Abstract
The use of bolted joints to connect the transition piece and monopile is nowadays widely applied in the offshore wind industry. Traditionally, grouted connections were used in the early generation of offshore wind turbines, but the experienced failures in such connections led to an increased tendency towards bolted flange connections to join the transition piece and monopile in the new generation of offshore wind turbines. The bolts used for this purpose have high strength and huge sizes, and are subjected to a preload that is applied during the tightening process. The present study is focused on the analysis of preload effects on stress distribution in M72 bolted connections by considering different friction coefficients between the bolt and nut threads. The bolt is considered to be made of grade 10.9 steel, whereas the nut is assumed to be made of grade 8.8 steel, which is a softer material. Using the finite element commercial software package Abaqus, numerical models were developed and analysed to establish trends for stress distribution and plastic strains during the bolt tightening process, and to quantify stress concentration factors in individual engaged threads.
Highlights
The connections between the offshore wind turbine (OWT) foundation and the transition piece (TP) are key mechanisms to enhance the stability and integrity of these offshore structures
One of the main structural integrity challenges in the offshore wind industry is the presence of wind, wave, and current forces, which are constantly acting on the structure, with variable amplitude cyclic loads causing fatigue and corrosion damage in OWTs [1,2], under adverse weather conditions such as thunderstorms and gales
There are three main connection types that have been developed during the last decades for MP-TP connections: (a) grouted connections, (b) slip joint connections, and (c) bolted flange connections
Summary
The connections between the offshore wind turbine (OWT) foundation and the transition piece (TP) are key mechanisms to enhance the stability and integrity of these offshore structures. In 2008, some problems started to appear related to the use of these grouted connections in wind turbines located at the Princess Amalia wind farm [5] due to vertical settlements at the grouted joint between the MP and TP This event led to significant repair, replacement, and financial loss of around €47 million. In order to prevent such failures related to grouted connections, conical shaped grouted connections were designed to avoid settlement issues [7], and it was proven that the implementation of shear keys efficiently reduced damage in the grout [8] These shear keys (see Figure 1) provide extra resistance against slipping and consist of circumferential welds on both TP and MP surfaces.
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