Numerical Analysis of Contact Stresses Between Mooring Chain Links and Potential Consequences for Fatigue Damage

Abstract

Design codes for offshore mooring systems recommend proof loading chain links to around 70% of the specified breaking load of the chain (API RP 2FP1, Lloyd’s Register). This is primarily to check that the chain will safely resist the service loads and will not excessively elongate. It is assumed that the proof load also generates compressive residual stresses at the interlink contact region and also at the point of the intrados (KT point) where a high stress concentration occurs during tensile loading. Tests have shown that proof loading improves the fatigue performance of chain under cyclic axial loads. Elastic-plastic finite element analyses of the proof loading have been performed. These analyses have shown that the proof loading also generates very high tensile residual stresses in the region surrounding the interlink contact zone. This region also experiences significant in-service cyclic stresses under cyclic tension or out-of-plane bending. The combination of the cyclic stresses and high tensile residual stress is of concern and it is proposed that the periphery of the interlink contact zone should be carefully reviewed. It is understood that chain link fatigue at present is only based on the risk of fatigue damage at the KT point. This paper presents and discusses results of finite element stress analyses of studless chains of different sizes and grades, and show the relative fatigue sensitivity of the KT and contact regions. The chain grade, dimensions and loading regime are shown to be important.

Multiaxial Fatigue Analysis of Mooring Chain Links Under Tension Loading: Influence of Mean Load and Simplified Assessment

“…This chapter continues the work presented by [27] This chapter is divided as follows. First, the mechanical properties of mooring chain steel are outlined.…”

“…The nominal dimensions of the chain are given as a function of the diameter [151], as reported in Figure 5.1. This figure also illustrates the fatigue failure locations of chains under tensile loading [27] [40], [145], [153], [154]. At both locations, the stress concentration factor (defined as the ratio between the maximal principal stress and the nominal stress; the latter computed by dividing the total external force applied at the chain link by its cross section) has a value of about 4 [153].…”

“…Therefore, the heat transfer analysis is independent of the stress analysis. The mechanical field is defined as follows: where εth and εp are the plastic and thermal strain tensors respectively, C is the fourth order [27], at higher temperatures they were estimated using the ratios published by Chen et al [158]. temperature.…”

“…Then, it must be determined whether the regions where the residual stresses are tensile are also subjected to high applied stress ranges. Bastid and Smith [27] showed that the region in the boundary of the contact zone experiences tensile residual stresses after proof loading; nevertheless, under tensile loading, the stress range is not sufficiently large to cause fatigue cracking.…”

Acoustic Emission Monitoring of Fatigue Crack Growth in Mooring Chains

“…Different AE sensors with different resonance frequencies and frequency bandwidths were tested: Vallen-VS150-WIC-V01 (resonant frequency 150 kHz, bandwidth 100-450 kHz), VS375-WIC-V01 (resonant frequency 375 kHz, bandwidth 250-700 kHz), and VS900-WIC-V01 (resonant frequency 350 kHz, bandwidth 100-900 kHz). It has been found [33,34] that chain failure most likely occurs at the point of the intrados (KT point) and crown positions, due to higher localised stresses in these areas ( Figure 2). Therefore, on each link, the sensor was placed 10 cm away from the weld on an accessible side ( Figure 3b).…”