Abstract

SCRs have been widely used in new field developments and tiebacks in deep water and ultra-deep water. However, riser fatigue at touchdown and SCR pipe - soil interaction is not fully understood. A number of experimental programs such as the STRIDE and CARISIMA JIPs have been carried out in recent years to get a better understanding of SCR touchdown behaviour in the context of riser fatigue at the touchdown and SCR pipe - soil interaction. Experimental studies were conducted using SCR sectional models of the touchdown region and by applying uni-directional displacements at the cut-off point to simulate SCR movement at the touchdown. However, application of uni-directional motions introduces significant errors compared to the true touchdown behaviour of a global SCR for a given sea state. This paper addresses how, by simultaneously applying surge and heave motions at the cut off point while removing the rotational constraint, a sectional SCR model can satisfactorily replicate the touchdown behaviour of a global SCR for any sea state in terms of riser displacements at the touchdown area, sag bend strain and SCR touchdown fatigue due to SCR pipe - soil interaction. As a result, more realistic SCR touchdown behaviours can be simulated in centrifuge model tests performed using sectional SCR models to investigate pipe - soil interaction at riser touchdown.

Introduction

To better understand the influence on the response of steel catenary risers (SCRs) on soil-structure interaction at the touchdown zone, a centrifuge testing program was initiated. The physical SCR model utilized in the centrifuge tests was truncated at a point equivalent in full scale to about 5m (16 ft.) above the sea floor. The results from this test program are discussed in a companion paper (Clukey et. al., 2011). The purpose of the centrifuge program was to examine the fatigue response for a model which realistically simulates the conditions throughout the touchdown region. However, before the centrifuge program could be initiated, extensive finite-element numerical studies were performed to determine:

  • the validity of a simplified local " sectional?? riser model to represent the complete global model;

  • the types of motions required at a mean position about 5m (16 ft) above the seafloor as simulated by the centrifuge tests; and,

  • the applied motions in the centrifuge to capture the random motions encountered in the field.

Figure 1 presents a schematic of the global and local sectional SCR models respectively. The cut-off location is 5m (16 ft) above the seabed as represented in the figure; and forms the " free end?? of the local sectional model.

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