The installation sequence of an offshore wind turbine depends on the foundation structure chosen. An offshore wind farm requires much closer integration of the design and construction activities than an onshore wind farm because of the additional challenges of operating at sea. Some basic principles, including construction, for typical offshore foundations are given in Table 5.1.1.
|
Foundation type |
Size (diameter) |
Weight |
Construction sequence |
|
Gravity base |
12 – 15 m |
500 – 1000 tonnes |
1. Prepare Seabed 2. Placement 3. Infill Ballast |
|
Monopile |
3 – 3.5 m |
175 tonnes |
1. Place Pile 2. Drive Pile |
|
Multipile |
0.9 m |
125 tonnes |
1. Place Base 2. Drive Pile |
|
Bucket (caisson) |
4 – 5 m |
100 tonnes |
1. Place Base 2. Suction Installation |
Table 5.1.1 Basic principles of typical foundations for offshore wind turbines [1]
Each type of foundation will be subject to construction constraints. A gravity base foundation requires the seabed to be prepared in advance and the toe of the structure to be protected against scour. An advantage is that the structure can be constructed onshore, thereby reducing offshore operations. The monopile is easy to install (drive) with proper equipment but large stones in the seabed can make it difficult or even impossible. If the pile needs to be driven into the bedrock (granite), expensive site works have to be undertaken. A comparison of the construction differences for monopile and gravity base foundations is summarised in Table 5.1.2.
|
Construction phase |
Gravity base foundation |
Monopile foundation |
|
Onshore construction |
Local to site |
No constraints |
|
Transport offshore |
More complex |
Lift onto barge |
|
Pre-placement activities |
Seabed preparation |
None |
|
Placement |
Lift or float-over |
Lift |
|
Fixing |
Grouting |
Pile driving |
|
Installation of tower / turbine |
Potential obstruction to lift |
No hindrance to lifting |
Table 5.1.2 Construction differences for monopile and gravity base foundation s [2]
Time delay at sea is the most significant problem related to offshore project engineering. As hired equipment is used for installation, all downtime will prove costly. Project developers try to minimise delays by pre-assembly and onshore testing of installation procedures. Any problem or design error detected at sea causes time delays and equipment downtime.
- At Middelgrunden some of the interconnecting cables were damaged when the foundations were installed. The problem was foreseen with spare cables available and a covering insurance.
- At Bockstigen downtime was caused by high winds preventing the jack-up barge from being operated. Jack-up barges cannot be safely deployed during heavy sea conditions.
Construction time for a driven pile foundation from a floating barge was initially shown to be less costly than using other methods. Due to weather downtime, the overall installation durations have been similar for gravity base foundations and driven pile foundations installed either from a jack-up vessel or floating barge.
The weather downtime allowance required for a 50 unit wind farm is considerable, approximately doubling the floating barge installation duration. It has been proposed to install the structure in two pieces (first the foundation unit followed by the assembled support tower, nacelle and rotor as one unit) compared to three pieces (installing each of the foundation, support tower and nacelle and rotor units in a separate operation) to save in construction time.
Offshore wind turbines are most likely to be installed from either a jack-up barge or a floating crane vessel. The choice will depend on the water depth, the crane capability and vessel availability. The crane must be capable of lifting the structures, with hook heights greater than the level of the nacelle to enable the tower and turbine assembly to be installed. Existing crane vessels have not been specifically designed for installing offshore wind turbines. For large offshore wind farms, greater than 50 units, significant time (and therefore cost) savings could be made by using an installation vessel purpose built for the task. This philosophy has been adopted elsewhere in the civil engineering industry.
So far, the installation process had held two phases. First the foundations are build and then the turbines are installed on top of the foundation. Usually turbines are erected as on land, i.e. first the tower in segments and then the nacelle and the rotor.
In the case of Middelgrunden, the first tower segment was pre-installed and transported on the foundation. The control board, switchboard and the transformer were located at the bottom of the tower during transportation and lifted in place, at intermediate floors, on site.
The total build duration for a multi-unit wind farm is likely to take several months. All installation operations will be subject to weather constraints and there will inevitably be periods of non-operation/weather down-time. This can be minimised by scheduling installation operations during the relatively calm summer months, when both wind speeds and wave heights are most frequently within safety limits.
The monopile foundation, i.e. a steel cylinder, is usually transported to the site on barges. Alternatively it can be capped and sealed at the ends and floated to the site.
At Vindeby and Tunø Knob, the caissons were floated to the site and filled with ballast. At Middelgrunden, the foundations were transported with a barge, that lifted the foundations several meters from the seabed and transported them one by one to the site.
The Opti-OWECS report suggests transporting the whole turbine in one piece. Two alternative tower and wind turbine transportation orientations were considered, i.e. a vertical and a near horizontal orientation. In the near horizontal orientation the barge space requirements govern the size of the barge required whilst in the case of the vertical orientation, the transportation stability requirements govern. Transportation in the vertical orientation is not regarded as feasible without substantial bracing to limit the bending moments at the base of the tower.
An amphibian vessel for transporting, installing and maintaining assembled wind turbines has been patented in the Netherlands [3].
All installation methods have their advantages as well as disadvantages. The decision will depend on assembly design, foundation structure, site conditions and to some part on the approach adopted for maintaining the structures.
