Mooring Forces Research Articles

Calculating the Mooring Force of a Large LNG Ship based on OCIMF Mooring Equipment Guidelines

Calculating the Mooring Force of a Large LNG Ship based on OCIMF Mooring Equipment Guidelines

자동계류시스템 고무 씰 유한요소해석을 위한 고무 소재의 온도별 기계적 특성 연구

An automatic mooring system for a ship consists of a vacuum suction pad and a mechanical part, enabling quick and safe mooring of a ship. In the development of a mooring system, the design of a vacuum suction pad is a key to secure enough mooring forces and achieve stable operation of a mooring system. In the vacuum suction pad, properly designing its rubber seal determines the performance of the suction pad. Therefore, it is necessary to appropriately design the rubber seal for maintaining a high-vacuum condition inside the pad as well as achieving its mechanical robustness for long-time use. Finite element analysis for the design of the rubber seal requires the use of an appropriate strain energy function model to accurately simulate mechanical behavior of the rubber seal material. In this study, we conducted simple uniaxial tensile testing of Chloroprene Rubber (CR) to explore the strain energy function model best-fitted to its experimentally measured engineering strain-stress curves depending on various temperature environments. This study elucidates the temperature-dependent mechanical behaviors of CR and will be foundational to design rubber seal for an automatic mooring system under various temperature conditions.

Wave-attenuation and hydrodynamic properties of twin pontoon floating breakwater with kelp

With the increasing awareness of environmental protection, more and more marine eco-structures are being built. Mangroves, corals and seaweeds can be combined with marine hard structures to resist marine hazards. As a kind of marine plant, kelp has the potential to weaken wave and current. In this paper, a twin pontoon floating breakwater with kelp (TPFBK) is proposed. Experimental and numerical studies are carried out in a wave flume. Firstly, comparative experimental studies of the wave-attenuation performance of TPFBK with different length kelp and the number of rows of kelp arrangements are conducted. Secondly, the mooring force of the TPFBK with different arrangements of kelp are compared exhaustively. Finally, a simplified fluid-solid coupled numerical model is developed to analyze the wave dissipation mechanism of the TPFBK with different kelp arrangements by velocity field, vorticity and turbulent kinetic energy distribution. The results indicate that the TPFBK exhibits better wave attenuation performance compared with the traditional twin pontoon floating breakwater (TPFB). At the same time, the kelp array is helpful to reduce the on-shore mooring force. However, it increases the off-shore mooring force which is very limited. The TPFBK with a single row of densely planted kelp improves wave attenuation performance mainly by reflecting wave energy, while the multi-row arrangement of kelp enhances wave energy dissipation. And the dissipation of wave energy is caused by the variations in velocity field, turbulent kinetic energy and the development and shedding of vortices.

Numerical investigation of the hydrodynamics of a submersible steel-frame offshore fish farm in regular waves using CFD

The hydrodynamics and non-linear interaction between the submersible steel-frame offshore fish farm “ShenLan 1” and regular waves are investigated using the open-source CFD toolbox REEF3D. The effects of the regular wave parameters, different net sheets, and structural variations on the motion responses and mooring forces of the fish farm system are studied. The numerical framework consists of a moored-floating body motion solver for the frame structure coupled to a fluid dynamics solver to simulate the two-phase propagation of the waves. The momentum loss of the flow over fixed net sheets is included in the solver. First, the chosen model is validated for a coupled rigid body motion and net hydrodynamics. Then, the solver is applied to the offshore fish farm “ShenLan 1”. It is shown that the surge motions are more sensitive to the wave parameters compared to the heave and pitch motions due to the restrain from the mooring system. In contrast to the vertical components, the horizontal loadings positively correlate with the wave heights and periods. The nets show minor influence on the motion responses, but the percentage of horizontal forces attributed to nets decreases from 55% to 10% in larger wave periods. The translational motions, as well as mooring restoring tensions, are particularly sensitive to shifts of the draught. Finally, the increase of the aspect ratio of “ShenLan 1” through enlarging the overall diameters results in declined motions and mooring forces. Thus, this study provides the first indications towards a systemic evaluation of the dependencies of different structural parts of a submersible steel-frame offshore fish farm for hydrodynamics.

