Interpretation of causation factors in IWRAP

By Markus Porthin, VTT, 18 February, 2010

Table 1, Interpretations of causation factors.

Accident_type	Scenario	Interpretation of causation factor (PC)	Remarks
Headon	1. On a leg segment, two ships sailing in opposite directions would collide, if sailing exactly as defined by the lateral distributions. 2. Ships fail to make evasive actions in order to avoid the collision.	Probability that the ships fail to make evasive actions, in a situation when they would collide if they would do nothing. The causation factor is a combination of the individual factors assigned to the two ships involved:	The theory on head-ons is solid and well documented. However, it should be noted that due to the modelling assumptions it is possible e.g. for a single ship to collide with itself.
Overtaking	1. On a leg segment, two ships sailing in same direction would collide due to different speeds, if sailing exactly as defined by the lateral distribution. 2. Ships fail to make evasive actions in order to avoid the collision.	Same as in Headon.	The theory on overtakings is solid and well documented.
Crossing	1. Two ships sailing in different directions are on collision course in a crossing situation (waypoint connecting four legs, or more). 2. Ships fail to make evasive actions in order to avoid the collision.	Same as in Headon.	In IWRAP MK II, one has to define to which leg the traffic continues in a waypoint. How is this accounted for in the calculation model (not defined in [1])?
Merging	1. Two ships sailing in different directions are on collision course in a merging situation (waypoint connecting three legs). 2. Ships fail to make evasive actions in order to avoid the collision.	Same as in Headon.	See Crossing.
Bend	1. Two ships sailing in opposite directions meet in a bend (waypoint connecting two legs). 2. One of the ships fails to change course at the waypoint, resulting in the ships ending up on collision course . 3. Ships fail to make evasive actions in order to avoid the collision.	Same as in Headon.	The probability of omitting to change course at the intersection is taken as 0.01.>BR> The exact calculation of this scenario is documented somewhat ambiguously in [1] (equation missing). The bend collision model does not account for the lateral distance between the traffic in opposite directions. This leads to too high collision estimates e.g. in TSSs. The possibility of collisions in a bend between two ships sailing in same direction is neither accounted for in the model.
Grounding (powered) -Category I	1. On a leg segment, the ship would run aground, if sailing exactly as defined by the lateral distribution. 2. Ship fails to make evasive actions in order to avoid the ground.	Omitting to avoid ground, in a situation when the ship would run aground if it would do nothing.	IWRAP Mk2 uses the same Pc for both Category I and II
Grounding (powered) -Category II	1. The ship would run aground, if continuing straight forward after the leg, omitting the waypoint. 2. Ship fails to change course at the waypoint. 3. Ship fails to notice the omitted change of course before running on ground, or notices the omission too late to be able to avoid the grounding.	Omitting to change course at waypoint.	The ships that notice the omitted turn before running aground are all assumed to be able to avoid the grounding. Another important parameter is the Mean time between checks.