Offshore Abandonment Activities: The Energy and Environmental Impacts
Authors: Brindley P.D. 
Published: December 2007

In an increasingly environmentally conscious world the forthcoming abandonment of North Sea Oil and Gas Platforms will need to be conducted in an environmentally friendly, and cost efficient manner. This study into some of the environmental costs of these activities evaluates four abandonment methodologies from an energy consumption, and CO2 production viewpoint, and identifies the problems associated with each from a 'local' and 'global' perspective.

The study shows that the selection of abandonment method can play a large part in determining of the overall energy and CO2 efficiency of the abandonment works, and also shows the difference between the 'local' costs to recover the structure, and the 'global' costs that result from those actions. It is also shown that the potential environmental savings from recycling materials are more than large enough to justify the extra environmental expense to recover all of a steel jacket and topsides, i.e. small costs for a large gain.

The end conclusion is that there is the potential to fully recover all the steel platforms in the North Sea in a financially and environmentally efficient manner, but in order to do this there will need to be a co-ordinated Inter Government, Inter Company (Operators and Contractors) drive, that will force a rethink of a lot of current practises.

Introduction

Although the abandonment of Offshore Oil and Gas platforms is not a new global phenomenon, the North Sea Industry is now having to address its attention to the forthcoming removal of redundant structures as fields become exhausted. To date a significant amount of attention has been paid to the financial cost, technologies needed, and'local' environmental impacts on marine life and fauna, of these activities.

The range of potential environmental impacts caused by abandonment operations, is quite large, ranging from local pollution of the air (short duration local impact particularly) through the slow degradation of the oils contained within drill cutting mounds, to the creation of diesel fumes from road transporting returned materials for disposal. This paper will however limit it's discussion to Energy expenditures, and CO2 production as a result of offshore and onshore abandonment activities for Oil and Gas facilities, and recycling.

For this paper an estimate has been made into the energy consumption, in Giga Joules, and CO2 production in Tonnes of gas to remove a 'typical' Central North Sea steel jacket platform, and process returned materials. A complimentary estimate into the potential energy savings, possible reduction in CO2 emissions that may be obtained by recycling returned materials has also been made.

Four different abandonment methods have been considered, such that a comparison can be made between the options. The environmental costs considered have been split into different activities, such that the energy expenditure, and resultant CO2 emissions, relating directly to the removal activities, and the indirect costs, such as replacing wasted materials with new materials can be assessed.

The 'Typical' Platform In order to gauge the impact of different abandonment options from a benchmark position, a 'typical' platform has been assumed. The platform is estimated to be some 370 Nautical Miles (Nm) from the Heavy Lift Vessel (HLV), and Accommodation Vessel bases, and 150 Nm from the local support base centre.

The platform is assumed to be a multi function structure, with drilling, production and accommodation facilities, and be based in the Central North Sea, at a water depth of 80m. This will allow all three removal options to be considered as viable abandonment options. The assumed total weight is 32,000 tonnes (excluding pile weights), the topsides being 19,800 tonnes, the jacket 12,200.

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