North Sea construction: Installing monopiles for the Dutch Q7 offshore wind farm

In October 2006, and under considerable time pressure, Dutch contractor Mammoet Van Oord began the installation of 61 monopile foundations for the 120 MW Q7 offshore wind farm. In mid-March Eize de Vries spent a day at the North Sea site, onboard the self-elevating jack-up installation barge, Jumping Jack, to experience first-hand the construction work in progress.

Our journey to the Q7 construction site outside the 12-mile Dutch territorial waters took a little over an hour onboard a supply vessel from the port of IJmuiden. As we departed, the contours of Q7 were already clearly visible on the vessel’s radar screen, in the form of a confined regular pattern of small dots.

The deck of the installation barge, Jumping Jack, towered high above the North Sea waves. We boarded the barge via a crane-mounted offshore people mover. Three people can be moved at the same time in this half-open yellow cabin (named the ‘Frog’), seated in a 120° arrangement securely strapped into their seat facing the sea.

Transition pieces lined up on board the Jumping Jack ready for installation at the Q7 wind farm all images mammoet van oord

The offshore industry has traditionally paid great attention to ensuring maximum safety under all operational conditions and during all activities. This includes stringent rules for vessel boarding and embarkation – rules which also apply in the growing offshore wind industry. For example, workers and visitors have to wear a special protective warm overall during a sea transfer to another vessel or structure – if someone does fall into the water, the fabric provides sufficient short-term protection against the cold water until a sea rescue can be mounted.

As we left the Jumping Jack, the pile-driving of the 48 monopiles was still in full progress – with each loud, sharp bang being carried over the water at quite some distance away from the barge.

Q7 offshore wind farm

The Q7 offshore wind farm is built in waters about 21-25 metres deep and about 23 kilometres from the port of Ijmuiden, outside Dutch territorial waters. It is the second Dutch offshore wind project to be built in a two-year period and consists of 60 Vestas V80-2 MW units and an additional offshore high voltage station (OHVS). The Q7 project follows on from the 108 MW Offshore Windpark Egmond aan Zee, which featured 36 Vestas V90-3 MW turbines, and became operational in autumn 2006. The unusual name of the project is a direct reference to block Q7 of the Dutch Continental Shelf.

Mammoet Van Oord (MVO), was established in April 2002 with three shareholders – Van Oord, Mammoet and ProDelta. Schiedam-based MVO partner, Mammoet, is a global heavy lift and heavy transport specialist which gained worldwide recognition for its key role in raising the Russian nuclear submarine, Koersk, in late 2001. Van Oord (VO) is one of the world’s two largest dredging, marine and offshore construction companies and is currently working on the spectacular Deira Corniche project in Dubai – a huge land reclamation project involving the construction of a new waterfront area in the shape of a falcon. It will form the basis for Palm Deira, a new self-contained urbanization for half a million residents.

The ‘Frog’ lifts crew safely on and off the Jumping Jack

In a new extended role, Van Oord has used its financial strength, wide experience and other capabilities to become one of the two Q7 project engineering, procurement and construction (EPC) contractors. In this capacity, Van Oord is responsible for all the engineering, procurement and installation activities related to:

  • the turbine foundations
  • electrical infrastructure (i.e. ‘infield’ and export cables and OHVS)
  • all rock-fill scour protection works required for the foundation and cables.

Van Oord contracted MVO to install the monopile type foundations and the OHVS for Q7 and Van Oord Offshore (VOO) for the installation and burial of the cables and the scour protection. The latter is based in Gorinchem and specializes in offshore installations and coastal protection works. Its other areas of expertise include offshore wind power and telecom cable installation, pipeline landfall and outfall installation, and deep-water gravel placement. VOO fall pipe vessels are being used for precision rock-fill (pipeline and cable protection) onto the seabed and onshore landings.

Scour protection and sea cable installation

On 7 April 2007, the last of the 61 monopiles on the Q7 project was successfully installed, meaning that VOO could begin cable laying operations. During the first week of April, a cable-laying vessel left for Q7 and began laying all 60 ‘infield’ cables inside the wind farm.

