Calculation Methodology of Out of Plane Bending of Mooring Chains

Abstract

Recently, the phenomenon of out-of-plane bending (OPB) fatigue of mooring chain links emerged as an important parameter in the fatigue assessment of mooring lines. Vessel motions induce a bending moment at the top chain of a mooring line. This bending moment induces alternating local stresses in the link and thus contributes to fatigue damage of those links. High pretension mooring systems are particularly sensitive to this phenomenon, since a small vessel motion combined with a high tension results in a relatively large bending moment in the upper mooring chain links. In mooring systems with high pre-tensions, this damage is of much greater magnitude than the fatigue damage induced by tension-tension loading only. An extensive study has been executed to investigate the fatigue life of mooring chain in deep water systems. This paper presents the calculation procedure to include the effects of local chain bending in the overall mooring line fatigue analysis. It was concluded that despite the complexity of the OPB issue, it is a phenomenon that can be incorporated in the mooring analyses by means of numerical procedures. The developed method is based on extensive Finite Element Method (FEM) analyses of chain links. Models of multiple chain links have been used that take into account the plastic-elastic properties of the material and contact friction between chain links. The FE models are used to derive empirical relations, between load angles, interlink angles, bending moments and stresses. These calculations were made for different combinations of line tension, interlink friction and chain size. The results were stored in a database to gain insight in the out-of-plane bending phenomenon. This database provides empirical formulas to lead to the local stress in different points on a chain link. These empirical formulas are used to translate floater (vessel or buoy) motions into local stress variations and fatigue damages in chain links. The long-term motion behaviour of the floater is known, the long term tension and bending stress ranges can be obtained and thus a fatigue damage of the chain links can be calculated.

An Out-of-Plane Bending Fatigue Assessment Approach for Offshore Mooring Chains Considering the Real-Time Updating of Interlink Bending Stiffness

“…Vargas et al provided procedures for calculating the stress concentration factors (SCF) of chain links subjected to OPB loads using finite-element analysis (FEA) [6]. Subsequently, many studies investigated the OPB fatigue phenomenon of mooring chains using either FEA or experimental methods [3,4,[7][8][9][10][11], and the phenomenon and failure mechanism of OPB were studied and interpreted, as shown in Figure 2 [12]. The relationship between OPB stresses in mooring chains and pretension, interlink angles, and mooring chain materials has been established in related studies [3]; moreover, it was also found that high OPB stresses will be induced in the proof loading test [8].…”

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].…”

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

“…Ter Brake et al [5] presented an S-N curve based fatigue analysis for mooring chains, in which the effects of the OPB on mooring chain stresses were determined by a FE analysis of chain links. Melis et al [6] presented experimental results of mooring chain stresses induced by OPB.…”