Riser-System Design in Water Depths Greater Than 3000 m
Authors: Carpenter Chris 
Published: January 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25840, “Frontier Deepwater Developments: The Impact on Riser-System Design in Water Depths Greater Than 3000 m,” by N. Saglar, B. Toleman, and R. Thethi, 2H Offshore, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed.

The offshore industry anticipates the need for production-riser systems in ultradeepwater fields. The development of these fields [this paper considers a field located in the central Gulf of Mexico (GOM)] leads to many challenges with respect to the selection of the riser concept; in some instances, such applications may require extending riser technology beyond its current limits. This paper evaluates the feasibility of a number of production-and export-riser configurations for ultradeepwater applications.

Riser-Wall-Thickness Sizing Please note that riser-design criteria, methodology, and data (riser functions and associated pipe sizes; riser internal-fluid properties; and riser-strength assessment) are provided in the complete paper.

Production Risers. Steel-catenary-riser (SCR) wall-thickness sizing is initially carried out when considering X65 line pipe. For a design pressure of 5 ksi, the wall thickness of the production riser is primarily driven by collapse because of external hydrostatic pressure. The maximum wall thicknesses required for 8-, 10-, and 12-in. pipes are 1.51, 1.85, and 2.17 in., respectively, and are driven by burst owing to the 15-ksi internal design pressure. It should be noted that these wall thicknesses are designed to resist only the burst and collapse pressures. The longitudinal-load and combined-load design checks are performed after the sizing is performed for collapse and burst. For ultradeep water, the longitudinal-load design criterion becomes a limiting requirement. For 3000-m water depth, the production risers meet both the longitudinal-load and combined-load design criteria. Buckling caused by combined bending and external pressure is also checked with the calculated wall thicknesses. The allowable bending strains of all the production risers are determined to be greater than the assumed maximum bending strain of 0.5%.

The field-proven thickest wall to date for conventionally welded X65 single-pipe wet-tree production-riser systems is 1.65 in. In this paper, the maximum wall thickness that can be welded for offshore fatigue-sensitive riser systems is assumed to be 1.9 in., on the basis of existing tests. Only the wall thickness required for a 12-in. production riser with a design pressure of 15 ksi exceeds the 1.9-in. limit.

Export Risers. SCR wall-thickness sizing is initially carried out considering X65 line pipe with a design pressure of 5,000 psi. The wall thickness of the export riser is first calculated to resist the burst and collapse pressures. It is determined that the wall thickness required for collapse is higher than the sizing based on burst for all water depths and outer diameters. However, the wall thickness driven by collapse does not meet the 0.5% allowable bending strain considering the combined bending and external-pressure criterion. The wall thicknesses of the export risers are increased to maintain the allowable bending strain of 0.5% in the pipe. Therefore, the wall thickness of the export risers is driven primarily by buckling because of the combined bending and external-pressure criterion. The wall thicknesses of the export risers are shown in Fig. 1. The maximum wall thicknesses required for 16-, 20-, and 24-in. pipes are 1.55, 1.95, and 2.32 in., respectively, and occur at a water depth of 4500 m.

When considering the longitudinal load from the static tension in the flooded condition at the top of the riser in water depths of at least 3750 m, the effective tension exceeds the 60%-of-yield tension capacity. For ultradeep water, the longitudinal-load design criterion becomes the limiting requirement.