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Copyright of Machine Tool & Hydraulics is the property of Guangzhou Mechanical Engineering Research Institute (GMERI) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Investigations regarding the dynamic response of a novel heave-compensated floating platform have been carried out. The concept consists of a semi-submersible floating platform with an air chamber below its hull, aiming to keep the static platform buoyancy constant, thus effectively reducing heave motion and enabling its use under harsh environmental conditions as in the North and Norwegian seas. This work develops a dynamic response model for the concept platform in which hydrodynamic coefficients calculated using potential flow theory and mooring lines effects are considered. Calculations in both the frequency and time domain are performed, and results show that the air chamber and effects of the mooring lines are the most beneficial scenario to reduce the dynamic response, especially in terms of heave motion reduction at lower wave frequencies. Another observed benefit is the change of the heave resonance peak away from the typical observed frequency range of sea waves. [ABSTRACT FROM AUTHOR]

Based on the key module-lifting arm system, based on the principle of similarity and the hydrodynamic experimental method of a multi-dimension vibration test platform, an experimental platform for dismantling equipment is designed and built. Subsequently, the motion control model of the six-degrees-of-freedom platform is established. The three-ring control model of a servo electric cylinder is established, and the active heave compensation control of a servo electric cylinder is realized by combining position control theory. Based on the co-simulation of ADAMS and Simulink, the co-simulation system of the integrated dismantling equipment experimental platform is designed and built, and the simulation system is tested and verified. Finally, simulation and experimental verification are carried out based on the experimental platform and co-simulation system. The results show that the heave compensation rate reaches 58.3% in third-class sea conditions, 61.2% in fourth-class sea conditions, and 62.4% in fifth-class sea conditions. The integrated dismantling scheme of super large offshore oilfield facilities is feasible but, in order to ensure the safety and reliability of the operation, a heave compensation system needs to be added. The error between the simulation results and the experimental results is about 15%. Based on the analysis of external interference factors in the experiment, the error results are within a reasonable range, which proves that the experimental platform, the co-simulation system of the experimental platform, and the heave compensation strategy are accurate and effective. This study, for the first time in China, provides an effective experimental platform and co-simulation platform for the design and optimization of the integrated dismantling equipment of super large offshore oilfield facilities and lays a good research foundation for the construction and engineering demonstration of subsequent equipment. [ABSTRACT FROM AUTHOR]

The vessel heave motion caused by wave action increases the difficulty of installing offshore wind equipment. Onboard wave heave compensation devices have therefore become increasingly critical in ensuring the stability and safety of the gangway and working platform. This study accordingly improves the compensation effect of such devices by developing a wave heave compensation model and designing an optimized backstepping control method. First, a model of the compensation system including the servo motor and electric cylinder is established by using the mechanism method. Second, a backstepping control method is designed to track the vessel heave motion, and particle swarm optimization is applied to optimize the control parameters. Finally, MATLAB/Simulink is used to simulate the application of the optimized backstepping controller, then regular and irregular heave motions are applied as input to a Stewart platform to evaluate the effectiveness of the control method. The experimental results show that the compensation efficiency provided by the proposed optimized backstepping control method is larger than 75.0%. [ABSTRACT FROM AUTHOR]

Mining ship is an essential Marine equipment in the exploitation of Marine resources. In order to reduce the heave motion, it is necessary to compensate the mining ship for heave compensation. A composite active and passive integrated heave compensation system based on composite hydraulic cylinder is proposed. In order to improve the control accuracy, an active compensation control strategy based on Mamdani fuzzy PID is proposed to reduce the compensation error under heave interference. Firstly, the heave compensation principle was introduced, and the calculation process of ship"s heave amplitude under different sea conditions was deduced based on Mohr"s empirical formula, then the promotion heavy compensation mathematic model and transfer function of the system were established, finally the proposed fuzzy control algorithm was used to control the system under the regular ocean wave and irregular wave and compare with the traditional PID control effect. The results show that under the two kinds of ocean wave, the error of the fuzzy control algorithm is less than the error of the traditional PID control algorithm, It can be seen that fuzzy control algorithm has obvious advantages. [ABSTRACT FROM AUTHOR]

