Perdido Development Project - Spar and Moorings
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 20885, ’Perdido Development Project - Spar and Moorings,’ by Curtis Lohr and Keith Smith, Shell International Exploration and Production, prepared for the 2010 Offshore Technology Conference, Houston, 3-6 May. The paper has not been peer reviewed.

A spar platform was selected for the Perdido development during system selection in 2005 to provide a stable platform for direct vertical-access drilling and producing operations in nearly 8,000 ft of water. A combination polyester/chain mooring system was selected to place less vertical load on the floating structure (compared to an all-steel mooring system).

Introduction

The Perdido spar was Shell’s first spar-host production platform, but had the benefits of being developed as a joint venture with Chevron and BP. Technip was selected as the spar and mooring contractor. The spar comprises a cylindrical upper section (hard tank); a trussed midsection (truss) formed from tubular legs and braces with horizontal stiffened-plate flats, or heave plates; and a bottom section (soft tank) used to carry solid ballast and provide flotation during installation. Some key characteristics of the Perdido spar are listed below:

Overall length—555 ft, draft 505 ft

Hard tank—118-ft outside diameter (OD)×264-ft length, with 214-ft hard-tank draft

Hull weight—approximately 20,000 tons including structural steel, appurtenances, and hull systems

Nine mooring lines

Two heave plates

Three strakes

A 46×46-ft moonpool

Six riser slots

Four-leg topside and truss

Octagonal soft tank

Six hydraulic tensioners supporting five production risers and one drilling/completion riser

The spar was designed to accept a single-lift topside by limiting the spar’s OD to 118 ft and by placing storage tanks for diesel fuel, methanol, dead oil, low-dosage hydrate inhibitor, and drill water inside the upper compartment of the spar rather than on the topside. The spar was fabricated in Finland. Fabrication began in November 2006, dry transport to the Gulf of Mexico began in May 2008, and the wet tow offshore occurred in August 2008.

One challenge that was unique to the spar fabrication was installation of components for the hydraulic riser-tensioning system in the main hull-fabrication yard. Tolerance for this system was extremely tight, and installation requirements were discussed well in advance to plan the work. Jigs to ensure that the installation of components was within tolerance were designed, fabricated, and used.

Mooring System

The Perdido mooring system uses nine polyester/chain lines averaging more than 2 miles in length, with the vast majority of each line’s length being polyester. The nine taut lines are oriented in a 3×3 pattern and consist of 9.68-in. polyester-rope main sections and short lengths of 5.28-in. chain at each end. The spar has an “active” mooring system to reposition it over drilling locations within an approximately 350-ft-diameter area. Because polyester is more difficult to handle in a winch, the top of each mooring line consists of a short length of chain, called the platform chain, which is connected to the spar. The platform chain runs through a below-water fairlead mounted on the outside shell of the spar. A dedicated chain jack controls each platform chain.