The offshore industry invests considerably in safety and environmental protection. These investments have resulted in a steady improvement in safety and environmental performance. Despite this – accidents and accidental oil spills still happen.

Ole Øystein Aspholm, Senior Principal Consultant Photo: DNV

One of the challenges when it comes to managing the risk of major accidents, such as large oil spills, is that the incidents are unlikely, although the potential impact is high. Examples include: Montara, Macondo, Bohai Bay and Frade. These are low probability – high consequence incidents (Montara, Macondo) and higher probability – lower consequence incidents. However, they all have potentially high consequences with respect to cost and reputation.

In order to manage the oil spill risk properly, it is important to understand all aspects of the risk; from the reservoir conditions that can increase the risk, via controlling measures, like a blowout preventer (BOP), that can reduce the risk, through the fate, dispersion and drift of oil in the water column and at the sea surface, all the way to the area’s environmental sensitivity and the risk of environmental and socioeconomic impacts.

The oil industry is searching for a way to improve oil spill risk management and methodologies for analysing the oil spill risk as well as the effect of risk reduction. They are looking into how a risk/hazard based approach can improve oil spill prevention and mitigation.

Oil spill risk – “from the well to the shoreline”
Traditional oil spill risk management is segmented into various disciplines, such as drilling, well control and oil spill response preparedness. The oil spill risk level is influenced by the reservoir conditions all the way from the well to the drilling rig and by the sensitivity of the ambient environment in which the operation takes place. There are a lot of external risk parameters that we cannot change, like the reservoir conditions, fluid type, weather conditions and occurrence of sensitive environmental resources. But the risk can be controlled by using preventive and mitigating measures. The risk of these measures failing is also part of the oil spill risk picture and thus an important part of the integrated oil spill risk management. This includes oil spill preventive measures in the well design and drilling operations as well as oil spill response and recovery measures.

DNV project/approach to ensure integrated oil spill risk management
DNV has developed and applies oil spill risk management methods and techniques that cover all aspects of the risk of major oil spill accidents – from the well to the shoreline.

Reservoir and operation-specific parameters are taken into consideration when calculating the blowout/leakage risk in terms of the probability of a blowout or leak occurring, the fluid characteristics and the flow rate and duration of the blowout/leak. In order to control and manage the risk, all potential blowout/leak scenarios must be considered, not only the worst-case blowout.

DNV takes a multidisciplinary approach to assessing the drilling or well operations, based on a set of predefined criteria for assessing the probability of a leak or a blowout. Well flow simulations are used and adjusted in order to assess the well-specific leak and blowout rates for the different operations. The potential leak and blowout durations are calculated using statistical models and taking into account the context of the drilling and well operations. This approach takes into consideration the field-specific reservoir challenges and the reliability of the oil spill prevention measures and technology, such as the BOP. One of the challenges is to quantify the human factors that influence well control. However, combining statistical blowout data with the technology-reliability data and risk of failure due to human factors gives an important understanding of the robustness of the oil spill preventive measures. The results are more accurate risk predictions and a better understanding of where to improve or add barriers and control measures.

Understanding the risk of a major oil leak or a blowout gives half the oil spill risk picture. The probability of impacting personnel and the environment and the potential consequences of this must also be included in the overall risk picture. DNV applies a state-of-the art oil drift modelling tool OSCAR (from SINTEF) to model the dispersion and drift of oil. The simulations give a misbalance of the oil in the water column, at the sea surface, at the shoreline and in the sediment as well as percentage evaporated and degraded oil compounds. The oil drift modelling results are combined with data on the abundance and distribution of environmentally sensitive resources and the sensitivity ranking of the resources. This shows the potential environmental impact of an oil spill. The risk is calculated by combining the potential impact with the probability of the spill and the probability of oil pollution of the sensitive environmental resources.

However, oil spill risk management is about more than just preventing oil spills and understanding the potential impacts of a spill. Mitigating measures, such as oil spill detection, oil spill recovery and source control, are an important part of oil spill risk management. The oil spill response mainly has five options; well kill (ultimately by drilling a relief well), subsea capping and containment, containment of oil at the sea surface with either in-situ burning or mechanical recovery, chemical dispersion of the oil plume at the seabed or of the oil slicks at the sea surface. The last option is shoreline cleaning and wildlife rescue if the other measures fail to stop oil from polluting a shoreline or animals. Oil spill response is a complex operation and it takes good planning to achieve an efficient and effective oil spill response. This planning is expressed in the operators’ and authorities’ oil spill contingency plans. Bringing in the various options for oil spill detection and oil spill surveillance makes the picture even more complex. So how is an operator to make the right decisions in order to develop an oil spill contingency plan that takes the operations’ oil spill risk into account?

To ensure that the oil spill response level is connected to the level of risk, a detailed oil spill contingency analysis (OSCA) is carried out in conjunction with an environmental risk assessment (ERA). The methodology includes estimating the Effective Daily Recovery Capacity (EDRC) based on relevant wind, wave and current conditions, as well as the efficiency of dispersant use. The total efficiency of the oil spill response is modelled and evaluated with respect to the oil spill’s environmental risk.

The oil companies’ challenge is to be able to understand the risk level, to define not only the probability of a blowout and the worst case scenario, but also more likely spill scenarios and the potential environmental impact of all the spill scenarios. Further, it is a challenge to estimate the oil spill response needed to sufficiently handle the potential oil spills.

An integrated oil spill risk management tool that takes into account the environmental risk level and effect of preventive measures as well as the effect of mitigating measures, such as oil spill detection and oil spill response, will be a helpful tool for the operators to better manage the oil spill risk.

DNV’s experience of oil spill/environmental risk assessment is that it is very complex and often not connected to the decision-making regarding risk reducing measures – both preventive and mitigating. By making a tighter connection between the elements, we will be able to assess/rank the risk level of various operations, including the risk reducing measures. This will give the operator a tool for understanding and evaluating the risk level and for considering whether there is a need for further risk reducing measures.