Displacements calculated in the user-subroutine can be added to or superimposed on displacements calculated from vessel motions. The major application for which this facility is intended is in modelling displacements due to TLP setdown. Consider, for example, a TLP tether subjected to 2D environmental conditions. The vertical motion of the top of the tether is due to two effects, the vertical motion of the TLP itself (heave) and the setdown due to the horizontal TLP motion (surge). In defining the boundary conditions for an analysis of this tether, you would specify that both the vertical and horizontal motion of the topmost tether node were to be calculated from vessel (TLP) motions via RAOs and phase angles. Then you would further specify a user-subroutine to be invoked for the vertical direction at the node. In the user-subroutine itself the setdown due to the instantaneous value of dispy would be calculated and added to dispx to give the total vertical motion of the node. This procedure is illustrated in the TLP Setdown Example to follow.