The unexpected heave motion of vessels generated by waves has always threatened the safe operation of mining vessels. The heave compensation system is a critical equipment for offshore cranes to ensure the safety and stability of the deep-sea mining vessel operation. Unfortunately, the nominal load of the mining vessel transporting minerals has become increasingly large, which has led to the dynamic performance of the heave compensation decline and a significant increase in energy consumption. To solve these problems, a semiactive heave compensation system and fuzzy logic-based controller are proposed in this paper. First, an active-passive hybrid heave compensation system is designed to reduce energy consumption greatly. The virtual prototype models of the proposed heave compensation system in a 500-ton deep-sea mining vessel are also built. Then, an adaptive tuning PID controller is developed based on the Transfer function of the heave compensation system and Mamdani-type fuzzy logic. Virtual prototype simulation results of AMESim-Simulink joint simulation illustrate that the accuracy of compensation for the four or six sea states reaches more than 90% with the proposed method and the power of the active compensation system is about 21% of the total power. Finally, the experimental results are included to demonstrate the effectiveness of the proposed semiactive heave compensation system. [ABSTRACT FROM AUTHOR]

Wave motion in the deep-sea threatens the safety and accuracy of ship operations, particularly the ship heave motion. Therefore, a ship heave compensation system based on the deep deterministic policy gradient (DDPG) and proportional–integral–derivative (PID) hybrid control method of the judge network is proposed to improve the control accuracy and application robustness. First, a transfer function model was established using the mechanism method. Subsequently, a hybrid control method was designed by combining PID and DDPG through the judge network, and an update method for the judge network based on the value function was designed. Finally, the compensation effect in a complex and variable environment was tested using ship heave motion in sea state level 3–6 Pierson–Moscowitz (P–M) spectrum simulations and hardware experiments. The results of the sudden sea state experiments show that the error of the hybrid compensation control method is minimal, indicating that it has good robustness. The compensation efficiency of the model trained by the proposed method can reach 98.08% under different sea states, which shows that it has excellent generalizability. The proposed control method has good dynamic tracking ability, and the control accuracy is up to 99.69%, making it applicable to ship heave motion environments. • A hybrid compensation control method based on Judge network is proposed for compensation control of ship heave system. • A Judge network based on value function update to reduce oscillations is proposed by combining DDPG and PID. • The hybrid control method has good generalization to different sea states and good robustness to changeable sea states. [ABSTRACT FROM AUTHOR]

With the depletion of land resources, demands for marine resources mining is increasing, the mining vessels which is known as one of the most important offshore equipment are widely concerned. The motions of wave surges in the marine environment will affect the operation of mining vessels, especially for the heave motion which will cause serious disturbances to the mining vessels during operation, and even the damage of the hull structure and related mining facilities. Hence, it is very necessary to compensate for the heave motion of the mining vessel, and a hydraulic cylinder-based active-passive integrated composite heave compensation system is designed in this paper. Firstly, the structural composition and working principle of the heave compensation system are introduced, and its mathematical model is also established. Secondly, the frequency domain and time domain analysis are carried out using AQWA software to obtain the heave displacement response of the ship under regular and irregular waves of six sea states as the desired compensation displacement. Finally, the controller of the active-passive heave compensation system is designed, and a fuzzy logic control strategy based on Mamdani type is proposed, and the control performance is analyzed. The results show that the accuracy of the fuzzy logic control algorithm is better than the traditional PID control algorithm, and the laws and methods of PID adjustment are obtained, which proves that the fuzzy logic control algorithm has obvious advantages for engineering applications. [ABSTRACT FROM AUTHOR]

