This white paper details the installation of wind turbines with a self-installing gravity base. The paper is a presentation given by ORCA Offshore managing director Herm Bussemaker to the Offshore Energy conference in 2012.
The MonobaseWind gravity base has been designed to offer a cost-effective and versatile foundation system for offshore wind turbines. Its main feature is an innovative installation system to transport the foundation with the wind turbine in one operation to the installation site and 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.
Conceptual designs for offshore wind turbine foundations
Orca Offshore has carried out 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 the 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 computational fluid dynamics (CFD)
- Recovery design
The MonoBase can be constructed using concrete, steel or a combination of the two. The choice of the building material depends on the project requirements and on the availability of materials. The design can be adjusted to meet local content requirements and restrictions regarding existing fabrication facilities.
In addition, the base structure could be a simple steel structure based on standard ship building technology. This will make it possible to mass produce these units at any ship building yard around the world. Transportation to the assembly site close to the installation site can be done with a heavy transport vessel.
Transportation of MonoBase to its installation site
The tow to the installation site will be done by floating on the buoyancy inside the tower and the base with the turbine pre-installed on the foundation. The typical draft will be around 10m, which makes the MonoBaseWind suitable for most large harbours. The floating MonoBase is not sensitive to normal waves and is capable of handling storm conditions with waves up to 6m. 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 offshore turbine foundations
At the installation site, the MonoBase will be installed by lowering the unit onto the seabed using ballast water. The Monobase consists of two concentric buoyancy elements, which can be subsequently ballasted down. 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, which 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. In addition, an observation vehicle (ROV) with an underwater camera can be deployed from the vessel to enable full control of the operation.
Generally, 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 consistency of the soil.
After-installation of the MonoBase, connection of the power cable can be carried out using a pre-installed J-tube in the base or tower. It may also be required 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.
Eco-friendly recovery of MonoBase units
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 achieved using compressed air or submerged ballast pumps.
After recovery of the MonoBase nothing is left behind, which is a major advantage of the system with regard to its environmental impact.
Cost-effective foundations for offshore wind turbines
The potential cost savings of this system compared with other foundation systems are:
- Economics of scale, scalable to large water depths and large wind turbine
- Only one offshore campaign per unit required
- Reduced weather risk transport and installation
- Recovery is simple and cost-effective
- No bespoke vessel required
- Mass production is possible
Environmental impact of MonoBaseWind system
The fabrication, transport, installation and recovery covering the full livecycle of the system has 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, reducing then environmental burden caused by equipment installation
- No loud subsea work is required, so it is marine life-friendly
- Minimum seabed disturbance and recovery of all materials after service-life
Touted to become the world's biggest floating wind farm, Hywind Pilot Park will be situated at Buchan Deep in the North Sea, approximately between 25km and 30km off the coast of Peterhead in Aberdeenshire, Scotland.
Orca Offshore has successfully completed the model testing of the MonoBaseWind concept.
Orca Offshore is happy to inform that we have successfully concluded the modeltesting of the Monobasewind concept.
MonoBaseWind has been awarded a free two-week model test slot at the MARIN maritime research institute.
MonoBaseWind has been nominated for the 'Vernufteling' election organised by The Royal Institute of Engineers (KIVI).
MonoBaseWind has been pre-qualified for an offshore windfarm engineering, procurement, construction and installation tender together with a large civil contractor.