Figure 47 Foundation monopile piling operations, courtesy of Menck.
Function
Foundation installation consists of the transport and fixing of foundation in position.
What it costs*
About £165 million for a 1 GW wind farm.
Who supplies them
Bauer Offshore Technologies, Boskalis, Foundocean (grouting only), Fred. Olsen WindCarrier, GeoSea (DEME Group), Jan de Nul, Jumbo Offshore (transition pieces only), Newwaves Solutions, Osbit, SAL Heavy Lift (transition pieces only), Saipem, Scaldis Salvage & Marine, SeaJacks, Seaway Heavy Lifting (Subsea 7), Swire Blue Ocean and Van Oord Offshore Wind.
Key facts
The process involved varies with the foundation technology employed. Offshore substation foundations may be installed in a similar way to turbine foundations but are substantially larger.
Monopiles
Monopiles may be installed from a jack-up vessel or a floating vessel. The transition piece is usually lifted and grouted or bolted in place from the same vessel but two-vessel strategies have been used successfully.
Monopiles (up to 10 m diameter) are generally moved into position using the main crane and upending tool and held in position by a gripper tool. They are the driven into the sea bed using a hammer and anvil system before mounting and grouting transition pieces.
Transition pieces are usually carried and installed by the same vessel, although a two-vessel strategy in which transition pieces are installed by a separate vessel has been used on several occasions. This focuses the utilisation of the monopile installation vessel, which is likely to have higher day rates than the transition piece vessel. A disadvantage is the costs of mobilising and demobilising two vessels.
Feeder strategies have been used for monopiles, notably with Van Oord’s Svanen, which has no useable deck space for transporting components. In this case, the monopiles are floated to site using tugs or transported using platform supply vessels.
An approximate timetable for installation of monopiles once at the wind farm site is:
- Transport and positioning: 2 hours for floating vessels; 4 hours for jack-ups
- Preparations: 1 hour
- Lifting and pile positioning: 1 hour
- Driving: 6 hours, and
- Grouting: 2 hours.
The full cycle time is 2-3 days per monopile, a figure that takes into account mobilisation and demobilisation, loading and waiting on weather.
Under some ground conditions, monopiles are grouted into a pre-drilled rock socket. Where boulders are present, a combination of drilling and driving is required.
Jackets
Jacket foundations may be installed by floating vessels or jack-ups. Installation usually involves pre-piling using a reusable template. The jacket is then lowered into place over the pin piles and grouted. Post-piling, in which the pin piles are driven (or lowered into pre-drilled sockets) through a sleeve on the jacket legs, may alternatively be used. Pre-piling has the advantage of decoupling the piling and jacket installation, enabling a lower cost vessel to be used for piling and maximising the use of deck space of the main jacket installation vessel.
An approximate timetable for installation of jackets once at the wind farm site is:
- Transport and positioning: 2 hours for floating vessels; 4 hours for jack-ups
- Preparations: 1 hour
- Lifting and pile positioning: 4 hours
- Driving: 8 hours, and
- Grouting: 2 hours.
The full cycle time is 3-5 days per jacket, a figure that takes into account mobilisation and demobilisation, loading and waiting on weather.
There is growing interest in suction bucket foundations, particularly under jackets. These are potentially faster and therefore cheaper, to install and avoid the need for expensive noise mitigation. This technology offers potentially lower installation costs because less equipment is needed. It also allows installation in areas with lower sediment depth to bedrock as the buckets do not need to go as deep. Suction buckets have been deployed commercially with jacket foundations but can be used with monopiles as well.
Gravity bases
Gravity base foundations may be installed by floating crane vessels (such as a sheerleg crane vessel) or specialist barges to support float out.
Concrete gravity foundations can weigh substantially more (3,000 t) than steel foundations and may be floated out to position before being submerged. The sea bed must be levelled to receive such foundations.
Large scale installation of gravity bases has not been attempted in UK waters. Cycle times are likely to be similar to jackets, but floating transportation can result in considerably more weather downtime and requires more onshore manufacturing space.
Floating foundations
To learn more about floating foundations, please see BVGA’s Guide to Floating Offshore Wind.
Scour protection
Scour protection is generally provided by dumping of rocks, bags of stones or other materials (such as tyres) around the base of the structure. Rock dumping may use a fall-pipe vessel that are widely used in the dredging industry. Bags are likely to be lowered into position using an offshore construction vessel.