Analysis of Tightening Sequence Effects on Preload Behaviour of Offshore Wind Turbine M72 Bolted Connections

Braithwaite Braithwaite,Mehmanparast Mehmanparast

doi:10.3390/en12234406

Abstract

Offshore wind turbines in shallow waters are predominantly installed using a monopile foundation, onto which a transition piece and wind turbine are attached. Previously, the monopile to transition piece (MP-TP) connection was made using a grouted connection, however, cases of grout failure causing turbine slippage, among other issues, were reported. One solution is to use bolted ring flange connections, which involve using a large number of M72 bolts to provide a firm fixing between the MP-TP. It is in the interest of offshore wind operators to reduce the number of maintenance visits to these wind turbines by maintaining a preload (Fp) level above the minimum requirement for bolted MP-TP connections. The present study focuses on the effect of the tightening sequence on the Fp behaviour of M72 bolted connections. A detailed finite element (FE) model of a seven-bolt, representative segment of a monopile flange was developed with material properties obtained from the available literature. Three analyses were made to examine the effect on Fp after tightening, including the initial Fp level applied to the bolts, the tightening sequence and the effect of an additional tightening pass.

Highlights

Offshore wind turbines using monopile foundations require two main connection points to secure them: the monopile to transition piece (MP-TP) connection and the transition piece to wind turbine (TP-WT) connection, both which often utilise bolted ring flange connections
A three-dimensional (3D) finite element (FE) model was developed based on the geometry of an offshore, monopile, A three-dimensional
FE the model was developedsequence based on geometry in ofM72 an offshore, bolted-flange connection, to(3D)

Summary

Introduction

Offshore wind turbines using monopile foundations require two main connection points to secure them: the monopile to transition piece (MP-TP) connection and the transition piece to wind turbine (TP-WT) connection, both which often utilise bolted ring flange connections. Turbine ring-flange connections are usually fastened with prestressed, high-strength bolt assemblies (HV-sets) with large diameters [1]. Due to the high loads experienced at the transition piece connections, bolts with large diameters of up to M72 are used. Even though applicable design standards do not explicitly exclude the application of large diameters, the current knowledge and experimental validation is limited to bolts with diameters of up to M64. The behaviour of such large bolts is mostly unknown, making them an important area of research for which further knowledge is required. Data from smaller bolts cannot be used accurately to estimate the behaviour of larger bolts due to the geometrical size effect, as the stress gradient from the threaded surface to the centre of the bolt increases with diameter [2]

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