Our experience with suction buckets jacket

Introduction

Monopiles (MPs) are currently the most commonly used foundation solution for offshore wind turbines with 82% of offshore wind turbines in European waters founded on MPs at the end of 2018 (EWEA, 2019). Where site conditions do not allow for an efficient or practical MP design, a number of alternative foundation solutions are available, including the suction bucket jacket (SBJ), piled jacket, gravity base or even a floating solution.

Therefore, the SBJ is one of a range of alternative foundation solutions to the more commonly used monopile foundation solution for locations where the MP solution is not appropriate. All of the above listed foundation solutions have successfully been used to support offshore wind turbine generators (WTGs), with the choice of foundation solution often dependent on site-specific conditions such as water depth, ground conditions and country specific requirements.

In general, there is limited industry experience in the design, fabrication and installation of SBJs compared to the more common MP foundation solution. Installing monopiles is a complicated undertaking, but due to the high degree of experience gained with this foundation type, the complexity has become well understood and manageable in practice. In contrast, the installation process for SBJ structures is yet to become standard practice and is thus considerably more complicated in practice than the installation process of monopiles. 

This memorandum provides a very brief background to the use of SBJs as a foundation solution for WTGs. It provides a brief description of suction buckets for windfarm applications, the limitations for the use of suction buckets and a summary of Ørsted’s experience with suction buckets as a foundation solution. 

Brief description of suction buckets for windfarm applications

Suction installed foundations, referred to as suction buckets, suction caissons, suction piles or suction anchors, have been widely used in the offshore industry since the early 1980’s for a range of applications. These foundations, normally made from steel or concrete, are installed using the principles of suction whereby the pressure difference generated between the inside of the bucket and the water surrounding it (at the seabed) leads to the structure being installed without any use of mechanical force. Therefore, a key difference between suction installed foundations and other foundation types is that the installation design, which must consider the soil type, soil strength, installation specific risks (for example, the presence of boulders or other hard inclusions) and the installation process (for example, the speed of installation), have a direct influence on the dimensions of the foundation. 

More recently, suction buckets have been deployed in the offshore wind sector with installations taking place at the Borkum Riffgrund 1 (2014; one position), Borkum Riffgrund 2 (2018; 20 positions) and Aberdeen Bay (2018; 11 positions) offshore windfarms. For this application, three suction buckets are used to support a ‘jacket’ structure, most commonly referred to as a ‘suction bucket jacket’ (SBJ).

SBJs for windfarm applications differ significantly from typical oil and gas suction assisted installations (such as suction anchors) as they:

  • are connected rigidly to a structure (such as a jacket)
  • are installed in relatively shallow waters (less than 100m water depth),
  • predominantly carry vertical loads (and relatively small moment and horizontal loads) which results in the behaviour being very similar to a shallow foundation, and
  • have a large overall footprint  and a low suction bucket ‘length to diameter’ ratio (L/D ratio), meaning that they generally cover a large spatial area whilst maintaining a small embedment into the soil.

Limitations summary

As the monopile is the most common foundation solution for supporting offshore WTGs, comparisons are often made between these two foundation solutions. In brief, the SBJ’s limitations when compared to the monopile are that:

  • they have a significantly larger footprint (approximately 30-40 m in diameter) and require more scour protection,
  • they are not necessarily able to be installed at locations with large sand waves or high seabed mobility (due to their shallow embedment),
  • there are installation challenges in shallow water (less than 20m),
  • the installation process is highly dependent on soil type and soil strength, 
  • the installation process is potentially riskier due to the larger volume of soil in contact with the structure (higher risk of ground variability, hitting a boulder or encountering a ‘hard inclusion’) and a lack of available proven mitigation options, 
  • installation experience is limited,
  • manufacturing experience and scale is limited; and
  • the overall cost may be higher. 

 

Ørsted experience with suction buckets

Whilst there is limited offshore wind industry experience relating to the design and installation of SBJs, Ørsted has been an industry leader in the development of SBJ technology for application in the offshore wind environment. Ørsted’s development of the SBJ foundation solution aims to provide flexibility in choosing the right foundation solution for any given offshore wind farm development.   

Ørsted installed the world’s first SBJ for an offshore WTG at the Borkum Riffgrund 1 offshore windfarm in Germany in 2014. 

Since the installation of the Borkum Riffgrund 1 SBJ, Ørsted has been involved in the design and installation of SBJs at the Borkum Riffgrund 2 and the design for Hornsea 1 offshore wind farms. At Hornsea 1, overall project timeline considerations and limitations of serial production capacities precluded the use of SBJs, and therefore the project chose an alternative foundation type. 

It is within this context that Ørsted has been an industry leader in the development of SBJ technology, through research and partnerships, to ensure a range of foundation solutions are able to be considered for each project. 

 

Ørsted publications

A significant volume of published information is available in the literature relating to suction buckets. The published information relates to both the installation of suction buckets and the behavior of suction buckets under different loading conditions. Initial research focused on the use of suction buckets for oil and gas applications (for example, Bye et al, 1995 and Erbrich & Tjelta, 1999), but more recently, the focus of the research has been targeted towards the use of suction buckets for offshore wind applications (for example, DTI, 2005, Houlsby and Byrne, 2005, Achmus et al, 2013, Tjelta, 2014 and Shonberg et al, 2017). References within this conference proceeding can be found in the full text available in the download box below.

The following publication relating to SBJs authored by Ørsted employees (or those directly associated with Ørsted) have been published over the past two years:

Harte, M., Shonberg, A. (2018)
“Reliability based installation design of a suction caisson in clay”
Proceedings of the 1st Vietnam Symposium on Advances in Offshore Engineering (Energy and Geotechnics), 1-3 November 2018, Hanoi, Vietnam (submitted, under review)

Shonberg, A., Harte, M., Aghakouchak, A., Andrade, M.P., Brown, C.S.D., Liingaard, M.A. (2017) “Suction bucket jackets for offshore wind turbines:
applications from in situ observations”, 
Proceedings of the TC209 Workshop at the 19th International Conference on Soil Mechanics and Geotechnical Engineering, 20 September 2017. Seoul, South Korea.  

Shonberg, A., Anusic, A., Harte, M., Schupp, J., Meissl, S., Liingaard, M. A (2017)
“Comparison of Self Weight Penetration Prediction Methods for Large Diameter Monopiles in North Sea Soils”, OTC-27763-MS. Proceedings of the Offshore Technology Conference, 2 – 5 May, 2017, Houston, Texas.

Surysentana, S., Byrne, B. W., Burd, H. J., Shonberg, A. (2017)
“Weighting functions for the stiffness of circular surface footings on multi-layered non-homogeneous elastic half-spaces under general loading”
Proceedings of the 19th International Conference on Soil Mechanics and Geotechnical Engineering, 17 - 21 September 2017, Seoul, South Korea. 

Suryasentana, S., Dunne, H., Martin, C., Byrne, B. W., Burd, H. J., Shonberg, A. (2018)
“Assessment of numerical methods for determination of shallow foundation failure envelopes”, Geotechnique (submitted for review).

Surysentana, S., Byrne, B. W., Burd, H. J., Shonberg, A. (2017)
“An elastoplastic 1D Winkler model for suction caisson foundations under combined loading”
Proceedings of the 9th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE), 25 - 27 June 2018, Porto, Portugal.

Surysentana, S., Byrne, B. W., Burd, H. J., Shonberg, A. (2017)
“Simplified model for the stiffness of suction caisson foundations under 6 DOF loading”
Proceedings of the SUT OSIG Conference, 12 - 14 September 2017, London, UK. 

 

Our experience with suction buckets