Steel catenary risers (SCRs) have been an attractive choice for deepwater field developments. However, standard SCRs may not be feasible for large motion vessels, especially in harsh environments. Titanium catenary risers (TCRs) or SCRs with titanium sections or components will expand the applicability of metal catenary risers.

This article summarizes a fully developed titanium riser concept for the Kristin project in the North Sea where the design of metal risers poses many design challenges with regard to buckling and fatigue. A feasible and robust riser configuration is presented, consisting of a TCR with varying weight coating to optimize dynamic behavior. A well-proven installation method is outlined consisting of onshore riser assembly, fabrication and sheltered launch. Riser launch over a chute, directly into a catenary configuration is proposed, avoiding any surface tow as well as minimizing use of buoyancy. This riser configuration is optimized for dynamic behavior and costs, by varying the weight coating and by adding steel cross-sections where feasible and the design can be extended for deepwater large diameter export risers, where traditional SCRs have design limitations.

Design

The riser design is to comply with the NPD (1990) regulations DNV-OS-F201 and DNV-RP-F201, meaning that the developed configuration is to meet ultimate, accidental and fatigue limit state criteria.
The riser is designed for a 100-year wave condition, combined with a 10-year current profile and the titanium chosen for the riser was ASTM Grade 23. Mildly sour conditions were expected and although this is not a NACE approved grade, Statoil was confident it would be suitable. It was also expected that mercury could occur in the export stream and test data from RTI showed that this would have no detrimental effect on the riser.

For assessment of fatigue response in the welded part of the riser, the S-N curve proposed by Marintek, of Norway, which was developed as part of Åsgard qualification study, is applied. This S-N curve is based on test results from welds from three suppliers using the lower bound curve of all data. Titanium riser is lightweight and hence a weight coating is applied to make it negatively buoyant. The density of weight coating is 3,000 kg/cu m and the weight coating thickness varies over the riser length.

Configuration

The riser configuration is shown in Figure 1. The riser is equipped with a keel-joint at the side of the sponson. The angle of the riser is 9? from the top end to the keel-joint. Below the keel-joint, the riser is designed with a larger static angle from the catenary shape.

Design challenges for a riser of this dimension at such shallow water depths are:

• Buckling and fatigue at touch down point (TDP)
• Fatigue at the keel-joint location

Titanium riser of this dimension for application of gas export needs a weight coating along its entire length. In this work a feasible riser configuration is achieved by varying the weight coating along the riser length, thereby optimizing stress distribution along the riser.

Response

In the strength analysis regular and irregular waves are used. The strength analysis is performed for the vessel in three positions, near, far and transverse positions.

The summary of the strength analysis results is:

• Tension:
• Static tension at top-end in mean position 813 kN
• Maximum tension in top-end in ULS 2192 kN
• Minimum tension in top-end in ULS 151 kN
• Maximum von Mises stress in ULS 492 MPa
• Maximum buckling utilization - in riser at TDP in ULS 0.94
• Maximum buckling utilization - in Taper Stress Joint in ULS 0.78

The fatigue response analysis is performed using a comprehensive procedure indicating the riser has sufficient fatigue capacity. The fatigue responses for the most critical sections are:

• At taper stress joint - non-welded cross-section 348 years.
(At the keel-joint location)
• At taper stress joint - welded cross-section 421 years.
(At a cross-section 26 ft (8 m) from keel-joint)
• Riser section with 16 mm wall thickness 928 years.
• At touch down area 18 mm wall thickness 1474 years.

Fatigue response due to VIV, calculated by using Shear7 software, is 4,787 years at the most critical location.

The riser body is manufactured from Grade 23 seamless pipe, which is rolled in a process similar to that used to manufacture steel line pipe. The manufacturing plan called for pre-welding the 32.8-ft (10-m) pipe lengths into doubles, shipping them to the pipe coaters, and from there to the fabrication yard at Gaupne.

Installation

The two export risers are planned to be launched and towed in a bundle, side by side. The water depth at Gaupne allows risers to be launched directly into the towing configuration, avoiding any surface tow. When arriving at the Kristin semisubmersible location, the risers are to be parked on the seabed and separated ready for pull-in, using the platform's winch assisted by a multi-service vessel (MSV) or lead tug.

Risers are towed in one operation and the maximum tow speed is based on static bollard pull (weight of risers and rigging) and the drag inherent in the risers and buoys during tow. The maximum bollard pull of MSV Nordica is 234Te. At 6 knots the tow duration is estimated to be 60 hours.

Risers will remain parked on the seabed until the export riser base is installed. The risers will be installed individually, where both vessels will lift either riser end off the seabed for final positioning, followed by hand over and pull-in to the Kristin topside. Pull-in will be completed using the Kristin topside winch. Wire from the platform 100 tonne pull-in winch is guided through the keel joint and attached to the top end of one riser while the trailing tug is attached to the subsea end to apply back tension.

The subsea end will be laid down by the tug and maneuvered into position approximately 49.2 ft (15 m) from the tie-in point. Pull-ins and connections will be performed using conventional tie-in tools for large diameter pipe.

Extended

The riser configuration here is for relatively shallow water in a harsh North Sea environment but similar concepts can be easily extended to deepwater conditions from large motion semisubmersible platforms. In deepwater conditions, a full titanium riser may not be necessary. A weight-optimized SCR with titanium sections in the heavily stressed top-end and at TDP will be the most optimum solution. We recommend that this should be investigated further.

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