Author(s):
Andrew Clegg -
Industrial Systems and Control Ltd
Background
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. We were selected to develop and supply the control system component of the TAS. The control algorithm runs on a 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 adjustment of hydraulic actuators.
Initially, dynamic simulations were used to investigate the control system strategy and sensor/hydraulic specifications to meet the target performance defined by Houlder. We then fully implemented the LabVIEW control software. The prebuilt functions of the LabVIEW Real-Time, LabVIEW FPGA, LabVIEW MathScript RT and LabVIEW Touch Panel modules made it easy to work with the different hardware targets and saved countless man-hours compared to building the functionality from scratch.
Before connecting to the real system, the algorithm functionality was tested using a software emulation of the system. We then began factory commissioning and testing the real system and exploring the performance of the control loops under many situations (different loads, different size and speed of motions and different orientations).
Turbine Access System Description
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/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. We do not need to compensate for any other boat motions since the boat remains in contact with the turbine tower by thrusting at the tower base.
Control System Components
The TAS is equipped with a CompactRIO with input/output (I/O) modules to interface to all of the TAS sensors and actuators as well as bridge and deck operator control stations. The CompactRIO controllers are approved by Lloyd’s Register for conformity to safety, electromagnetic compatibility and environmental rules for use in marine environments.
The bridge and deck operator control stations use a FOX-121 touch panel computer (TPC), waterproof to IP67 (NEMA6), with a rugged aluminium chassis and a high-brightness screen. These TPCs run a Windows XPe operating system and provide the TAS operator interface, which we also built with LabVIEW.
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.
The control system functionality includes the following:
- I/O - Serial read from MRU and analogue and digital I/O from other components (see Figure 2)
- Inverse kinematics to calculate the cylinder lengths and maintain the gangway tip fixed in space. This is implemented using MathScript Nodes in LabVIEW to type in complex textual algorithms (see Figure 4)
- Real-time control featuring cascade control with feedforward and nonlinear compensation. Figure 3 shows the interconnections between control, operating logic, monitoring and operator interface subsystems.
- Operational logic implemented as state machine code architectures for different modes and submodes of operation (such as inactive, idle, approaching, compensating, stowage and maintenance)
- Extensive monitoring of analogue I/O, digital I/O and internal calculations to quickly and appropriately respond to faults (Some fault actions change with current operating mode. A separate, hard-wired safety relay circuit stops the TAS for certain primary faults, and the control system monitors and reacts to this as well.)
- A watchdog timer implemented on the CompactRIO FPGA automatically initiates an emergency stop if the embedded control software is unresponsive, with extremely high reliability and response speed inherent in the FPGA hardware architecture
- An operator interface built as a LabVIEW stand-alone executable running on the two TPCs, and interfaces to the CompactRIO control software through shared variables over Ethernet
- Limited data logging is performed by the cRIO. The development PC used during factory testing and sea-trials captures a more comprehensive set of performance related data
Results
The two plots on the left of Figure 4 show cylinder length error for both the early simulation-based work and from the CompactRIO controller when testing with the real system. The graph scaling has been removed for confidentiality, but it is consistent for both plots and shows that the simulation was a reasonable representation of the final system. The plot on the right of Figure 5 shows the calculated residual motions of the gangway tip in the x-z plane with 20 minutes of irregular boat motions applied to the real system. Again the scaling has been removed for confidentiality, but the performance of the motion compensation is within the accuracy required.
Overall, the TAS control system development has been successful, moving from a blank sheet of paper to implementation and factory testing in a little over a year. The resulting performance of the final system makes the TAS a viable solution for extending the range of sea conditions under which engineers can be transferred from service boats onto offshore wind turbines.
Conclusion
The combination of LabVIEW and CompactRIO was ideal for this application because it provided the computational power to handle low-level control and inverse kinematic calculations whilst running the operational logic and monitoring function. The small, low-power nature and flexible interface to the I/O are perfectly suited to the marine environment.
According to Frederic Perdrix[JG1] of Houlder, Ltd., “In realising the TAS, Houlder and BMT required significant control system expertise. The system ISC delivered using NI LabVIEW and CompactRIO hardware very accurately compensates for work boat heave, pitch, and roll motions, and exceeded our expectations. The TAS is now well-placed to significantly improve the operation of offshore wind farms.”
Sea trials will show the TAS working in its intended environment and confirm that the range of motion compensation achievable is in line with that expected from the final factory testing. Whilst the main focus is now on selling and building more TASs , there are already ideas to extend the capability of the original design.
Videos showing TAS:
Author Information:
Andrew Clegg
Industrial Systems and Control Ltd.
Email: andy@isc-ltd.com
Author Information:
Andrew Clegg
Industrial Systems and Control Ltd
36 Renfield Street
Glasgow G2 1LU
United Kingdom
Tel: 0141 847 0815
andy@isc-ltd.com
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