Matterhorn Steel Catenary Risers: Critical Issues and Lessons Learned for Reel-Layed SCRs to a TLP
Authors: Kavanagh W.K. , Harte G. , Farnsworth K.R. , Griffin P.G. , Hsu T.M. , Jefferies A. , Desalos A.P. 
Published: October 2008

This paper presents key issues and lessons learned from the design, fabrication, weld qualification, fatigue testing and determination of defect acceptance criteria for the Matterhorn field export SCRs, the first reeled SCRs installed to a TLP in the Gulf of Mexico.

The Matterhorn TLP, located in 2850ft water depth in Mississippi Canyon 243, exports oil and gas via 8-inch and 10-inch SCRs, operated by ChevronTexaco and Total.

The design of these SCRs was one of the first to consider The Cold-Core Eddy Current (or submerged current) in the Gulf of Mexico and the design of the SCRs and other catenary risers was strongly influenced by its introduction.

Reeled installation of the SCRs also affected the design, fabrication and fatigue testing process, particularly the pipe welding and matching process and the fatigue test and fracture mechanics analysis strategy.

Both SCR designs were shown to be robust by early consideration of sensitivity to a wide range of design inputs. Designing to several such sensitivities was found to be a very valuable input to achieving robust SCR designs when managing change to base case assumptions in as-built SCRs and components.

Being among the first to consider the Cold Core Eddycurrent in the Gulf of Mexico and the first reeled SCR to a GoM TLP, lessons learned have significant relevance to future SCR projects.

Introduction

The Matterhorn field consists of a SeaStar mini-TLP in 2,850ft of water in Block 243 of the Mississippi Canyon area of the Gulf of Mexico. The riser system consists of an array of top-tensioned production risers combined with steel catenary risers (SCRs) for gas and oil export with an additional flexible riser and steel tube umbilical riser to an injection well. The 10" gas export SCR is operated by Total and the 8" oil export SCR is operated by ChevronTexaco. Figure 1 illustrates the SeaStar TLP steel catenary riser and subsea layout.

SCR Design and Construction Process

The design, procurement and installation process for deepwater SCRs is a combination of interrelated tasks in several disciplines the interface between which is a critical prequesite to achieving a feasible, optimized and consistent SCR design. The maintenance of the interface between these disciplines and functions is an essential part of the SCR design process which requires effective interface management throughout the project.

Figure 2 presents a flowchart of the SCR design and construction project process which illustrates the interfaces between the key SCR project stages:DesignProcurement of pipe and ancillary devicesWeld qualification and testingCoating and constructionInstallationOperation

Also illustrated by this flowchart is the interrelationship between these processes and the important feedback loops that exist back to the design stage of the project.

Although most inputs to design are typically defined by the project Design Basis, design is also influenced by key assumptions made at the design stage. Key elements of this feedback to design are provided by the pipe procurement (pipe weight), flexjoint procurement (flexjoint stiffness), pipe matcing (stress concentration factors), fatigue testing (design S-N curve).and installation (fatigue damage) stages of the project.