4. Collisions with jacket platforms
Jacket braces are most exposed to bow collisions. Although the legs are less prone to damage, they should not be neglected in damage prevention schemes. A high-speed, central bow impact is a potentially serious event because the legs play the primary role of carrying the functional loads. Contact to the legs is also likely to occur in sideways impacts, whereas both the legs and braces are exposed in stern collisions.
A design impact event cannot be allowed to result in the direct collapse of the jacket, and the damaged platform must be capable of resisting the design environmental actions (where all of the partial safety factors are equal to unity). Further, impact on the risers and the conductors may result in environmental pollution, and the accident may escalate into a catastrophic fire. Hence, the risers and conductors in the collision prone zone should not be located towards the outer perimeter of the jacket, but rather at a safe distance inwards to ensure that the impacting vessel can be stopped before environmental damage or fire is likely to occur. Collision design places heavy demands on a single or a few braces if these braces are assumed to dissipate most of the collision energy and may become major concerns if the bow collision energy is increased in the revised NORSOK N-003 as expected.