Development and Application of Environmental Models in the Assessment of Exploratory Drilling in a Sensitive Coastal Region, Isle of Man, UK M.C. Rymell, BMT Marine Information Systems Ltd., and Z.A. Sabeur, BMT Marine Information Systems Ltd, and M.O.Williams, BMT Marine Information Systems Ltd., and D.M. Borwell, Elf Caledonia Ltd., and A.O. Tyler, SPE

Abstract

Contamination of the seabed by drilling operation discharges and the potential for accidental oil spillage remains a significant environmental concern. A project commissioned by Elf Caledonia Ltd. has led to the development of a series of models that allow assessment of the environmental impact of contaminants associated with offshore drilling and prediction of possible oil spill scenarios.

Three environmental modelling systems have been implemented. A current flow prediction system has been set up to provide a primary source of data on the hydrodynamics of the region. An oil spill model allows prediction of the trajectory and fate of oil in the event of an accidental spillage. The third model considers the drilling mud and cuttings discharges made during exploration. The three models use the same base set of hydrodynamics/bathymetry and run under the same base GIS user-interface.

This paper discusses the development of the various databases underlying the models, development of the models and the use of the models in planning the drilling programme. Development of models for future applications is discussed.

Introduction

Prior to exploratory drilling activities in the Irish Sea, Elf Caledonia Ltd. commissioned the development and configuration of a number of software applications to aid the Environmental Impact Assessment studies.

Exploratory offshore drilling is of environmental concern for several reasons. Discharges made during exploratory drilling include both solid cuttings and liquid drilling muds. Potential for environmental impact on surrounding benthic communities exists because of the toxicity of the discharges, possible depletion of sediment oxygen by organic enrichment and because the seabed may become smothered. In addition there is a risk of accidental oil spillage inherent with such activities.

Oceanography. The Elf Caledonia licensed acreage lies off the northeast coast of the Isle of Man (Figure 1). Water movement in the licensed acreage is tidally-dominated, although persistent winds can produce a wind-drift current. Within the region, offshore sand banks orientated ESE-WNW strongly influence local flows. At maximum flood and ebb flow, currents within the region are primarily forced by flow through the North Channel but currents show influence from St. Georges Channel streams at other tidal states. The strongest currents occur between the Point of Ayre and the Scottish mainland to the north (maximum spring currents of over 2m/s). Eddies form off of the Point of Ayre during both flood and ebb tides.

Most of the sediments within the immediate area of the drilling site are of firm clays and sands, overlain by up to 1.5m of softer sediments. To the northeast of the zone softer clay is overlain by up to 3m of sand.

Natural Environment. The local marine environment contains a rich and diverse community of bivalves, crustaceans and fish. Large areas of the sea bed are covered with beds of horse mussels and brittle star. The Manx coastline adjacent to the licensed acreage contains a number of Nature Conservation Trust Reserves, important seabird colonies and seal breeding grounds.

Objectives. The applications used in the study aimed to address the environmental sensitivities outlined above, whilst providing a structure on which further developments could be made: for example, during the production phase.

Three applications were utilised in the project:

  • A current flow prediction system providing a tool for validation and visualisation of hydrodynamic data.

The system was used to characterise the local oceanographic regime.

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