Riser-System Design in Water Depths Greater Than 3000 m
This paper evaluates the feasibility of a number of production- and export-riser configurations for ultradeepwater applications.
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.
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.
After wall-thickness sizing is carried out, an optimum water depth is calculated for each SCR configuration while meeting all of the American Petroleum Institute (API) RP 1111 criteria. Water-depth limitations of each riser are determined considering both X65 and X70 steel grades. X70-grade pipes result in thinner wall thicknesses compared with risers with X65-grade pipe. X70-grade pipe also results in a lighter static riser weight at the top of the riser because of a thinner wall thickness and hence extends the water-depth limit. The maximum water depths for the X65 8-, 10-, and 12-in. production risers considering a 5,000-psi design pressure are 3617, 3495, and 3422 m, respectively. As the riser design pressure increases, the water-depth limit decreases. The maximum water depth for the production risers is 3,903 m considering X70 8-in.-outer-diameter (OD) pipe and 5,000-psi design pressure.
As with production risers, higher water depths can be achieved with X70 pipe, with a maximum water depth of 3340 m. The maximum water depth does not change significantly for different ODs. It should be noted that these water-depth limits are obtained for the riser-top angle of 12°.
SCRs are constructed from conventionally welded line pipe. A typical steel grade used in the SCR applications is X65. However, X70 pipe is also used in some SCR projects in the GOM. X80 welded line pipe has not been used to date for the conventionally welded SCR systems. Weldability of thick-walled X80 line pipe offshore is not qualified in the industry yet for sour-service conditions; therefore, it is not considered in this paper.
Riser-wall-thickness sizing performed on the basis of API RP 1111 indicates that an advantage can be gained by increasing wall thickness along the top section of the riser instead of specifying a single wall thickness along the entire riser.
Strength analysis is carried out for the following three riser configurations:
- X65, 8-in. production SCR, 0.67‑in. wall thickness, 5-ksi design pressure, 3617-m water depth.
- X65, 10-in. production SCR, 1.85‑in. wall thickness, 15-ksi design pressure, 3283-m water depth.
- X65, 16-in. oil-export SCR, 1.17‑in. wall thickness, 5-ksi design pressure, 3100-m water depth.
Preliminary analysis shows that the SCRs exceed the allowable stress ratio of 0.67 in 10-year-winter-storm operating conditions. This is because 60% of the pipe-stress use is already taken by the static effective tension of the riser. With the addition of bending moments caused by vessel dynamic motions and hoop stress caused by operating pressure, the SCRs do not meet API RP 2RD criteria in the maximum operating environment.
The SCRs do not meet the allowable stress ratios in the 100-year-hurricane condition. For the 100-year hurricane survival condition, only the 8-in. production SCR does not meet the criteria. During the 1,000-year-hurricane condition, the stress in the riser exceeds the yield capacity of the pipe at the touch-down point (TDP) because of high bending moment induced by compression loads. The SCRs experience compression in the touch-down zone because of the high heave motions of the deep-draft semisubmersible at the longer wave periods in a 1,000-year-hurricane condition. In addition, an 8-in. production SCR is a relatively lightweight riser and extends into 3617-m water depth, which makes the riser very slender and susceptible to the heave motions of the floater. Because of this, excessive compression and stresses above yield are observed in the touch-down zone of an 8-in. production SCR.
On the basis of the dynamic response of the risers, it is seen that an X65-grade SCR attached to a deep-draft semisubmersible with a flexible joint presents a significant challenge to provide acceptable strength behavior in water depths greater than 3000 m.
Steel-Lazy-Wave-Riser (SLWR) Strength Assessment
Because of the challenges faced by traditional SCRs to stretch into water depths greater than 3400 m and to provide acceptable strength behavior, an assessment is performed with an SLWR. This is a special SCR with a segment of its length equipped with external buoyancy modules; its upward buoyancy force in water is greater than its downward gravity force. Because of this buoyancy force, the top tension of the riser at the vessel hang-off reduces significantly.
As with the SCR strength analysis, a preliminary strength analysis is also carried out with an SLWR configuration for the 10-in. production riser with 15‑ksi design pressure in 4500 m of water. A buoyancy section of 5,250 ft is installed above the TDP, which isolates the porch-heave motions from the TDP. On the basis of the analysis results, the SLWR meets the design-strength criteria for all load conditions considered. The maximum von Mises stress does not exceed the yield strength for a 1,000-year hurricane, demonstrating the robustness of the SLWR system. No compression is observed along the lazy-wave riser.
On the basis of the dynamic response of the risers, it is seen that the SLWR configuration leads to more-viable global response in water depths greater than 3000 m because of its lower top tension and its ability to control the dynamic heave response along the riser.
The 12-in.-OD production risers constructed from X65-and X70-grade pipe with a design pressure of 15 ksi result in wall thicknesses greater than 2 in. and are not feasible for a welded-line-pipe solution. 24-in.-OD export risers lead to wall thicknesses greater than 2 in. because of high external collapse pressures and very high static axial loads; hence, they are not feasible for conventionally welded SCR applications in ultradeep water. The maximum water depths obtained by large-OD (16- to 24-in.) export risers with conventionally welded SCRs are less than 3400 m. It is determined that SLWR systems are a feasible option for water depth beyond the reach of SCRs because of additional uplift forces obtained from the buoyancy modules.
As the water depth increases, the mixed-mode axial and bending excitation frequencies of the risers tend to move into the range of wave energy and increase the likelihood of exciting the risers in small-period sea states that tend to drive long-term fatigue life. If a floater has a heave response even with small amplitudes at a mixed-mode excitation frequency matching that of the riser, it would be sufficient to induce significant dynamic motions in a suspended riser and increase fatigue damage.
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. Copyright 2015 Offshore Technology Conference. Reproduced by permission.