It is often anticipated that tower units complete with the nacelle and rotor could be installed as a single unit at a rate of two per day (24 hour working) during the summer months (May-August). Under these circumstances vessel downtime of around 50% is anticipated i.e. a rate of 1 tower per day accounting for downtime with a total installation period inclusive of mobilisation of 4 months. However, the temporary storage of the turbines to be installed may constitute a problem.
The Opti-OWECS report [4] presents a good summary of the options available for installation of the tower (inclusive of nacelle and rotor etc.):
Jack- up Installation
Jack-up lift appears at first glance to be the obvious method of installing the tower, nacelle and rotor. It forms a stable base from which to carry out the operation and is the preferred choice for carrying out the piling operation. However, its inherent stability and hence lack of manoeuvrability poses problems for the installation of the tower. Offloading tower elements from a floating barge and lifting them into place will most likely require a form of piecemeal construction with the tower, nacelle and rotor all installed as separate items. The same jack-up barge can be used for driving the monopile and for installing the turbine.
Semi-Submersible Installation
Lifting from a vessel is in principle most straight forward method of installation. Semi-submersible crane vessels represent the most stable floating platform from which to carry out offshore construction work. Existing vessels, however, are designed for more remote offshore operation and have difficulties operating in shallow water depths.
Ship Shaped Vessel, Flat Bottom Barges and Land Based Cranes
Ship shaped vessels and flat bottom barges offer appreciably less stability for carrying out construction work and are consequently subject to weather delays. Ship shaped vessels with rotating cranes offer the best performance. As a result, they are in heavy demand and are attracting appreciable day rates. Flat bottom barges with sheer leg cranes of a suitable size are in far greater supply and offer a cost effect approach to tower installation despite weather delays. One way of combining the benefits of rotating crane with adequate reach but at a lower day rate is to use land based cranes. Such a system is adopted quite satisfactorily in sheltered locations.
Float-Over Installation
The Opti-OWECS report presents a float-over installation, where the tower is erected and floated out in the vertical orientation before being floated-over then lowered down onto the pre-installed pile. The tower is erected at the quay side on a dummy pile and is stabilised by a pin which is housed in the tower and lowered into the pile. The tower is secured to a barge in the vertical orientation ready for transportation. The vessel required for this operation may need to be specially built although modifying an existing vessel is also an option. The vessel takes-on the tower at the quay side where it is moored adjacent to the tower and securely seafastened. Then, possibly on a rising tide, the barge is deballasted allowing the tower to be detached from the dummy pile. Once in a safe water depth, the barge is ballasted for the tow. On arrival at the site the vessel is deballasted, if necessary, and safely moored over the offshore installed pile. Then follows the operation of ballasting the vessel down so as to safely transfer the support for the tower onto the pile. The sea-fastening is then released leaving the vessel to be towed away.
Offshore wind energy structures and their foundations must be designed to accommodate exposed weather and equipment workability, with support towers designed to be compatible with the available construction equipment. Additional work is required in:
– Improved dissemination of knowledge of offshore marine related construction procedures and techniques amongst designers/developers.
– Optimise the cost-effectiveness of offshore wind structure installation operations by making use of novel construction sequences and scenarios.
– Investigation of reducing fatigue loading by introduction of inherent flexibility, i.e. flexible towers, compliant couplings, etc.
– Reduction of fatigue loading through more sophisticated control. (Benefits of greater sophistication to be balanced against potential reliability problems.)
– Investigation of the technical and economic feasibility of ‘re-useable’ foundations.
– Identification of suitable European test sites with offshore type conditions, e.g. islands.
The highest uncertainty in offshore installations relate to time delays and costs in use of rented equipment. Also, it is important to minimise the time needed for offshore operations as any unscheduled downtime. There is a clear need for installation vessels that can withstand more severe weather conditions and operate for longer periods of the year. Special installation vessels, designed for installing offshore wind turbines are possible, and perhaps a necessity, when offshore wind energy installation becomes a continuous all-year activity. Cost control efforts should be focused on the overall installation process, and dissemination of areas for economic improvements identified.
A longer term objective should aim for an integrated design, where the foundation and the turbine is installed as one piece. The installation procedure should at least be simplified and include a minimum of operations offshore.
The projected overall cost for an offshore wind farm should account for decommissioning costs which include an allowance for shifts in environmental ground rules or other fluctuating cost factors. The offshore oil and gas industry is currently facing the issue of decommissioning offshore installations and subsea wellheads, the cost of which exceeds previous conservative estimations.
1. Watson, Gillian (Ed.) OWEN workshop on Structure and foundations Design of Offshore Wind Installations. Final Report. http://www.owen.org.uk/workshop_3/ws3final.pdf
2. ibid.
3. J.F. Rikken & J.Klop, “Studie naar goedkopere concepten voor de ondersteuning van een offshore windturbine” (Dutch language). KEMA Report No. 99560396-KPS/SEN 00-3035. November 2000.
4. Martin Kühn et al. Opti-OWECS Final Report Vol. 4: A Typical Design Solution for an Offshore Wind Energy Converting System. Delft University of Technology. Report No. IW-98140R The Netherlands August 1998
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