Fatigue Design Methodology for Large Mooring Chains Subjected to Out-of-Plane Bending

Abstract

The present paper presents a fatigue life prediction method for chains subjected to tension and Out-of-Plane-Bending (OPB). The investigation was carried out on a high strength mooring chain segment with a diameter of 165 mm and a steel quality R5. Such chains are used in mooring systems for large Floating Production Storage and Offloading (FPSO) units in harsh weather condition. Due to the fact that the mooring chains are pre-tensioned the wave induced displacements will introduce secondary bending effects about the weak axis in the chain links, particularly in the bilge hang-off area. In such chains both conventional tension fatigue and fatigue damage accumulation due to Out-of-Plane-Bending (OPB) have to be analyzed. Results from full scale behavior tests, Finite Element Analyses and a case study with simulation of in-service loading are included in the present study. Finally, fatigue life predictions and an operational strategy are presented for a case study for a floater in the Barents Sea. With a target service life of 30 years, the endurance with respect to fatigue damage is a major design criterion for the mooring chain in this case.

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

“…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.…”