Analysis of Spill Probability

Spill Probability Analysis

In helping clients develop appropriate marine oil spill contingency plans and countermeasures, the first step at SL Ross is to summarize the facilities and situations in which spilled oil may enter marine waters. The possibilities usually involve one or more of the following sources:

i) large spills from storage facilities (depending on actual location vis-a-vis marine waters);

ii) subsea pipeline spills;

iii) loading spills;

iv) large tanker spills, both local and from nearby shipping lanes;

v) small spills from support vessels, land sources, etc.

The facilities in each of the above categories are studied and described in terms of operation, location, and type and quantity of material handled, transported or stored.

The next step is to determine the chances of various spills occurring. This helps to identify the particularly high risk spill situations that should be the focus of attention for contingency planning. The spill probability information is useful in assessing and putting into perspective the various costs options for spill control systems that will be recommended ultimately. In the probability analysis the following spill size categories are usually considered. Note that 1 barrel = 0.159 m3.

Category	Spill Size (Barrels)
I	1 - 49
II	50 - 999
III	1000 - 9999
V	10,000 - 149,999
V	150,000 -500,000
VI	500,000 +

Although the analytical approach we use is very simple (for example, for tanker spills it relies on a risk exposure of oil volumes transported), it has proved to be a reasonably good predictor of spill frequencies in various areas studied (all regions of Canada, U.S. at-large, California, U.S. Gulf of Mexico, Alaska, Trinidad, etc.). An example of the output of the risk analysis is shown in the following table.

Estimated Crude Oil Tanker Spill Frequencies For U.S. Gulf of Mexico Ports (Spills/yr)¹
Port	Bbbl Transported/Yr	Spills>1000 bbl	Spills>10,000 bbl	1000<Spills<10,000 bbl	Spills>150,000 bbl
Loop	0.331	0.228	0.136	0.092	0.0165
Galveston	0.329	0.228	0.136	0.092	0.165
Miss. River Ports	0.277	0.188	0.112	0.076	0.014
Port Arthur	0.127	0.089	0.053	0.036	0.0065
Lake Charles	0.114	0.079	0.047	0.032	0.0056
Corpus Christi	0.111	0.079	0.047	0.032	0.0056
Pascagoula	0.098	0.067	0.040	0.027	0.0049
Freeport	0.039	0.027	0.016	0.011	0.0019
Mobile	0.012	0.008	0.005	0.003	0.0006
Total GOM	1.44	0.99	0.59	0.40	0.072
		one every year	one in 1.7 yrs	one in 2.5 yrs	one in 14 yrs

1. Calculated by assuming 50% of tanker spills occur on inbound journey
and 50% on outbound journey

Source of table: Evaluation of Capabilities to Respond to 10,000-Barrel Tanker Spills in the Gulf of Mexico (Marine Industry Group (MIRG), July 1991)

For further information, please e-mail
info@slross.com.