Earth-Fixed Heave Compensation

Abstract

With a global growing demand of energy, more offshore wind farms are installed, further away and in deeper waters. Nowadays, offshore wind turbines are mostly installed by jack-up installation vessels. Most existing jack-up vessels have legs that are becoming too short for these water depths while offshore wind turbines are increasing in size year after year. Since 2014, the turbine capacity of newly installed wind turbines has increased by 16% every year. Therefore, larger jack-up crane vessels are needed and installation by floating crane vessels is being considered. Heerema Marine Contractors aims to be one of the leaders in the offshore wind installation with her large crane vessels such as HLV Aegir, SSCV Thialf and SSCV Sleipnir. The challenge of floating installation of offshore wind turbines is that motions of the vessel are transferred to the rotor-nacelle assembly (RNA) while low tolerances apply for the installation of an RNA. Vertical motions can be reduced by means of a heave compensation system. Such systems are available but come with certain drawbacks: they are difficult to retrofit to a vessel, use a large amount of energy and are rather expensive. A possible solution to these problems could be a novel concept called earth-fixed heave compensation. In this concept, the crane wire is connected to the seabed via a transmission on board of the vessel, transforming an upward motion of the vessel into a downward motion of the RNA and vice versa. At present, it is unclear if such a system is technically feasible. In this research, an analytical model of an earth-fixed heave compensation system is developed. The objective of the model is to gain insight in the influence of design parameters such as the transmission and stiffness of the system. The model is set up in three stages: Stage 1 comprising one degree of freedom for the transmission; Stage 2 comprising two additional degrees of freedom for the sheaves that connect the earth-fixed wire from the seabed to the transmission; Stage 3 comprising all other sheaves, crane reeving and payload are added, resulting in a sixteen degrees of freedom model. A first finding is that wire damping has a negligible influence on the results because natural frequencies of the system are found to lie outside the wave frequency range. However, the first natural frequency is close to the wave frequency range, resulting in a larger response amplitude for both transmission and vertical payload motion in the higher frequencies. Furthermore, it is shown that the inertia of the sheaves of the earth-fixed wire can be neglected for a range of transmission inertias and earth-fixed wire stiffnesses. On top of this, the model confirms that the inertia of the other sheaves can be modelled by means of an equivalent inertia block, to simplify the model for time-domain simulations. The losses in wire tension due to wire-sheave interaction were approximately 5% and it is shown that they can be modelled accurately by means of a sigmoid function. Frequency-domain simulations showed that the heading of the vessel, or wave direction, has a significant influence on the vertical RNA motions. On top of that, increasing peak periods generally result in larger vertical RNA motions. A time-domain simulation for typical North Sea environmental conditions, head waves and a fixed crane slew angle without the heave compensator is made to compare results with. These simulations have shown that it is in principle possible to reduce vertical RNA motions. For a given sea state, 80% motion reduction is achieved by tuning the transmission ratio of the system. It is shown that the location of the earth-fixed wire has an impact on the vessel and payload motions. When located on the starboard side of the vessel, rather than reducing the vertical payload motions that are induced by vessel roll, those motions of the vessel itself are actually increased. Further simulations show that the stiffness of the earth-fixed wire and the losses occurring due to wire-sheave interaction determine the performance for a large part. Although the overall performance of the system can be considered promising, additional research is needed to confirm whether earth-fixed heave compensation can be competitive relative to existing passive and active heave compensation systems with a performance of 80% to 95%. Also, it is recommended that the behavior of the system for different sea states is investigated. Sensitivity analyses already show that shorter wave periods result in a significant drop in performance. The vessel heading seems to have a limited influence on the performance of the heave compensator.