Abstract

The design, materials, testing and installation of a large diameter steel catenary riser system were some of the major challenges of the Independence Trail Project in the Gulf of Mexico. This paper will address global design considerations including, analysis of strength, fatigue (wave-induced, riser VIV, hull VIM, heave-induced VIV) and interference loading conditions; also welding, validation testing used to ensure a safe operating environment.

The Independence Trail 20-inch SCR is the largest riser installed to date in this water depth. The global design of the riser system was heavily influenced by the fatigue performance of the riser touchdown zone and hang-off region. The interface with the floating production unit is made up of a support structure and flexible joint assembly to absorb the local dynamic bending moment.

The 20-inch SCR that connects the export pipeline system to the Independence Hub floating production facility underwent an extensive design review and validation testing, using full scale and laboratory techniques to verify the fatigue design of the system.

As the largest diameter deepwater SCR operating from a semi-submersible floating production unit to date, the challenges met and the solutions employed as part of the delivery of a robust and safe riser system provide important lessons learned and have significant relevance to future SCR projects.

Introduction

The Independence Hub development consists of a DeepDraft SemiTM moored in approximately 8,000 ft of water in Block 920 of the Mississippi Canyon area of the Gulf of Mexico (GoM). The floating production facility is a hub for numerous gas fields in the Eastern GOM deepwater. The hub will provide real estate and gas processing excess capacity to tie-in future gas finds in the area. The gas product will be exported from the platform via a 20-inch diameter Steel Catenary Riser (SCR) that transitions into the 24-inch diameter Independence Trail Pipeline which runs northwest for 134 miles, terminating at a fixed platform in 115 ft of water in Block 68 of West Delta area. The 20-inch Independence Trail SCR and 24-inch Independence Trail Pipeline are 100% owned and operated by Enterprise Product Partners LP, while Enterprise owns 80% of the DeepDraft SemiTM. Figure 1 presents the DeepDraft SemiTM, SCR and subsea layout.

Design and Construction of Gas Export SCR

The design and construction of the export riser for Independence Hub required that multiple parties work and interface in order to execute the project of designing the world's deepest export system. With many disciplines interfacing, an Integrated Project Team (IPT) was formed to ensure the timely flow of information and ultimate safe, robust project design and delivery. Figure 2 presents a typical flowchart of the SCR design and construction process per Kavanagh et al. [5], highlighting the key activities and interfaces for the project:

  • Design

  • Procurement

  • Weld qualification and testing

  • Construction

  • Installation

  • Operation

Figure 2 also illustrates the cycles and iterations that may be required and undertaken during project execution to achieve a feasible riser system design.

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