During conceptual engineering for a deepwater Gulf of Mexico field development project, the option of a long distance dual flowline tie-back to a semi-submersible platform was assessed. Pipe-in-pipe (PIP) steel cantenary risers (SCRs) were considered for connecting the PIP flowlines to the platform, to satisfy stringent thermal insulation requirements.
Preliminary analysis results for the PIP SCRs based on an equivalent single pipe finite element (FE) model and use of generic correction factors to account for PIP behavior indicated marginal performance for both fatigue and extreme hurricane loading conditions. Therefore, to improve response prediction accuracy, application-specific correction factors were established by comparing results from the single pipe model and a detailed PIP model, for selected static and dynamic load cases. This paper describes the analysis models and methodology, the load cases assessed, and summarizes the results and conclusions.
Hydrodynamically loaded PIP structures have been in use since the first offshore platforms were installed more than fifty years ago. In the case of such platform conductors, Stahl and Baur1 reported that when an inner pipe (casing or tubing) is not ideally centralized, it provides additional contribution to the global bending moment by an amount equal to the product of its tension and the offset (or eccentricity) of its centerline from the centerline of the outer conductor pipe. This contribution can be significant for platform conductors when assessing their global stability, particularly for deep reservoirs when the tension in the casings and tubing is large.
Top-tensioned risers also often consist of two or more concentric pipes. For computational efficiency, such risers are conventionally analyzed using an equivalent single pipe model.
SCRs, on the other hand, have historically tended to be single pipe structures and therefore have not required such considerations. However, recently, PIP flowline SCRs have been used for deepwater developments in order to satisfy stringent flow assurance requirements. As with vertical risers, it is evidently advantageous to use a single pipe model for the majority of design analyses. However, in order to safely use this approach, it is first necessary to establish appropriate and reliable means to correct for PIP effects that are not captured in the single pipe model.
The water depth at the host platform location considered was approximately 6000 feet. The PIP riser consisted of an 11 3/4‘outside diameter steel outer pipe, and a 7 5/8’ outside diameter steel inner pipe. Figure 1 presents a sketch of the PIP arrangement, with discrete centralizers.
It was proposed that the inner and outer pipes be connected by bulkheads at the top of the riser (immediately below the flexible joint) and at a location on the seabed beyond the extreme touchdown point.
The centralizers were spaced at six-foot intervals over the full length of the riser between the two bulkheads. The outer diameter of the centralizers was taken to be equal to the drift diameter of the outer pipe, to facilitate installation.