Offshore Structure Loads and Strength - Offshore Structures: Design, Construction and Maintenance - page 75

2.7 COLLISION EVENTS
If a rigorous impact analysis requires, accidental (collision event) design should be
established representing bow, stern and beam-on impacts on all exposed
components.
The collision events should encompass both fairly frequent conditions, dur-
ing which the structure would suffer only insignificant damage, and rare events,
where emphasis is on avoiding a complete loss of integrity of the structure.
Two energy levels should be considered:
1. Low energy level, representing the frequent condition, based on the type of
vessel that would routinely approach alongside the platform, such as a sup-
ply boat, with velocities representing normal maneuvering of the vessel as it
approaches, leaves or stands alongside the platform.
2. High energy level, representing a rare condition, based on a vessel operating
in the vicinity of the platform and drifting out of control in the worst sea
state in which it is allowed to operate close to the platform.

In design for both collision situations, the first energy level represents a
serviceability limit state for which the owner can set the requirements based
on practical and economical considerations. The second energy level represents
an ultimate limit state in which the structure is damaged but progressive
collapse should not occur.
In both cases, the analysis should account for the vessel smass,itsadded

mass, orientation and velocity. Effective operational restrictions on vessel
approach sectors can limit the collision exposure of some areas of the structure.
The vertical height of the impact zone should be established based on the dimen-
sions and geometry of the structure and the vessel, and it should account for tidal
ranges, operational sea state restrictions, vessel draft and motions of the vessel.

'

2.7.1 Vessel Collision
Accidental damage should be considered for all exposed elements of an instal-
lation in the collision zone. The vertical extent of the collision zone should be
assessed on the basis of visiting vessel draft, maximum operational wave height
and tidal elevation.

Accidental Impact Energy
Total Kinetic Energy
The total kinetic energy involved in collisions can be expressed as:

½amV 2

E = (2.30)

where m is the vessel displacement (kg); a is the vessel added mass coefficient,
which is 1.4 for sideways collision and 1.1 for bow or stern collision; and V is
the impact speed (m/s).