The cables are concentrated into eight strings, which will enter the OHVS as eight power input lines, explains VOO’s cable laying and rock fill scour protection operations manager Hidde de Boer: ‘The 33 kV infield cable has an outside diameter of 115 mm and the mass per metre is about 19 kg. In contrast, the 150 kV OHVS output cable that feeds the power to shore has a diameter of 195 mm and its mass is no less than 69 kg per metre. To handle a cable with such a huge specific mass and exterior dimension, we have to employ a different vessel – one fitted with a dynamic turntable for precise cable positioning and deployment. In addition, all cables are buried one metre into the seabed surface for which we apply an integrated submerged trenching device with rotating chain cutter.’ The final three-kilometre section of the output cable will be buried three metres into the seabed and the very last part pulled to shore near IJmuiden with the aid of land-based equipment. The cable will be passed through the sea dunes with the aid of a horizontal drilling operation (HDD) before being finally connected to the onshore high-voltage network. De Boer plans to complete all cable works by August.

At the end of April MVO commenced with installing the OHVS, and in May VOO is due to commence with monopile scour protection works. Rock-fill scour protection around the monopiles serves as an erosion control measure; within the first 24-hours after installation the sea current ‘eats away’ a circular dent into the seabed around the pile.

Loading the monopiles in harbour

Van Oord’s comprehensive EPC contractor responsibilities will end with the electrical connection of all 60 turbines to the grid. Once operational, Q7 will produce about 400 GWh of electricity annually.

Vestas Wind Systems of Denmark is Q7’s second EPC contractor and is responsible for the installation of the 60 topsides (wind turbine towers and nacelles). Starting in May 2007, A2SEA of Denmark will finally install the Vestas topsides.

Project completion is planned for later this year.

Spreading project risks

The splitting of a large offshore wind project into separate contracts to spread the project risks over two or more large and independent (EPC) contractors represents a substantial change for the offshore wind industry. The development is also seen as a necessary and appropriate solution to the contract situation commonly used between 2002 and 2005 when often only one party (e.g. the wind turbine supplier) served as the sole EPC contractor. This single party then had to bear the full project risks. This unfavourable situation contributed in recent years to delays to several UK offshore projects.

A tug follows the barge to its destination

MVO’s project manager for Q7, Adriaan van Oord, says that companies like his mainly work as sub-contractors and are usually called in, and contracted, at a very late stage of an offshore wind project. ‘That gives us insufficient time for proper project engineering and all necessary preparations. Furthermore the real weather-related and other offshore risks lie with the installation activities. In the ‘old’ situation, these risks were passed on entirely to sub-contractors like MVO. Besides facing enormous (weather-related) risks in a small emerging market, we had to try to make the best possible risk assessment for each individual project but, at the same time, having only limited means to influence lowest offer lump sum contract prices.’

The Jumping Jack with legs extended

Looking back at several recent offshore projects, Adriaan van Oord says that many of the first pioneering wind park initiatives were the brainchild of creative entrepreneurial individuals. ‘However, these initiators often – despite their good intentions – lacked the financial leverage to push their initiative to a successful completion. Next on the scene arrived a variety of financial investors, this time with sufficient money but who generally lacked the necessary relevant technical expertise. Third in line came a new and later unnecessary layer of so-called middlemen. This group claimed to possess all necessary expertise to manage the complex offshore projects. But, in reality, they passed on project risks to third parties and pushed up price levels without added value. Most of these first-hour offshore wind entrants have now largely gone, and offshore wind park developments are now more and more carried out by mature companies – utilities/oil companies and the like. These players are capable of carrying out these projects based on multi-contracting and this results in a better allocation of project risks between the parties involved. This in turn offers new opportunities for large professional companies such as Van Oord and other construction companies with sufficient size and leverage.’

Stable working platform

MVO has built up a diverse offshore wind installation record with experience based largely on the use of the Jumping Jack, which became operational in 2003. But the much-delayed development of the European offshore wind market means that the new barge largely lay idle in Schiedam harbour during 2004 and 2005 waiting for contracts.

