An experimental study on an auxiliary towing system for an FPSO using active thrusters

It is ideal to design the best hull shape for a floating production storage and offloading unit (FPSO) that meets all performance criteria from an engineering point of view. However, in reality, it is difficult to satisfy all the criteria at the same time. If one of the performances cannot meet the criteria, then additional equipment or systems are installed to improve that performance. In this paper, when an FPSO cannot meet the towing stability inherently, we attempt to establish a procedure for determining an auxiliary towing system. Firstly, various systems that can be applied to the FPSO are compared and reviewed to improve the towing stability of the FPSO. Among them, using three active thrusters was chosen as the auxiliary towing system. Improvement of towing stability was verified from a towing model test conducted at the Maritime Research Institute Netherlands (MARIN). However, thruster cavitation was raised as a key problem when using thrusters for a purpose for which they were not intended. A thruster cavitation model test was additionally performed to check the safety of the thrusters. Based on these results, a safe operating range under extreme condition was confirmed by observing the occurrence of thruster cavitation. The predicted maximum unit total thrust of each thruster was compared with the observed safe operating range. Finally, the effectiveness and safety of the auxiliary towing system was verified.     

Active heave compensation on drill-ships in irregular waves

This paper investigates a possible method for heave compensation on board deep-water drill-ships subjected to irregular-wave excitation. The proposed system exploits favorable interaction of coupled oscillators to achieve the desired results. This study examines an actively controlled compensator which performs well over a large wave-frequency bandwidth. Performance under certain operating conditions is investigated using a dynamic model. Simple mathematical arguments and frequency-domain computations in an irregular wave spectrum show the proposed heave compensation system to be effective within the bounds of linearity.     

Model-based dynamic positioning of underwater robotic vehicles: theory and experiment

This paper addresses the trajectory tracking problem for the low-speed maneuvering of fully actuated underwater vehicles. It is organized as follows. First, a brief review of previously reported control studies and plant models is presented. Second, an experimentally validated plant model for The Johns Hopkins University Remotely Operated Underwater Vehicle (JHUROV) is reviewed. Third, the stability of linear proportional-derivative (PD) control and a family of fixed and adaptive model-based controllers is examined analytically and demonstrated with numerical simulations. Finally, we report results from experimental trials comparing the performance of these controllers over a wide range of operating conditions. The experimental results corroborate the analytical predictions that the model-based controllers outperform PD control over a wide range of operating conditions. The exactly linearizing model-based controller is outperformed by its nonexactly linearizing counterpart. The adaptive controllers are shown to provide reasonable online plant parameter estimates, as well as velocity and position tracking consistent with theoretical predictions-providing good velocity tracking and, with the appropriate parameter update law, position tracking. The effects of reference trajectory, "bad" model parameters, feedback gains, adaptation gains, and thruster saturation are experimentally evaluated. To the best of our knowledge, this is the first reported comparative experimental study of this class of model-based controllers for underwater vehicles.     

An accurate four-quadrant nonlinear dynamical model for marinethrusters: theory and experimental validation

This paper reports two specific improvements in the finite-dimensional nonlinear dynamical modeling of marine thrusters. Previously reported four-quadrant models have employed thin airfoil theory considering only axial fluid flow and using sinusoidal lift/drag curves. First, we present a thruster model incorporating the effects of rotational fluid velocity and inertia on thruster response. Second, we report a novel method for experimentally determining nonsinusoidal lift/drag curves. The model parameters are identified using experimental thruster data (force, torque, and fluid velocity). The models are evaluated by comparing experimental performance data with numerical model simulations. The data indicates that thruster models incorporating both reported enhancements provide superior accuracy in both transient and steady-state responses     

Feedback Stabilization of High-Speed Planing Vessels by a Controllable Transom Flap

This paper focuses on the mitigation of porpoising instability of high-speed planing vessels using controllable transom flap and dynamic feedback. A control oriented model that captures both steady-state and dynamic characteristics is presented and used to facilitate the model-based control design. A nonlinear controller is developed based on the feedback linearization method to achieve asymptotic stability of the planing boat, thus avoiding porpoising at high speeds. We first show that the full-state nonlinear dynamic model describing the ship motion is not feedback linearizable. A state transformation is then constructed to decompose the model into a linearizable subsystem and a nonlinear internal dynamic subsystem. A reduced order state feedback is shown next to stabilize the planing vessel motion around the equilibrium point. Analysis of the region of attraction is also performed to provide an assessment of the effective safe operating range around the equilibrium point.     

