Jacket or Lattice Structures

Article published on Wednesday, 12 June 2013

There are many variants of the three or four-legged jacket/lattice structure typically consisting of corner piles interconnected with bracings with diameters up to 2m. The soil piles are driven inside the pile sleeves to the required depth to gain adequate stability for the structure. The tubular joints are welded.

These types of structures are considered well suited for sites with water depth ranging from 20-50m according to the DNV. The minimum is 3.5m at the South Korean offshore wind farm Tamra and the maximum depth for an operational project is 45m on the Beatrice Demonstration project. Other projects in the plannng pipeline are suggestion using jackets in water dephs up to 60-70m but these have yet to be consented.

Jacket Support Structure

Jacket Support Structure

The transition piece forms the connection between the main jacket and the tower of the wind turbine. Loads are transferred through the members mainly in axial direction. The large base of the jacket structure offers large resistance to overturning.

The secondary steel includes the work platform, ladders and stairs, access systems, J-tube, cables, and corrosion protection systems.

Proponents cite the advantages of the jacket structures as:

• Low wave loads in comparison to monopiles (the jacket structure is very stiff and the area facing the wave movement is smaller than monopiles) • Fabrication expertise is widely available, in part due to Offshore Oil and Gas industry supply chain

Others cite disadvantages as:

• High initial construction costs and potentially higher maintenance costs • Transportation is moderately difficult and expensive

Installation

In general, the jacket structures are attached into the seabed through piles. There are two methods to install the foundation of the jackets: pre-piling and post-piling.

Pre-piling

Prepiling jacket foundations

Prepiling jacket foundations

During pre-piling, subsea templates are used to ensure the correct positions of the piles, including inclination. Once the piles are inserted into the seabed, the jacket is lowered and fitted into the piles.

Proponents of pre-piling cite advantages as:-

• Pile sleeves and mud mats are eliminated on the jacket • Piles can be installed well in advance of the jacket and wind turbine generator installation • Fabrication of jacket can be conducted in parallel with piling operations • Seabed variations can be levelled by varying pile stick-up • Seabed levelling ensures standardisation of jackets • Verticality of jacket is easily arranged

Alpha Ventus was the first wind farm where the pre-installed pile method was used. A centre template was used to secure the position on seabed, after which a slot was moved around the centre template for the four pile locations. The slot provides vertical stability for the piles and the piling hammer can be lowered on top of the pile.

Slot template used on Alpha Ventus

Slot template used on Alpha Ventus

Piles were loaded on-board and up-ended prior to lifting the piles into the slot on the seabed. The piles were hammered down using a subsea hammer.

Alpha Ventus helped establish that pre-installed piles increases the operational weather window and reduces installation time and risks. Other example projects where jacket foundations have been used are Thornton Bank Phase II, Ormonde and Nordsee Ost Offshore wind farms.

Post-piling

This method is the installation of piles through the sleeves located on the jacket legs. Grouting is usually used to fixate the sleeves to the piles and provides the connection to transfer the loads from the jacket leg to the pile. As well as the sleeves mounted on the jacket, the post-piling process requires mud-mats which transfer loads to the seabed acting as a support foundation while the piles are being installed.

The post-piling process has not been commonly adopted for use on offshore wind farms. The post-piling process has only been used at the Beatrice Demonstrator Offshore Wind farm.

Beatrice Demonstrator Project

The Beatrice Demonstrator Project involved two 5MW turbines situated in 45m water depth. The export cables link the turbines to the nearby Beatrice oil field production platform.

The turbines were installed in two phases as shown in the figure below. The jacket support structure was transported to the offshore location on a barge. On site, the heavy lifting vessel Rambiz used its two cranes to lift the 730t structure off the barge, tilting it into an upright position. Subsequently, the support structure was lowered onto the seabed and levelled.

The second part of the installation procedure involved installing the entire wind turbine, including the turbine tower, in one lift. The turbine was pre-assembled onshore on top of a soft landing system and lifted off the quayside using a specially designed lifting frame. At the offshore location the turbine was joined with the support structure, where the soft landing system compensated the motion of the turbine assembly during the set down phase. Finally, the lifting frame and the soft landing system were removed to complete the installation procedure.

Beatrice Demonstrator was the first wind farm in using jacket foundations.

Various stages of the installation at the Beatrice Demonstrator Project

Various stages of the installation at the Beatrice Demonstrator Project

Jacket Design Variations

OWEC Quattropod®

OWEC Quattropod® is a complete wind turbine substructure (transition piece and jacket foundation) particularly suitable for larger turbines, deeper water and demanding soil conditions. It has been developed by OWEC Tower.

OWEC Quattropod® is:

• Installed in four offshore wind farm projects • Considered fabrication friendly and proven for serial production • Designed for adaptability to alternate seabed fixing including suction buckets

OWEC Quattropod®

OWEC Quattropod®

Offshore wind farms where it has been installed are: Beatrice Demonstrator (2 units), Alpha Ventus (6 units), Ormonde (30 units) and Thornton Bank II (48 units + 1 offshore transformer substation).

Hochtief Solutions

The Hochtief solutions was developed on the EnBW Baltic II project. The foundation had a water depth of 23-44m and was designed as part of Hochtief's input into the Joint Venture HGN (Hochtief Construction, GeoSea, Nordsee Naßbagger) which won the contract for design, production, transport and installation of foundations.

Hochtief Foundation

Hochtief Foundation

ATKINS/BIFab Jacket

This jacket was developed as part of an initiative with Scottish and Southern Energy PLC (SSE) for “a framework of key suppliers to help to drive efficiencies in its offshore wind programme”. Atkins, in combination with Burntisland Fabrications Limited (BiFab), developed a transition piece concept for wind turbine jacket foundations which proponents argue is intended to cut costs by 30-40%. Developed in 2010, the connection detail features shear plates to join the mast transition to the jacket legs, distributing bending moments into the tubular jacket. The concept is suitable for all current and envisaged turbine sizes and any water depth in excess of 15m.

ATKINS/BIFab Jacket

ATKINS/BIFab Jacket

Twisted Jacket Foundation

The twisted jacket foundation designed for 30-60m water depths, has become the first design to be installed through the OWA (Carbon Trust’s Offshore Wind Accelerator) in a demonstration project. It was jointly funded by Mainstream Renewable Power, DONG Energy and the Carbon Trust.

The ‘twisted jacket’ uses less steel than a conventional jacket and has fewer components. Installation costs could well be lower because potentially more units can be fitted onto and installation barge due to its relatively small footprint.

The foundation has been used to support the first met mast to be installed for Round 3 on the Hornsea site.

ATKINS/BIFab Jacket

Twisted Jacket Foundation

Jacket Structure Concerns

Jackets were seen to overcome the perceived design limitations of monopiles. However monopile designers have “fought back” leading to developers of some consent authorised projects deciding to use supersize monopiles instead of jackets structures. An example of this is RWE Innogy at Innogy Nordsee I offshore wind farm.

The experience in increasing monopile diameters has so far been positive, making larger monopiles a potentially more cost effective solution, according to the fabricator G&G. This trend is contrary to the industry expectations a few years ago where it was considered that monopiles could not support the increasing weight of the larger turbines.

It is difficult to say if the jacket structure will have an increasing demand in the future, although some developers like DONG Energy foresees more wind farms planned with jacket foundations in the next five years.

References

Offshore Standard: ‘Design of Offshore Wind Turbine Structures, DNV-OSJ101’, Det Norske Veritas (2010) Project UpWind Foundation Types - E.ON Beatrice Demonstration Project Twisted jacket - Carbon Trust OWEC Quattropod®

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