Gas, liquid and multiphase leaks
Hydrocarbon leaks can be divided into three categories: gas leaks, liquid leaks or multiphase leaks (oil-gas). Gas leaks have the greatest potential for causing damage due to the explosion risk from the dispersion of gas clouds.
In the Risk Level Project, hydrocarbon leaks are categorised into three groups according to the leakage rate: Greater than 10 kg/s (kg per second), 1 – 10 kg/s and 0.1-1 kg/s. Even a gas leak with the lowest recorded leakage rate (0.1 kg/s) has a considerable accident potential – corresponding to the amount released by 2000 gas burners.
The Piper Alpha accident
Hydrocarbon leaks have a major accident potential well illustrated by the total loss of ’Piper Alpha’ in 1988 on the British continental shelf, leaving 167 dead. A medium-sized gas leak caused a gas cloud which was ignited, the explosion caused breakage of a pipe transporting oil and a fire which raged unchecked due to water not being available for the firewater system.
In time, this caused the other large pipes connected to gas- and oil pipelines (risers) to burst, which in turn caused a total loss. The large number of fatalities was partly due to the failure of all evacuation and rescue measures.
Through the Trends in Risk Levels Project, all leaks of more than 0.1 kg/s are recorded. There are four to five leaks each year on the Norwegian shelf of a size equal to the one which started the ’Piper Alpha’ accident.
Along with serious well incidents, damage to supporting structures and marine systems, and ships on a collision course, such leaks accounted for more than 80 per cent of total major accident risk on the NCS in 1996-2004.
That has been established through investigations carried out by the trends in risk level project (RNNP).
Studies have also shown that some 40 per cent of hydrocarbon leaks occur in connection with normal operation, whilst the remainder arise during other manual work on/at the facility.
Prevention of leaks related to normal operation can be further strengthened through better choice of equipment and improved inspection, maintenance and experience transfer routines. However, an even greater prevention potential is likely to be offered by better planning, preparation and execution of other manual work.
The Norwegian authorities took an initiative with the industry in the summer of 2003 to reduce the number of leaks, and a project was launched by the Norwegian Oil Industry Association (OLF).
This aimed to cut the number of hydrocarbon leaks larger than 0.1 kilograms per second (kg/s) by 50 per cent before the end of 2005, using the average for 2000-02 as its baseline.
The project focused attention on management involvement, personnel behaviour and measures to improve the expertise of people involved in the work. It also reviewed technological conditions on/at the installations, and directed greater attention at potential sources of leaks.
The goal of a 50 per cent cut was achieved, and the industry set a new goal of reducing leaks bigger than 0.1 kg/s by a further 50 per cent from the 2005 level before the end of 2008.
This collective effort represents a valuable contribution to cutting the level of risk on the NCS. It also underlines the importance of constant attention being paid to key issues by everyone involved.
Norway and the UK
The authorities have compared part of the NCS – the North Sea above the 59th parallel and the southern Norwegian Sea – with the UK continental shelf for hydrocarbon leaks above 0.1 kg/s. This study shows that such incidents are considerably more frequent in the relevant area of the NCS than in the corresponding part of the UKCS.
Britain launched a campaign as early as 2000 to reduce the number of these events, which could be one reason why it achieves better results than Norway for large hydrocarbon leaks.
The big difference between the NCS and the UKCS shows that a substantial improvement potential remains in the Norwegian offshore sector.
A number of incidents involving hydrocarbon leaks from wells, subsea installations and pipelines have been recorded in recent years. These occurrences highlight the need for better understanding of the way hydrocarbon emissions behave under water and the risks they present.
This requirement was underlined by the submarine gas blowout on the Snorre A installation in 2004. The industry and the government have accordingly launched a project to assess problems associated with underwater gas leaks.
This work aims to document the challenges and describe proposed measures, and will ensure a good understanding of these phenomena.
Hydrocarbon leaks at land-based plants
The biggest contributions to major accident risk at the land-based plants will be hydrocarbon leaks with consequent fires or explosions. Such plants often feature production processes which involve large flows of hydrocarbons, and frequently have considerable storage capacity.
A major escape of hydrocarbons on land could accordingly be larger than a big leak on an offshore installation. And processes at a plant are often more complex, with high temperatures and pressures.
As a result, the consequences of a fire or explosion could be very large. However, the risk of personal injury will be lower on land than with a similar incident offshore.
This is because evacuation opportunities are better in the former case, while the number of people close to the process area will not be as high during normal operation.
When modifications are being carried out in parallel with full production, however, that picture could change and a larger number of people may be exposed.
Experience from foreign plants indicates that major maintenance tasks represent a clear increase in risk.