A Novel Control Algorithm for Interaction Between Surface Waves and A Permeable Floating Structure

An analytical solution is undertaken to describe the wave-induced flow field and the surge motion of a permeable platform structure with fuzzy controllers in an oceanic environment. In the design procedure of the controller, a parallel distributed compensation (PDC) scheme is utilized to construct a global fuzzy logic controller by blending all local state feedback controllers. A stability analysis is carried out for a real structure system by using Lyapunov method. The corresponding boundary value problems are then incorporated into scattering and radiation problems. They are analytically solved, based on separation of variables, to obtain series solutions in terms of the harmonic incident wave motion and surge motion. The dependence of the wave-induced flow field and its resonant frequency on wave characteristics and structure properties including platform width, thickness and mass has been thus drawn with a parametric approach. From which mathematical models are applied for the wave-induced displacement of the surge motion. A nonlinearly inverted pendulum system is employed to demonstrate that the controller tuned by swarm intelligence method can not only stabilize the nonlinear system, but has the robustness against external disturbance.     

The dragged surge motion of tension-leg type fish cage system subjected to multi-interactions among wave and structures

In this paper, a typical tension-leg type net cage system is analyzed, and a set of motion equations for the cage system in the simplified two-dimensional case is derived. Subsequently, a close form analytical solution is obtained when all of the multi-interactions between the top platform structure, the incident wave, and the motion of both the tether and net are considered. The parameter of solidity as a ratio of the twine diameter to the mesh size of the cage is also taken into account in the analysis. The purpose of this paper is to investigate the dragged surge motion of the tension-leg type cage system when subjected to incident waves and flow drags on the net-cages. Furthermore, the dynamic behavior of the platform is compared to the case when the net-cage drag is not considered. Numerical examples are carried out in a range of periods of waves, and the results are discussed focusing on the motion of both the platform and cage. It was found that there is a significant influence of the net-cage drag on the surge motion of the entire structural system. Various parameters such as the material property, twine diameter, mesh size and the dimension of the net-cages were considered. It was found that the vibration of the net-cage system might be significantly overestimated if the drag effect of the net and tethers is neglected.     

Dragged surge motion of tension leg platforms and strained elastic tethers

This paper presents an analytical solution for the dynamic behavior of both the platform and tethers in the tension leg platform system when the platform system is subjected to the wave-induced surge motion and the flow-induced drag motion. Along with the analysis the coupling problem of a two dimensional tension leg platform interacting with a monochromatic linear wave train in an inviscid and incompressible fluid is being considered. The scattering problem and radiation problem are first solved independently and then combined together to resolve for all unknowns. The dynamic behavior of the platform and tethers was further solved based on these solutions. The material property and the dimensional effect for the tether incorporated in the tension leg platform system are both taken into account in the analytical analysis. Corresponding to the variation of material properties and tether dimensions, it was found that the dynamic behavior of both the tether of tension leg platform and the platform itself is closely related to the material property and the dimension of the tether.     

Model test of the surface and submerged vehicles with the micro-bubble drag reduction

This study is based on the effective experiment observation and measuring technology to discuss the interaction influence between liquid turbulent boundary layer and a crowded group micro-bubbles. It is in order to understand and quantify the micro-bubbles clouds inside the turbulent boundary layer to eliminate the capacity of skin friction drag. Whenever the micro-bubbles are over supplied, pile up effect happened which makes micro-bubbles to integrate to each other as a large-size air film. Although they still have the drag reduction effect, the efficiency of drag reduction slowed down at this transition period. In the experiment of vertical type circulating water tunnel, when 1 μm porous medium is at 7 m/s flow speed, the C_v value at 0.056 has the best drag reduction efficiency of 26%. While 10 μm porous medium is at the same flow speed, the drag reduction efficiency is only around 23%.     

Non-linear analysis of a dynamically positioned platform in stochastic seaway

Safety is the topmost priority considered by the designers of floating systems or any other structural systems. Reliability, economy and environmental pollutions and the liability of the structure in the case of accidents are also considered by the designers. The paper concentrates on the non-linear behavior of a moored floating platform in stochastic seaway generated using the Pierson–Moskowitz spectrum. Second-order wave forces (slow drift force) acting on the structure is considered as they contribute to a major percentage of the excursion of a large platform. Wave drift damping and skin friction damping have also been considered.It has been shown that the principal frequency of the second-order motion of the platform due to drift forces closely matches the natural frequency of the system in surge motion. This has been subsequently used in tuning a PID (proportionate integral and differential)-based control system for the surge mode, where reduction in the order of 90% has been observed.     

