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Contact with the seabed
It is possible to define a sea floor composed of different plane surfaces with different slopes. Friction elements may be created by the user to connect some points of the structure and the sea floor. These elements generate the reaction and friction forces when there is contact.
The seabed contact is ruled by contact elements connecting a node of the riser with a node on the seabed. Following the relative positions of these two nodes, each contact element generates reaction and friction forces.
Normal Reaction Force
The normal reaction force is proportional to the penetration, except close to Z=0 where a quadratic formula is used to ensure tangent continuity. The nodal reactions must balance the apparent weight of all the elements connected to each node. For a binodal elements, if Lo is the length and Wo its apparent weight, the nodal penetration is approximately be given by: Z = WoLo / (2Kn) where Kn is the normal stiffness.
Friction Coulomb Model
As for the lateral displacements, two friction coefficients are introduced along with two given directions u 1 and u 2. The total friction force is Ffric = F 1+ F 2 and the Coulomb criteria is separately checked on both directions. The Coulomb law is slightly modified for numerical reasons. A small reversible lateral displacement is allowed even when the lateral force is lower than the Coulomb threshold FN. The contact therefore element acts like a spring with a given stiffness Kt. The threshold displacement Th is linked with Kt through \(Th = \mu Fn/Kt\), where is the friction coefficient.
Effect of Trenches
The lateral friction force model may include the additional peak resistance loads exerted along the line when embedded in a trench. Trench geometrical properties such as depth, bottom and upper width together with associated lateral resistance loads characteristics can be defined from the Contact Type window as illustrated below :
