ABSTRACT The work presents an expression to deal with the stiffness non-linearity of polyester mooring cables. The basis for that is a PhD thesis of the second author and full scale testing of five inch diameter polyester mooring cables. On one hand, weak dependence on the frequency has been detected. On the other hand, the mild to significant dependence on the average load and load amplitude has become clear. Suggestions on how to deal with the expression in a mooring design are also addressed.
INTRODUCTION The use of polyester in mooring lines of floating structures is a very recent issue due to the oil exploitation in deeper and deeper water. It is nevertheless a reality. After several years of research and field evaluation, PETRO BRAS decided to use polyester lines in several Floating Production Systems (FPS) in Campos Basin, offshore Brazil. Two of them have been installed in 1997 and others are planned to 1998. The basic research that sustained the feasibility of the lines is consolidated in (Del Vecchio 92), where a comprehensive series of testing and analysis is summarized. Typical yarns and small diameter (less than an inch) cables were then studied. Later on, several acceptance tests as required by PETROBRAS have also been performed. Both of these activities yield a fundamental basis for the understanding of the polyester ropes mechanical behavior. The highly nonlinear characteristics have come up clearly as shown in (Fernandes and Castro 97) and summarized next.
ACTUAL LARGE DIAMETER LINE TESTING According to PETROBRAS" specification (PETROBRAS 96) rope markers have to perform acceptance tests with samples taken from the production line as part of the scope of their supply. The results discussed below refer to the rope purchased for the IMODCO-3 monobuoy installed in 830 m of water.