Tc 67/sc 4 n date: 2005-03-9 iso/wd XXXXXX ISO tc 67/sc 4/wg 6 Secretariat: Design of dynamic risers for offshore production systems Élément introductif — Élément central — Élément complémentaire Warning
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Global AnalysisPrevious sections have discussed use of a riser analysis in design, and modeling of environmental effects lead to applied loads and vessel motions. The objective of this section is to provide general guidance on analyses techniques and modeling practices typically used in industry. The treatment is intentionally generic, because many techniques apply to a wide variety of riser configurations and can be used to generate basic data from which further detailed results (e.g., component stress, tensioner stroke, riser clearance) can be derived. More detailed guidance for specific analyses and riser types is provided in subsequent sections. The following presents the basic equation of motion fundamental to riser analysis and covers several key issues that need to be properly addressed in every riser analysis, including effective tension, stiffness, mass, buoyancy and hydrodynamic loads. Next, a discussion of typical boundary conditions is presented, followed by an introduction to various solution techniques. Finally, several special modeling considerations are discussed. For simplicity in presentation, the following discussion is restricted to the case of planar, small angle, linear strain analysis of an initially straight riser modeled as a tensioned beam. The static equilibrium equation for lateral displacement of a tensioned beam is:  ...(35)
where z = Spatial coordinate along the beam axis, u = Transverse displacement in the direction of the load, EI = Bending stiffness of cross section, r = Applied lateral load, not including applied hydrostatic and pressures Te = Effective tension. The beam’s resistance to deformation is provided by flexural stiffness and more significantly, geometric stiffness arising from axial tension. The dynamic equation of equilibrium can be obtained from Equation 35 by incorporating inertia forces and a mechanism for energy loss (damping). Applying D'Alembert's principle, assuming viscous damping, and simplifying leads to:   ...(36)
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