Analysis of Inertial Influence and Damping of Mooring Lines on Fatigue Damage

Resumo

The high demand for oil has driven its exploration into increasingly deeper water depths, which required significant scientific and technological advancements. In this context, the floating production and offshore system is a type of platform, based on ship, anchored by mooring lines arranged by chains/steel cables and polyester cables with the objective to restrict the platforms movements in a design region. Offshore platforms are exposed to environmental loads of waves, wind, and currents, which are responsible for generating hull motions and tension variation in the mooring lines. This tension history, in turn, is directly associated with fatigue failure of the lines. Despite efforts to estimate the fatigue life of mooring lines to ensure their integrity throughout the platforms operation, premature failures in the mooring system have been observed over the past decades. This serves as an alert that the fatigue damage estimated during the design phase may be less than the actual damage occurring in the field. Several factors influence the initial estimation of fatigue life, for instance, the analysis method used to represent the lines behavior, such as quasi-static and dynamic analysis. The first one, that ignore the inertial and damping effects, was widely used in design of mooring lines in the 90s and the beginning of this century due its low computational cost, while the dynamic analysis, that represents rigorously the mooring line dynamic behavior, is used increasingly nowadays. The use of these two different approaches can affect the fatigue damage. Therefore, the present work aims to evaluate the influence of the inertia and damping of moorings in the fatigue damage, comparing the quasi-static and dynamic approaches studied with a floating unit numerical model, with 910m water depth, that uses a conventional catenary mooring system. In this way, a critical analysis is carried out based on the comparison between the fatigue life resulting from the two forms of analysis.