Fatigue Life Prediction of Mooring Chains Subjected to Tension and Out of Plane Bending

Abstract

The present paper describes the behaviour of chain segments that are subjected to pretension and a rotation angle at the segment end. The behaviour such segments has been investigated both experimentally and by finite element modelling. The purpose is to carry out fatigue life predictions. A full scale test with a studless chain segment with a diameter of 125 mm has been carried out to shed more light on the behaviour. The test corroborated the assumption that the chain segment behaves semi-rigidly under the given conditions due to locking of the inter-link hinge mechanism. The influence of various important parameters on the bending effect was studied. The chain segment has a significant flexural stiffness and an intra-link bending curvature for links that are in a flat position with respect to the inflicted end rotation. The associated Out of Plane Bending (OPB) stresses are significant for critical links close to the end hang-off. This effect must be taken into account when carrying out fatigue life predictions. There is still no guidance given in rules and regulations for this case. In the present work it is suggested that a hot spot method is applied for fatigue life predictions. Under a combined loading mode, defined by tension and OPB, the maximum principal stress range in the link bend area must be determined by refined finite element analysis. Subsequently, the fatigue life can be predicted by using an appropriate hot spot S-N curve. The paper points out how a good fatigue design of the hang-off area can reduce the effect of OPB. A Buoy Turret Loading (BTL) is used as a case study to demonstrate both the design proposal and the fatigue life prediction methodology.

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

“…Traditionally chain links were considered to be perfectly hinged together by a rolling or sliding mechanism at the contact region between each link [17]. In other words, it was considered that the bending stiffness of the chain could always be neglected.…”

“…Lassen et al [17] proposed similar recommendations and suggested reducing the pretension of the mooring line whenever possible. As already noted, a JIP was formed in order to have a better understanding of the OPB mechanism.…”

Fracture mechanics analysis for a mooring system subjected to tension and out-of-plane bending

“…For Scenarios 2 and 3, the most critical location of mooring chain links prone to fatigue damage moves to the middle of crown section (D-D cross-section) as shown in Figure 8 [6][7][8]. Figure 9 shows a typical normal stress distribution over the cross-section described in Figure 8.…”

“…Vargas and Jean [7] compared mooring chain stresses calculated by FE method with experimental results of Melis et al [6]. Lassen et al [8] and Lassen et al [9] performed a mooring chain fatigue life using an S-N curves based approach, and the behaviour of chain links subjected to tension and OPB was investigated by both experimental and FE methods. In this paper, a fracture mechanics analysis for the mooring system of a semi-submersible accounting for OPB is presented.…”

Computational fatigue assessment of mooring chains under tension loading

“…The rupture occurred eight months after the installation and was not predicted by industry standards. Therefore, this loading mode attracted the attention of several research programs and multiple publications can be found in the literature, see for example [7,9,[22][23][24][25][26][27]. Among this effort in understanding and quantifying Out-of-Plane Bending, it is worth mentioning a Joint Industry Program (JIP) gathering 28 different companies whose objectives were a better understanding of Out-of-Plane Bending fatigue and propose fatigue design recommendations, their main findings can be found in [7,9].…”