内孤立波过陆架陆坡地形的数值模拟研究

内波是海洋中普遍存在的波动形式。内孤立波是典型的非线性内波,多发于陆架边缘海,如南海等海域,对陆架海域有重要影响。本文针对内孤立波在陆架地形上的传播问题,先基于弱非线性与全非线性数值模型,模拟了不同振幅、地形高度条件下内孤立波的演化的过程,探讨了动力系数对内孤立波演化过程的影响,对比了两模型的模拟结果在内孤立波演化过程、能量分配以及能量耗散的差异,后分析了南海的动力系数分布特征。结果表明,在内孤立波不发生破碎的情况下,弱非线性模型与全非线性模拟结果相近。当发生破碎过程时,弱非线性模型可准确模拟头波,但无法通过强非线性的破碎过程耗散能量,只能以裂变的方式辐射能量。在弱非线性模型中,随地形高度增加,频散系数减小到零,平方非线性系数由负转正,立方非线性系数绝对值增大一个量级,并主导陆架地形上内孤立波的演化过程。通过对比南海夏季与冬季非线性内波动力系数空间分布,发现内孤立波在传播过程由于夏季平方非线性效应、立方非线性效应与频散效应较强的影响,其在夏季更易发生陡化与裂变,波列发生频率高。     

An experimental study of a pulse compression scatterometer system

The ideal scatterometer, operating from either an aircraft or a satellite platform, should be capable of making rapid, accurate estimates of the sea backscatter cross sectionsigmaover as wide a range of grazing anglespsias possible. Efficient operation over a large range of grazing angles is desirable because 1)sigmabehavior for90deg geq psi geq 70degyields rms gravity wave slope information and is an indicator of sea state and 2)sigmabehavior for70deg geq psi geq 0degyields data on surface wind magnitude and direction as well as information about the power spectrum of the sea. A "hybrid" estimation procedure has been developed for pulse compression radars which uses both frequency and spatially decorrelated samples ofsigmato provide an unbiased estimate ofsigmahaving minimum variance over the entire range of grazing angles for which radar reception is not noise-limited.     

The optimum control of thruster system for dynamically positioned vessels

An offshore vessel with a dynamic positioning system (DP system) needs fast response to produce thrust to counteract the environmental forces acting on it for the purpose of maintaining its position and heading as close as possible to the working position. Therefore, quick and effective modulation of the thrust is the problem to determine the thrust and the rotation angle of the thruster devices of the ship. This paper presents an effective optimum control for a thruster system, using the sequential quadratic method to achieve economical and effective modulation of the thrust and the direction of the thruster. An optimum control study of a 2280 tons DP coring vessel with five rotary azimuth thruster marine positioning is studied in detail, which can quickly and exactly estimate the thrusts and angles of direction of all the thrusters. The results can provide a valuable thruster system for a dynamically positioned vessel.     

Development, comparison, and preliminary experimental validation ofnonlinear dynamic thruster models

This paper examines dynamical models for bladed-propeller-type marine thrusters. Previously reported thruster dynamics models are reviewed, and a simplified model is proposed. Experimental testing of both the transient and steady-state performance of a marine thruster corroborates previously reported data, validates the simplified thruster model, and raises new questions. A companion paper reports preliminary experiments in the design and testing of thrust controllers which incorporate the dynamical thruster models described in the present paper     

Coupled dynamic analysis of deep-sea mining support vessel with dynamic positioning

Deep-sea mining (DSM) is an advanced concept. A simulation method of coupled vessel/riser/body system in DSM combined with dynamic positioning (DP) is proposed. Based on the three-dimensional potential flow theory, lumped mass method, and Morison’s equations the dynamic models of the production support vessel, riser and slurry pump are established. A proportion integration differentiation (PID) controller with a nonlinear observer and a thrust allocation unit are used to simulate the DP system. Coupled time domain simulation is implemented with the vessel operated in two DP modes. Results of the vessel and pump motions, riser tension, and thruster forces are obtained. It shows that the pump will be lifted by the riser when the vessel is chasing the next set point. Riser tension is influenced by the wave frequency motions of the vessel in positioning mode and low-frequency motions in tracking mode. The proposed simulation scheme is practical to study the DSM operation.     

