Effect of Polyester Mooring Stiffness on SCR Design for FPSO Application
Authors: Chang Gwo-ang , Tan Pao-Lin , Pathi Souma , Kwan Tom 
Published: April 2012

Polyester rope is a visco-elastic material and its stiffness is affected bymean tension, tension amplitude, loading period, and loading history. There isa perception that rope stiffness significantly affects vessel offset, which inturn affects riser performance and cost. However, a systematic investigation onthe impact of rope stiffness on riser stress and fatigue life has not beenpublished for wide variety of design conditions. This study carried out such aninvestigation and provides specific guidance to designers for the prudentselection of rope stiffness model and values to achieve safe and cost effectiveriser design. This study investigated polyester rope stiffness modeling and theeffect of polyester rope stiffness on steel catenary riser (SCR) design for anFPSO sited in a variety of water depths. A parametric study of stiffness modelsfor polyester rope was conducted to validate the use of a 2-slopestatic/dynamic stiffness model in global analysis to provide motion responsesfor the riser analysis. Time domain riser analysis was performed to obtain themaximum riser stress and fatigue damage under various conditions. Based on theresults of the investigation of riser and mooring analysis, guidance on theimpact of rope stiffness to the riser design has been developed.

Introduction

As the industry progresses into deep and ultra deep waters, the use ofpolyester mooring has increased significantly, and can now be considered themost attractive option for deepwater operation. Compared to a steel wiremooring, polyester rope is a visco-elastic material and its stiffness isaffected by mean tension, tension amplitude, and loading period. It is " softer" at static and LF loading conditions, which will have significant impact on theoffset and motion response characteristic of an FPSO and its SCR design(strength and fatigue).

The impact of rope stiffness on riser stress and fatigue has not beenquantified, and the impact can be different for different riser types. Therefore, the sensitivity of fiber rope stiffness on riser stress and fatiguelife is investigated for an SCR hung from an FPSO in various water depthsconsidering West African environmental conditions. In the investigation, the2-slope static/dynamic rope stiffness model validated by parametric study isused; this stiffness model is as proposed in the recently issued ABS GuidanceNotes on the Application of Fiber Rope for Offshore Mooring" [1].

The results of the study related to SCR design will be given later in thispaper. First, the results obtained from the parametric study on fiber ropestiffness modeling for FPSO will be addressed considering environmentalconditions offshore Brazil.

Parametric study of polyester mooring stiffness model for FPSOapplication

A parametric study of polyester rope stiffness modeling was conducted in theDeepStar 6403 study [2] for a SPAR under hurricane conditions in which wavefrequency (WF) motions are dominant. A parametric study for a FPSO which lowfrequency (LF) motions are dominant has not yet been investigated. In thisstudy, an upper/lower bound model, 2-slope (static/dynamic) model, and 3-slope(static/LF/WF) model were investigated for FPSO application. Since the 3-slopestiffness model can be the best method for simulating the mooring stiffness, the response (vessel offset and mooring tension) determined from this model isused as the base case in the parametric study.

AQWA frequency domain (FD) analysis method was used for this study forcomputational efficiency. An analysis procedure similar to that presented inAPI RP 2 SK [3] for steel wire rope mooring is used to calculate the maximumvessel offset and mooring line tension.