Design and Installation of a Banded Riser System for a North Sea Development
Authors: Song Ruxin , Clausen Terje , Struijk Paul 
Published: October 2008

A banded riser system consisting of production risers, test riser and heat-up risers was selected for the Gullfaks field development in the Norwegian North Sea. The risers were banded together for transportation purpose and fabricated and tested onshore, towed to the field using surface tow and Control Depth Tow Method (CDTM) and pulled in through designated J-tubes individually. The risers were connected to the flowline bundle via rigid spools at individual riser bases.

This paper describes the work performed for the EPCI riser system project and focuses on design and installation. The major challenges of this project are highlighted.

The design of the banded riser system mainly covers wall thickness design, thermal expansion analysis, riser strain analysis and fatigue analysis. It includes the novel temporary banded riser structural design including flat pack design, buoyancy tank design, and ballast chain design. The banded riser installation covers load out, towing, pull-in operation, and connection. Comparisons are made between the towing engineering analysis and the actual towing measurements.

It is concluded that the banded riser system is a good concept for the riser system and can be designed and installed successfully.

Introduction

The banded riser tow concept was selected for the phase two development of the Gullfaks Satellite Field located in the Norwegian North Sea. The risers are pulled-in individually through existing J-tubes. The field is developed using subsea facilities with production and injection templates tied back to the existing production platforms. Fluid transport back to the GFC platform is through bundled flowlines, see Figure 1.

The banded riser tow system consists of 2×12" production risers, 1×8" test/production riser and 2×4" heat-up risers. The riser system together with four riser bases was fabricated and tested onshore and towed to the field using surface tow inshore and CDTM offshore. After arrival at the GFC platform, located at water depth of 220 m, the banded riser system was repositioned, disconnected and pulled in individually through designated J-tubes at the south side of the platform. They were then connected to the flowline bundle via rigid spool pieces at individual riser bases.

This paper describes the work performed for the EPCI riser system project with focus on design and installation. The risers were successfully installed in the Gullfaks field in April 2000. The major challenges and features of this project IncludedBanded dual heat-up risers pulled in through one J-tube;Largest possible 12" riser pipes (OD=314.3mm) pulled through 16" J-tubes (ID=368.2mm);Novel flat pack concept was used first time for towing;Combined towing methods were used for the first time for the transportation;Riser fatigue governs the towing installation;Riser strain and expansion governs in-situ design.

The design issues addressed in the paper cover mainly wall thickness sizing, thermal expansion analysis, riser strain analysis and fatigue analysis. The paper also describes the design of the novel banded riser temporary structure: the flat pack, buoyancy tank design, and ballast chain design. The riser analysis has revealed that the governing parameter is riser strain.