Back-Span Stress Joint
Authors: Marshall P.W. , Edwards C.D. , Pijfers J.G.L. , Webb C.M. 
Published: October 2008

The need to accommodate increased rotation of a hull with respect to the upper end of a riser is a limitation inherent in the smaller spar-type designs which may be considered for increasing the profitability of deep water developments. The back-span stress-joint increases riser flexibility as required to accommodate increased rotations by having most of the riser tension taken out at the lower hull support.

INTRODUCTION, SUMMARY& CONCLUSIONS

The idea of tapered stress joints for risers is well mown. They allow a continuous hard-piped bore for veils and pipelines, while accommodating the lending angles required for compliant structures, e.g., 17LP's. However, their usual application is at the bottom of risers, where tension is low and the required angle is only 3 to 6 degrees. Other types of floating production systems, which pitch more than a TLP, need to accommodate relative angles up to 11 degrees at the keel interface, near the top of the riser. Furthermore, this must be done in the presence of full top tension, which tends to concentrate bending strains, making it more difficult to achieve the required angle without overstressing the riser. If riser tension is taken out at the lower hull support, e.g., through an elastomeric flex element supporting back-to-back stress joints, this creates a substantially lower tensioned inboard span which can more easily flex. The net riser angle is achieved mostly in the upper tapered joint inboard of the flex element (i.e. the backspan), plus some from the lower (outboard) tapered joint, with the total angular capacity being 4 to 12 times that of a conventionally fixed stress joint, and 2 to 6 times that of a fully tensioned back-to-back arrangement.

Rigid risers with back-to-back and backspan stress joints have evolved at Shell in studies in connection with several production system concepts, each providing topside wellhead access for frequent well intervention in 3000-ft. water depth. These include;the "Longneck" tethered spar,10,000-ton and smaller mini spars,larger spar with an onboard riser/completion/workover rig,S.P.S.O. (semisubmersible production-storage-offloading unit).

Earlier riser designs used a single outboard stress joint, then evolved to top tensioned risers with back to back stress joints at the keel. Finally, the backspan concept was introduced to accommodate even larger predicted pitch angles. While vertical motions are normally low, tracked buoys provide top tension through extreme stroke ranges of up to 60 ft.

Design methods range from simple generic equations to fully coupled hull and riser dynamic analyses with Shell's WAMIT-COSMOS suite of programs. The backspan stress joint has been shown to be workable in all of the cases studied so far. For example: for a 9 5/8 inch diameter dual-casing riser with 500 kips top tension, the backspan stress joint accommodates 11 degrees relative pitch/roll with a modest 1" thick tapered joint, while a fully tensioned back-to-back arrangement requires 3-inch thick gunbarrels. A backspan stress joint with the latter proportions would be able to achieve over 20 degrees rotation.