偏航工况下海上浮式风力机基础运动响应及锚泊载荷影响分析

海上浮式风力机受风、浪、流等外部载荷影响,运营期间经常处于偏航工况,给风力机基础运动响应和锚泊载荷带来重要影响.基于经典叶素动量理论及势流理论,建立海上浮式风力机水—气动力耦合分析模型,对在非定常风、不规则波浪联合作用下,风力机偏航时基础运动响应及锚泊载荷等进行分析.研究发现,额定风速工况下,风力机偏航对平台纵荡和纵摇运动影响较大,偏航30°时纵荡和纵摇平均值比偏航0°时分别下降20.68％和37.36％,垂荡运动响应受风力机偏航影响较小;锚泊载荷变化趋势与平台运动及锚链布置有关,平台纵荡对锚泊载荷影响较大,偏航30°时锚链#1有效张力平均值比偏航0°时下降12.98％.     

透水cosine-type同心圆柱波浪绕射研究

在势能流及微小振幅波理论假设下,采用复合边界元素法(CBEM)数值解析规则波通过单根外壁透水cosine-type型同心圆柱结构物绕射。将cosine-type外壁不透水条件改为透水外,并在内部设有一半径为b的不透水内圆柱共同组成同心圆柱,为了验证CBEM数值模式正确性与可行性,将其退化成圆柱,均得到合理结果。数值计算条件包含波浪正向入射凸端、透水参数及绕射参数,探讨波浪通过单根cosine-type同心圆柱体四周水面波动变化情况,并与双重圆筒同心圆柱作比较。计算结果显示结构物外壁采用透水型式,可以大幅降低cosine-type同心圆柱体四周整个波浪绕射场的水面波动。     

Study on coupling effects of ship motion and sloshing

This study considers the coupling effects of ship motion and sloshing. The linear ship motion is solved using an impulse-response-function (IRF) method, while the nonlinear sloshing flow is simulated using a finite-difference method. The IRF method requires the frequency-domain solution prior to conversion to time domain, but the computational effort is much less than that of direct time-domain approaches. The developed scheme is verified by comparing the motion RAOs between the frequency-domain solution and the solution obtained by the IRF method. Furthermore, a soft-spring concept and linear roll damping are implemented to predict more realistic motions of surge, sway, yaw, and roll. For the simulation of sloshing flow in liquid tanks, a physics-based numerical approach adopted by Kim [2001. Numerical simulation of sloshing flows with impact load. Applied Ocean Research 23, 53–62] and Kim et al. [2004. Numerical study on slosh-induced impact pressures on three-dimensional prismatic tanks. Applied Ocean Research 26, 213–226] is applied. In particular, the present method focuses on the simulation of the global motion of sloshing flow, ignoring some local phenomena. The sloshing-induced forces and moments are added to wave-excitation forces and moments, and then the corresponding body motion is obtained. The developed schemes are applied for two problems: the sway motion of a box-type barge with rectangular tanks and the roll motion of a modified S175 hull with rectangular anti-rolling tank. Motion RAOs are compared with existing results, showing fair agreement. It is found that the nonlinearity of sloshing flow is very important in coupling analysis. Due to the nonlinearity of sloshing flow, ship motion shows a strong sensitivity to wave slope.     

Hydrodynamic design of a TLP type offloading platform

The hydrodynamic aspects of a design study of a TLP type offloading platform, that will operate in the north-east of Marmara Sea are presented in this paper. The main scope of the paper is to discuss the general properties of the platform and environmental conditions prior to the calculation of hydrodynamic forces based on Morison’s equation and the resultant motion responses. The non-linear quasi-static effect is taken into account due to set-down phenomenon in the calculation of surge responses. Comparisons between the designs with two different drafts and the results of their spectral analysis are presented. With decrease in the draft of the platform, surge responses increase in the low frequency region. Since the differences between the surge responses of two different designs are not significant, the shorter platform has been adopted for the location due to its low cost. The non-linear set-down effect is not found to be significant because this platform has relatively short taut chains compared to those of conventional type.     

