Hydraulic system calms rough seas
Erecting offshore wind turbines is one thing, but maintaining them in often rough seas is quite another. In offshore wind farm support, transferring engineers and technicians to and from the turbines for maintenance is vital.
As wind turbines are installed further offshore, sea conditions become more extreme, reducing the availability to affect such transfers whilst maintaining safety. The subsequent increase in wind turbine downtime degrades the overall economics of building offshore wind farms.
The Turbine Access System (TAS), jointly developed by Houlder and BMT, is designed to address this problem. This system is designed to provide a safer means of offshore transfer between workboats and offshore wind turbine structures. Developed with BMT Nigel Gee, it operates via a motion-compensated gangway that, once raised into place, maintains a fixed transfer position in comparison to the wind turbine structures, providing personnel with a secure point from which to transfer.
The TAS does not require any dynamic positioning and at no point attempts to actually fix or connect to the turbine structure, thus mitigating any risk of damage to its outer surface. It also serves as an important safety measure as, if a problem occurs, the system and boat are free to move away immediately.
The mechanical structure consists of a gangway mounted on a hydraulically-articulated base. The gangway is attached to the base such that it can move up and down (pitch cylinder) whilst the base moves forward/back (surge cylinder) and port/starboard (roll cylinder). These three hydraulic cylinders compensate for boat motions in roll, pitch and heave. There is no need to compensate for any other boat motions, since the boat remains in contact with the turbine tower by thrusting at the tower base.
With most alternative transfer systems limited to operating in wave heights of up to 1.5m, the TAS is designed to increase operability to 2.0m resulting in significantly higher cost efficiency. The predicted total savings of utilising a system to improve operability are calculated at just below 200,000 per year. In terms of installation, the TAS does not require a large vessel to support it, in fact, it can be used on smaller workboats ranging from just 18m in length.
The control system component of the TAS relies on a control algorithm runs on a National Instruments CompactRIO embedded controller, which processes the boat motions measured by a Motion Reference Unit (MRU) sensor and performs active heave, pitch and roll compensation through the adjustment of hydraulic actuators.
The Motion Reference Unit (MRU) measures the motion of the vessel and transmits data for positions and angles via a serial data link to the main processing unit. The MRU itself is a complex device and required careful selection and configuration to ensure suitable performance.
Houlder and BMT Nigel Gee have recently announced that the TAS has been awarded additional funding by the Carbon Trust under the Offshore Wind Accelerator – Access programme. The additional backing will be used to develop an integrated bow roller mechanism to further enhance its operability as a safe, reliable and predictable transfer platform.
The hydraulically-damped rollers will allow the vessel to push up to the boat landing on the turbine foundation structure in the conventional manner and the integral damping system will reduce and control the vessel motion thereby eliminating the sudden movements of the vessel which can occur with conventional fender systems. The development of the integrated bow roller unit will also enable shipbuilders to install TAS rapidly for both new build and retrofit Windfarm Support Vessels.
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