LESSONS LEARNED FROM DEEPWATER-SPOOL DESIGN ON AASTA HANSTEEN

摘要

Spools are frequently used to tie-in pipelines to subsea structures with the purpose of absorbing expansions at the pipeline end, in addition to the connection between pipeline and structure. Deepwater rigid spools have to be designed to accommodate expansion movements resulting from high product temperatures, low strength soils and phenomena such as pipe walking. They also have to accommodate spool fabrication and installation tolerances. These requirements drive the spool geometry and can lead to complex spool geometries that are difficult to install. The design code usually used for spool analysis is DNV-OS-F-101 [1] which is a primary design code for pipeline design. While using this design code for straight-linepipe and bends, careful considerations should be made because the bends tend to behave different from a straight pipe. Especially the ovality response under in-plane bending is of interest for establishment of a design criteria. The most accurate way to establish this design criteria is by utilising a 3-D non-linear finite element (FE) analysis. The bending moment at failure should be compared with the corresponding bending moment under design conditions. By using FE analyses, more flexible spool can be designed giving lower reaction forces and moments. In addition there are other design-optimization methods to reduce the required spool size and complexity which are further described in this paper. The present paper shows the reduction in wall thickness that is possible through design by analyses. The spool design process is also outlined, and a comparison made on different spool size optimization concepts.