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.
Tie-in systems for deepwater field developments have grown in complexity from the simple dogleg assembly of pipes and bends traditionally used in shallow water (Figure 1). This complexity poses a number of design and installation challenges for deepwater projects where the tie-in spools need to accommodate large end expansions, fitup tolerances and differential pressures while at the same time minimize the interface loads. Using an allowable stress design (ASD) approach, thick pipe sections may be needed to accommodate the load effects generated by the large fit-up tolerances but this increases the rigidity of the spool and consequently the interface loads.