Flexcom > Theory > Model Building > Contact Surfaces > Dynamic Analyses and Timestep Size

Theory

By its nature, intermittent contact between risers and flat/cylindrical guide surfaces involves modelling impact of the riser on the guide surface. Impact situations tend to cause structure response at high frequencies. This problem is exacerbated by the fact that, in general, the element length around the area of contact tends to be small, which can lead to increased structure response at these high frequencies. This difficulty may be alleviated in a number of ways:

•A small timestep may be required to successfully model the riser/guide surface impact. This should be borne in mind when selecting the timestep algorithm in dynamic analyses. If you are using a fixed timestep, then it should be sufficiently small to model the riser/guide impact. Similarly, if you are using a variable timestep, then the minimum permitted timestep should also be sufficiently small to model the riser/guide surface impact. For most applications, the variable timestep is recommended, as this allows the program to automatically select a small timestep when required to model impact.

•Even if you wish to model an effectively rigid surface, caution is advised against the specification of excessively large contact stiffness values. Using very high values tends to increase high frequency noise, resulting in the requirement of increasingly small time steps, and computationally intensive analyses.

•Specify a non-zero Contact Ramp, as described in Contact Modelling. Adopting a non-linear approach tends to lessen the effects of instantaneous impact, reduce high frequency noise, and consequently allows the solution to proceed at larger time step increments.

•Zero-gap guides may be used as an alternative to flat guide surfaces – particularly in situations where riser/guide contact is expected to be constant rather than intermittent. Unlike flat guide surfaces, constraints are not removed until sufficient relative motion in the axial direction has occurred. Models which use the zero-gap guide contact algorithm tends to be more dynamically stable as there is little or no high frequency noise introduced by the riser/guide contact.

Relevant Keywords

•*TIME is used to define time parameters for an analysis.