Based on the key module-lifting arm system, based on the principle of similarity and the hydrodynamic experimental method of a multi-dimension vibration test platform, an experimental platform for dismantling equipment is designed and built. Subsequently, the motion control model of the six-degrees-of-freedom platform is established. The three-ring control model of a servo electric cylinder is established, and the active heave compensation control of a servo electric cylinder is realized by combining position control theory. Based on the co-simulation of ADAMS and Simulink, the co-simulation system of the integrated dismantling equipment experimental platform is designed and built, and the simulation system is tested and verified. Finally, simulation and experimental verification are carried out based on the experimental platform and co-simulation system. The results show that the heave compensation rate reaches 58.3% in third-class sea conditions, 61.2% in fourth-class sea conditions, and 62.4% in fifth-class sea conditions. The integrated dismantling scheme of super large offshore oilfield facilities is feasible but, in order to ensure the safety and reliability of the operation, a heave compensation system needs to be added. The error between the simulation results and the experimental results is about 15%. Based on the analysis of external interference factors in the experiment, the error results are within a reasonable range, which proves that the experimental platform, the co-simulation system of the experimental platform, and the heave compensation strategy are accurate and effective. This study, for the first time in China, provides an effective experimental platform and co-simulation platform for the design and optimization of the integrated dismantling equipment of super large offshore oilfield facilities and lays a good research foundation for the construction and engineering demonstration of subsequent equipment.

Heave compensation systems are of great importance to the safety and efficiency of marine operations subject to irregular-wave excitation. While many efforts have been made to improve the displacement compensation control, only a few of researchers pay their attention on tension compensation control. This paper presents an adaptive robust integral sliding mode control (ARISMC) based on the back-stepping method to realize constant tension control of hybrid active-passive heave compensator (HAHC) applied to heavy deep-sea towing systems. The proposed ARISMC is intended to overcome the effects of parametric uncertainties, uncertain nonlinearities, and external disturbances in the electro-hydraulic system of HAHC. The stability of the whole system is proved using common Lyapunov method. To verify the effectiveness of the proposed controller, we carried out a number of simulations and experiments using the measured heave motion data under different sea conditions. The results demonstrate that the ARISMC controller presented in this paper has better advantages than the traditional PI controller in the accuracy and robustness of tension compensation.

In the marine industry, heave compensation systems are applied to marine equipment to compensate for the adverse effects of waves and the hydraulic system is usually used as the power system of heave compensation systems. This article introduces importance theory to the opportunistic maintenance (OM) strategy to provide guidance for the maintenance of heave compensation systems. The working principle of a semi-active heave compensation system and the specific working states of its hydraulic components are also first explained. Opportunistic maintenance is applied to the semi-active heave compensation system. Moreover, the joint integrated importance measure (JIIM) between different components at different moments is analyzed and used as the basis for the selection of components on which to perform PM, with the ultimate goal of delaying the degradation of the expected performance of the system. Finally, compared with conditional marginal reliability importance (CMRI)- based OM, the effectiveness of JIIM-based OM is verified by the Monte Carlo method. [ABSTRACT FROM AUTHOR]

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.

For the purpose of researching the displacement control system of heave compensation for offshore drilling platform, a set of crane type active and passive combined heave compensation device are designed on the basis of the similarity principle. As it known that platform heaving will be caused by the wind, sea wave and ocean current when conducting the drilling operations on the offshore drilling platform, which then will disturb the drilling operation. Therefore, the compensation device must be adopted to keep the vertical relative motion between the drill string system and the drilling platform to zero during the operation. Meanwhile, to improve the real-time performance of the heave compensation, the control system is optimized by establishing the simulation model of the active-passive combined crane, and LS-SVM (Least Squares Support Vector Machine) is improved by the artificial immune algorithm to predict the motion trend of offshore platforms. Eventually, in order to acquire the best control scheme, the Proportion Integration Differentiation (PID), fuzzy PID, BP neural network PID control method are utilized to carry out the simulation analysis, and the BP neural network PID control is found to be the optimum. Experiments showed that after using the BP neural network PID control algorithm, the displacement compensation rate of hook for active-passive combined crane device is more than 90%, the performance of the heave compensation is better, and the control is in time.

