Installing monopiles from a vessel operating on DP will require the use of a new generation of motion-compensated handling equipment
Pile grippers have long been an important piece of equipment used to install monopiles from jack-up vessels, but with a transition underway towards installation from floating vessels operating in dynamic positioning (DP) mode, and the first such operations due to be undertaken shortly, the equipment needs to evolve.
DP is a method used to automatically maintain a vessel in its position and/or heading or a predefined track by using its thrusters and/or rudders. Installation on DP has a number of potential advantages, not least that it will be quicker, because a DP vessel can move quickly between locations and the time needed to jack-up and then retract the legs when installation is complete is eliminated.
Seaway 7 recently completed trials with its vessel Seaway Strashnov that demonstrated the ability to install monopiles on DP and plans to install the monopiles for the Hollandse Kust 1 & 2 and 3 & 4 offshore windfarms in the Netherlands while on DP.
Offshore Heavy Transport’s new Alfa Lift crane vessel, which is currently under construction, will use a fully automated system to transport monopiles from their storage position on deck into an upending frame, after which a pile gripper frame will take control of the structure. During installation, the Alfa Lift vessel will rely on its DP system, which will work closely with a pile gripper and its control system to keep installation tolerances within required limits.
DEME is due to take delivery of its DP3-class installation vessel Orion, shortly, and another leading contractor, Jan De Nul, recently ordered two installation vessels: a jack-up, Voltaire, and a floating crane vessel, Les Alizés, which will operate on DP.
How ‘smart’ grippers are varies from vessel to vessel and application to application, but they are all able to handle and hold in position a monopile.
As Jan De Nul senior superintendent Ben Casier explains to OWJ, grippers can have three main functions: assisting during lowering of a monopile from the deck of a vessel through the splash zone to the seabed; positioning it correctly when it is being hammered into place, ensuring verticality; and, if required, rotating a monopile on its vertical axis, to ensure entry holes for cables are correctly positioned.
“The diameter of a monopile is obviously very important, and the larger the diameter the larger the forces it exerts and the greater the drag the system experiences,” he explains. “Wind is a factor, but not a critical one in the process. More important are currents and wave action that affect the motion of the vessel and induce loads on a monopile and on the gripper, which can lead to resonance in the foundation around its centre of gravity.”
Unlike a moored vessel or jack-up, a heavy lift vessel maintaining station on DP will experience variation in its position, as the vessel’s thrusters and DP control system try to maintain it in a fixed position. That is challenging for existing, static pile grippers. It could, potentially, also be challenging for a DP system, if it cannot take into account the movement and forces the process imparts to it.
On a jack-up, a gripper may be a fairly simple tool that does not have to deal with much movement, other than tides and currents, because the vessel is connected firmly to the seabed, but working on DP changes that and requires a much greater level of integration between the control system for the gripper and the ship’s DP system.
“We already have experience of unloading objects into the sea while in DP mode, such as during cable-lay and rock-dumping operations, but installing a monopile on DP is something the industry only has limited experience of,” says Mr Casier. “On a floating vessel, because of the size of the foundations that need to be installed, environmental loads on the pile will be much more important to the success of the operation and the DP system will need to take those loads into account.”
Responding to the move towards installing using DP, equipment manufacturers have developed more capable, motion-compensated equipment.
Royal IHC sales manager handling equipment Jurgen Zijlmans tells OWJ residual vessel motion due to DP inaccuracy was an important issue motion-compensated systems need to address. “Using a motion-compensated gripper, these movements can be counteracted and a force can be applied to the pile to counteract wave loads,” he explains, “and the inclination of the pile during pile driving can be controlled.”
Royal IHC has been working on the concept of a motion-compensated system for several years. The result is a dynamic outrigger frame and a control system that “works in harmony” with the DP to achieve active motion compensation.
“Technology-wise, this was a challenging project because it involved so many disciplines,” says Mr Zijlmans. These included simulation, vessel dynamics, pile-soil interaction, control systems and an understanding of the hydraulic and electrical behaviour of the system. The key to success is the ability to deal with ever-changing pile-in-soil conditions during the installation process and the pile-driving process itself.
The dynamic outrigger frame is a hefty piece of equipment – weighing close to 700 tonnes – but can be moved in the X and Y axes to ensure a monopile remains in position and unwelcome surprises are avoided during project execution.
Another well known designer and builder of this kind of equipment, UK-based Osbit, has also developed a motion compensated gripper, or ‘MoCo Gripper.’ Osbit director Robbie Blakeman tells OWJ, “The challenge when operating from a moving vessel is to ensure not only can the gripper apply the necessary forces to correct the verticality of the monopile throughout the installation process, but also the position of the gripper head in space remains constant.
“To achieve this the connection between the gripper ring and the vessel must adjust depending on the motions of the vessel due to environmental and piling loads acting on it. This means the MoCo Gripper must compensate for vessel motions such as surge and sway, and any change in the vessel heading during the installation cycle.”
In the MoCo Gripper, this is achieved by having various actively moving hydraulic cylinders and linkages to ensure the vessel can move relative to the stationary gripper head. This meant tailoring the performance of the system to the motions and DP response of a given vessel. This is done by combining the monitoring of loads exerted on the pile, gripper head position and inclination, with the position of the vessel and its dynamic motions.
MacGregor and Kongsberg Maritime also developed a pile gripper intended for use on a floating vessel. The company highlighted that a pile standing on the seabed will self-penetrate to a certain extent, but it is not stable and wave forces can be significant. They said the new pile gripper they developed also uses motion-compensation technology, integrated with a ship’s DP system.
It includes a motion-compensated, hydraulically-operated gripper; a gripper control system that works in tandem with the DP system and co-ordinates the interaction between the gripper and the DP system; a pile-gripper servo control system that translates vessel motion and monopile position data into movement in the actuating cylinders and monitors loads and the status of the pile for feedback into the guidance system; and an ‘abandon monopile’ capability.
Working together, Temporary Works Design (TWD), Barge Master and DEME also studied the development of a motion-compensated pile gripper. TWD has extensive experience designing pile handling tools and motion compensation systems and was responsible for the structural and mechanical design of the gripper and upending tool used with it. Barge Master was responsible for the systems’ drives and control/safety system. The system will be installed on Orion, which has also been contracted to install the foundations for the Moray East offshore windfarm and Ørsted’s Hornsea Two offshore windfarm, both of which are in the UK.