While industry partners have joined forces with the aim of updating existing knowledge of design practices for large diameter grouted connections between monopiles and transition pieces with shear keys, Norwegian Trelleborg Offshore has sold 2,120 elastomeric bearings to offshore wind farms. OceanWise takes a look at how these bearings work.
By Tea Tramontana
When grouted connections between monopiles and transition pieces some three years ago were found to slip in some offshore wind farms, an elastomeric spring bearing was developed to improve the load carrying capacity. The company behind the bearing, Trelleborg Offshore Norway, has supplied bearings to five sites so far.
“If no bearings were fitted in the construction phase and slippage in the grout has occurred later, the elastomeric bearings can be retrofitted by welding new brackets to the inside of the transition piece”, says Erhard Christensen, sales manager at Trelleborg Offshore Renewables. He explains that the solution is used on installed wind farms where grout problems occur but also as a precaution:
“At the start of the construction process, the bearings can be fitted so that they are unloaded and just resting at the top of the monopile. If slippage in the grouted connection occurs at a later stage, the bearings will then gradually be loaded as they assist the grout in supporting the weight of the transition piece and tower assembly.”
A third function of the bearing is preloading so that they carry the static vertical load from the start with the aim of preserving the strength of the grout.
The idea behind an elastomeric bearing is to accommodate axial, shear and rotational movement. The bearing developed for the support of the grouted connection on offshore monopile foundations consists of a number of interleaved stainless steel plates, vulcanized to layers of synthetic rubber. This construction gives the offshore bearing a high load carrying capacity in the vertical direction, while remaining flexible in the lateral direction. In order to preserve the stability of the structure some of the interleaf plates are connected to two anti-buckling bars at the front of the bearing. Graphic: Trelleborg Offshore
The bearings are located inside steel brackets welded to the inside of the transition piece. For a normal monopile foundation six bearings are needed. Graphic: Trelleborg Offshore
Over 2,000 bearings offshore
The 88-turbine site Sheringham Shoal in the North Sea which will be commissioned later this year was trapped in the 2011 revision of the design standards for offshore monopile wind turbine structures. A joint industry project led by the certifying body DNV came out with a non-recommendation of the cylindrical shaped design of grouted connections without additional support arrangements reducing the axial load.
The solution to keep the Sheringham Shoal project on schedule was the elastomeric bearings. Four other sites followed: Belwind with its 55 turbines in the North Sea off the Belgian coast. The site was commissioned in 2010. The 140-turbine site Greater Gabbard had 120 bearings delivered to the site which will be commissioned later this year, while 960 bearings are in production for the 160-turbine site Gwynt y Môr in the Irish Sea due for commissioning in 2014. In addition, Trelleborg Offshore is producing 158 bearings for Rhyl Flats which is a 25-turbine site in the Irish Sea, commissioned in 2009.
DNV-conducted evaluation of design standards
DNV is currently heading a joint industry project where partners from industry evaluate the design of large diameter grouted connections with shear keys. Shear keys are circumferential weld beads on the outside of the monopile and the inside of the transition piece in the grouted section. The purpose of the shear keys is to increase the sliding resistance between the grout and steel so that no settlement occurs. The existing design standards for such connections are based on limited test data for alternating dynamic loading, thus industry partners are now developing a design practice for shear keys in order to properly incorporate them into design standards.
“Shear keys have been used in the oil and gas industry for years. Hopefully the work in the industry group will show that shear keys are a solution and the result will be a description of how to design a large diameter grouted connection with shear keys”, says Claus Fridtjof Christensen, Head of DNV Wind Energy Certification.
DNV expects the report from the joint industry project in the coming months and the update of the OS -J101 standard on design of offshore wind turbine structures before spring 2013.