Page 555 - Marine Structural Design

Chapter 30 Risk Assessment Applied to qffshore Structures 53 1

Pcpp = Frequency of powered passing vessel collision
PcpD = Frequency of collision due to a passing vessel in drift
The frequencies of powered and drifting vessel collisions are generally dependent on the
location of ship routes relative to the platform. This information may be obtained by assessing
a database of ship routes or by performing a localized survey for the area.
Powered Ship Collision
A powered ship collision may occur when the following three conditions are met:
The ship is on a collision course to the platform.
0
The navigator is not aware of the situation early enough before the ship reaches the
platform.
The ship and the platform both fail to normalize the situation.
The basic mathematical expression for powered ship collision frequency can thus be written
as:
Pcpp = N- Pcc * PFSR PFPR (30.2)
*
where,
N = Annual number of passing vessels
Pcc = Probability of passing vessel on a collision course
PmIR = Failure probability of ship initiated recovery
PFPIR = Failure probability of platform initiated recovery
The probability of being on a collision course, Pcc , is a geometric factor. It is based on the
composition and position of the traffic flow. For a vessel, which has not taken pre-planning to
avoid a site, it may be assumed that the vessel will be normally distributed about the route
center. The fraction of vessels on a collision course can be found based on the route details
and the collision diameter presented by the platform. For a vessel that has taken deliberate
steps to avoid a platform or to use it for position fixing, re-modeling is needed to modify the
traffic distribution. A skewed distribution can normally be observed instead of normal
distribution.
The ship-initiated recovery from a collision course is divided into two cases: Early recovery
and late recovery. Early recovery is a normal operation under sound command. The ship is
recovered hm a collision course in the early recovery zone. Late recovery occurs under the
condition that early recovery fails. This reflects recognition of an emergency situation and
quick response recovery. The failure frequency of ship-initiated recovery may be calculated by
fault trees involving a number of factors, such as watch-keeping failure mode, visibility, vessel
type and size categories, traflic-planning group, and vessel flag etc.
The platform-initiated recovery is to alert the ship in time by platform or standby vessel to
prevent a collision. The failure probability of platform-initiated recovery is highly dependent
on the reason for the failure of ship-initiated recovery. It may be estimated by event trees
based on whether the following actions are taken in time:
Identification of the vessel as a possible threat
Attempt to inform the vessel on radioNHF