A Complete And Consistent Methodology For The Assessment Of Mooring Systems
Authors: lsnard J.L. , Leblanc L. , Wilczynski H. 
Published: October 2008

Ten years ago, the capacity of mooring systems was usually assessed using a static approach. Simplistic computational models were made to suit the computer technology available at that time.

Conservative safety factors were used in order to balance the uncertainties associated with these calculationmethods. The whole process was nevertheless coherent and experience has shown that an adequate level of safety was thus achieved.

Since then, safety codes have evolved at the same tempo as theoretical advances.

Nowadays, computem are so fast that it is possible to perform very sophisticated analyses. Simulations of the mooring system behaviour that would include the 3D-dynamics of each line could probably be petformed, even for complex mooring patterns.

However, the actual dynamic response of mooring lines in full scale conditions has not yet been fully elucidated and calibration of such computer software would therefore be difficult. Furthermore, the interpretation of results derived from such a complex calculation would be dreadful.

Subsequently, the most recent codes recommend performing a quasi-static analysis of the whole mooring system, to be completed by a dynamic analysis of the most critical line in its average vertical plane.

These codes provide revised safety factors that are dramatically smaller than those used in the past, on the grounds of reduced uncertainties associated with partial dynamic considerations.

This paper explains why such new practices are not necessarily consistent or appropriate.

This paper also presents a comprehensive and consistent methodology for mooring system assessments based on "quasi-dynamic" principles.

Calibration and validation have been derived from a very large number of cases, thus ensuring optimization between over-scantling, safety and cost effectiveness.

Equations used to compute loads and motions are given in detail together with damping coefficients for each type of unit. Safety factors are also specified for Maximum Design Condition.

An alternative mooring analysis methodology is proposed which is complete, consistent and efficient.

EVOLUTION IN THE ASSESSMENT OF MOORING SYSTEMS

The assessment of environmental loads on a floating body and the derivation of design mooring loads are generally based on an engineering approach combining both model tests and numerical simulations. Provided they are properly carried out, model tests allow for the accurate consideration of a large number of parameters such as theinfluence of the current, the effect of the floating body velocity on the wave drift forces, the wave drift damping, the line dynamics and, in some instances, the effects of risers.

In return, model tests remain expensive and the number of combinations of environmental and vessel loading conditions that it is feasible to test is necessarily small compared to the number of actual possible combinations. Hence, numerical simulations appear to be the best way to complete the database derived from model tests.

Until the recent past, unable to account for all possible phenomena, and more particularly for the dynamics of the line itself, high safety factors were used to determine the acceptable breaking load of the mooring lines and thusbalance the huge amount of uncertainties.