A skewed eddy of Batchelor-modon type

A Batchelor-modon eddy is a highly specialized nonlinear vortex pair, whose potential vorticity depends linearly on the stream function viewed from the coordinates moving with the translation velocity of the eddy. To generalize it, a skewed model is developed by introducing a cubic nonlinearity in addition to the linear term. A perturbation analysis shows that the eddy region is no longer a circle but is elongated longitudinally or transversely according as the sign of the cubic term. Moreover, the eddy is slightly flattened or steepened. The cubic term increases or decreases the translation velocity, if the average radius and the amplitude are fixed. A numerical experiment on anf-plane is carried out to show that these skewed eddies retain their initial forms even after they turn a corner of the basin; they are as stable as (first-mode) standard Batchelor-modon eddies. The present skewed model gives a reasonable qualitative interpretation of deformed eddies which result from merging of two eddies or from initially Gaussian eddies near the boundary.     

Nonlinear heave radiation forces on two-dimensional single and twin hulls

A study of nonlinear heave radiation of two-dimensional single and double hulls has been carried out in the time domain. The problem is analyzed by means of a fully nonlinear mathematical model, referred to as the mixed Eulerian–Lagrangian (MEL) model, which is based on an integral relation formulation coupled with time-integration of the nonlinear free-surface boundary conditions. The integral equation solver is based on a cubic-spline boundary-element scheme in which both potential and velocity continuity conditions can be enforced through the intersection points. The body undergoes periodic forced heave oscillation. By implementing effective wave-absorbing beaches at the two ends of the rectangular numerical tank, long-term steady-state force-histories could be achieved consistently in all computations.Results in terms of radiation forces for rectangular and triangular single- and twin-hull geometries are presented and discussed. Linear hydrodynamic forces in terms of added-mass and damping are validated for the rectangular hull. The Fourier-analyzed results reveal the extent of nonlinear (higher-order) components in the force-signals over different parameters which include the amplitudes of oscillation, hull-spacing for the twin-hulls and water depth.     

一类受持续扰动的非线性时滞系统的扰动抑制

研究一类受已知动态特性的外部扰动作用的非线性时滞系统的扰动抑制问题。基于内模原理构造能够抵消扰动影响的伺服补偿器,然后利用极大值原理及灵敏度法设计带有补偿器的最优控制律,使得系统在无扰动作用时以1种最优方式运行,而有扰动作用时,控制律能够消除扰动引起的稳态误差,实现无静差扰动抑制。最后,通过仿真算例验证提出算法的有效性。     

Preliminary experiments in model-based thruster control forunderwater vehicle positioning

This paper reports comparative experiments with two novel and one conventional thrust control algorithms for the unsteady (transient) control of thrust generated by conventional bladed-propeller marine thrusters. First, comparative experiments with three different thrust control algorithms over a wide range of unsteady operating conditions suggest that model-based control algorithms offer transient thrust-control performance superior to that of their nonmodel-based counterpart. Second, hybrid simulations combining actual real-time experimental thruster responses with simulated one-dimensional real-time vehicle dynamics suggest that model-based thrust control algorithms offer vehicle position control superior to that of its nonmodel-based counterpart     

Multichannel system for the acquisition of high-frequency acoustic information

A 14-channel system for the acquisition of acoustical data is described which operates in the frequency range 1–20 kHz with smooth retuning over the entire frequency range. The effective bandpass is 700 Hz; the dynamic range is 80 dB. The system operates on-line under the control of the microcomputer ELECTRONIKA-60.Translated by Mikhaik M. Trufanov.     

Tide and surge dynamics along the Iberian Atlantic coast

The eastern Atlantic barotropic dynamics (in a region spanning from 20° N to 48° N and 34° W to 0°) are studied through numerical modelling and in situ measurements. The main source of data is the tidal gauge network REDMAR, managed by Clima Marítimo (Puertos del Estado). The numerical model employed is the HAMSOM, developed both by the Institut für Meereskunde (Hamburg University) and Clima Marítimo.In this paper, tidal and storm surge dynamics are studied for the region, focusing particularly on the nonlinear transfer of energy between the different forcings.The results of tidal simulations show good agreement between semidiurnal harmonic components and the values obtained from the tidal gauges (both coastal and pelagic) and current metres. The nonlinear transfers of energy from semidiurnal to higher order harmonics, such as M4 and M6, were mapped. Those transfers were found to be important in only two areas: The French continental shelf in the Bay of Biscay and the widest part of the African shelf, south of Cabo Bojador. The modelled diurnal constituents show larger relative differences with measurements than semidiurnal harmonics, especially in data concerning the phase.A method to isolate the nonlinear transfers of energy between tidal and atmospheric forcing during a storm surge was developed. These transfers were found to be significant in the same areas where tidal nonlinear activity was present. The effect of short period wind generated waves on sea surface elevation was also investigated. The magnitude of the spatial derivatives of radiation stress was compared with wind-induced stress. As a result of this comparison, we found the inclusion of a forcing term that depends on radiation stress in ocean model simulations at this scale and resolution to be not essential. The effect of computing wind-induced stresses, with a formulation that explicitly depends on sea state, was also explored by means of a coupled run of the HAMSOM and the spectral wave model WAM for a storm surge event in the Spanish coast. This formulation was not found to be an improvement over a classical parameterisation which only depends on wind fields.     

