Page 314 - Marine Structural Design

290 Pari II Ultimate Strength
- S P V

v >v

vP

Figure 14.1 Collision between Supply Vessel and Semi-submersible
Platform

14.3.3 Conservation of Energy
In a central collision, all the kinematic energy dissipates as strain energy (elastic or plastic) in
the ship (E, ) and in the platform (E, ).
In the case of an eccentric collision, some of the kinematic energy will remain as rotational
energy of the vessel or of the platform after the collision.
The equation of conservation of energy assuming central collision reads:
1 1 1
-rn,v: +-m v2 =- h +m,k +E, +E, (14.10)
2 2 pp 2
where,
E, = Strain energy dissipated by the ship
E, = Strain energy dissipated by the platform

Combining this equation with the equation for the common velocity, v, the following
expression for the dissipated strain energy emerges:

1
E, +E, = -m,v: (14.11)
2

The wave induced platform motion is often small compared to the ship velocity and can be
neglected. Otherwise, the characteristic velocity should be based u on a stochastic evaluation
of the relative collision velocity between the ship and the platform b, - vp ) accounting for the
phase lag between the respective motions.