Mad Dog Polyester Mooring - In Service Inspection and Rope Manufacturing Quality Control
Authors: Petruska D.J. , Geyer J.F. , Ayers R.R. , Craig M.J. 
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 evermade being in the 2000 mT range. As such, the in-service inspection plan and quality control exercised over the rope manufacturing was important to the project team. This paper will review the inspection plan intended for the service life of the mooring and also the quality control measures put in place for the project to ensure high quality ropes were consistently produced, with particular emphasis on splicing the end terminations.

The information in this paper can have direct application on future projects that use polyester moorings. The in-service inspection plan has been approved by the USCG, MMS and ABS, the first such approval on the OCS of the Gulf of Mexico. The quality control imposed was also a deviation from what has been used for other projects and resulted in several improvements to the splicing process that was proven effective by the numerous break strength tests conducting over the course of manufacturing the ropes. Results of these tests will be presented showing an improvement was obtained.

Introduction

The Mad Dog project was facing a significant hurdle in trying to keep development cost down so BP Exploration & Production Inc. ("BP") and the partners, BHP Billiton Petroleum (Deepwater) Inc. ("BHP Billiton"), and Union Oil Company of California ("Unocal"), could sanction the project. In order to reduce 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 world'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. The base case was to use sheathed spiral strand for the riser section of the mooring but polyester offered the opportunity to reduce weight and improve overall mooring system performance. Early design studies showed that a polyester mooring system could reduce vertical loads on the hull by 1500-2000 tonnes while also reducing offsets. The lower weight and the compliance offered by using polyester resulted in a lower pre-tension and maximum in-place loads than for an all steel system. It also resulted in smaller platform and ground chain, and smaller chain jacks and fairleads, which also produced cost savings beyond the impact on the hull. With these benefits and the risks of using polyester being evaluated and determined to be manageable, the project team endorsed the decision to proceed with polyester.

Two issues for both the project team and regulatory authorities before proceeding with the use of polyester were:development of adequate assurance that the mooring could be safely operated over the field life andassuring a high quality product could be manufactured for all the rope segments and test inserts particularly since the spliced end termination is a hand made component.