Abstract
The installation of offshore wind turbines has achieved rapid and substantial progress worldwide and further increase is predicted by enhanced technologies that will reduce costs and increase service time. Secondary structures applied to the primary structure, as the transition piece of a monopole, can be e.g. cable support, boat landing or anode systems. These structures are often welded, which leads to problematic notch effects and hydrogen embrittlement, especially for underwater applications. Also the handling of technical equipment as power current or artificial housings for scuba divers for underwater welding is challenging. Adhesive bonding will lead to cost reduction as the mentioned negative aspects can be avoided. The corrosion protection coating and the primary structure will no longer be damaged and therefore do not need a subsequent coating. This article focuses on the area which is permanently exposed to water. A critical point is how the capability to form adhesion and cohesion, will be influenced by the application process under water. Therefore, stud bonding fasteners are designed that enable the injection of adhesive to the bonding area under water. The load capacities for different adhesives, surface pre-treatments and the degradation by exposure to artificial sea water were investigated. Adhesion was achieved with two different adhesives, which were able to cure and realize reasonable strength under water. Furthermore, two selected coating systems were able to improve the performance of the adhesive bond.
Highlights
Due to the considerable development of the offshore wind energy industry in recent years [1,2,3], especially in Germany and Great Britain [4], the question arises whether load-bearing underwater bonds can further optimize offshore support structures by replacing welded joints
How the glass transition temperature and stiffness under water develop over a longer period of time remains an open question, as none of both adhesives had fully saturated with water after 125 days of storing
The results have shown that adhesive bonding under water is feasible, and that it might represent a good alternative to commonly used joining techniques
Summary
Due to the considerable development of the offshore wind energy industry in recent years [1,2,3], especially in Germany and Great Britain [4], the question arises whether load-bearing underwater bonds can further optimize offshore support structures by replacing welded joints. Secondary structures (e.g. ladders, railings, cable ducts, etc.), if they have to be retrofitted under water, are attached to the main supporting structure by welding, bolting or riveting [5]. These joining techniques lead to notches on the main load-bearing structure and cause unfavorable local stress peaks. Other methods, such as friction stitch welding or laser welding, are still under development; these can be performed automatically by underwater robots [6,10]. Mechanical clamping - with clamping pieces - is an alternative to welding, but for geometric reasons it is of limited use and highly impracticable, due to large diameters needed for offshore wind structures
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