Suction Bucket or Caisson Foundations

Article published on Thursday, 07 January 2016

Suction caissons are best described as upturned buckets that are lowered into marine sediment, to anchor structures. This is done by pumping water out of ‘bucket’ to lower the pressure inside bucket skirt, this negative pressure and weight of foundation causes the foundation to sink into sea floor. This process is easily reversed for removal of foundation.

This type of foundation has been used in the oil and gas industry since the early 80’s to anchor floating structures to seafloor. Some installations taking place at depths of over 1500m

Benefits include:-

  • Low noise during installation
  • Reduced weight
  • Increased installation speeds
  • Ease of decommissioning
  • Little to no seabed preparation needed
  • Suitable for deep waters and large turbines
Two variants are currently being developed for the offshore wind sector, Mono Bucket by Universal Foundation and Suction Jacket Foundation by DONG energy

Mono Bucket (Universal Foundation)


Source:
Universal Foundation

The all-in-one Mono Bucket structure consists of a large monopile type structure that is welded to the top of a single giant suction bucket.

The Universal Foundation concept was one of four foundation designs selected in The Carbon Trust’s Offshore Wind Accelerator programme.
Installation

The Mono Bucket foundation is installed using a jet and suction system as the driving force. Lowering the pressure in the cavity between the foundation and the seabed generates water flow, which lowers resistance around the edge of the foundation’s skirt. This reduces resistance and allows seabed penetration.

A patented jetting system controlling vertical alignment ensures the foundation is installed within prescribed tolerances, removing the need for a transition piece to adjust verticality.

Source:
Universal Foundation

Benefits:-

  • Can be installed in a wide variety of site conditions, including sand, silt, clay and layered strata.
  • A patented jetting system controlling vertical alignment ensures the foundation is installed within prescribed vertical tolerances, removing the need for a grouted transition piece to adjust verticality.
  • Installation does not (in most cases) require seabed preparation.
  • Installation is noise free and offers no risk to marine mammals.
  • The foundation can be easily removed by reversing the process used during installation.
  • Single-lift or self-floating installation
  • Designed for water depths of up to 55m
Track Record

2002 August - Frederikshavn, Denmark

Prototype of suction bucket installed onshore to support a Vestas V90 3MW turbine. This project was delivered by MBD Offshore Power (Universal Foundation) and ELSAM (DONG Energy) in cooperation with the Aalborg University.

  • Bucket diameter 12m
  • Height of bucket 6m
  • Total weight 135 tonnes
2009 March - Horns Rev 2 Meteorological Mast, Denmark

Universal Foundation in cooperation with DONG energy installed a mobile meteorological mast with a suction bucket foundation at Horns Rev 2 site. The foundation was fabricated by Bladt Industries in Aalborg in 2008. Foundation was floated out to location, prior to installation.

Met mast decommissioned and completely removed from site on 7th July 2015, as mast was no longer required by DONG energy. Removal method was to reverse the installation method, by increasing pressure within the bucket structure of foundation.

2013 September - Dogger Bank 2 x Meteorological Mast, UK

In October 2011, Universal Foundation was awarded the contract for the turnkey installation of two meteorological masts at Forewind’s Dogger Bank offshore wind farm.

Both of the Mono Buckets were installed successfully within the prescribed tolerances and achieved cost, time saving and noise reduction targets.

2014 September North Sea Trial Installation, UK

As part of The Carbon Trust’s Offshore Wind Accelerator (OWA) programme trial installations took place at three UK offshore wind sites - Dogger Bank, Dudgeon and Hornsea. 29 installations were carried out in 24 days.

The trial was to assess foundation penetration, verticality, water injection impact on soil plug, forces and stress in skirt structure and internal soil levelling capability.

It is understood that the Mono Bucket coped very well with multiple installation-retrievals and the differing sediments, as soil characteristics within the three sites varied from soft clay, moraine clay, boulder bank clay – with sand spikes and layers, clay crust, sand and silt.

Two suction bucket foundations were used in trial, one measuring 8m in diameter with a 6m skirt and a reference structure of 4m diameter with 6m skirt.

Suction Bucket Jacket (DONG energy)

The suction bucket jacket design consists of three legs welded together in a lattice jacket structure that stand atop of three giant suction bucket foundations.

Structure is lightweight and requires a single operation when installed offshore.

The Suction Bucket Jacket was designed by DONG Energy and based on ideas from the SPT Offshore foundation, a finalist in a global competition run by the Carbon Trust to find lower cost foundation solutions.

 
Source
: Dong Energy
Benefits:-

  • Significant reduction of the foundation cost, due to minimal seabed preparation.
  • Environmentally friendly installation process.
  • Transport and installation in one procedure.
  • Suitable for water depths of 30-60m
Installation process
1.        The suction bucket jacket is lowered to the seabed
2.        An under pressure is applied to the buckets by individual suction pumps
3.        The hydrostatic pressure difference and the weight of structure causes the buckets to penetrate the soil
4.        The suction pumps are recovered to the installation vessel
5.        A thin layer of concrete is injected between seabed and bucket lid
Track record

2014 October Borkum Riffgrund 1 Germany

Full scale test with Siemens SWT-4.0-130 turbine at Borkum Riffgrund 1 offshore wind project in Germany, site was selected due to its dense sand seabed, which created a challenging installation situation. Installation was carried out in water depth of 25m.

DONG energy and The Carbon Trust have collaborated on this full scale demonstrator, and received 6m GBP in funding from The Carbon Trust through the Offshore Wind Accelerator programme (OWA)

Borkum Suction Bucket Jacket Specifications

  • Weight
    • Transition Piece: 130 t
    • Jacket: 372 t
    • Buckets: 250 t
    • Installed weight: 752 t
  • Footprint: 23 x 23 m
  • Top: 16 x 16 m
  • Buckets: 8 x 8 m
  • Total height: 56.6 m

Call +44 (0)1502 307 037 for more information

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