MVO’s first large offshore wind project involved the installation of 80 monopile-type foundations for Horns Rev I (160 MW) in 2002. However, the Jumping Jack was not operational at the time and other installation barges had to be employed. The maiden project for the Jumping Jack was Arklow Bank in 2003. This shallow water project in the Irish Sea involved the installation of seven monopile type foundations and seven 3.6 MW GE topsides. It was followed by monopile foundations for 30 2 MW Vestas turbines at Scroby Sands off the UK in 2003 and early 2004. Last year, MVO installed 25 3.6 MW monopile foundations at Burbo Bank off the UK and will erect the 3.6 MW Siemens topsides during this spring and summer.

The Jumping Jack’s large elevated deck space offers a safe and stable working platform for the 36 members of the crew in water depths up to 32-35 metres. Most of the crew members on board are specialists and everybody works in 12-hour shifts, seven days a week, for a one-month period followed by a month off-duty.

A hydraulic ram hammers the monopiles into the seabed

The new generation barge consists of a 91m x 33m x 7m steel ‘flat box’ body, with four expandable legs and a 1200 tonne ringer type crane. Jack-up barges typically do not have a propulsion system thus enabling them to operate in shallow water. On the other hand, it means that jack-ups have to be towed to loading stations in order to take in new cargo such as foundations and/or nacelles and that tugs are needed each time the barge has to be moved to a new position.

The Jumping Jack can raise itself out of the waves on its 49 metre legs while carrying a cargo of 4000 tonnes on its deck; the crane capacity is sufficient to hoist even complete nacelles in the new 5-6 MW super class. The top head mass (nacelle + rotor) of such giant multi-megawatt class turbines (Multibrid M5000, REpower 5M, Enercon E-126, Bard Engineering VM) is typically between about 320 and 530 tonnes. In comparison, the much larger 130.5 metre long x 38 metre wide self-propelled installation vessel (former MPI), Resolution, has six legs but, according to its specifications, this giant has a crane capacity of ‘only’ 300 tonnes.

A2SEA owns and operates two vessels that were originally feeder-type container ships but which were converted in 2001 to self-elevating crane ships for the emerging offshore wind farm market. A stable working platform is created after the jacking-up but, in contrast to jack-ups, the hull of the self-propelled A2SEA vessels remains largely submerged. According to company information, one vessel has been upgraded with longer legs and an improved jacking system to enable it to operate in water depths of up to 27 metres.

Significant wave height

The Jumping Jack is capable of operating in marine conditions with a significant wave height (~60% of the maximum wave height) up to 1.5 metres. But offshore operations in its elevated position have to be halted when weather forecasts indicate a prolonged period of a minimum of 72 hours with expected significant wave heights (Hs) of over 1.5 metres unless, depending on location and water depth, jacking-up into survival mode is possible.

If on-site conditions are deemed unsuitable for operations, the barge returns to a sheltered harbour and waits for the weather to improve. This is because, during the operational conditions described above, the hull structure could be exposed to complex wave induced loads.

Installing scour protection around the base of a monopile

Adriaan van Oord explains: ‘Under normal operating conditions, rolling waves pass the elevated hull undisturbed from underneath. Higher waves might hit the hull sideways, as well as partly pass the hull from underneath. These combined wave forces (in excess of 1.5 m Hs) could impose very high loads on both the expanded legs and the hull. For example, when operating in deeper water without a sufficient air gap, a massive wave much higher than the Hs of 1.5 metres might lift the entire barge off the seabed to be smashed back again on its legs moments later.’ In January 2007, maximum wave heights of 11 metres were recorded on two occasions in the Q7 area, although at the time the Jumping Jack was safely in the port of IJmuiden.

The physical size of a barge is important with regard to the operational weather window because, only with large new-generation jack-ups, is it possible to continue work offshore during winter. For the Q7 project, it proved necessary to begin work in time with the foundation installation activities and to continue during the difficult winter period as the building permission expires at the beginning of 2008, explains Adriaan van Oord. A second important point is the need for a stable deck area during installation of the monopiles and the transition piece while handling weights of 320 tonnes in a safe manner and without needing to apply specially developed guiding equipment. This is achieved by jacking up the barge (partly) out of the water. Sometimes this requirement cannot be met when the water is too deep and a floating barge has to be employed as the only alternative. Such a situation occurred, for instance, at the Beatrice project off the Scottish coast where a 5 MW REpower 5M turbine was erected in a record water depth of 44 metres during 2006.

