Steel catenary risers (SCRs) have been an attractive choice for recent deepwater field developments. However, design standard SCRs for large motion vessels face two major challenges.

SCRs are an attractive choice for high-temperature, high-pressure applications and large-diameter export lines from semisubmersibles in deep water. Application of SCRs to semisubmersible floating production units presents design challenges because of the large vessel motions, resulting from waves, and large vessel offsets from wind, current and slow-drift wave actions. Large heave motions cause buckling at the touch-down point (TDP). And fatigue problems arise from vessel motions and soil-riser interaction.
These problems can be addressed with a lightweight coated steel pipe in the TDP area. The effects of applying this technique have been studied with non-linear time-domain analysis in both North Sea and Gulf of Mexico conditions.

Applying a comprehensive, irregular wave, time-domain procedure analyzes fatigue. The procedure reveals an improvement in the dynamic behavior of SCRs with lightweight coatings. The von Mises stress at TDP is reduced considerably, and the effect of soil-riser interaction on the fatigue response of lightweight coated SCRs is greatly reduced compared to a bare SCR. Hence, the coated SCR presents an attractive solution for deepwater applications with large vessel motions.

Coating availability, limitations

Polymeric external coating is generally used on flow lines and risers for corrosion protection, mechanical protection and thermal insulation. The most common coating systems used in the oil and gas industry are multi-layer and foamed PP/PE; polyurethane/syntactic polyurethane; and rubber coating.

Weight and buoyancy requirements

Dynamic riser behavior is controlled by distribution of weight and buoyancy along the riser. Weight is obtainable by increasing the steel wall thickness and/or by applying a weight coating. For large diameter risers, weight coating is often required, depending on the limitations in metal pipe production (maximum wall thickness). Weight coating with a density of 3,000 kg/cu m and thickness up to 124 mm are available.
Buoyancy can be obtained by attaching buoyancy modules or by applying a low- density coating (preferred due to the complexity of offshore installation of buoyancy modules). However, if the length of riser requiring buoyancy is significant, a low density coating system can be applied more easily.

Design data

The riser design must comply with the NPD (1990) regulations and API RP2D/API RP1111, meaning that:
• the developed configurations are to fulfill PLS, ULS and FLS limit state criteria;
• the extreme stresses are to be checked by the working stress method; and
• the design lifetime shall be obtained using a factor of 0.1 on the calculated fatigue lives. The minimum fatigue life required for the risers is 25 years.

Environmental conditions

The riser is designed for the 100-year wave condition in combination with 10-year current profile. The design conditions are shown in Table 1.

Analysis procedure

Riser configuration is developed by meeting the ULS design conditions. The basic configurations are obtained by performing non-linear dynamic response analysis using the dynamic analysis program RIFLEX.

A comprehensive non-linear time domain fatigue analysis is performed and the response is obtained by completing a non-linear dynamic response analysis using random irregular waves, applying the Airy wave model.

Results: North Sea

For the North Sea, a feasible configuration with acceptable strength and fatigue life is achievable by varying the weight along the riser length with a long section of lightweight coated riser section at TDP. The diameter of the riser is 16-in. (406.4 mm). A heavier section in the middle is needed to reduce the dynamics, and the top angle is 17 degrees.

Strength and fatigue analyses are performed for the configuration with both the lightweight coated pipe and the bare pipe. Results, shown in Table 2, indicate the lightweight coating decreases the extreme von Mises stress at TDP by about 40%. Without coating, von Mises stress was 1.22 times yield stress. The results indicate top tension is reduced by nearly 10% with a lightweight coating.
Fatigue life improves by more than three times when lightweight coating is applied, corresponding to approximately 50% improvement in stresses within sea states, which mostly contribute to fatigue. The influence of soil-riser interaction on fatigue response is reduced.

Gulf of Mexico

Varying weight along the riser length allows a feasible configuration for the Gulf of Mexico with acceptable strength and fatigue life. The outer diameter of the riser is 16 in. (406.4 mm), and the top angle is 13.8 degrees.

The 1,312-ft (400-m) long section of lightweight coated riser section is necessary at TDP to achieve a feasible configuration. Strength and fatigue analyses are performed for the configuration with both lightweight coated pipe and bare pipe. Results in Table 3 indicate lightweight coating decreases the extreme von Mises stress at TDP by approximately 20%. Fatigue life improves by more than 70%, corresponding to a 20% improvement in stresses within sea states.

References

API RP 2RD, "Design of Risers for Floating Production Systems (FPSs) and Tension Leg Platforms (TLPs)," first edition, June 1998.
API RP 1111, "Design, Construction, Operation and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design)," third edition, July 1999.
Karunakaran, D.; Dutta, A.; Clausen, T.; and Lund, K. 2002. "Steel Catenary Riser Configurations for Large Motion Semisubmersibles with Lightweight Coating," Proceedings of Deep Offshore Technology Conference, New Orleans, 2002.

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