Collision risk in the approaches to the Strait of Juan de Fuca : an analysis of circumstances and traffic routing
Date
1992
Authors
Judson, Brad
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Abstract
Marine traffic casualties in the approaches of the Strait of Juan de Fuca risk the accidental release of oil or other hazardous liquids which threatens the loss of property, human life and the spoiling of the coastal zone. Traffic in the approaches to the Strait has increased at three percent annually from 15,828 vessel transits in 1980 to 19,887 transits in 1989. This traffic carries as fuel about 40 percent of the total volume of crude oil transported from Valdez, Alaska to Puget Sound by about 333 tankers annually. Collisions are of particular concern as they accounted for 83 percent of those casualties caused by human error. The problem of marine collisions in the approaches to the Strait of Juan de Fuca was investigated with the purpose of identifying causal and contributory factors, modelling collision risk as a Poisson distribution, and designing an alternate traffic routing scheme intended to reduce casualties.
Initially, the causal and contributory factors to marine collisions in the study area were identified to direct the investigation towards practical and applicable solutions to the specific problems encountered. Traffic patterns were reconstructed by using a Geographic Information System to plot traffic positions sampled from radar position information obtained from Tofino Vessel Traffic Services and fishing vessel positions surveyed by Fisheries and Oceans. This enabled the calculation of traffic density, courses and speeds. It was found that through vessel traffic nearly doubles in frequency during summer months coincident with the fishing season, and, while vessel traffic tends to separate into inbound and outbound sectors, average traffic routes pass through the most active fishing areas.
Collision reports indicate problems of excessive speed, low visibility and high traffic density. A detailed analysis of the climatological and navigational conditions associated with vessel traffic and the circumstances of collisions confirmed the significance of these contributing factors. Speed was found to be excessive in eight out of ten collisions, seven collisions occurred in an area of high traffic density and six occurred in restricted visibility. Furthermore, the finding that mariners proceed through the approaches to the Strait uncontrolled at an average speed of 12.5 knots, without a significant reduction in vessel speed where prudent, indicates that an inappropriate level of risk is accepted.
It was demonstrated that the collision rate in the approaches to the Strait of Juan de Fuca is Poisson distributed with a probability of 0.25 per 5,000 transits. Since the collision rate is correlated with average traffic densities encountered and ship-miles, it can be predicted and reduced through the modification of traffic patterns. The benefit of traffic routing in the approaches to the Strait was demonstrated by a 68 percent reduction in absolute risk of collision during the peak traffic month of July and an annual reduction of 40 percent.
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UN SDG 14: Life Below Water