Flexcom > Theory > Model Building > Contact Surfaces > Guide Surface Friction Modelling

Theory

Friction on flat or cylindrical guide surfaces is modelled using a very similar approach to that adopted for seabed friction (refer to Seabed Friction Modelling Algorithm for a detailed discussion of the seabed friction model). Specifically, friction is effectively modelled using non-linear springs. In an ideal (that is, Coulomb) friction model, each of these non-linear springs would completely prevent motion until the total force exceeds the limiting friction force, at which point free motion is allowed subject to a resistance which is equal to the limiting friction force. The main difficulty with implementing such a friction model in a finite element program such as Flexcom, which solves for deflections, is that the stiffness of the ideal spring is effectively infinite in the region corresponding to zero deflection. This would make it virtually impossible for the program’s iterative solution scheme to converge on the correct solution. Instead, this spring characteristic has the region around the zero-deflection point replaced by a section of very high (but not infinite) stiffness. The stiffness of this section of the force-deflection curve is given by the expression:

(1)

Here Lc is what is known as the ‘characteristic length’. Clearly, the value of Lc affects the stiffness of the non-linear spring – the smaller this value, the greater is the stiffness and the closer the friction model is to an ideal model. However, reducing Lc makes it harder for the program to converge on a solution.

The guide surface friction model differs from the seabed friction in terms of characteristic length selection. In the seabed case, the value of Lc is based on the element length, as this gives a degree of scalability to the model. You can also specify maximum characteristic lengths in both the longitudinal and transverse directions (both of which default to 3.047m/10ft if unspecified). Guides are typically used to model contact over relatively short contact lengths – a typical application might be that of a Spar, where intermittent contact is modelled between risers and sections of the Spar hull. As the surfaces are typically short, the characteristic length is unrelated to the element lengths, and simply defaults to 10% of the guide ‘size’, though you do have the option to override this. This ‘size’ is the flat surface height or cylindrical surface length. Note also that the same characteristic length is used in both the longitudinal and transverse directions.