Compared with using ships with anchor spreads or jack-up vessels, installing monopiles from ships with dynamic positioning (DP) could significantly reduce the time it takes to complete that part of the installation process and reduce costs.
DP is a technique used to automatically maintain a vessel in position and/or heading or a predefined track by using its thrusters and/or rudders. Using DP, a vessel’s position can be quickly set up and maintained.
In contrast, installing monopiles using a moored vessel takes time because anchors have to be run before a monopile is upended and raised from the deck of a vessel into position. Once a pile has been driven into the seabed and installation is completed, the vessel has to recover the anchors.
On a jack-up, a lot of time is spent lowering the vessel’s legs to the seabed before piling commences and raising them again afterwards. On a windfarm with 60-70 foundations to be installed, that would result in a lot of unproductive time for the vessel. In fact, one of the leading operators in this field, Seaway 7, estimates that when installing a monopile, more than 40% of a moored vessel’s time is spent running and retrieving anchors.
Seaway 7, one of a growing number of vessel owners transitioning to working on DP, is a step ahead of its competitors in the transition, having conducted a demonstration in Dutch waters late last year.
But as Seaway 7 manager, specialist engineering and R&D Jeroen Regelink tells OWJ, installation on DP is a technically challenging process that requires using significant amounts of data and developing and modifying control systems if it is to be successful.
As OWJ revealed late in 2019, having completed the demonstration project, Seaway 7 was selected by Vattenfall to install the monopile foundations for the Hollandse Kust Zuid 1 & 2 and 3 & 4 offshore windfarms in the Netherlands.
The demonstration was the culmination of a multi-year innovation programme at Seaway 7, including extensive data collection during numerous monopile installation campaigns with floating crane vessels, developing and validating a simulation model and modifying vessel software, all with the aim of confirming the feasibility of installing large monopiles using dynamic positioning (DP).
“Installing a monopile on DP might sound easy,” says Mr Regelink. “We have been installing pipelines from our vessels for years using DP but installing a monopile and driving it into the seabed on DP is a very different process. We analysed a lot of data and conducted a lot of simulations before we got to the point where we were ready to prove that it works and demonstrate to clients that it is possible.”
The demonstration off the Dutch coast saw Seaway 7’s vessel Seaway Strashnov install and remove a 6.5-m diameter, 600-tonne monopile while in DP mode, remaining within inclination tolerances using a motion-compensated gripper.
One of the keys to the success of the demo was the development by Seaway 7 and its industry partners of a ‘coupled system’ that enables the DP control system to ‘talk’ to the control system in the motion-compensated gripper that held the monopile in place while it was being driven into the seabed. There are other potential challenges involved in installing on DP, such as thruster failure – which is rare – and human error, but potential instability in control systems is probably the biggest challenge.
“You need the DP system and the gripper control system to interface with one another so they do not interfere with one another,” Mr Regelink explains. “Our simulations showed it is possible to install monopiles on DP without using a coupled system, but there were limits to workability, and it was not possible to install every pile on DP unless the control systems work together.
“We needed to ensure the DP control and the control system in the motion-compensation system would work together to ensure workability was not limited in any way.
“When you are driving a pile into the seabed, its inclination is very important. It has to remain within strict tolerances. Waves and currents can affect that, as can the seabed conditions. It is the job of the pile gripper to apply a force and ‘push back’ to maintain correct inclination of the pile when it is subject to these forces, but you do not want the DP system to react to what it perceives as an external load being applied to the ship and apply a force to counteract it.
“If it did, that could affect piling and inclination. In the worst case, if the DP and the control system in the gripper cannot talk to one another, it can compound unwanted movement and lead to unwanted inclination of the pile.
“If you are installing a foundation from a moored vessel, external loads are directed into the seabed,” Mr Regelink explains. A similar situation applies when installing from a jack-up, which has its legs firmly attached to the seabed.
“But on a floating installation vessel, loads are counteracted using the ship’s thrusters. Without ‘coupling,’ loads applied by the gripper that were detected by the DP system could cause the vessel to make an unwanted adjustment to its position. Hence the work we did with the supplier of the DP system on the vessel and with Bosch Rexroth, and our use of the simulation system we developed.”
Seaway 7’s scope of work under the contract for Hollandse Kust Zuid 1 & 2 and 3 & 4 includes transporting and installing 76 monopile foundations. The company is expected to assign Seaway Strashnov to the project and has begun work to identify what modifications might be required to the technology used on it to customise for conditions at the sites.