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
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
• 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
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.
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
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
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
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.
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
Bank Phase II, Ormonde
Ost Offshore wind farms.
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
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.
Demonstrator was the first wind farm in using
Various stages of the installation at the Beatrice Demonstrator
Jacket Design Variations
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
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).
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.
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.
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
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
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
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.
Standard: ‘Design of Offshore Wind Turbine Structures, DNV-OSJ101’, Det
Norske Veritas (2010) Project
Types - E.ON Beatrice
Demonstration Project Twisted
jacket - Carbon Trust OWEC