The general focus in environmental evaluation of offshore drilling is the discharge of cuttings and chemicals to local marine environment, in some cases including emissions to air from offshore turbines and engines. However, significant impacts occur as a result of processes upstream and downstream from drilling operations, in the manufacture of chemicals and materials, in supply and transport operations, and in treatment of drilling wastes.

Life-cycle assessment (LCA) is the structuring, aggregation and evaluation of environmental impacts in a cradle-to-grave scope. We present an LCA model developed through a series of case studies for offshore drilling operations. Model components include a fleet of drilling rigs and supply vessels, a library of drilling fluids and chemical products, various cuttings treatment technologies, top-hole abatement techniques and well operations.

The LCA model is used to evaluate technologies in historical, current and future best practice for offshore drilling. One measure under evaluation for the future is the development of smaller, fit-for-purpose vessels (Cat B), which are shown to potentially reduce emissions from the drilling vessel itself by 50 %.

Slim-hole drilling is another technology with several potential benefits, including time savings, reductions in steel and concrete in installing and cementing casings, less drilling fluid required, and less cuttings waste logistics and treatment. In all, these provide 30-50 % reduction when compared with conventional well designs, in greenhouse gas emissions and other air emissions, and in harmful releases from manufacture of materials and chemicals and treatment of wastes.

Relevant for operations outside European waters, the LCA shows that production and waste treatment of drilling fluid may pose a significant source for impacts unless fluids are managed properly by reuse of drilling fluid and recycling of oil from cuttings waste. These measures provide a reduction of 30 % of the total environmental footprint, illustrating the benefits from current best practice.

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