How to model impact with fixed elongation coupling
1. Intro
This method is used to model an impact with only plastic deformation. This is a simplified method since the Forced Elongation Coupling (FEC) is not direction dependent. Here a 3D body is used to model the vessel. It is accelerated up to a fixed velocity (2 m/s) and the impact starts at 100 m. The impact is limited to plastic deformation and no elasticity.
2. Fixed Elongation Coupling  damping exponent

The fixedelongation coupling is using the same definition as the stationkeeping forces. The hysteresis mode is used to model the plasticity, however this can give large differences in force for small velocities, and can give numerical problems.
From the stationkeeping force (4.4.4 Forceelongation with hysteresis):
4. FEC  Velocity Limit
To avoid the numerical problems, the velocity limit can be used. For the smallest velocities, the linear damping model will be used.
5. Velocity limit = 0
In the top figure the impact force is shown, and in the picture below, the velocity of the vessel (red) and the impacted body (blue) is shown. It is visible from the velocity plot that there is three impacts where the bodies change velocities, however the impactforce is only shown for the first one.
There is no impact force even if there is a change in velocity of the vessel and the body.