WHO DAT Project - Polyester Mooring System Design, Analysis, and Installation
Authors: Mullet Craig , Phillips Chris , Huntley Mark , Garrity Robert , Orson Jon , Pasternak Jason 
Published: April 2012

This paper presents the design, analysis, and installation of the suction pilesand polyester mooring system for LLOG's OPTI-EX Semi-submersible productionfacility installed in 3,100 ft water depth at Mississippi Canyon. Mooringsystem design began in October 2010 and the facility was hooked-up " storm safe" in July 2011, with the mooring system and suction pile design, CVA (CertifiedVerification Agent) approval, fabrication, and installation occurring inparallel and completed in ten (10) months.

Non-linear polyester stiffness curves

Also presented in the paper is the design and project-specific load-elongationtest procedure for the polyester rope. The results from the load-elongationtest procedure were used to derive the static and dynamic non-linear polyesterrope stiffness curves as well as the load dependant permanent elongation fordirect input into the mooring analysis.

Fabricated suction pile padeyes

Due to the fast-track schedule of the project, cast suction pile mooringpadeyes, with typically long lead times, were deemed as impractical for thefacility. Instead, fabricated suction pile plate padeyes were utilized andexamined in detailed Finite Element Analyses (FEA), accounting for non-linearchain shackle - padeye interactions and non-linear material properties.

Pre-lay mooring lines / Facility hookup

The entire mooring system, including the polyester rope, was preset on theseafloor using LLOG's existing contracted Anchor Handling Vessel (AHV). Thepolyester rope was approved for pre-lay on the seafloor due to its constructionand dual jacketing system which provides soil filtration for the load bearingsubropes. When the facility was towed out to location, all twelve (12) mooringlines were hooked-up in seven (7) days. The ability to preset the entiremooring system reduced critical path time for hook-up of the facility andminimized the facility's exposure to tropical storms in a partially mooredcondition during installation.

Information presented in this paper will be of interest to polyester mooringsystem designers and engineers, polyester mooring system and platformoperators, polyester rope manufacturers and users, and regulatoryagencies.

Introduction

The OPTI-EX Facility was built by Exmar Offshore Company on speculation. Thehull was delivered to Kiewit Offshore Services Shipyard in Ingleside, TX andmated there with the topsides in 2009. In August of 2010 LLOG agreed topurchase the Facility from Exmar, stipulating an aggressive schedule with theFacility to be towed out and installed on location by July 2011. Delmar wasawarded the engineering, procurement, construction, and installation (EPCI)mooring contract in late September, 2010, and work on the mooring systembegan.

The mooring system is a twelve (12) line chain-polyester-chain mooringconfiguration in approximately 3,100-ft water depth. The final mooring designconfiguration resulted in 107mm R4 studless platform and ground chain. Thepolyester rope is Whitehill's VETS 370 construction rope with a MBL of 1,100-mTand an average overall diameter of ~205-mm. Delmar's patented subsea connectorwas used to connect the ground chain to the suction piles.