Modeling of coupled tide–wave–surge process in the Yellow Sea

A coupled wave–tide–surge model has been developed in this study in order to investigate the effect of the interactions among tides, storm surges, and wind waves. The coupled model is based on the synchronous dynamic coupling of a third-generation wave model, WAM cycle 4, and the two-dimensional tide–surge model. The surface stress, which is generated by interactions between wind and wave, is calculated by using the WAM model directly based on an analytical approximation of the results using the quasi-linear theory of wave generation. The changes in bottom friction are created by the interactions between waves and currents and calculated by using simplified bottom boundary layer model. In consequence, the combined wave–current-induced bottom velocity and effective bottom drag coefficient were increased in the shallow waters during the strong storm conditions.     

波流组合作用下SPAR平台垂荡-纵摇耦合运动研究

探讨SPAR平台在波流组合作用下的运动响应机理。运用Matlab程序分析了SPAR平台产生内共振的运动过程,运用刚体动力学理论建立了平台垂荡-纵摇耦合方程,对波流共同作用下Spar平台的垂荡、纵摇运动进行数值模拟,结果表明:流的加入对于内共振的影响并不明显;同时采用AQWA软件研究了流的加入对于纵荡二阶慢漂力的影响,研究发现,当波流同向时能大大增加Spar平台的纵荡运动,而波流反向时却能明显削弱纵荡运动。本文研究成果对于指导深海浮式结构设计开发具有一定的理论借鉴作用。     

Nonlinear coupled response of offshore tension leg platforms to regular wave forces

Among the compliant platforms, the tension leg platform (TLP) is a vertically moored structure with excess buoyancy. The TLP is designed to behave in the same way as any other moored structure in horizontal plane, at the same time inheriting the stiffness of a fixed platform in the vertical plane. Dynamic response analysis of a TLP to deterministic first order wave forces is presented, considering coupling between the degrees-of-freedom surge, sway, heave, roll, pitch and yaw. The analysis considers nonlinearities produced due to changes in cable tension and due to nonlinear hydrodynamic drag forces. The wave forces on the elements of the pontoon structure are calculated using Airy's wave theory and Morison's equation ignoring diffraction effects. The nonlinear equation of motion is solved in the time domain by Newmark's beta integration scheme. The effects of different parameters that influence the response of the TLP are then investigated.     

Pore pressure loads on gravity structures

It is common practice to compute wave-induced loads on the immersed surface of gravity structures exposed to the wave motion and disregard the pore-water pressure variation on the foundation surface. However, when the soil is porous, wave-induced pressures propagate within the soil under the structure and result in a rather significant contribution to overall loads. This paper describes a practical method for numerical modeling of the pore pressure under a gravity platform foundation for compressible water and a rigid, but porous soil. The porous soil may be bounded by an impermeable horizontal layer at some arbitrary depth.

The paper outlines the basic boundary element procedure for pore pressure analysis and presents numerical results for a typical gravity structure as well as results for comparison with an existing analytical solution for a vertical circular cylinder.     

Analytical solution on the surge motion of tension-leg twin platform structural systems

In this study the wave induced surge motion of a twin platform of a pretension leg structural system is investigated. A set of equations along with boundary conditions are derived, and solutions are obtained analytically. The tension-legged twin platform structural system might better represent a platform system in the practical application in either idealized two-dimensional analysis or actual three-dimensional practice than a single platform system. In the analysis the coupling problem of a two-dimensional tension leg twin platform interacting with a monochromatic linear wave train in an inviscid and incompressible fluid is considered. The problem is considered as a combination of the scattering and radiation. These two boundary-value problems are first solved independently and then combined together to resolve for all unknowns. The analysis is focused on the wave induced surge motion of the twin platform and the reflection coefficient. Other than the wave-related parameters, the dimensional effect of the twin platform structure is examined and presented in numerical examples. From the analytical results, it is found that the behavior of the tension-leg twin platform is significantly influenced by dimensional factors of the platform system such as the dimension of each single platform and the spacing between two platforms. It is also realized that to simplify the analysis of a twin platform system into a single platform, it tends to underestimate the wave-induced response and then reduces the safety of the platform system.     

