Summary
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.
Introduction
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.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
2 February 2008
- Meeting paper published:
5 May 2008
- Revised manuscript received:
14 March 2008
- Manuscript approved:
14 March 2008
- Version of record:
15 September 2008
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