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Challenge

Grouted connections are widely used in support structures for offshore wind turbines. The grouted connection relies on high-strength grout filling the annulus between jacket legs/piles, monopiles/transition pieces, etc. Early Age Cycling (EAC) refers to the relative displacement due to wave and current loading during curing of the grout material.

Excessive EAC may compromise the grout material performance and consequently the asset integrity of the wind turbine foundation. The effect of EAC is a relatively immature and undeveloped topic in the industry, and few dedicated tests have been conducted to understand the phenomena. The database used for evaluating the effect of EAC is still limited and often based on small scale tests not applicable for the increasing size of the offshore wind turbine structures.

Objective

Currently DNV-ST-0126 limits the effect of EAC by specifying that relative movement during initial phases of grout curing shall be limited to maximum 1 mm. Preventing excessive EAC displacements may lead to significant additional costs and prolonged installation times for offshore wind projects. There is a large push from the offshore wind industry that the 1 mm criterion is relaxed, while at the same time limited conducted EAC has demonstrated that EAC influences the grouted connection capacity. In addition, there are currently several different offshore grouted connection types and a lot of variables that all affect the extent of damage due to EAC, such as geometries, loads, displacements, materials, temperatures, etc.

It is therefore our objective through this JIP to update EAC guidelines currently outlined in DNV-ST-0126 standard for the most used grouted connections in the industry. DNV also see the need to develop a reliable small scale testing methodology for screening of grout materials to ensure material properties are adequate for EAC.

Deliverables

Based on input from our partners, DNV is set on determining the most efficient way to acquire the necessary knowledge to improve upon existing EAC guidelines. DNV propose that a large-scale testing regime is a viable approach to isolate the effects of the effect of the critical factors influencing EAC.

We plan on carrying out the JIP in several stages, where the first stage (Stage 1) will be to establishing key objectives, the testing scope and schedule with the end goal of having an established test plan to be executed for the next stage (Stage 2). The end goal of the JIP is to establish an analytical design approach that accounts for the effects of EAC for displacements larger than 1 mm for the most used grouted connections. A design approach will limit the need for expensive and time-consuming project specific testing for typical offshore grouted connections constructed today.

Project details

Grouted connections are the cornerstones of offshore wind turbines as they secure the structure to its foundations, and we see a huge interest from the industry to join our JIP. At the launch of the JIP, participants from CoP, COWI, DEME, ITW PP, Kent, MBS, Rambøll, RES, SAIPEM, Steisdal, Swancor Renewable Energy and Ørsted expressed the need for and importance of this JIP. DNV would therefore like to invite relevant parties to join in the discussion to define these variables in such a way that it benefits the most used offshore grouted connection designs.