ABSTRACT

The circumferential welds on steel tubular berthing monopiles, namely pile splices, are potential fatigue hot spots. This paper presents the latest progress in linear elastic fracture mechanics (LEFM)-based fatigue safety assessment for welded splices of steel berthing monopiles. Particularly, the fatigue assessment against vessel impacts is studied at the current stage. Practical methods for determining stress intensity factor (SIF) and stress concentration factor (SCF) at the circumferential welds given bending moments are reviewed and summarized. This paper also proposes a new modification factor for Paris law, as well as a Beta distribution for characterizing the hot-spot stress range caused by vessel impacts, which is often doubly bound by operational water levels. The findings of this paper provides practical information for performing LEFM-based fatigue safety assessment of splice welds of steel berthing monopiles.

INTRODUCTION

Steel marine piles often work against cyclic and harsh operational and environmental loadings. Due to welding flaws or defects (undercuts, cracks, incomplete penetration, and gas pores), circumferential welds on steel piles, also known as splice welds, transversal welds, or girth butt welds, have been recognized as potential fatigue hot spots, given the frequently repeated loadings. Insufficient fatigue strength of welded pile splices may cause severe structural failures (Dailey et al., 1987a, 1987b; Weidler et al., 1987). This paper studies the fatigue reliability of circumferential welds of steel berthing monopoles. Fatigue loadings for a berthing monopile include berthing impacts, wave effects, current oscillations, etc. In many cases, for example well-sheltered harbours, the berthing impact dominates as a major fatigue loading for splice welds. The berthing impact normally leads to substantial bending moments on a monopile. This paper focuses on the cases dominated by berthing impacts. Typically, hot-spot fatigue strength can be assessed with either S-N curves or linear elastic fracture mechanics (LEFM).

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