Recommendations |
 
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Recommendations |
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It is recommended that in analyses in which the bending response of a flexible riser is characterised by hysteresis, that the seabed contact model used be elastic (rather than rigid). Experience has shown that the convergence characteristics of analyses which include hysteresis are greatly improved if an elastic seabed is used. Refer to Elastic Seabed Profile for options available for defining an elastic seabed bathymetry
Based on simple test cases performed during the original implementation of the hysteresis modelling feature in Flexcom, it was felt preferable initially to use the Constant (rather than the default Updated) damping formulation, where damping effects were included. Refer to Damping for further information on the various damping options in Flexcom. The rationale was that as the damping matrix is directly proportional to the stiffness matrix, instantaneous changes in damping forces could be introduced by the slip-stick nature of hysteretic response, which could potentially affect the solution. However, based on experienced gained with realistic flexible riser models since then, using the Updated formulation is now recommended practice. The stiffness of a flexible riser includes a significant contribution from the effective tension distribution, and as the effective tension varies during a dynamic analysis, so too does the riser stiffness. Consider the case of a flexible riser in a simple catenary configuration – if effective compression occurs intermittently in the touchdown region, the geometric stiffness contribution can alternate between substantial and negligible values. Furthermore, if the model is susceptible to buckling, the orientation of elements in the touchdown zone can vary quickly and significantly. Both of these issues will effectively be ignored in the assembly of the structural damping matrix if a Constant formulation is used.
•*SEABED PROPERTIES is used to specify properties such as seabed type (i.e. rigid or elastic) and friction parameters.
•*DAMPING FORMULATION is used to specify the damping formulation to be used in a time domain dynamic analysis.