Modelling and simulation of traditional hydraulic heave compensation systems

Sanders, R.V. (2016) Modelling and simulation of traditional hydraulic heave compensation systems.

Abstract:The complexity of hydraulic heave compensation systems lies in the many different components, each with their own dynamics. In this report two methods of heave compensation systems are analysed, modelled and simulated. The methods distinguish in actuation, where one uses a hydraulic cylinder to adjust the cable length, the other does so by directly rotating a drum using a hydraulic motor. The goal of this research is to compare and quantify the performance and efficiency of the two systems. Both systems are modelled in exactly the same fashion in order to make a fair comparison. Both systems are subjected to a heave motion that originates from a wave model using superposition of different sine frequencies and amplitudes to generate a wave pattern. The heave motion of the vessel is assumed to be measured at a motion reference unit at the base of the crane. The respective heave motion of the crane tip is derived using transformation matrices, the crane is assumed rigid in this process. Simulations show that the translational system is able to achieve the highest accuracy and efficiency. Extended with a passive cylinder connected to a nitrogen accumulator, it has the ability to counter static forces like the inertia of the load. This technique is often used in practice and is therefore also taken into account. Translational systems with accumulator can be applied on a much wider range of operating conditions, with high efficiency and without becoming too large. The rotational system uses only the components that are already present in a standard configuration for raising and lowering a load. Also it does not require extra guiding sheaves for the cable as in the translational system, which greatly increases the cable lifetime. The translational system is unable to compensate for heave motions beyond the cylinder stroke, which often results in over-dimensioned systems that require relatively large operating space, which is scarce aboard a vessel. These arguments cannot be measured in a simulation and are therefore left open for discussion. Simulations have shown that the rotational system has relatively more trouble at lower operating depth, caused by the inertia of the remaining cable on the drum. Translational systems have no issues here as they are not influenced by the drum. Velocity feedforward is applied which significantly improves the accuracy of both actuation systems. Stability of the system has been improved using tension feedback and especially the position error of the load has been greatly reduced. Without the tension feedback the load attached to a cable shows increasingly large oscillatory motion of the load for greater operating depths as the cable stiffness decreases. In frequency regions close to the natural frequency of the cable this oscillatory behaviour caused resonance in the cable and made the system unstable. This problem no longer occurs when using tension feedback.
Item Type:Internship Report (Master)
Hycom BV, the Netherlands
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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