Soft support structure

During the installation of the 25 foundations at Burbo Bank in summer 2006, the Jumping Jack stayed at the construction location all the time. The ready piles and transition pieces were delivered on-site on coasters directly from the Hoboken manufacturing plant in Belgium (see boxed text on page 106).

Conditions at Q7 were quite different as work to install the foundations began in the autumn and lasted during winter – periods of the year characterized by frequent spells of bad weather. The component supply strategy used for the Burbo Bank foundations was therefore considered too risky and the foundations are instead supplied from Hoboken to a quay at IJmuiden harbour via inland barge transport. On each re-supply journey to IJmuiden, the Jumping Jack loads three monopiles (each weighing 320 tonnes and 54 metres long) and three so-called transition pieces each about 19 metres long (see Box 1). The piles are stowed horizontally on the deck and the transition pieces are locked in a vertical position during transport to the construction site.

The deck of the Jumping Jack is further crammed with a range of fixed and mobile structures dominated by the huge 1200 tonne main crane at the centre. There are also support cranes and a clever, patented, pile-handling device. Prominent on the deck is a huge hydraulic hammer (MHU 1900) from Menck of Germany, which is operated by a company technician from a separate deck-based container cabin.

Other fixed deck structures include the crew accommodation, a washing and dining room, and an elevated operational command centre full of computer screens and additional maritime and other equipment. Compartments underneath the deck accommodate a diesel power plant, giant cable reel systems for operating the four expandable legs, anchor drives, etc.

All operations are guided from the central command centre where the Bargemaster (captain) is in charge of all maritime operations and a separate installation manager co-ordinates the foundation installation activities.

The position of each individual monopile foundation within the wind farm is mapped precisely and the barge is pulled to each new location with the aid of a tug and/or by its anchors. Precise positioning is performed with the aid of a four-anchor system using a global positioning system (GPS) to guide the activity closely. Once the programmed position is reached, the barge is raised to its elevated operational position. A clever innovation is that the four huge 49-metre long legs are lowered with the aid of steel cables and cable winches, instead of the more common rack-and-pinion drive system used for smaller jack-ups.

Laying power cables to connect the wind farm

‘A huge advantage of the cable-winch solution is that it works much faster compared to rack-and-pinion drive’, says Adriaan van Oord. ‘Speed is of crucial importance in the transitional phase between floating and the barge standing firmly on its legs onto the seabed. This period should be as short as possible to make as much use of the small available good weather windows, especially in the winter period.’

A related advantage of the cable-winch system is the less rigid cable connection between the hull and the legs, which reduces the load impact on the barge. This in turn guarantees superior operational behaviour, Adriaan van Oord adds. Readings on a computer screen show the exact co-ordinates of each new pile at the seabed and these are used as a tool to guide the crane during final pile positioning. A deck-mounted pile-gripping device, which is freely movable in ‘x-y’ directions, helps to guide each new pile in position.

Pile driving

The pile diameter measures ‘only’ four metres, and has a relatively large and varying wall thickness for reasons of system dynamics. The total length of the support structure is measured from the pile ‘clamping point’ close to the seabed surface up to the tower top. The combination of total length, the diameter and wall thickness of the pile, and the tower diameter plus wall thickness mean that the Q7 solution is characterized as a so-called ‘soft support structure’ (see above).

‘As part of a steep learning curve, the Q7 installation time record stood at three complete foundations within 48 hours’, said a satisfied Adriaan van Oord in the command centre overlooking the operations.

Each 54-metre long monopile has to be brought into the vertical position with the aid of clamping device mounted on a mechanical-hydraulic crane on one end and a second mechanical gripper on the other. It takes up to an hour to bring a pile from the stationary horizontal position into a ‘dynamic’ vertical position (up-ending).

The crane then moves the pile into the right location on the seabed. The pile is open at both ends and, once freed from the crane, sinks 4-4.5 metres into the seabed with its own mass. The next step is to slide the hydraulic hammer over the top of the pile. This added mass pushes the pile a bit further into the seabed. After this stage, the pile installation engineer in charge measures the vertical position with a ‘common’ digital spirit level and any inclination from the vertical is corrected, if necessary, with the aid of the pile-gripping device.

