Implications for Seabed Modelling |
 
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Implications for Seabed Modelling |
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The major implication of this approach to friction modelling is that a node in contact with the seabed has to move a distance equal to the mobilisation length before the full limiting friction force is mobilised. Below this level of deflection, the friction force is proportional to the deflection. This means that the rate of tension dissipation along a length of pipeline on the seabed due to the presence of seabed friction may, in some circumstances, be a function of the mobilisation length. Specifically, there is a transition region in which the tension dissipation will gradually increase from zero (where no friction forces are mobilised) up to the theoretical maximum rate of tension dissipation (where friction forces are fully mobilised).
For the majority of applications, this is of little consequence. There are, however, some situations where it is desirable to be able to limit the maximum value that the characteristic length can take on. Traditionally, Flexcom provided a Maximum Characteristic Length input for this purpose, and while this feature is still retained for compatibility with earlier versions, it has effectively been superseded by an actual Mobilisation Length entry. Both entries basically allow you to control the stiffness of the non-linear springs used to model seabed friction in both longitudinal and transverse directions.
An option is provided to suppress the effects of friction. Very occasionally, it may be desirable to exclude seabed friction, for example, during the initiation of seabed contact in a pipeline analysis. By default frictional effects are modelled (provided at least one non-zero friction coefficient is specified), and this is suitable for the majority of analyses.
•*SEABED PROPERTIES is used to specify properties such as seabed type (i.e. rigid or elastic) and friction parameters. Specifically, the FRICTION= input is used to specify friction data such as friction coefficients and mobilisation lengths.
•*NO FRICTION is used to suppress the effects of friction.