Arms across the ocean: Giant grippers on the MPI Discovery jack-up vessel will hold the piles steady
No one should doubt the skill of the installation teams who build the offshore wind turbines that seem to spin with such gentle serenity when viewed from the safety of dry land.
On board gigantic jack-up vessels, those responsible for the erection of these towering structures oversee a process whereby gigantic hydraulic hammers drive 700-tonne monopiles into the seabed up to 40m below. Despite their colossal size, the steel monopiles have to be installed with ultra-accuracy, to within a small fraction of a degree to the vertical.
The jack-up vessels are fitted with gripper arms that hold the piles steady while they are driven into place. But these arms have often proved inflexible to the differing diameters of wind-turbine structures, sometimes resulting in damage such as crushing of the steel piles.
Step forward a small British engineering company that is helping to make the insertion of monopiles a simpler, faster and more reliable process. Glasgow-based ISC has developed an intuitive control system to position and stow hydraulic gripper arms. The system gives installation teams far greater control of the powerful arms, so monopiles can be handled better and driven into the ground more precisely, with operators using joysticks to inch them into position.
The gripper arm control system is helping offshore installation teams to reduce construction delays caused by inaccurate positioning of the monopiles. And, in a sector where the costs of chartering a jack-up barge can run into tens of thousands of pounds a day, that is an important consideration.
Steel for sea: Monopiles await transport offshore on the MPI Discovery
ISC’s managing director Andy Clegg says: “We were approached by Houlder, the gripper arm designer, as we had previously worked together successfully on the development of a motion-compensated gangway to improve access to offshore wind turbines. Houlder wanted a company that could deliver a complete solution, from early simulations right through to the final system, and all done within four months.”
There were very specific challenges to the development of the system. Three main hydraulic cylinders actuate each gripper arm. One cylinder is raised and lowered from the vertical stowage position, and the other two provide motion in the X-Y plane.
There was a requirement from the customer for individual or synchronous steering of the arms so that the vertical inclination of the pile could be adjusted with the required levels of accuracy.
“The piles have to be inserted to within a few fractions of a degree of vertical,” says Clegg.
“They cannot be inserted off-centre, as it would risk damage to the nacelle at the top.”
It was also critical that the control system prevented the gripper jaws from crushing the pile, so a preset minimum gap constraint had to be observed at all times. An additional challenge lay in the sheer physical size of the system. Typically, 70-tonne arms are needed to manoeuvre larger piles, which meant that ISC could only test the final control system once fully installed on the vessel.
The solution came with the development of a gripper arm control system based on National Instruments’ LabView and CompactRio hardware. At the outset, ISC designed an algorithm combining the forward and inverse kinematics of both gripper jaws to compute the hydraulic cylinder lengths required to move the arm in a desired direction. A simple, proportional-integral control minimised cylinder position errors.
Constraint-handling logic maintained appropriate behaviour at the edge of the operating envelope, allowing arm retraction for stowage, and limited permissible jaw diameter to prevent pile damage. Operational logic, monitoring and fault actions were also executed on the CompactRio system and accessed through a touch-panel computer with a joystick and a set of buttons/LEDs in the operator’s chest pack for ease of use.
The embedded controller was equipped with numerous I/O modules to interface with the sensors, actuators and the operator’s chest pack. The structure of the gripper arm control software included the main realtime application, a field-programmable gate array (FPGA) program that included a ‘watchdog’ emergency-stop function, and the human-machine interface software running on the computer.
“Everything runs on the CompactRio controller, with realtime applications,” says Clegg. “Within the controller chassis there is a separate FPGA with a watchdog that monitors the software. If it detects that it has become inactive, it will trigger an emergency stop. So there is a safety system built in.”
Arms at the ready: Monopile is lowered towards the robotic grippers
Houlder liked the proposed solution, particularly as it had a small footprint, both physically and from an electrical power point of view. Clegg says: “On these jack-up boats, if you have a system dissipating a lot of heat then you also need some form of ventilation system. The CompactRio doesn’t produce much heat and can be handled in an entirely watertight case.”
The control software had to be capable of deploying and stowing the gripper arms using two lifting hydraulic actuators. Two pairs of hydraulic cylinders perform the X-Y position control, which the operator guides using the chest-pack joystick. The software included inverse kinematics, calculating the cylinder lengths required to move the gripper arms to a given position in the horizontal plane.
In terms of realtime control, the software is capable of interpreting operator joystick movements as the commanded direction and arm movement speed. This ‘speed demand’ is internally converted to the X-Y position set points using inverse kinematics and the actual X-Y positions of the arms. The set points are rate-limited, then passed through inverse kinematics to convert into the corresponding cylinder lengths, taking into account their minimum and maximum limits.
The individual cylinder length dynamic controllers compute the control signals to drive the corresponding hydraulic servo valves. Individual cylinder controllers also feature feed-forward paths to improve tracking performance, which allows different gains for cylinder extension and retraction to be applied.
Operational logic was an important consideration, says Pawel Majecki, ISC’s lead engineer on the gripper arm control software project: “This was implemented in the LabView state machine code architecture, which made possible different modes and submodes of operation – initialisation, inactive, deploy/stow, single arm, synchronous. Extensive monitoring of analogue I/O, digital I/O, and internal states and calculations ensures that faults are trapped and receive a quick and appropriate response, whether just a warning to the operator or a controller-initiated emergency stop.
“An external safety relay circuit responds to both external emergency-stop buttons and the gripper arm control system initiated faults. The operational interface was built as a standalone LabView executable running on the touch-panel computer, which communicates with the CompactRio control software through shared variables via Ethernet.”
Owing to the size of the mechanical components, it was not possible to fully assemble the gripper arms in ISC’s factory. So a decision was made to display expected overall behaviour virtually via a software emulator, which mimicked the behaviour of the arm. This allowed ISC to test the software fully in the office to make sure that all the functionality was achieved.
Majecki says: “We developed the emulator to extensively test the operating logic and fault response. The emulator ran on a computer connected to the CompactRio system via Ethernet. Network-published shared variables were defined to exchange data between the emulator and the main application. In addition, the emulator replaced any component that was not physically present during factory test.”
This proved to be an extremely successful approach, he says: “The gripper arms were so large that the only time they would get assembled for testing was on the boat. We had an ever-decreasing window of time to commission the work – fewer than four days. We identified that this would be a problem early on.”
To mitigate this, ISC used its software emulator of the gripper arm system to check that the operating logic worked properly. Majecki says: “We could inject faults into the gripper arm emulator to make sure that the software worked as it should do. The emulator software was probably as big as the realtime application itself. Now, when required to make operational software changes, we use the emulator to prove that the changes work. That is done in the office before deployment.”
Installation site: The MPI Discovery arrives at Humber Gateway windfarm
The gripper arm and its software were installed and commissioned on the MPI Discovery jack-up vessel earlier this year. The vessel then set sail for the Humber Gateway windfarm, 8km off the coast near Grimsby, to install four monopiles and transition pieces. The windfarm will consist of 73 turbines when completed in 2015.
ISC engineers were present for the successful installation of the first two monopiles. The system worked well and matched expectations from the factory testing, with just a few items requiring fine-tuning on the day, says Clegg.
“The control system development was very successful. We moved from a blank sheet of paper to implementation, to factory testing and finally successful sea trials in just a few months,” he says.