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Heave compensation systems are widely used to decouple the load motion from wave-induced vessel motion for the equipment handling on the ocean. Researches have been made to achieve successful compensation, yet few of them discusses the inherent constraints of the systems, such as bounded compensator's stroke and max actuator's velocity. This paper presents a solution for active heave compensation systems with such constraints by means of variable structure control. The controller's complexity on design procedures and effectiveness are compared with a trajectory planning control method which turns out that the variable structure controller is more suitable to apply to the active heave compensators. The back-stepping method is used to robustly stabilize this variable structure system and for the aim of a decrease on the high robust gain due to uncertain friction term, a modified decoupled friction observer is used which is also verified by both theoretical and experimental analyses. To compensate for the time delay of the motion reference unit (MRU), a heave prediction algorithm is used. The experimental results show that most heave motion can be compensated when the motion and its velocity are feasible, while no hit occurs otherwise.

The article presented here described the design of a test stand for hydraulic active heave compensation system. The simulation of sea waves is realized by the use of hydraulic cylinder. A hydraulic motor is used for sea waves compensation. The hydraulic cylinder and the hydraulic motor are controlled by electrohydraulic servo valves. For the measurements Authors used displacement sensor and incremental encoder. Control algorithm is implemented on the PLC. The performed tests included hydraulic actuator and hydraulic motor step responses.

This article presents an offshore-simulated loading and unloading of a payload from a floating platform to a fixed structure. The experiments are performed in a dry-lab, where a Stewart platform is used to simulate the motion of the vessel. A hydraulically actuated vehicle loader crane is used to perform the tasks of payload transfer. The crane includes a hydraulic winch where the wire force is measured by a load cell. A mathematical model of the winch is derived and is experimentally verified. The control strategies include a heave compensation and a constant tension mode. A motion reference unit is used to generate the reference motion of the moving platform. Experimental results show the wire force while performing the load cases. This paper shows the advantage of using a reference motion as a feed forward control reference, instead of only relying on the constant tension.

O mecanismo Plataforma de Stewart é um manipulador do tipo paralelo, com seis graus de liberdade, boa relação peso/carga e alta rigidez. Tais características conferem a este tipo de manipulador propriedades superiores de precisão em relação aos manipuladores seriais. Neste trabalho, o controle de um Manipulador Plataforma de Stewart (MPS) acionado por atuadores hidráulicos é estudado com o objetivo de compensação de movimentos para viabilização de transferência de cargas e pessoas em ambiente naval.Visando ao desenvolvimento de um protótipo experimental, o manipulador é estudado considerando a situação em que se encontra sobreposto a um segundo MPS que tem por objetivo simular o movimento da maré, sendo ambos MPS considerados desacoplados dinamicamente. Neste contexto, o estudo envolve a análise cinemática e dinâmica do manipulador incluindo, também, a dinâmica dos cilindros hidráulicos. Além disso, são estudadas unidades de medição inercial (IMU) utilizando-as como instrumento para medição do movimento da base a ser compensado. O projeto do controlador do sistema de atenuação de movimento faz uso da técnica de Torque Computado (TC). A análise de estabilidade, feita separadamente para o sistema mecânico e hidráulico, baseou-se da teoria de Lyapunov. Simulações realizadas considerando trajetórias similares às do movimento de um navio são utilizadas. Para compensação do movimento são utilizados, também, sinais provenientes de uma IMU. Por meio de simulação, comprova-se que o sistema proposto é capaz de compensar adequadamente os movimentos da base estudados. The Stewart platform mechanism is a parallel manipulator with six degrees of freedom, high load/weight ratio and high stifness. These properties give them a better accuracy when compared to serial manipulators. This work focuses on study of electrohydraucally Stewart Platform Manipulators (MPS) to enable compensation of vessels motions for load and personell transfer in sea. Aimed at developing an experimental prototype, a second MPS is placed underneath the rst MPS to simulate vessels motions and so both manipulators are considered dynamically decoupled. In this sense, the kinematics and dynamics of this manipulator are presented, as well as a mathematical model of the hydraulic actuator. Furthermore, special attention is given to the study of inertial measurement units (IMU) which is used as an instrument for measuring the motion to be compensated. Controller design for the compensation system is developed considering compute torque theory which consider the system separated in two: mechanical and hydraulic. The Lyapunov criteria is used to guarantee closed loop stability for each subsystem. Simulations are performed considering similar vessel motions. Signals provided from a comercial IMU are used for motion compensation. The control compensation performance is veri ed by means of computer simulations.