Objectives Predicting heave motion is helpful for improving the performance of the heave compensator and reducing the disturbance of waves on operating equipment. To improve the accuracy and stability of the heave prediction model, a real-time prediction method for ship heave motion is proposed in this paper.Methods Based on the Nonlinear Autoregressive with eXogeneous inputs (NARX) neural network, a single sea-state prediction model is established. The simulated heave motion of the vessel is obtained using the Marine Systems Simulator software tool to verify the model. The prediction model based on NARX is compared with prediction models based on Kalman and BP. On this basis, a multi sea-state prediction model is established by improving the single sea-state model.Results The prediction accuracy requirements of the multi sea-state prediction model are satisfied, and its stability is better than the single sea-state model, with a maximum prediction error of less than 10-4 magnitude in the range of sea state from 2 to 5.Conclusions The simulation results verify the good adaptability of the NARX neural network to the complex wave environment. Its prediction speed and accuracy are higher than the common back-propagation neural network and the traditional filtering method. It still maintains high prediction accuracy under high sea state.

Active heave compensation systems are typically single-degree-of-freedom systems that operate vertically to attenuate the vertical effects of wave motion. To apply compensation to towed bodies, which experience significant multiple-degree-of-freedom disturbances, a generalized motion compensation system using a “reference-point algorithm” is required that determines the length of a tow cable that should be reeled in or reeled out by an on-board winch system in response to external wave motion. This paper proposes, implements, and assesses four different reference-point algorithm approaches in both simulation and experimental environments. Of the proposed reference-point algorithms, the method that directly accounts for the tow cable angle proved to be most effective in both simulation and experimentation. It was found that there was little improvement between real-time measurements of the tow cable angle and a constant nominal tow cable angle—indicating a potential cost saving opportunity through the avoidance of real-time sheave-angle measurement. [ABSTRACT FROM AUTHOR]

A heave compensation system combined with a vibration absorber and accumulator is proposed, with regard to the requirement for poly-metallic nodule mining systems at an ocean depth of 6,000 m in China. The principle of the compensation system was analyzed, its main parameters were designed, and its mathematical model was established. The system performance was simulated and analyzed in MATLAB, and the frequency domain characteristics and compensation rate of the system with different volume accumulators, and with or without vibration absorbers, were compared. A simulation test of the compensation system was conducted. The proposed compensation system had a better compensation effect for a random wave interference under level-6 sea conditions, and the compensation rate being more than 72% with optimal system parameters. [ABSTRACT FROM AUTHOR]

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.

The heave compensation system is essential for the installation of subsea equipment. A new semi-active heave compensation system is designed and the control system is analysed, with the aim of solving heave movement problems during deep-water installation. The heave compensation is implemented with a pulley block system and actuated by a compound hydraulic cylinder. The control system can switch between passive compensation mode and semi-active compensation mode. The state-space equations of both modes are set up based on the mechanical and hydraulic system models of the compensation system. With the state-space equations, three control strategies are compared based on the classic Proportion Integration Differentiation (PID) control, and the limitations of PID control are also analysed. In order to avoid defects of the PID controller, the active disturbance rejection control (ADRC) technology is introduced, and a newly developed second-order ADRC controller is applied in the tension control strategy for heave compensation, which results in improved compensation effect. [ABSTRACT FROM AUTHOR]

Copyright of Journal of China University of Petroleum is the property of China University of Petroleum and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In order to reduce the influence of waves heave to the vessel motion for the security, a compensation system based on the secondary control unit was proposed. The compensation principle and transport operation mode were described. And a mathematics model of the compensation system was established. The adaptive control based on load feedback was employed to control the speed of the secondary control unit. The simulation results show that the system can successfully implement the heave compensation method for supplying operation with both efficiency and security. [ABSTRACT FROM AUTHOR]

This paper provides a comprehensive review of vertical heave motion compensation systems used on ocean vessels from the early 1970s up to, and including, modern systems. Specifically, this review provides details on passive heave compensation, active heave compensation, hybrid active–passive heave compensation systems, and wave synchronization systems along with detailed explanations of the most common motion actuation methods, control schemes, and heave motion decoupling potential found with each. Based on the results of this review, it is recommended that more experimental work be carried out on real-world systems to experimentally validate the active heave compensation controllers being designed and simulated in literature. It is also suggested that future work involving model-predictive control may be used to further improve upon the performance of the current active heave compensation systems. [ABSTRACT FROM AUTHOR]