An assessment of mooring systems' forces of ships berthed at dolphins

In the current practice of structural design for berth structures, the mooring forces (tension in lines and compression in fenders) are typically determined through simplified linear static analyses, and then magnified by dynamic amplification coefficients. It is known, however, that the mooring's stiffness presents nonlinearities (both physical and geometrical) that may at times become relevant, raising the issue of whether linear analyses are really adequate. Also, the environmental loads induce oscillatory behavior whose peak values may not always be satisfactorily estimated through dynamic amplification coefficients, which are far from being well defined in the technical literature. This work presents an investigation on the forces developed on mooring systems of vessels berthed at dolphins, and the consequences of the current design practice's simplifications. Different hierarchical models are defined (linear static, nonlinear static and linear dynamic in the frequency domain) for a case study of a Floating Storage and Regasification Unit (FSRU) permanently berthed at dolphins at the Barcarena's Liquefied Natural Gas (LNG) terminal in Pará, Brazil. The vessel's displacements and their peak values are determined through stochastic and statistical theories. The corresponding forces on the mooring system are computed and the dynamic amplification coefficients therefrom obtained. A thorough discussion on the results is provided. The methodology presented may be a useful tool to ascertain the dynamic amplification coefficients required in the simplified methodologies from the international standards.

Experimental study on hydrodynamic characteristics of the box-type floating breakwater with different mooring configurations

The box-type floating breakwater is simple and durable, which is one important type of floating breakwaters. In this paper, a systematic experimental study on the hydrodynamic characteristics of the box-type floating breakwater is performed in a wave flume, and the influence of the mooring system on the wave attenuation performance of the box-type floating breakwater is studied. The transmission coefficients, reflection coefficients, energy dissipation coefficients, motion responses and mooring forces of the box floating breakwater under the fixed condition and three different mooring configurations are compared and analyzed. The experimental results show that the box-type floating breakwater under the fixed condition has better wave attenuation performance than that under the mooring condition. The cross mooring and parallel mooring configurations without section lying on the seabed are more effective in reflecting and dissipating longer period waves. The results also indicate that there is obvious difference in motion responses of the breakwater under different mooring configurations, and the mooring forces are affected by the motion responses. A reasonable choice of mooring configurations is more conducive to ensuring the safety of floating breakwaters and improving the wave attenuation performance.

A Simplified Modeling Approach of Floating Offshore Wind Turbines for Dynamic Simulations

Currently, floating offshore wind is experiencing rapid development towards a commercial scale. However, the research to design new control strategies requires numerical models of low computational cost accounting for the most relevant dynamics. In this paper, a reduced linear time-domain model is presented and validated. The model represents the main floating offshore wind turbine dynamics with four planar degrees of freedom: surge, heave, pitch, first tower fore-aft deflection, and rotor speed to account for rotor dynamics. The model relies on multibody and modal theories to develop the equation of motion. Aerodynamic loads are calculated using the wind turbine power performance curves obtained in a preprocessing step. Hydrodynamic loads are precomputed using a panel code solver and the mooring forces are obtained using a look-up table for different system displacements. Without any adjustment, the model accurately predicts the system motions for coupled stochastic wind–wave conditions when it is compared against OpenFAST, with errors below 10% for all the considered load cases. The largest errors occur due to the transient effects during the simulation runtime. The model aims to be used in the early design stages as a dynamic simulation tool in time and frequency domains to validate preliminary designs. Moreover, it could also be used as a control design model due to its simplicity and low modeling order.

