Abstract

Increasing fuel cost and concern regarding emissions have incurred focus on energy conservation on offshore oil and gas installations. On the Norwegian continental shelf there has been an increased commercial value of natural gas and a CO2-tax on fuel consumption the last 10 years. For the time being the alternative value of natural gas is in the range 0 to 0.06 EUR/Sm3 and consumption of fuel is taxed with approx. 0.09 EUR/Sm3 burned. This has brought forward significant changes in operational procedures and technology development on the Norwegian oil and gas installations compared to installations in other oil and gas regions throughout the world. Focus has increased on keeping the power requirement down, avoiding unnecessary fuel consumption and designing more energy efficient systems. The most efficient way of achieving this is by installing cold process flares and steam cycles on existing or new gas turbines to form Combined Cycles (CC). In Norway three steam cycle plants fueled by gas turbine exhaust are under completion on offshore oil and gas installations. The overall electrical efficiency of these plants is around 50%. When steam is extracted for process heat the overall thermal efficiency is higher. Compared to conventional simple cycles the combined cycle reduces consumption of fuel gas and emissions of CO2 and NOX with minimum 25%. When designed as alternative to conventional simple cycle solutions, a combined cycle represents an alternative investment reducing the size and/or the number of gas turbines needed. This gives a good contribution to the overall Life Cycle evaluation in favor of the combined cycle option when compared to the simple cycle option.

Introduction

The growing environmental concern with respect to emissions of greenhouse gases caused the Norwegian government to define a self imposed goal of stabilizing the national CO2-emissions on 1989 level by year 2000. This goal has not been achieved. However, there is a new internationally agreed goal defined in the Kyoto agreement which allows Norway a 1% emission increase of greenhouse gases compared to 1990 in the period 2008 - 2012. Compared to 1996this is an decrease of about 7%. As an economical incentive to install more energy efficient technology to achieve reduced emissions, a CO2-tax on hydrocarbon fuel was introduced in 1990. For the oil and gas industry, this was a significant cost increase. Due to the large amount of projects in the NorthSea in the period 1990 until today, the absolute and relative contribution ofCO2-emissions from the oil and gas industry has increased. The national annual emissions of CO2 have rised from 35 to 42 mill tonnesi. In the same period the contribution from the offshore oil and gas industry has increased from approx.20% to 22%. 80% of the offshore emissions comes from roughly 250 gas turbines installed to produce electricity or drive compressors and pumps directly. The efficiency of these machines are in the range of 15 to 35%. The remainingCO2-emissions offshore comes mainly from flaring. The increase in emissions has caused a considerable pressure to develop technology to reduce this trend of increased CO2-emissions.

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