This paper presents a constrained finite horizon model predictive control (MPC) scheme for regulation of the annular pressure in a well during managed pressure drilling from a floating vessel subject to heave motion. In addition to the robustness of a controller, how to deal with heave disturbances despite uncertainties in the fric-tion factor and bulk modulus is investigated. The stochastic model describing sea waves in the North Sea is used to simulate the heave disturbances. The results show that the closed-loop simulation without disturbance has a fast regulation response, without any overshoot, and is better than a proportional-integral-derivative control-ler. The constrained MPC for managed pressure drilling shows further improved disturbance rejection capabilities with measured or predicted heave disturbance. Monte Carlo simulations show that the constrained MPC has a good performance to regulate set point and attenuate the effect of heave disturbance in case of signif-icant uncertainties in the well parameter values. [ABSTRACT FROM AUTHOR]

A new underwater two-part towed system is proposed and studied for the test of near-surface towed vehicles. The near-surface towed vehicle is towed by submarines or the large displacement unmanned undersea vehicle, and plays the role of the communication relay with the onshore station. To deal with the influence of the harsh wave environment and create a stable test platform, a fuzzy-PID control method and an active heave compensation device are applied in the towed system. The mathematical model of the two-part towed system is built. In the model, the nonlinear aspects are considered, such as the interaction of the multi-body, the flexibility of the cable, the wave force, the heave compensation. Based on the model, the simulation results indicate that the Fuzzy-PID control method can significantly reduce the depth fluctuation of the towed vehicle under different sea state. Besides, it is also demonstrated that the active heave compensation device can obviously improve the stability performance of the two-part towed system by greatly reducing the depth fluctuation of the ballast, and create a very stable test environment similar to the submarine or large underwater vehicle. • A novel two-part underwater towed system for near-surface towed vehicle test is proposed. • The integrated mathematical model is built and the hydrodynamic coefficient is obtained by experiment. • The active heave compensation device can greatly improve the stability of the towed system. • The control method can greatly reduce the depth fluctuation of the towed vehicle under different sea states. [ABSTRACT FROM AUTHOR]

To keep the remotely operated vehicles ( ROVs) motion unaffected by the ship's motion in the vertical direction, a nonlinear active heave compensation controller is designed in this paper. The compensation system consists mainly of an electro-hydraulic system driven by a double rod cylinder. Based on the external model principle, an adaptive observer is proposed to asymptotically estimate the ship's heave motion represented by a set of sinusoidal signals. The hybrid control development and stability analysis are based on the Lyapunov's direct method and backstepping technique. The proposed controller forces the tracking error to globally asymptotically converge to a small zone around zero that can be reduced arbitrarily. Simulation results illustrate and validate the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Active heave compensation systems are usually employed in offshore and deep-sea operations to reduce the adverse impact of unexpected vessel's vertical motion on the response of underwater instruments. This paper presents a control strategy for an active heave compensation system consisting of an electro-hydraulic system driven by a double rod actuator, which is subjected to parametric uncertainties and unmeasured environmental disturbances. Adaptive observer and discontinuous projection type updating law with bounded adaption rate are presented firstly to estimate the uncertain system parameters. Then a similar estimation algorithm is designed by using a multiple delayed version of the system to enhance the performance of parameter observation. A reduced order observer is also introduced to estimate unknown wave disturbances. Using the obtained uncertainty information, the resulting control development and stability analysis are implemented based on the Lyapunov's direct method and back-stepping technique. The proposed controller guarantees the heave compensation error convergent to a bounded neighborhood around the origin. Simulations illustrate the effectiveness of the proposed control system. [ABSTRACT FROM AUTHOR]

In order to validate the simulation model and develop heave compensation control strategy, heave compensation model tests were performed. The model test installation includes the mining ship motion simulator, the heave compensation system, the lifting pipe simulator, the buffer simulator and the water pool. The tests of mining ship motion simulator show that it is able to perform under the predetermined attitude path smoothly and can meet the requirements of the mining ship motions. The heave compensation effect is more than 60% under random wave and the goal is set to be 50%. The model test results indicate that this heave compensation system is effective and feasible. [ABSTRACT FROM AUTHOR]