Numerical Study on the Mooring Force in an Offshore Fish Cage Array

The mooring force in a fish cage array subjected to currents and waves is investigated using the finite element method. Firstly, the numerical model of a fish cage array with six gravity cages is built by Ansys/APDL. Collars and bottom rings are simulated with pipe and beam elements while the rest structure is simulated with link elements, including the net and mooring cables. Thus, the weight and hydrodynamic load on the cables can be considered. The initial shape of the mooring ropes is calculated based on mooring dynamics. Since each component is a slender structure in the cage array, the Morison equation is used to calculate the hydrodynamic load. Secondly, the mooring forces are assessed for the system in different sea states. The locations of the maximum mooring force on different parts in the mooring system are found. The mean values and amplitudes of maximum mooring forces on different parts are calculated. The main ropes have the maximum mooring forces under all sea states. The mean values of the maximum mooring forces increase with the current velocity and wave height. When the attack angle is 0° and 90°, the two adjacent bridle ropes do not play the role of pulling the cage together. One is pulled tight and the other one is slack.

Numerical investigation on the hydrodynamic behavior of a floating breakwater with moon pool through a coupling SPH model

The moon pool opened in a floating structure significantly affects its wave attenuating performance. In this paper, the hydrodynamic behaviors of several floating breakwaters with moon pool were investigated preliminarily by the Smoothed Particle Hydrodynamics (SPH) method. For a comparative study, wave interactions with a rectangular floating breakwater and a cylindrical dual pontoon floating breakwater were also simulated. The wave was generated with a relaxation zone method which had displayed its accuracy as well as applicability. In addition, the lumped mass method was adopted to provide mooring forces for the floating breakwater. The coupling SPH model was validated by a physical model test and the numerical results were in good agreement with the experimental data. After that, the wave coefficients, such as transmitted wave coefficient, reflected wave coefficient and dissipated wave coefficient, were computed to evaluate the wave attenuating performance of floating body models. Meanwhile, for each model, the motion responses, including sway, heave and pitch, were analyzed to investigate their hydrodynamic performance. The numerical results and analysis are useful for the design of floating breakwaters, and the conclusions could be further applied in the design of wave energy converters.

Analytical Investigation of Tension Loads Acting on a TLP Floating Wind Turbine

The purpose of this study is to scrutinize the coupled hydroaeroelastic problem for a TLP design of a floating structure consisting of multiple vertical truncated cylinders which support a 10 MW wind turbine (WT). The platform is exposed to the combined effects of water waves and wind loading. The motions of the platform are examined for seven different directions of the incident waves. The hydrodynamic problem was solved analytically by combining the physical idea of multiple scattering and the method of matched axisymmetric eigenfunction expansions in order to obtain analytical representations of the velocity potential around the cylindrical members of the platform, while the contribution of the WT is considered within the six degrees of motion of the floater. Numerical results are initially presented for the exciting forces acting on the platform, the added masses, and the hydrodynamic damping coefficients, as well as the RAOs of the platform’s motions. Furthermore, the shear forces and bending moments are calculated at the point where the WT is assembled with the floater. Finally, results for the total mooring forces are given.

Experimental investigation on the wave-attenuating performance and shape optimization of water ballast type floating breakwater

Floating breakwaters (FB) play an important role in protecting the safety of marine structures, and Box-type FB is widely used due to its simple shape, among which, the ballast water type FB has good wave-attenuating performance. However, the research on the influence of the connection mode and the inner ballast water on the wave-attenuating performance is still insufficient. In this paper, firstly, two different connection modes (catenary type and tensely type) of water ballast Box-type FB is experimentally investigated, the results show that when compared with tensely type, the mooring force of catenary type is significantly reduced while maintaining similar wave-attenuating performance, indicating its suitability for engineering. Secondly, to further improve the wave-attenuating performance of catenary type, the shape of Box-type FB is optimized and a new type of I-shaped FB is proposed. After that, the wave-attenuating performance, motion response, inner ballast water movement, and maximum mooring force of catenary water ballast I-shaped FB are experimentally compared with that of catenary water ballast Box-type FB, and the better performance of the I-shaped FB is verified. Furthermore, the influences of initial ballast water depth and wing length are experimentally investigated. The results indicate that within the scope of this study, the catenary water ballast I-shaped FB present a superior wave-attenuating performance and simultaneously exhibit the smallest mooring force, when the lower edge of the upper wing is initially flush with the water surface and also the wing length equals 1.5 times of the wing thickness. This paper could provide references for the optimization of catenary water ballast floating breakwater.

