The fabrication and installation of foundations for an offshore wind farm can represent a significant proportion of development spend. With ambitious UK Round 3 plans now well underway there is an industry wide recognition that foundation costs must be brought down to ensure economic viability of projects going forward. A number of innovative foundation designs that offer potential savings in weight and installation time (and thereby in cost) have emerged in recent years and these are being evaluated by industry.
In 2011, Mainstream, with the support of the Carbon Trust elected to be the first to use an innovative foundation concept (the Keystone ‘Twisted Jacket’) as the foundation for a met mast at the Hornsea site being developed in partnership with Siemens Financial Services (SMart Wind). The Twisted Jacket was one of four winners selected from 104 entries in the Carbon Trust’s Offshore Wind Accelerator (OWA) foundation competition.
In taking this lead, Mainstream received a grant award from the Carbon Trust towards the project. In collaboration with University College Dublin (UCD), the company also received a research grant from Enterprise Ireland to monitor the performance of the foundation. The monitoring was also supported by DONG Energy, an OWA member who participated as an observer in the project under an industry collaboration promoted by the Carbon Trust.
The project timeline was very ambitious. A Letter of Intent was given to Hochtief AG in early 2011 to allow designer Keystone to commence the design works. The design was to accommodate the 30m water depth at the site with a 50-return period design maximum wave height of 12m and ground conditions comprising stiff clay over dense sand as revealed in a borehole conducted at the location in 2010. An EPCI contract was signed with Hochtief and fabrication of the foundation commenced at Bladt Industries in Denmark.
The central element of the design features a lattice steel ‘Guide Structure’ (yellow in the
image – click image to enlarge) that fits over a central pre-driven pile termed the ‘Caisson’ (brown in the image – click image to enlarge). The Guide Structure sloping legs act as sleeves through which piles are installed and driven into the sea-bed. The final stage of the operation involves grouting the annuli between the Guide Structure and the caisson and between the piles and the pile sleeves. Because of their length the sloping piles come in two overlapping parts for ease of transportation and handling. During the fabrication works at Bladt, structural monitoring instrumentation was attached to the foundation components to allow monitoring of the stresses and strains in the foundation in operation, and thereby study the performance of the innovative structure.
The fabricated structure elements shipped out on a barge from Aalborg in early September and linked up with the installation vessel the JB-114 at Rotterdam before commencing the final leg of the journey to the Hornsea site. Generally favourable weather conditions were encountered for the installation that commenced on 19th September. Marine mammal observers confirmed the absence of any marine mammals in the vicinity before commencing of pile driving. Unforeseen ground conditions are seen as a major risk in offshore construction, however the 64.5m long, 95T weight central caisson pile was driven to the design depth of 25m below sea-bed without difficulty, validating the geotechnical investigation work in advance of construction. The guide structure, approximately 30m high was then lifted over the central caisson. The lift was a difficult operation but was well planned and executed. The P piles, each with a total length of 70.8m when connected were then driven in turn through the pile sleeves to provide rigidity to the structure. The final operation of grouting the structure was confirmed successful when grout emerged through a port at the top of the piles.
The completed structure (pictured) is a great tribute to the team that worked to a very tight timescale. At the time of writing, the met mast is providing valuable wind data. The early indications from structural monitoring instrumentation are showing that the foundation is behaving as expected. Commenting on the installation of the Met Mast, Phil de Villiers, Offshore Wind Accelerator Manager for the Carbon Trust said ‘We have been very impressed with the commitment shown by all parties in delivering the project, and we would like to thank Mainstream and SMart Wind for demonstrating the Twisted Jacket.”
More photos of the ‘Twisted’ jacket met mast can be sourced from Mainstream’s flickr account here.
Bernard Casey
Head of Civil Engineering – Offshore
Mainstream Renewable Power