During offshore installations in harsh sea conditions, the involved crane system must satisfy rigorous requirements in terms of safety and efficiency. The forces resulting from the vertical motion of the vessel have an extensive effect on the overall crane structure and its lifetime. Moreover, vessel motion handicaps the operator during fine positioning of the payload. Hence, an active compensation system for the vertical vessel motion is proposed. An important point to consider for such systems is the time delay between the sensors and actuators, which diminishes performance. To compensate the dead times in the system, a prediction algorithm for the vertical motion of the vessel is proposed in the first part. In the second part, an inversion-based control strategy for the hydraulic-driven winch is formulated that considers the dynamic behavior of the drive system. A feedforward controller compensates the vertical-motion disturbance using the predicted motion. The proposed controller together with the prediction algorithm decouple the motion of the rope-suspended payload from the vessel’s motion. The active compensation approach is evaluated with simulation and measurement results. [ABSTRACT FROM PUBLISHER]

passive heave compensator is designed to enhance the operation safety of a 4.5 km remotely operated vehicle (ROV). This paper proposes a novel idea of designing a compensator with relatively low natural period to optimize gas volume and while adding a special device to remove the problem of ineffectiveness and resonance in long seas. Numerical simulations are done based on serious dynamic model of the whole system, including the compensator, the umbilical tether and the vehicle, solved by the fourth-order Runge-Kutta scheme. The compensator provides great attenuation of motion and tension in most sea states. As the working depth increases, the system natural period decreases, resulting in the occurrence of risk of resonance. By regulating the system damping, the compensator can be effective in these situations. [ABSTRACT FROM AUTHOR]

Abstract: Heave motion of a vessel or a rig has an adverse impact on the response of a drill-string or a riser. To compensate for heave motion, passive and active devices are usually used. Active heave compensators, in which a control system is an essential part, allow conducting operations under more extreme weather conditions than passive ones. This paper presents a constructive method to design a nonlinear controller for an active heave compensation system using an electro-hydraulic system driven by a double rod actuator. The control system reduces the effect of the heave motion of the vessel on the response of the riser by regulating the distance from the upper end of the riser to the seabed. The control development is based on Lyapunov''s direct method and disturbance observers. The paper also includes a method to select the control and disturbance observer gains such that actuator saturations are avoided. Stability of the closed loop system is carefully examined. Simulation results illustrate the effectiveness of the proposed control system. [Copyright &y& Elsevier]

A parallel force/position controller is proposed for the control of loads through the wave zone in marine operations. The controller structure has similarities to the parallel force/position control scheme used in robotics. The parallel force/position controller is tested for crane control in simulations and model experiments and the results are presented in this paper. To evaluate the performance of the proposed controller, we study three different control strategies for control of loads through the wave zone: active heave compensation, wave synchronization, and parallel force/position control. The parallel force/position controller gave improved results, in particular, a significant improvement of the minimum value of the wire tension, which is important to avoid snatch loads that may break the wire. The three strategies are tested and compared in simulations and experiments. [ABSTRACT FROM AUTHOR]

A new strategy for active control in heavy-lift offshore crane operations is suggested by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimizing variations in the relative vertical velocity between payload and water using a wave-amplitude measurement. Wave synchronization is combined with conventional active heave compensation to obtain accurate control. Experimental results using a scale model of a semi-submerged vessel with a moonpool shows that wave synchronization leads to significant improvements in performance. Depending on the sea state and payload, the results indicate that the reduction in the standard deviation of the wire tension may be up to 50%. [ABSTRACT FROM AUTHOR]

This article describes the design, operation, and field testing of the depthimeter. The depthimeter merges heave and acoustically derived vessel depth to form estimates of instantaneous vessel depth and instantaneous sea surface height, both relative to mean sea level. Results from sea trials held in December 1997 demonstrate successful operation of the system. [ABSTRACT FROM AUTHOR]