Master's thesis in Offshore technology A typical installation of a subsea structure with mono hull offshore construction vessel is studied. Focus has been on the vessel motions, lifting arrangement and the planning of this operation. The behaviour of vessel motions are studied with the available transfer functions of the vessel at centre of gravity. The key transfer functions for Heave, Pitch and Roll are discussed when the wave propagates in head and beam seas. The transfer functions used are qualitatively verified for their correctness and usage in this study. Then, the heave motion at two different points far away from the CoG of the vessel are studied to see the difference in heave motion. Transfer functions at cranetip are derived, as this is the main excitation for the dynamics on the crane wire. The derived transfer function at the crane tip is verified against the results from SIMA software. In the lifting arrangement study, transfer functions for the vertical motion of the suspended load are derived by assuming linear relationship between crane tip motion and motion of the mass. Then, using that transfer function, the transfer function for the dynamic tension on the crane wire is derived for the wave in head sea and beam sea. To improve the lifting arrangement system, the heave compensation system is introduced with a discussion about its efficiency. To obtain the limiting sea state for this operation, a deterministic and a stochastic approach have been followed. In the stochastic method, JONSWAP spectrum is used for the total sea case which propagated in head sea and beam sea respectively. Then, combined sea spectrum i.e. double peak spectrum for the wind sea with head sea direction introduced and swell sea which here is simplified to always propagate between the direction of head and beam seas, is used to obtain limiting sea state accounting the effect of crossing sea approximately. Respective response spectrum is then derived based on linearity of the response and assumption that response of swell sea is not effected by the wind sea from a different direction and vice versa. The discussion is made about the comparison of the approaches followed to obtain the limiting sea state. Sensitivity study on the probability of exceedance of critical response is shown as well. For planning of this operation, 60 years of hindcast data are used. The hindcast data taken are corrected based on the wind speed measurement available. And the discrete values of the spectral peak period in the data are uniformly distributed. After randomization of Tp, annual and monthly scatter tables of Hs and Tp, and plots have been produced to see the percentage of times available for a sea state during a year and monthly. Likewise, for Hs ≤ 2m, the average percentages of non-exceedance of this sea state are produced for various months. Since the scatter table do not give any info regarding the duration of the sea state, the average durations of sea state for Hs ≤1 m, 2 m, and 3 m are calculated. Then, since number of available windows are useful for planning an operation, the average number of windows available for Hs ≤ 2m, and duration 6, 12, 24, 48, 72 hours are screened through the hindcast data and presented. Finally, dividing an operation into smaller phases, here 72 hours operations divided into 48+24 hours, 24+24+24 hours, are studied and discussed about the change in percentage of successful operation and average number of days to complete that operation, in a month using the 60 years of hindcast data.

Ploveće dizalice, koje prenose koristan teret tijekom izvođenja radova na otvorenom moru, moraju zadovoljiti rigorozne zahtjeve u pogledu učinkovitosti i sigurnosti u teškim uvjetima, dok ih iznenadno gibanje ploveće dizalice prouzročeno valovima, čini još težim. U radu se analizira dinamika ploveće dizalice i upoznaje s aktivnim sustavom za kompenzaciju dizanja i spuštanja broda (heave compensation system - HCS) s kojim se postiže pomicanje tereta nezavisno od gibanja broda prouzročenog valovima u uspravnom smjeru. Točnije, uvodi se virtualna tehnologija stvaranja prototipa za sustave plovećih dizalica s ciljem istraživanja stvarnog djelovanja HCS-a. Stvoren je virtualni mehanički model ploveće dizalice kako bi se analizirala dinamika dizalice. Uz to, konstruiran je aktivni HCS za potpogonjeni sustav ploveće dizalice. Za bitan dio HCS-a predlaže se zakon samopodesivog PID upravljanja zasnovan na poboljšanom genetskom algoritmu. Zajednička simulacija i eksperimentalni rezultati pokazuju da se pomicanje tereta zbog vertikalnog gibanja broda može značajno smanjiti u teškim morskim uvjetima., The floating cranes, which transfer payloads during offshore constructions, need meet rigorous demands for efficiency and safety in serious sea conditions while the unexpected motion of the floating crane caused by wave make it even harder. This paper analyses the dynamics of a floating crane and presents an active heave compensation system (HCS) to allow the payloads motion decoupled from the wave-induced ship motion in the vertical direction. Specifically, virtual prototyping technology is introduced for the floating crane systems to investigate the real behaviour of the HCS. A virtual mechanical model of the floating crane is created to analyse the dynamics of the floating crane. In addition, an active HCS is designed for the under actuated floating crane system. A self-adaptive PID control law based on improved genetic algorithm is proposed for the essential part of the HCS. The co-simulation and experiment results demonstrate that the payload motion due to the ship’s vertical motion can be reduced significantly in harsh sea conditions.