Research on a motion-inhibition fuzzy control method for moored ship with multi-robot system

Compared with the conventional mooring arrangements, robot mooring systems have better adaptability to sea conditions, higher operating efficiency and security. Due to strong random disturbances of wave, wind, currents and the mechanism near-singularity induced by the line distribution of mooring robots along the pier, PD control or other conventional control methods for the mooring robots are difficult to obtain a superior motion-inhibition effect. In this paper, a motion-inhibition fuzzy control method for a moored-ship with a kind of multi-robot system is proposed. By analyzing the motion-inhibition effect of the mooring restoring forces and damping forces, a three-dimensional (surge, sway and yaw) mooring force fuzzy controller (MFFC) is developed. According to the force mapping relationship from mooring force space to the driving force space, adjustment factors for the outputs of the MFFC are given to decease the largest driving-force of robot actuators induced by the mechanism near-singularity. Numerical simulations are conducted to obtain the motion responses of a 60000DWT bulk carrier moored by a multi-robot mooring system (MRMS) considering different conditions and parameters. The results show that the MFFC can inhibit adaptively the ship motions for different sea conditions.

An aero-hydro coupled method for investigating ship collision against a floating offshore wind turbine

Ship collision hazards with floating offshore wind turbines (FOWTs) has been identified and the dynamic responses of FOWT under ship collision scenarios are necessary to be investigated. This paper develops a numerical solver to address the challenge of considering wind-wave loads in the ship-FOWT collision analysis. The user-defined load subroutine, LOADUD, in LS-DYNA is utilized to model the wind, wave and mooring loads. A simplified aerodynamic model considering blade pitch strategy is used and the hydrodynamic loads are calculated based on potential-flow theory with a linear wave kinematic model. The mooring forces are considered with a linearized model. The implementation of the coupled solver is verified by conducting free decay test, wave-only and wind-only analysis, respectively and comparing them with the reference values. Good agreement has been achieved, indicating that the aero-hydro coupling effects can be accurately simulated by the numerical solver. With the developed method and solver, cases of ship collision with a spar-type floating wind turbine are simulated and both local structural deformation and global motions in 6DOF are obtained. The influence of velocity, flexibility of tower, deformability of the ship, wind-wave loads are further discussed. It is found that the impact velocity can greatly affect the structural responses and a high-speed impact can directly lead to tower collapse. By comparing the cases of ship collision with FOWT, and those with fixed-support wind turbine, the flexibility of wind turbine tower is found to rarely influence the energy dissipation under collision scenario of FOWT, while it is important for the collision scenario of fixed-support wind turbine. The wind-wave condition can be more hazardous as a smaller impact velocity can lead to structures collapsing. This is because, even though the structure can withstand the ship collision, the residual strength may not be enough for the following wind loads, which can result in the structure to finally collapse. Additionally, the proposed fully coupled method is used to validate one of the authors’ previous studies, which is a decoupled method regarding the ship-FOWT collision analysis [1]. The external dynamic results from two methods are compared and discussed. Good agreement is reached between two methods for low-velocity impact but for high-speed collision or collisions with serious deformation, limitations of decoupled method can be found because some basic assumptions of them are violated. The proposed method and solver can be used to evaluate the crashworthiness of FOWT during the engineering design phase.

