Computational Fatigue Assessment of Chains Working in Twisted Conditions

Chains are extensively used in the Oil and Gas industry for mooring vessels or floating platforms, both for temporary and long-term moorings. Moreover, recently they found a new application in the mooring of floating wind turbines. The integrity of mooring systems is of great importance for both industries. Failure Records [1]–[2] indicate that fatigue of mooring chains is one of the main causes of failure, and therefore their lifetime prediction represents an important challenge for industry. Chains are intended to work under Tension Loading, however, anomalous loading modes such as Out-of-Plane Bending (OPB) or Twisting can appear. For example, OPB caught the attention of engineers after the Girassol incident; where it was identified as the main cause of failure of several chain links eight months after the installation [3]. Since this failure, OPB has attracted the attention of several research programs and multiple publications can be found in the literature (For example, [4]–[5]). Twist may occur during the installation of a chain or during service due to torque generated in the near-by elements of the mooring (for example wire cables). This paper presents a computational fatigue assessment of mooring chains working in twisted conditions. It is based on a two steps analysis: a mechanical and a fatigue analysis. The mechanical analysis accounts for the initial residual stress state of the chain after manufacturing and subsequent proof loading. The result is the stabilized cycle of the multiaxial stress field, which is obtained through an elastic-plastic three-dimensional Finite Element Analysis (FEA). Then, the fatigue analysis is performed to predict the failure location and determine the lifetime. The predicted failure location has shown good agreement with fatigue cracks found in recovered chain links in the North Sea after more than 15 years of service.