Before the actual pile-driving procedure begins, all crew present on the deck put on their hearing protection aids and others are issued with compulsory protective earplugs. With the first blow of the internal hydraulic hammer weight coming down, the pile sinks about 0.3 metres into the seabed. However, the distance becomes gradually less during each following blow.

Following the first blow, the procedure of checking and correcting the vertical pile position is repeated, followed by a second blow, etc. After a number of sequences, a pile will have already sunk metres into the seabed and further inclination checks are not necessary.

The complete pile-driving takes about 2.5 hours, with increasingly sharper-sounding bangs as the shaft friction between soil and steel pile skin area increases continuously. Once a pile reaches the required depth, the shaft friction (a phenomenon of soil mechanics) keeps it firmly in position during its operational lifetime.

The next step is to fit the lower submerged section of a so-called J-tube assembly intended for guiding the electrical cables. The assembly slides into a final locked position with the aid of a patented double-rail system welded to the piles. According to Adriaan van Oord, the solution represents a genuine deep-water offshore wind technology innovation. Foundation supplier, Smulders Group of the Netherlands, developed this lower/upper J-tube solution specifically for Q7.

The transition piece is then put over the monopile top with a loose fit. It is essential that the upper J-tube section fitted to the transition piece and the lower section already in place alongside the monopile are positioned exactly in line. This is achieved with the aid of a guiding camera placed temporary in the upper J-tube section during the positioning operation.

A transition piece itself is a highly complex engineering component, with substantial added value content. Its function is to compensate for a 0.5° (maximum) inclination of a monopile that has not been hammered exactly vertically into the seabed. The wind turbine tower bolted on top of the transition piece needs, for various reasons, to be exactly vertical. Adriaan van Oord says that the Jumping Jack crew has now mastered the pile-driving technique to such an extent that the majority of the monopiles are already driven in at an almost exact vertical position.


In its ready form, a transition piece has a service platform at the outside on top which is assembled separately during installation. In addition, three J-tube flanges for guiding the electrical cables are fitted, together with a ladder for mounting the turbine after boat landing plus several additional assemblies and (sub) systems.

A number of temporarily hydraulic cylinders were installed. Inside each transition piece. These aim to force the huge component into an exact level position on the monopile. After levelling, the position is fixed by tightening a number of welded set screws (so-called ‘stud bolts’) spaced evenly around the circumference of the transition piece.

After removing the hydraulic cylinders, a fast-curing liquid concrete with specific properties (grout) is pumped into the annular gap between pile and transition piece from below (bottom-up). A distinct feature of the transition piece design is a huge seal in the bottom section. This seal closes the annular gap between pile and transition piece, and thus prevents liquid grout from leaking away into the water during the filling-up operation and curing.

Cathodic foundation protection is applied as an offshore anti-corrosion measure. This package consists of several heavy pieces of zinc clamped together as a ‘wide belt’ or otherwise around the circumference of the transition piece.

As well as cathodic protection, all exposed surfaces and a small section of each monopile receive long-lasting surface treatment with a marine-type finish coating which has a high content of anti-abrasive ceramic particles. Before applying the marine coating, all steel wall surfaces are made smooth and freed from imperfections such as welding spots. The coating has to last the entire 20 year operational lifetime of the installation – even under harsh North Sea offshore conditions.

New owners for Jumping Jack

Early in February 2007, the Jumping Jack was sold to A2SEA but hired back temporarily to complete the Q7 and Burbo Bank (UK) projects. Adriaan van Oord says that the offshore wind market is finally picking up, offering new challenges. ‘With regard to MVO’s future prospects, a company with only one barge at its disposal is relatively vulnerable and therefore lacks the necessary feasibility in the medium to long-term.’ Independent growth as well as a strategic co-operation with a third party was therefore considered.