Experimental investigation of a point-absorber wave energy converter response in different wave-type representations of extreme sea states

The experimental results of a 1:30 scaled wave tank experiment of a point-absorber in extreme sea states and intermediate water depth are studied. The effect of the PTO damping parameter and different non-linear phenomena in extreme wave conditions such as wave breaking and overtopping are investigated with the focus on the maximum line (mooring) force in the presence of an upper end-stop. In the comparison of different wave-type representations, i.e. irregular, regular, and focused waves, of the same sea state, not all the wave types necessarily yield to the same peak line force. Moreover, there exists an optimum damping value for each sea state in which the smallest peak force is achieved. Both end-stop spring compression and wave breaking slamming result in peak line forces which may be compensated by overtopping water pressure. Large surge motion is obtained for waves with a long wavelength which can contribute to higher and more damaging line forces.

Floating Behavior of a Composite Bucket Foundation under the Combined Action of Wind and Waves

In this study, a composite bucket foundation (CBF) was investigated by taking into account the reflection and refraction effects of waves. A numerical model for the motion responses of the structure under different wave heights and wave periods was built based on the 3D potential flow theory. The model was verified against the test results for analyzing the stability and seakeeping performance of the foundation under the action of waves. Under the combined action of wind and waves, the possible scenarios of floating under wind and waves of different scales and directions were simulated, and the mooring force and motion response peaks and laws of the structure were established, aiming to improve the safety of composite bucket foundation transport and provide technical support for floating construction.

Discrete Element Simulations of Ice Load and Mooring Force on Moored Structure in Level Ice

Moored structures are suitable for operations in ice-covered regions owing to their security and efficiency. This paper aims to present a new method for simulating the ice load and mooring force on the moored structure during ice-structure interaction with a spherical discrete element method (DEM). In this method, the level ice and mooring lines consist of bonded sphere elements arranged in different patterns. The level ice model has been widely validated in simulation of the ice load of fixed structures. In the mooring line simulation, a string of spherical elements was jointed with the parallel bond model to simulate the chains or cable structure. The accuracy of the mooring line model was proved by comparing the numerical results with the nonlinear FEM results and model towing experiment results. The motion of the structure was calculated in the quaternion method, considering the ice load, mooring force, and hydrodynamic force. The hydrodynamic force comprised wave-making damping, current drag, and buoyancy force. Based on the proposed model, the interaction of a semi-submersible structure with level ice was simulated, and the effect of ice thickness on the ice load was analyzed. The numerical results show that the DEM method is suitable to simulate the ice load and mooring force on moored floating structures.

Dynamic behaviors of a hinged multi-body floating aquaculture platform under regular waves

This paper numerically studies the dynamic behaviors of a hinged multi-body floating aquaculture platform under regular waves. A coupled scheme that integrates the boundary element method (BEM) and the lumped-mass model is used to model the frames and the moorings/netting simultaneously. The effect of the hinge-joint rotational stiffness on the motion responses, mooring and hinge-joint forces at different wave steepnesses are examined. The results show that both the wave period and the hinge-joint rotational stiffness affect the pitch response of the multi-body aquaculture platform. However, the hinge-joint rotational stiffness has a predominant influence on the maximum pitch response. Unlike the pitch response, the stiffness has little effect on the heave response. Moreover, with the increase of stiffness, these two individual cages also show distinctive dynamic behaviors. The mooring force decreases with the stiffness and a 14.5% reduction has been obtained at the maximum stiffness. Under the same wave period, the y-direction rotation moment increases alongside the stiffness. When the wavelength is less than 0.9 times the structural span, the increased wave period decreases the y-direction rotation moment. In contrast, the stiffness rarely affects the z-direction hinge-joint force.

