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OIL SPILL STATISTICS
Not unlike other information, oil spill statistics are gathered and used to serve specific and often predetermined purposes. Statistics that have been gathered for purpose A, may not be suitable for purpose B, and vice versa. The main source of statistical data about offshore oil spills in the UK is DECC. DECC receives oil spill reports and collates these into annual lists. These lists may not be suitable for oil spill contingency planning, but since they are the only source of information, they often are. As I have suggested in the page about oil spill risk assessment, more information than is provided by the spill records is necessary to fully assess oil spill risk. The other side of the coin is that the statistical analysis of the DECC spill records as presented in many oil spill plans have ignored valuable information embedded in these records.
In 2002, I undertook to analyze historic UKCS (United Kingdom Continental Shelf) oil spill records. Spill records of the 27 year period 1995-2001 were analyzed against a number of operational criteria; thus expressing the oil spill risk of the industry as a function of these criteria. In 2006, I updated the study and it now covers data from the 31-year period 1997-2005. Amongst others, the studies established relationships between the size and number of historic oil spills and the volume of past oil and gas production; the number of fields in production; the number of installations in operation, the types of installations in production and the age of the installations.
For members of Oil & Gas UK the full report is available on their website. A spin-off study for DECC resulted in a better method for setting spill targets that I hereby recommend.
Fourty-six percent of all spills were crude oil spills and 18% of the spill reports related to diesel spills. The other types of oil were: condensate (2%), hydraulic oil (8%), OBM and base oil (8%) and unknown types of oil and oily water discharges (12%). The relative number of diesel, condensate and hydraulic oil spills increased over the last 15 years of the study period. The analysis also shows that most OBM spills occurred during the short period of 1987-1991, indicating a decline of these spills well before OBMs were phased out. Even though 46% of the number of spills relate to crude oil, possibly 79% of the tonnage of oil spilled to the environment is crude oil. OBM comes in second with 15% and diesel third with 4.2%. Although these figures are UKCS specific they may also serve to inform the oil spill risk assessment of the offshore oil and gas industry in general.
Estimating the size of an oil spill can be difficult and a certain amount of subjectivity is an integral part of spill records and this has been confirmed by the analysis of the UKCS spill records. Only order of magnitude conclusions can therefore be drawn from this page.
Spills per production volume
On average, every million tons of oil equivalents (TOE) produced on the UKCS resulted in 0.94 spills and with those the discharge of 3.06 tons of oil. Over time, an upward trend occurred in the period 1975-1990, a downward trend in the period 1991-1995 and an upward trend thereafter. The latter is a reflection of the fact that less production is achieved with increasing numbers of smaller fields.
The higher amounts of oil spilled in 1977, 1980, 1986, 1988 and 1989 were caused by a limited number of large oil spills.
Spills per field
The following figure includes five Bell curves: one for the 7-year period 1975-1981 and one for each of the 6-year periods thereafter. Over the years, a shift can be observed towards smaller spill volumes and a reduction in the number of spill reports per field per year. In the period 1975-1981, most spills were between 1 and 10 tonnes. In 2005, most were between 10 kg and 100 kg. This indicates that the oil spill risk (a function of likelihood and spill size) of the UK offshore oil and gas industry has reduced over the years.
Spills per installation
The following figure shows how the performance of the industry has also improved over time when using a spill performance indicator of spills per topsides per year. Note that, as in the previous figure, the bell curves have moved to the left over time and have also become wider as a result of reporting smaller spill sizes.
Spills per type of installation
The following figure shows that overall (i.e. over the whole 31-year dataset; it is most likely that oil is spilled from the topsides of a fixed oil production installation and least likely from a gas producing installation. Floating oil production facilities spilled less than fixed oil producing installations.
Installation age
Most oil spills have been caused by oil producing installations installed in the period 1976-1984; i.e. those of earlier designs. The spill risk of the newest installations is significantly lower than that of the oldest ones. The difference between old and new installations has become less significant in recent years. The number of years of operation is not a contributing factor to spill risk.
OSCP 1 2 3 4 5
