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.

Offshore wind turbines are considered one of the most promising solutions to provide sustainable energy. The dominant majority of all installed offshore wind turbines are tied up to the seabed using monopile foundations. To predict the lifetime of these structures, reliable values for shape function and stress intensity factor are needed. In this study, finite element simulations have been performed for a wide range of monopile geometries with different dimensions, crack lengths as well as depths to evaluate shape function and stress intensity factor solutions for monopiles and an empirical equation is developed. The new solutions have been verified through comparison with the existing solutions provided by Newman & Raju for small hollow cylinders. The empir ical shape function solutions developed in this study are employed in a case study and the results have been compared with the existing shape function solutions. It is found that the old solutions provide inaccurate estimations of fatigue crack growth in monopiles and they underestimate or overestimate the fatigue life depending on the shape function solution employed in the structural integrity assessment. The use of the new solution will result in more accurate monopile designs as well as life predictions of existing monopile structures.

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