Nonlinear Static Finite Element Stress Analysis of Pipe-in-Pipe Risers

Owing to the complexity of the pipe-in-pipe (PIP) riser system in structure, load and restraint, many problems arise in the structural analysis of the system. This paper presents a new method for nonlinear static finite element stress analysis of the PIP riser system. The finite element (FE) model of the PIP riser system is built via software AutoPIPE 6.1. According to the specialties of a variety of components in the PIP riser system, different elements are used so as to model the system accurately. Allowing for the complication in modeling the effects of seabed restraint, a technique based on the bilinear spring concept is developed to calculate the soil properties. Then, based on a pipeline project, the entire procedure of stress analysis is discussed in detail, including creation of an FE model, processing of input data and analysis of results. A wide range of loading schemes is investigated to ascertain that the stresses remain within the acceptable range of the pipe material strength. Finally, the effects of the location of flanges, the thermal expansion of submarine pipelines and the seabed restraint on stress distribution in the riser and expansion loop are studied, which are valuable for pipeline designers.     

A three-dimensional theory for the development and migration of deep sea sedimentary waves

A perturbation model is presented for a velocity field of a bottom current flowing over a sinusoidal topography or an obstacle. The model extends existing theory by taking into account the three-dimensional Coriolis vector and an initial horizontal velocity vector at any orientation. One possible mechanism of the development of sedimentary waves in the vicinity of an obstacle by an arbitrarily oriented initial horizontal current is analyzed in detail. Space-stationary fluid particle oscillations are initiated on the downstream side of an obstacle, which can result in sedimentary waves. The model shows that their wavelength depends on latitude, water depth, obstacle width and orientation as well as the initial current direction and intensity. The model defines intervals for current velocities normal to the wave crest, for which the sedimentary waves grow (or are destroyed) or migrate in a certain direction. Information derived from bathymetric and seismic surveys, such as wavelength, height, orientation and migration direction of mudwaves, can be used to calculate the velocity component across the wave crest and to estimate the current direction, as is demonstrated for an example from the Argentine Basin (Project MUDWAVES, Site 5).     

Incorporating irregular nonlinear waves in coupled simulation and reliability studies of offshore wind turbines

Design of an offshore wind turbine requires estimation of loads on its rotor, tower and supporting structure. These loads are obtained by time-domain simulations of the coupled aero-servo-hydro-elastic model of the wind turbine. Accuracy of predicted loads depends on assumptions made in the simulation models employed, both for the turbine and for the input wind and wave conditions. Currently, waves are simulated using a linear irregular wave theory that is not appropriate for nonlinear waves, which are even more pronounced in shallow water depths where wind farms are typically sited. The present study investigates the use of irregular nonlinear (second-order) waves for estimating loads on the support structure (monopile) of an offshore wind turbine. We present the theory for the irregular nonlinear model and incorporate it in the commonly used wind turbine simulation software, FAST, which had been developed by National Renewable Energy Laboratory (NREL), but which had the modeling capability only for irregular linear waves. We use an efficient algorithm for computation of nonlinear wave elevation and kinematics, so that a large number of time-domain simulations, which are required for prediction of long-term loads using statistical extrapolation, can easily be performed. To illustrate the influence of the alternative wave models, we compute loads at the base of the monopile of the NREL 5MW baseline wind turbine model using linear and nonlinear irregular wave models. We show that for a given environmental condition (i.e., the mean wind speed and the significant wave height), extreme loads are larger when computed using the nonlinear wave model. We finally compute long-term loads, which are required for a design load case according to the International Electrotechnical Commission guidelines, using the inverse first-order reliability method. We discuss a convergence criteria that may be used to predict accurate 20-year loads and discuss wind versus wave dominance in the load prediction. We show that 20-year long-term loads can be significantly higher when the nonlinear wave model is used.