We here investigate the frequency and intensity of oscillations in oceanographic data within intraseasonal time scales using spectral analysis of surface wind and wave time-series data collected at off-island weather stations or moored buoys around Taiwan. Data from marine weather stations were used to trace atmospheric conditions, while we used buoy data to examine sea states. The spectra and wavelet scalogram of the wind fields revealed oscillations with a period of around 20–33 days, and the energy density of the wind field at the off-island stations was stronger than that at the data buoy stations. However, the wavelet scalogram of the wave height measured at the buoy stations was stronger than its associated wind field. This long-period oscillation is consistent with the wavelet scalogram of the wind field calculated from the off-island weather stations. About 20–33 day oscillations exist within intraseasonal variations, which are closely linked to the atmospheric environment and to wind and ocean wave fields. Oscillations with a period of 5–10 days are a pronounced feature over northeastern Taiwan waters during the winter season and can be interpreted as the wave pattern following synoptic weather systems.     

A probabilistic assessment of design sloshing pressure time histories in LNG tanks

The violent motion (sloshing) of liquefied natural gas (LNG) in cargo tanks has attracted significant attention. Transformations of the LNG market have led to the increased transport of LNG in partially filled tanks, but established technology is mainly based on engineering experience with completely filled containers. This paper investigates a large sample of sloshing pressure measurements. It focuses on the magnitude of individual sloshing impact events, and their associated temporal and spatial patterns. The durations of these impacts are comparable to the natural frequency of an LNG container wall, so the details of their time histories are important in determining the structural response. Experiments are performed on tanks with high (92.5%) and low (30%) filling levels, for various wave headings. The common post-processing approach of representing impact pressure histories by a triangular profile is studied, and an alternative approach is presented. Two statistical models are used to describe the distribution of maximal pressures in sloshing impacts: a three-parameter Weibull model and a generalized Pareto model. The latter is found to be of questionable utility due to small sample sizes. It is observed that for low filling levels the sloshing impacts are of greater magnitude, having longer durations, smaller ratios of rise time to duration, and larger spatial extents. All these factors should in principle increase the structural response.