Interaction Model for Steel Compliant Riser on Soft Seabed
- Charles Paul Aubeny (Texas A&M University) | Giovanna Biscontin (Texas A&M University)
- Document ID
- Society of Petroleum Engineers
- SPE Projects, Facilities & Construction
- Publication Date
- September 2008
- Document Type
- Journal Paper
- 1 - 6
- 2008. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 1.2.2 Geomechanics, 4.2.4 Risers, 4.1.5 Processing Equipment, 4.3.4 Scale, 4.2 Pipelines, Flowlines and Risers
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The use of catenary steel-compliant-riser (SCR) systems has increased as hydrocarbon production has moved progressively farther offshore and into deeper waters. The issue of fatigue damage caused by cyclic interaction of a riser with the seabed has gained prominence with the widespread use of SCRs and with the lengthening of the spans. The problem involves a number of complex factors, including trench configuration, nonlinear soil stiffness, breakaway of the riser from the seafloor, and degradation of soil resistance during cyclic loading. This paper presents a soil-interaction model capable of modeling these complexities, using input parameters that can be obtained with reasonable expenditure. Model simulations for typical offshore soft-soil conditions indicate that the model is capable of realistic predictions of cyclic bending moments. The degradation of soil resistance has a major effect on cyclic bending moments, particularly when uplift motions at the riser touchdown point (TDP) are large.
The introduction of these compliant floating systems for offshore hydrocarbon production has led to the development of new designs for the riser pipes, with the SCR often being the system of choice. Fatigue stresses associated with extreme storms, vessel movements, and vortex-induced vibrations are critical to SCR performance. The zone at which the SCR contacts the seabed, the touchdown zone (TDZ) (Fig. 1), often proves to be a spot where bending stresses are largest and, therefore, is a critical location for fatigue (Bridge et al. 2003; Bridge et al. 2004). Analyses typically show fatigue damage to be sensitive to seafloor stiffness, which currently cannot be estimated with much reliability. It should be noted that the vertical scale in Fig. 1 is exaggerated for illustrative purposes. Typically, the ratio of trench depth to trench length is small (approximately 1%), so the problem may be reasonably treated within the framework of a small-deflection-beam analysis. However, uplift at the TDP is sufficiently large that the end moment associated with axial tension in the riser should be considered in the analysis.
This paper presents an analytical framework for soil/riser interaction on the basis of a model constituting a linearly elastic pipe supported by nonlinear springs. This model accounts for effects of nonlinear soil load-displacement behavior and separation of the riser from the seafloor. Degradation of soil resistance during cyclic loading is implicit in the model, in that soil stiffness during reloading (laydown) is always less than soil resistance during unloading, behavior supported by experimental evidence. This paper first presents a model for spring stiffness. The spring-stiffness model is then incorporated into a riser pipe/soil spring interaction model. Finally, the model is applied to a test case for typical offshore soft-clay conditions.
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