Monopiles Support Structures

Article published on Wednesday, 05 June 2013

The monopile support structure is a relatively simple design by which the tower is supported by the monopile, either directly or through a transition piece. The monopile continues down into the seabed. The structure is made of a cylindrical steel tube.

The pile penetration depth is adjustable to suit the actual environmental and seabed conditions. A limiting condition of this type of support structure is the overall deflection (lateral movement along the monopile) and vibration, and are subjected to large cyclic, lateral loads and bending moments (due to the current and wave loads) in addition to axial loads (e.g. vertical loads due to the transition piece). Monopiles are currently the most commonly used foundation in the offshore wind market due to their ease of installation in shallow to medium water depths. This type of structure is well suited for sites with water depth ranging from 0-30m.

The diameter of the monopile ranges up to 6m and the wall thickness is as much as 150mm. The weight of the monopiles is usually up to 650t as in the London Array offshore wind farm.

The standard method of installation of piled structures is to lift or float the structure into position and then drive the piles into the seabed using either steam or hydraulic powered hammers. The handling of piles and hammers can require the use of a crane vessel (revolving or shear leg crane), however jack-up are the most commonly used vessels for the installation of the monopoles in general. For example the Stanislav Yudin was used on Greater Gabbard and Rambiz was used on Ormonde and the jack up vessel SEA WORKER was used on EnBW Baltic 1 OWF and MPI Resolution was used on Kentish Flats.

Monopile Foundation

Monopile Foundation

A transitional piece is placed on top of the monopile and has the function of adding a flange for connecting and levelling the tower. Grouting is used to seal and connect the transition piece to the monopile.

Grouting issues

In 2009, grouting failures were detected between the monopile and the transition piece on some wind farms such as Horns Rev I. One reasons given for such failure was that the grouting connection was insufficiently engineered to enable the transfer of the bending moment from wind loading. As part of a response, DNV in 2010 established a joint industry project involving key industry players to revise and improve the basis for calculating the axial load capacity of large diameter grouted connections. The joint industry project concluded that a cylindrical shaped design of grouted connections without additional support arrangements for axial load is not recommended. Instead of this, a conical shaped connection design has been developed where the monopile and the transition piece are fabricated with a small cone angle in the grouted section. If the bonds between the steel and the grout are broken during in-service life, some slight settlement of the transition piece will occur. This settlement introduces compressive contact stresses between the steel and grout which, together with friction provide sufficient resistance against further settlement. This concept has already been put into use in the London Array offshore wind farm and Lincs offshore wind farm.

Conical connection between the transition piece and the monopile (DNV)

Conical connection between the transition piece and the monopile (DNV)

Examples of wind farms using monopile foundation and support structure are Horns Rev I, Robin Rigg, Rhyl Flats and Barrow.

A recent market development has seen some projects initially planning use of jacket foundations switching to large monopile foundations. Reasons given have been that it is part of the optimisation of wind farm design in order to achieve cost reductions in installation and fabrication. Wind farms where ‘supersized’ monopiles are planned are Innogy Nordsee I and Gode Wind I, and Gode Wind II and we are seeing much heavier units for example, 910t for Gode Wind I and 1,050t for Gode Wind II, with a transition pieces of 580t. These monopiles have been designed to support the next-generation wind turbines (6-8MW).

Monopile variations

Drilled Monopile

This method of installation was used at the Bockstigen offshore wind farm in Sweden and was proposed in order to save costs. The foundation consists of steel monopile fixed with concrete in a deep hole drilled in the seabed rock. This method helps keeps the weight of the foundation relatively low (typically 43t). The foundation can be towed to the site by a normal tug boat and lifted into place by a relatively low lifting capacity such as 35t.

The foundation hole can be drilled using a suitable vessel such as a jack-up barge. A hole of 2.4m diameter and around 30m depths is drilled into the rock. Drilling can be performed into any kind of rock hard enough to make the structure self-supporting. After drilling, the tug boat tows out the monopile which is then lifted into the hole. The monopile is grouted into position by filling the gap between the monopile and the rock with concrete.

This method was also used at the Barrow offshore wind farm, where compound of steel monopiles with an outer diameter of 4.75m of varying length, were installed. The monopile foundations penetrated the seabed by a range of depths between 30.2m and 40.7m using a combination of driving and drilling.

Drilled Concrete Monopile

This methodology was developed by Ballast Nedam and MT Piling for the Vattenfall research project “Foundation Concepts Krieger's Flak Wind Farm”. This concept consists of prefabricated concrete monopiles installed using a vertical drilling method adapted from the horizontal tunnel-drilling methods and avoids the use of a transition piece.

The reasons given for developing this method were:

• Prefabricated concrete monopiles can be less expensive than steel monopoles (requires volume) • Using concrete as base material has the proposed advantages of: - Less vulnerable to market related price fluctuations - Concrete fabrication capacity is considerably higher than steel production capacity - Supply is not dependent on a few main suppliers as is the case for steel

At sites with very high seabed resistance and where there is a likely occurrence of boulders, there is no requirement for having two work methods in place (driving and drilling). The Drilling Concrete Monopile integrates the two work methods in one which the proposers states results in:

• Lower risk profile for installation • Lower mobilisation cost • No intermediate cost for changing from driving to drilling equipment

The concrete monopiles were designed for 3.6MW turbines with a hub height of 76m MSL (Mean Sea Level) and 5MW turbines with a hub height of 86m MSL in water with a depth of up to 30m. The monopile foundations have an interface level at +3.5 m MSL The top of the monopile is fitted with a concrete ice cone.

Dimensions
3.6MW
5.0MW
Outer diameter (mm) 6,500 6,900
Wall thickness (mm) 500 700
Pile length (m) 61 64
Weight (t) 1,450 2,200

Concrete monopile dimensions by Ballast Nedam

Fabrication

The monopiles are made of pre-cast reinforced concrete ring elements and are fitted with a steel cutting shoe to ‘cut’ through the seabed, creating an overcut. This overcut is filled with self-hardening drill fluid. The monopiles are transported by floating them to the offshore site.

Work method in brief

The floating monopile is upended by the Heavy Lifting Vessel Svanen using a proven work method and is positioned on the seabed within Svanen's guiding frame. The monopile settles several metres into the seabed, after which the drilling machine is installed inside the monopile. Drilling starts and the monopile is continuously lowered until the required depth is reached. At this point the drill is removed and the drill fluid hardens. The ice cone is placed and grouted on the top of the monopile.

Drilling method by Ballast Nedam

Drilling method by Ballast Nedam

References

Offshore Standard: ‘Design of Offshore Wind Turbine Structures, DNV-OSJ101’, Det Norske Veritas (2010) Project UpWind Foundation Types - E.ON Monopile failures put grout in doubt - Wind Energy Update Offshore Wind Farm Bockstigen - Installation and Operation Experience Drilled Concrete Monopile - Krieger's Flak R&D Project

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