Initially, the A2SEA shareholders and the three MVO shareholders considered combining their two fleets but, in the end, the decision was made to sell the Jumping Jack to A2SEA. Adriaan van Oord explains: ‘That decision implies the discontinuation of MVO as a separate company specialized in offshore wind installations. However, with its shear size, international experience and financial leverage, Van Oord will continue its active role in the growing offshore wind industry. But this involvement will be in a new and much wider role as dedicated offshore wind farm EPC contractor. This far expands its previous subcontractor role that focused just on installing foundations, topsides, cables and scour protection.’

I greatly enjoyed the visit to the Jumping Jack during the Q7 foundation installation work and was impressed by the way the vessel and its installation equipment were operated in a safe, controlled and professional manner. By working in this way, the crew achieved high productivity laying with as many as seven foundations per week.

The professional contribution of installation firms like MVO and Van Oord and the substantial offshore wind farm installation experience that has been built up means that the entire offshore wind industry has been able to benefit and mature.

Eize de Vries is Wind Technology Correspondent for Renewable Energy World

On behalf of Renewable Energy World, I wish to thank Mammoet Van Oord’s director Jan Kranenburg for his efforts in organizing the long-awaited visit to the Q7 wind farm. I am also grateful to Q7 project manager, Adriaan van Oord, for accompanying me during the interesting and inspiring journey. And to Van Oord Offshore BV’s Q7 operations manager, Hidde de Boer, for providing me with valuable information on the cable-laying operation and scour protection works.

Last, but not least I would like to thank the captain, operations manager and crew of the Jumping Jack for the pleasant stay on board, everybody’s openness in answering my questions and their combined efforts to look after me amidst the hectic offshore installation operations.

Q7 transition piece

The monopile-based offshore wind turbine support structure used in the Q7 project consists of three main components:

  • The actual monopile (diameter 4.0 metres) is an open steel pipe of varying wall thickness that is partly hammered or drilled into the seabed. Once at its final depth, it sticks a couple of metres out of the water.
  • The transition piece (diameter of around 4.25 metres) fits ‘loosely’ over the top of the monopile like a jacket and is fixed to the monopile by means of grouting.
  • After the grout is completely cured, the tower is bolted to a completely level transition piece top flange.

After five years and multiple applications, this method has developed into the semi-standard solution for the emerging offshore wind market.

The Dutch Smulders Group is one of Europe’s largest manufacturers of fabricated steel products and the market leader in monopile type foundations. These have been produced from its Hoboken plant near Antwerp in Belgium since 2001. With this former shipyard, Smulders acquired a manufacturing facility with several unique features including a huge portal crane and direct access to the river Schelde.

All (unfinished) tubular pipes for the monopiles and the open steel pipes for the transition pieces are manufactured by SIF. This renowned roller mill specialist is based in Roermond in the Netherlands (Limburg province) and operates in a partnership with Smulders. SIF has the capability to cold roll steel pipes from steel sheets with a thickness up to 180 mm. Unfinished pipes are put on a ship for delivery to the Hoboken plant for further processing.

The monopile-transition piece combination was first applied during the Horns Rev project as a joint design effort by the Danish main contractor and offshore specialist, MT Højgaard, and Tech-Wise, the engineering consultancy division of Danish utility Elsam. A Belgian subsidiary of Smulders carried out the transition piece product engineering.

Financing the Q7 wind farm

Q7 is jointly owned and developed by subsidiaries of the Dutch utility, Eneco Energie, and renewable energy developers, Econcern and Energy Investment holdings (EIH).

Q7 project finance is provided by three international banks (Dexia, Rabobank and BNP Paribas) on a non-recourse basis. This means, in practice, that the banks rely solely on the project to generate the revenues needed to service the interest costs and principal repayment of the financing, with very limited additional sponsored support. They thus act as so-called Mandated Lead Arrangers (MLA). The Danish export credit agency, Export Krediet Fonden, supported the export of the Vestas turbines by participating in engineering the overall financing structure for Q7.

The financing arrangement also covers the wind turbine construction phase. According to a press release issued by Econcern on 3 March 2007, the new solution is unique in that it has resolved a serious bottleneck whereby the contractors had to bear the full (weather and technical) risks during the construction phase. And which in the past years has bogged down the realization of several offshore wind farms in Europe.

The total investment budget for the Q7 project is €383 million.


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