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OIL SPILL RISK ASSESSMENT
Oil spill risk assessments can be improved by clearly defining their scope. It is almost impossible to assess the oil spill risk presented by the offshore oil and gas industry as so few spills have ever reached the beach and cause significant damage. And although a spill to sea is a significant performance indicator, it is not an indicator of overall spill risk.
Oil spill risk assessments for offshore oil and gas platforms consist of three parts (see Figure).
- The first part of the risk assessment determines the risk of a leak, i.e.: the probability of failure of primary and secondary containment measures and the volume of any leak.
- The second part of the risk assessment determines the risk of a spill to sea, i.e.: the probability of a spill to sea and the volume of such a spill.
- The third part of the risk assessment determines the risk of environmental damage, i.e.: the probability of a spill damaging the environment and the severity of such damage.
In all three steps, the formal meaning of risk is being assessed; i.e. the probability and severity of an adverse effect / event occurring, rather than the perceived risk. It is important to distinguish between these risk levels and to not compare probabilities and severities from different levels. It is, for instance, not possible to assess the risk of environmental damage by looking at spill frequencies or leak frequencies without considering the natural dispersion of oil in the open sea.
Environmental damage would occur if:
- A leak had occurred on the platform, and
- The primary and secondary containment measures would fail, and
- Any tertiary containment measures would fail, and
- A spill to sea has occurred, and
- The environment exposed to the spill is sensitive to its properties, and
- The natural dispersion forces in combination with the oil spill contingency measures have failed.
Risk of a leak
A leak is not the same as a spill. Offshore platforms are designed to contain leaks, and a leak would only result in a spill to sea if the platform containment systems would fail. The risk of a leak is a function of the spill potential and the reliability of the platform containment systems. The leak potential is determined by the types of oil, their amount, the type of system and their pressures. Leak statistics are maintained by platform operators to prevent recurrence, but are not generally published. Oil inventories of offshore platforms are much smaller than those of oil tankers. Gas platforms contain in the range of 50 tonnes of oil and oil platforms in the range of 1,000 tonnes.
Risk of a spill to sea
A leak would result in a spill if the containment systems would fail, including any tertiary containment systems. Tertiary containment systems are leak trays, closed decks and drain systems. Tertiary containment systems are absent with bunkering activities and with sub sea equipment. The volume of oil spilled is determined by the duration of the spill and its flow rate. Oil spills need to be reported and spill statistics are maintained by the relevant regulatory agencies.
Risk of environmental damage
Whether an oil spill results in notable environmental damage, depends on the local environmental sensitivities, the time of the year, the distance of the spill location to sensitive shorelines, the weather conditions and the effectiveness of a spill response. Most offshore platforms are located far offshore. An oil spill from an offshore platform is most likely to disperse naturally before it can reach a shore.
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