Traditional steel pile foundations for offshore turbines will become unsustainable as wind farms move further out to sea. Jan Groot, project manager for MonoBase Wind, discusses how a new type of gravity-based structure offers easy installation and the necessary support for tomorrow’s developments

The offshore wind industry is facing a dilemma: deeper waters and larger turbines offer a clear path to increased energy production, yet the piles typically used as foundations are not viable past a certain size.

"The steel piles used at the moment have a diameter of around 6m, which is already enormous, but there are plans to expand this to 9m," says Jan Groot, project manager at MonoBase Wind.

"As we move towards deeper water, this is going to become a major problem. You’ll end up using foundation parts that are so huge, and therefore so expensive, that you reach a technical limit."

Current pile foundations are only financially viable in water depths of 30m or less. Beyond this, gravity-based structures (GBSs) are the only possible solution.

GBSs, however, are notoriously tough to put in place. Stability during installation and towing is the main challenge, with current practices requiring a range of expensive equipment, including tugboats and jack-up barges with large cranes.

"There is also a long list of complicated steps you have to go through to complete the foundation: transition pieces, grouting, the central column and then the turbine. Doing all that while you’re offshore can be very difficult indeed," says Groot.

Designed with installation in mind

Many of these challenges are the result of poor design planning. All too often, engineers focus on how the structure will perform once it is in place, rather than considering the installation process itself.

MonoBase Wind – a partnership between Orca Offshore, PBConsultants and Venture Counsels – has developed a new form of GBS that overcomes these challenges. By splitting the system into a tower and a slideable doughnut-shaped base that can float during the transportation phase, it eliminates the need for jack-up barges or other expensive lifting equipment, requiring only three tugboats.

"The floating doughnut keeps everything stable during the transport phase. When you go down with the monopile, it stays up, keeping everything steady," Groot explains. "It’s only once the central column is anchored to the sea floor that the doughnut goes down, and it does this automatically – it’s totally hands-free."

The MonoBase design also allows installation of the turbine into the base onshore. Not only does this make the entire process far simpler, but it also minimises downtime due to poor weather conditions. The foundation is fabricated from concrete or steel, or a mixture of the two. Decommissioning costs are low, and nothing is left behind. "As far as we know, it’s really the only GBS that has a technically feasible installation method," says Groot.

Eyes forward – and down

Designed for water depths of 30m or more and 5MW+ wind turbines, the MonoBase is geared more towards a future – rather than the existing – offshore market. Groot, however, is confident that demand will grow.

"Even if you just search on the internet, there are dozens of projects in water depths of well over 30m," he says. "There is no question over whether or not this technology will be needed; the only issue is when. We estimate around 2017."

Despite Groot’s confidence, the partnership is not aiming to become a large manufacturer of the MonoBase product. Rather, it plans to supply design and installation knowledge, letting a bigger construction firm get on with the actual building process.

"All three of the MonoBase Wind partners worked in the oil and gas sector for Heerema – the biggest heavy-lifting company in the world," says Groot. "Having worked there for 25 years, we have a distinct advantage over the many onshore builders who are just trying to move onshore technology to an offshore environment. We have a much stronger knowledge base."

Already designed and patented, the MonoBase product is now in what Groot describes as the "studying phase". Computer analysis has been carried out, and, in October, two weeks of model testing will begin at the Marin facility in the Netherlands.

"There are so many places that lack offshore installation equipment like jack-up barges. I know of some sites in the US that aren’t going ahead simply because of this problem," says Groot. "But, with MonoBase, you just need a construction yard to build the GBS, and then the tugboats to install them.

"It’s certainly not limited to the North Sea or any other area. This is a concept that can be applied worldwide."

The MonobaseWind gravity base has been designed to offer a cost-effective and versatile foundation system for offshore wind turbines. The main feature of the system is an innovative installation system to transport the foundation with the wind turbine in one operation to the installation site and to install it on the seabed.

The MonoBaseWind has been developed by Orca Offshore and Naval Dynamics. Both companies have a track record in renewable energy and offshore installation technology.

Developments

ORCA Offshore has done several conceptual designs based on experience with various oil and gas offshore platforms. Several numerical studies have been performed to analyse the dynamic behaviour of the system during transport and installation. Design studies covered the following issues:

  • weight engineering
  • fabrication methods
  • transport accelerations on nacelle
  • operational limits and workability analysis for installation
  • risk assessments for transport and installation
  • foundation and scour protection design
  • environmental impact assessments
  • lowering analysis using CFD (computational fluid dynamics)
  • recovery design.

Fabrication

The MonoBase can be constructed from concrete, steel or a combination of the two. The choice of the building material depends on the project requirements and on the availability of the materials. The design can be adjusted to meet local content requirements and restrictions regarding existing fabrication facilities.

The base structure could be a simple steel structure based on standard shipbuilding technology. This will make it possible to mass produce these units at any shipbuilding yard around the world. Transportation to the assembly site close to the installation site could be done with a heavy transport vessel.

Transport to the installation site

The tow to the installation site will be done floating on the buoyancy inside the tower and the base with the turbine pre-installed on the foundation. Typical draft will be around 10m, which makes the MonoBaseWind suitable for most large harbours. The floating MonoBase is not very sensitive to normal waves and is even capable of handling storm conditions with waves up to six metres (significant wave height). Due to the nature of the design, motions will be limited and acceleration loads on the nacelle will be very low.

Installation of the MonoBase

At the installation site the MonoBase will be installed by lowering the unit to the seabed using ballast water. The Monobase comprises two concentric buoyancy elements that can be ballasted down subsequently. This will ensure the stability and control over the unit, and will also reduce the motions of the system due to waves during set down.

To install, the MonoBase will be connected to three tugs that will control the position of the foundation; a fourth vessel is required to control the lowering and set down. This will be done with an umbilical connecting the ballast system of the Monobase to a control unit on the vessel. Also from the vessel an observation vehicle (ROV) with an underwater camera will be deployed to have full control over the operation.

In general, bottom preparations are not required (the foundation system of the MonoBase will accommodate normal seabed unevenness) while the combination of on-bottom weight and skirts under the base will be designed to suit the strength of the soil.

After installation of the MonoBase, hook-up of the power cable can be done by utilising a pre-installed J-Tube in the Base or tower. It may also be necessary to install a scour protection system around the base. A system is available pre-attached to the base, which can be installed using a work-class ROV only.

Recovery

The recovery after the service life of the unit can be done by reverse installation. This will require de-ballasting of the compartments, which can be done using compressed air or submerged ballast pumps. After recovery of the MonoBase, nothing is left behind, a major advantage of the system with regard to the environmental impact issues.

Cost savings

The potential cost savings of this system compared to other foundation systems include:

  • economics of scale, scalable to large water depths and large wind turbines
  • only one offshore campaign per unit required
  • reduced weather risk when transporting an installation
  • recovery is simple and cost effective
  • no bespoke vessel required
  • mass production possible.

Environmental impact

The fabrication, transport, installation and recovery covering the full live span of the system have been reviewed for environmental impact. The main observations are:

  • fabrication can be done using existing facilities and existing methods, with proven HSE systems and experience
  • the transport and installation has reduced weather risks, which will minimise the time required for transport and installation, and reduce the environmental burden caused by the installation equipment
  • no loud subsea work is required, so mammal friendly
  • minimum bottom disturbance and recovery of all materials after abandonment.