Steel catenary riser

A steel catenary riser (SCR) is a common method of connecting a subsea pipeline to a deepwater floating or fixed oil production platform. SCRs are used to transfer fluids like oil, gas, injection water, etc. between the platforms and the pipelines.

In the offshore industry the word catenary is used as an adjective or noun with a meaning wider than is its historical meaning in mathematics. Thus, an SCR that uses a rigid, steel pipe that has a considerable bending stiffness is described as a catenary. That is because in the scale of depth of the ocean, the bending stiffness of a rigid pipe has little effect on the shape of the suspended span of an SCR. The shape assumed by the SCR is controlled mainly by weight, buoyancy and hydrodynamic forces due to currents and waves. Thus, in spite of using conventional, rigid steel pipe, the shape of an SCR can be closely approximated^[1] with the use of ideal catenary equations, used historically to describe the shape of a chain suspended between points in space. A chain line has by definition a zero bending stiffness and those described with the ideal catenary equations use infinitesimally short links.

The rigid pipe of the SCR forms a catenary between its hang-off point on the floating or rigid platform, and the seabed. A free-hanging SCR assumes a shape roughly similar to the letter 'J'. A catenary of a lazy wave SCR consists in fact of at least three catenary segments. The top and the seabed segments of the catenary have negative submerged weight, and their curvatures 'bulge' towards the seabed. The middle segment has buoyant material attached along its entire length, so that the ensemble of the steel pipe and the buoyancy is positively buoyant. Accordingly, the curvature of the buoyant segment 'bulges' upwards (inverted catenary), and its shape can also be well approximated with the same ideal catenary equations. The positively and negatively buoyant segments are tangent to each other at the points where they join. The overall catenary shape of the SCR has at those locations points of inflection. Lazy Wave SCRs were first installed on a turret moored FPSO offshore Brazil (BC-10, Shell) in 2009,^[2] even though Lazy Wave configuration flexible risers had been in a wide use for several decades beforehand.

The SCR pipe and a short segment of pipe lying on the seabed use 'dynamic' pipe, i.e. steel pipe having slightly greater wall thickness than the pipeline wall thickness, in order to sustain dynamic bending and steel material fatigue associated in the touch-down zone of the SCR. Beyond that the SCR is typically extended with a rigid pipeline, but a use of a flexible pipeline is also feasible.^[3][4] The risers are typically 8-12 inches in diameter and operate at a pressure of 2000-5000 psi.^[5] Designs beyond those ranges of pipe sizes and operating pressures are also feasible.

Free hanging SCRs were first used by Shell on the Auger tension leg platform in 1994 which was moored in 872 m of water.^[6]

References

1. Wajnikonis, Christopher J., Robinson, Roy, Interactive Deepwater Riser Design, Analyses and Installation Methodology, IBP42400, 2000 Rio Oil & Gas Expo and Conference, 16-19 October, 2000, Rio de Janeiro, Brazil
2. Wajnikonis, Christopher J., Leverette, Steve, Improvements in Dynamic Loading of Ultra Deepwater Catenary Risers, OTC20180, Offshore Technology Conference, 4-7 May, 2009, Houston, Texas, USA http://www.spe.org/jpt/print/archives/2009/09/11NewFieldDev.pdf
3. "Steel Catenary Risers". Tenaris.
4. "Steel Catenary Risers". 2H Offshore.
5. Howells, Hugh. Advances in Steel Catanery Riser Design (PDF). DEEPTEC'95.
6. Mekha, Basim (November 2001). "New Frontiers in the Design of Steel Catenary Risers for Floating Production Systems". Journal of Offshore Mechanics and Arctic Engineering 123 (4).