ABSTRACT

This paper identifies the offshore hydrocarbon operations which contribute to planned and unplanned objects being placed on the seafloor by the oil and gas industry Examples of underwater equipment and debris likely to be encountered are described The interaction and co-operation required to reduce mutual disturbance and prevent damage to the environment when utilising the seabed are examined An introduction is given to a joint Fishing and oil industry initiative to provide fishermen with a comprehensive electronic spatial database of oil and gas assets for overlay on fishing plotters and the "Fishsafe" warning system

INTRODUCTION

Prior to 1964, the seabed in areas adjacent to the United Kingdom and Ireland supported few activities A small number of communication cables had existed for both military and civil purposes Naval minefields had been cleared and areas identified as munitions and spoil dumping grounds were well charted These, along with the wrecks of merchant and naval shipping, fishing vessels and aeroplanes, covered an extremely small percentage of the seafloor around our coasts Such a small quantity of objects could easily be avoided by the largest user of the seabed, the fishing industry The seas and oceans of the world were still regarded as the great cleansers of all waste thrown Into them How things have changed in the last 20 years, with a far greater awareness of the damage that can be caused to the environment and of the fine balance that supports seabed flora and fauna

Little did anyone realise, when the first North Sea mining licences were granted in 1964, that the seabed would soon support a myriad of structures and transportation networks Enormous concrete and steel structures would be required to support the drilling, processing and living accommodation for hundreds of men and women working around the clock, 365 days a year Pipeline systems would be laid crossing one another under and over the seabed for hundreds of kilometres linking the remotest oil and gas fields to shore terminals all around the North Sea The developments in technology which now allow us to modify the seabed topography with ploughs, dredges, remote vehicles and gravel dumping in order to bury and protect the seabed installations, were the engineering fantasies of the early days of exploration

So where do we begin to unravel the background to the use of the seabed by the hydrocarbon industry?

PLANNED OPERATIONS

If we look at the various phases of oil and gas development we can see how the seabed might be affected by the various operations

Exploration

The earliest work will be carried out by seismic survey vessels, which traditionally tow their gun sources and streamer arrays without Interference with the seabed More recently however seismic techniques to better define the reservoir and it's depletion over time, have used ocean bottom streamer cables to capture both pressure waves and shear waves The cables may be towed to predetermined locations on the seabed, their positions accurately fixed, before a source vessel firing gun arrays, passes parallel to the bottom streamers

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