Advances in Titanium Risers for FPSO's
Authors: Baxter Carl , Pillai Sitham , Hutt Graham 
Published: October 2008

Titanium alloy Grade 29 is shown to be suitable for dynamic riser application. Results, including fatigue and fatigue crack growth testing at 150°C in sea water, are presented for GTAW welded samples. Orbital GTAW welding is shown to be suitable for onshore construction and tow-out of such risers, but impractical for offshore use due to the time required to complete a welded connection. Progress on the development of rotary friction welding is presented. This process is many times faster than GTAW and thereby provides a safe and costeffective method for offshore construction and installation. An example installation process is presented.

Introduction

A combination of increased use of floating platforms together with more aggressive high temperature and high temperature well fluids has encouraged the development of alternatives forms of dynamic risers using various metallic and compositematerials. Among these materials are titanium alloys which have a unique combination of resistance to well fluids and seawater, lightweight, flexibility and high strength. These properties have lead to their growing use for downhole tubulars and topsides piping. Riser components where strength and flexibility are required have also been produced such as the taper stress joints for the production risers on the Neptune Field Spar Platform in the Gulf of Mexico and the drilling riser for the Heidrun TLP in the North Sea.

There is interest in applying titanium for production risers connected to ship and semi-submersible based floatingproduction systems recently, particularly in Norway. Work here has shown that a system is feasible but expensive. It was found in the studies that a lot of the expense involved in producing an installed riser system arose not from the cost of the material itself but from the conversion of titanium pipe into a riser, and installing it. It was also realized that costs in these areas could be dramatically reduced as the knowledge increased, and a system could be produced that is both cost competitive with alternative materials and offered other advantages such as lighter hang-off weights and the ability to cope with high temperature and/or aggressive well fluids.

Traditionally, flexible risers have been made of un-bonded, reinforced elastomeric and polymeric materials. The flexibility in bending of this form of riser makes it very suitable forshallow water. As water depths increase metallic risers become feasible because the flexibility requirements decline. Carbon steel is a feasible material in deep water provided well luids are sufficiently benign. However, titanium has an elastic modulus 60% that of steel and can operate in the most aggressive well fluid environments.

There will be therefore, applications in which well fluid composition dictates that titanium risers are required; and a range of water depths where steel has insufficient flexibility. n addition, as water depths increase and well fluids allow, titanium pipes can be profitably used in conjunction with steel pipes to give the most safe, economic and durable solution.