Hydrodynamic Performance of a Multi-Module Three-Cylinder Floating Breakwater System under the Influence of Reefs: A 3D Experimental Study

As the technical and theoretical research of floating breakwaters is becoming increasingly mature, the floating breakwaters are now being utilized, especially in offshore reefs. Therefore, it is of practical significance to study the hydrodynamic performance of a multi-module floating breakwater system under the influence of reefs. In this study, a 3D model experiment was carried out on a system consisting of eight three-cylinder floating breakwater modules under the influence of reefs. A wave attenuation mesh cage was incorporated at the bottom of the model. The floating breakwater system was slack-moored in its equilibrium position, and each module was connected by elastic connectors. The reefs were modeled on a bathymetric map of existing reefs in the East China Sea. In this experiment, the wave transmission coefficients, motion responses, and mooring forces of the floating breakwater system were measured. The results showed that the three-cylinder floating breakwater in the beam waves (β = 90°) has excellent wave attenuating performance under the influence of reefs, especially for short-period waves. However, under the influence of the reef reflection wave and the shallow water effect, the motion responses in the three main stress directions of the floating breakwater were large, and there was some surge and pitch motion. Under the influence of the aggregation and superposition of reflected waves on both sides of the reefs, the peak mooring forces in the middle position of the floating breakwater system were the largest at large wave height. The three-cylinder floating breakwater exhibited satisfactory hydrodynamic performance under the influence of reefs. It has broad application prospects in offshore reefs.

Numerical study on hydrodynamic responses of a single-point moored vessel-shaped floating aquaculture platform in waves

A vessel-shaped floating aquaculture platform has numerous advantages over traditional high-density polyethylene gravity cages. Combined with a single-point mooring system, the unique structural form of the vessel shape can significantly reduce the environmental loads transmitted from the bow. This study used potential flow theory and dynamic composite cable theory to simulate the hydrodynamic responses of a single-point moored vessel-shaped floating aquaculture platform in waves. The motion response, mooring force response, and reaction force of the nets were simulated in regular waves and compared with experimental values. The numerical and experimental results agree well. After the validation, the response amplitude operators of the floating frames of the platform were calculated and analyzed in the frequency domain. Then, time-domain calculations were carried out to investigate the hydrodynamic characteristics of the platform. The results show that the heave and pitch responses of the front frame are significantly higher than those of the rear frames of the same platform. The increase in the number of floating frames decreases the motion responses and increases the mooring force response of the platform. The X and Z components of the hinge joint force between the floating front frames are significantly higher than those between the rear frames. Similarly, the reaction force of the nets of the floating front frame is higher than that of the rear frames under the same wave conditions.

Hydrodynamic analysis of a hybrid modular floating structure system and its expansibility

ABSTRACT This paper reports a hybrid modular floating structure (HMFS) system preliminarily designed for mild marine environment. It is composed of a certain number of inner semi-sub modules and outermost box-type modules. The box-type modules are connected to adjacent semi-sub modules with hinges and additional Power Take-Off (PTO) damping systems. The hydrodynamic characteristics of several representative simplified serially connected HMFS systems under typical sea conditions are compared. The functions of the outermost module, the effects of different outermost module types and the expansibility of the HMFS system are emphatically analysed. The results indicate that compared with the outermost semi-sub module, the outermost box-type module is more effective in reducing the motion responses of inner semi-sub modules, connector forces and mooring forces. The additional PTO damping systems can produce substantial wave energy. The HMFS system shows good expansibility for mild sea zones. Abbreviations: HMFS, Hybrid Modular Floating Structure; VLFS, Very Large Floating Structure; RMFC, Rigid Module and Flexible Connector; MOB, Mobile Offshore Base; FEM, Finite Element Method; PTO, Power Take-Off; MFS, Modular Floating Structure; Ks, Pitch Elastic Stiffness; Kp, Pitch Damping Coefficient; DOF, Degree of Freedom; Fx, Horizontal Force of Connector; Fz, Vertical Shear Force of Connector; My, Pitch Bending Moment of Connector