Stochastic stability of tethered buoyant platforms

A moment method for analysing the stochastic stability of the surge motion of a tethered buoyant platform (TBP) in a random sea is examined. In the differential equation describing the surge motion the variation of tether tension caused by the vertical component of the wave forces is random-time dependent in form. The asymptotic moment behaviour of the solution is determined and approximated in terms of an integral equation. Under the assumption of a narrow band process imposed upon the random coefficient, the stability results are obtained with the aid of deterministic stability theory. The mean square stability is studied and criteria for stability are obtained in terms of the damping coefficient and the auto-correlation function of the random sea.     

A coupled model of submerged vegetation under oscillatory flow using Navier–Stokes equations

This work presents a new model for wave and submerged vegetation which couples the flow motion with the plant deformation. The IH-2VOF model is extended to solve the Reynolds Average Navier–Stokes equations including the presence of a vegetation field by means of a drag force. Turbulence is modeled using a k–ε equation which takes into account the effect of vegetation by an approximation of dispersive fluxes using the drag force produce by the plant. The plant motion is solved accounting for inertia, damping, restoring, gravitational, Froude–Krylov and hydrodynamic mass forces. The resulting model is validated with small and large-scale experiments with a high degree of accuracy for both no swaying and swaying plants. Two new formulations of the drag coefficient are provided extending the range of applicability of existing formulae to lower Reynolds number.     

A synchronously coupled tide–wave–surge model of the Yellow Sea

A coupled wave–tide–surge model has been established in this study in order to investigate the effect of tides, storm surges, and wind waves interactions during a winter monsoon on November 1983 in the Yellow Sea. The coupled model is based on the synchronous dynamic coupling of a third-generation wave model, WAM-Cycle 4, and the two-dimensional tide–surge model. The surface stress generated by interactions between wind and waves is calculated using the WAM-Cycle 4 directly based on an analytical approximation of the results obtained from the quasi-linear theory of wave generation. The changes of bottom friction factor generated by waves and current interactions are calculated by using simplified bottom boundary layer model. The model simulations showed that bottom velocity and effective bottom drag coefficient induced by combination of wave and current were increased in shallow waters of up to 50 m in the Yellow Sea during the wintertime strong storm conditions.     

深水半潜式平台系泊系统动力特性研究

探讨张紧式系泊系统的基本特征,比较张紧式和悬链线式系泊半潜平台运动和动力响应特性,分析拖曳力系数变化对张紧式和悬链线式系泊动力效应的影响,以及张紧式和悬链线式系泊缆节点运动特点,从而为改善半潜平台系泊系统动力效应提供参考.     

Floating pontoon breakwaters

The hydrodynamic properties of a pair of long floating pontoon breakwaters of rectangular section are investigated theoretically. The structures are partially restrained by linear symmetric moorings fore and aft. The fluid motion is idealized as linearized, two-dimensional potential flow. The breakwater motions are assumed to be two-dimensional, in surge, heave and pitch. The solution for the fluid motion is obtained by the boundary integral equation method using an appropriate Green's function. Numerical results are presented that illustrate the effects of the various wave and structural parameters on the efficiency of the breakwaters as barriers to wave action. It is found that the wave reflection properties of the structures depend strongly on their width, draft and spacing and the mooring line stiffnesses, while their excess buoyancy is of lesser importance.     

Water wave interaction with an array of bottom-mounted surface-piercing porous cylinders

The interaction of water waves with arrays of bottom-mounted, surface-piercing circular cylinders is investigated theoretically. The sidewall of each cylinder is porous and thin. Under the assumptions of potential flow and linear wave theory, a semi-analytical solution is obtained by an eigenfunction expansion approach first proposed for impermeable cylinders by Spring and Monkmeyer (1974), and later simplified by Linton and Evans (1990). Analytical expressions are developed for the wave motion in the exterior and all interior fluid regions. Numerical results are presented which illustrate the effects of various wave and structural parameters on the hydrodynamic loads and the diffracted wave field. It is found that the porosity of the structures may result in a significant reduction in both the hydrodynamic loads experienced by the cylinders and the associated wave runup.