Mad Dog Polyester Mooring - Prototype Testing And Stiffness Model for Use in Global Performance Analyses
Authors: Petruska D.J. , Geyer J.F. , Ran A.Z. 
Published: October 2008

The Mad Dog Floating Production System (FPS) will be the first truss spar to use polyester for a permanent mooring system. The breaking strength of the polyester ropes is also the largest ever-made being in the 2000 mT range. As such, prototype testing to validate the breaking strength capacity of the spliced ropes was important along with gathering performance data of the ropes to be used in the mooring design and global performance analyses of the FPS. Since this is the first spar to use a polyester mooring, and since loop currents typically govern the mooring of a spar in the Gulf of Mexico, a better understanding of the "static drift stiffness" or extension of the rope was required. Thus far, polyester moorings have been predominately used by Petrobras in the Campos Basin for semi-submersible FPS and Floating Production, Storage and Offloading (FPSO) units and thus only the dynamic and drift stiffness has been important. Much data is available for these two stiffness, but very little on the static drift stiffness was available. Consequently, a model had to be developed along with procedures to test the ropes to derive this stiffness.

This paper will discuss the prototype test plan, which basically follows API RP 2SM but with several deviations, in particular to obtain more dynamic stiffness and static drift stiffness (extension) data over a range of mean loads, load ranges and rate of loading. In addition, axial tensioncompression fatigue testing was conducted explicitly to the mean load, range of loading and number of cycles expected to occur to the mooring while in-service to confirm this is not a problem since tension did fall below 5% of MBL. Finally a stiffness model will be presented that can be used for mooring design / global performance analyses for FPS using a polyester mooring system.

Information presented in this paper will help designers of polyester mooring systems and also should impact the future revision of API RP 2SM.

Introduction

Petrobras has designed and installed numerous polyester mooring systems to semi-submersibles FPS and Floating Production, Storage and Offloading (FPSO) systems (Costa, 2001). However, to date, polyester has not been used in a permanent mooring system outside of the Campos Basin. BP Exploration & Production Inc. ("BP") and the Mad Dog project partners changed that when the taut-leg polyester mooring system was installed on the truss spar in early 2004.

The Mad Dog project was facing a significant hurdle in trying to keep development cost down so the company and the partners, BHP Billiton Petroleum (Deepwater) Inc. ("BHP Billiton"), and Union Oil Company of California ("Unocal"), could sanction the project. In order to control cost, it was important that the hull be fabricated and transported to the Gulf of Mexico as a single piece. The size and weight of the hull was already challenging the capabilities of the worldâ??s heavy lift vessels and in addition, payload was increasing to meet topsides requirements. Thus the project team investigated using a taut leg polyester mooring system.