Extreme Surge And Mooring Load Of Fps Structures

Abstract

ABSTRACT Offset is an important parameter in a compliant structure mooring design. To evaluate maximum offset, commonly adopted methods are based on the assumption that the combined linear and Iow-frequency wave induced off sets are Rayleigh distributed. This assumption could result incorrect, particularly in those cases, as for instance large volume tension leg platforms and tankers at single point mooring (SPM) where the low frequency component dominates the horizontal motion response. Aim of the paper is to discuss this assumption and to evaluate the sensitivity of the extreme response of an SPM tanker to the damping, non-linearities in the mooring system as well as to sea state parameters. To do this, a systematic analysis has been performed and the peak distribution of surge motion and mooring loads have been derived by means of a simplified numerical procedure specifically developed for extreme value evaluation. The results show that the Rayleigh assumption is not suitable and that particular care is needed in the selection of environmental parameters, including the spectral shape. Even if the analysis has been limited to simple conditions (one degree of freedom, slow drift force only, long crested head sea) the conclusions are believed to hold true in most general cases. INTRODUCTION The use of floaters for production end storage is being more and more adopted for both marginal fields and deep water exploitation. For marginal fields, tankers at single point mooring have became so popular that ad hoc guidelines and regulations have been put in force or are under development with e.g. API, ISO TC7 and IMO MSC Design and Equipment Committee. As far as the safety of the SPM is concerned, one of the most critical aspects is the mooring system: its loss of holding capacity could lead to the tanker loss with extremely heavy consequences in terms human lives, environmental pollution of investment. For this reason, it is essential that reliable predictions of the extreme mooring load can be made at the design stage. Mooring loads are of random nature. Therefore, in order to predict their extreme values, a reasonably accurate, and suitable for design, stochastic model is needed which is not an easy goal to achieve. As a contribution to the solution of this problem, a sensitivity analysis has been performed with the aim of investigating some still controversial points in presently adopted guidelines, namely the assumed Rayleigh distribution of SPM response peaks and the effects of sea state parameters, in particular the wave spectrum shape and the influence of wave peak period. NUMERICAL METHOD According to the simplified character of the analyses to be made, a single degree of freedom system has been adopted to represent the dynamic behaviour of the moored tanker the following equation of motion has been considered.