Application of Design By Analysis Procedures For Deepwater Subsea Tie-In Systems

The Saipem Group has been involved in the design, construction and installation of subsea systems in water depths exceeding 2000m. In most of the cases, the design verification of the tie-in spools represented a critical element of the project execution largely due to the absence of clearly defined design criteria for deepwater pipeline components. In the absence of such criteria, which take into account the particularity of the loading regime where the main load is due to end expansions and fit-up tolerances rather than external mechanical loads, the verification using allowable stress design can lead to complex if not unfeasible spool configurations. The complexity is further increased when the design relies on a deterministic approach to accommodate the fit up tolerances generated from different sources of uncertainties. To this end and as the industry moves into deeper waters, attention has to be paid to the development of design criteria specific for pipeline components as was the case in recent project experience. Here, design by analysis procedures and reliability methods were used to define a strain based design criteria which was successfully employed to produce viable spool configurations. This and other aspects of the design of tie-in spools are discussed in this paper with emphasis on the design of induction bends.