Numerical model for analysis of the load attenuation on mooring lines in catenary system

Resumo

With the advancement of oil exploration at greater depths, the solution to maintain offshore platforms
in place has changed from a traditional rigid system to a flexible system formed by mooring lines. Usually, a
mooring line consists of an anchor chain attached to the floating unit in the upper end and the anchor or foundation
element at the lower end, which implies that, along its length, the chain is not only submerged in water but also
embedded in soil. Experimental investigations conducted on reduced scale have shown that the load applied at the

embedment point located on the seabed surface does not reach the same magnitude at the anchoring point. The so-
called “load attenuation” is related to the inverse catenary shape developed by the mooring line embedded in the

soil, providing a transversal reaction force and longitudinal friction force in the soil-chain interface. In order to
evaluate the problem from a numerical point of view, an initial 3D finite element model of the anchor chain
embedded in marine soil was built in the ANSYS software. As a starting point, the anchor chain behavior is adopted
as elastic, the soil is considered perfect elastoplastic with Drucker-Prager yield criterion and the contact interface
is simulated by Coulomb’s law of friction. The load attenuation levels obtained for several cases are compared to
reference results, both experimental and numerical, and the stress disturbance in soil is presented in terms of von
Mises equivalent stress.