Offshore Gas Turbines with Reduced Pollution / Experience and Challenges
Authors: Skorping R. , Carstensen C. 
Published: December 2007

Statoil has a philosophy of being at any time at the forefront of applying the best available technology with respect to the environment. This paper describes important efforts to reduce emissions to air from gas turbines offshore.

The paper outlines further the experience gained so far and challenges for future applications in conformance with recent revisions of power plant air emission guidelines.

Gas Turbines are used in the offshore oil and gas production primarily for power generation and gas compression. Statoil has over 50 aeroderivative gas turbines in operation, with a total power output of more than 1,5 billion horsepower.

Exhaust from the turbines are a major contribution to the nitrogen oxide (NOx) and carbon dioxide (CO2) emission offshore. Several gas turbine manufacturers started years ago a comprehensive technology development program in order to reduce the NOx emission levels. This technology is now available to gas turbine users, and Statoil was one of the first oil companies that implemented this technology offshore.

Theoretically, application of the Dry Low Emission (DLE) technology will reduce the NOx emission by more than 80%. However, experience so far has proved that there are still technical and operational challenges to overcome in order to meet these ambitious emission levels during regular operation.

In addition, high efficiency gas turbines combined with waste heat recovery units are put into operation in order to utilise energy content of the flue gas and hence reduce the CO2 emission levels.

Introduction

In the recent years, the emission of NOx and CO2 has been under increased attention. The authorities / legistative has supported / initialised R&D activities, and national and international bodies are working on directives for the regulation of these substances.

The Norwegian authorities introduced years ago (1991) a "CO2 tax" for offshore installations. The tax is proportional to gas turbine fuel consumption, which promote the users to implement high efficiency turbine technology.

The paper is intended as an overview of Statoil's dry low emission (DLE) / high efficiency gas turbine systems and the experience so far.

Statoil's experience in gas turbines extends over 20 years. With the operation of more than 50 gas turbines of the type LM2500 at 12 installations, Statoil is one of the largest operators of gas turbines in the north sea, and the largest commercial operator world wide for this type of turbine. For the LM2500 turbines the number of operating hours has exceeded 3 Million in 1999. With this fleet, Statoil is the worlds largest offshore power producer. Statoil selected mostly aeroderivative GT's due to the higher thermal efficiency and lower weight of these turbines in contrast to their industrial counterparts.

Description of the DLE system

The combustion process of natural gas results in production of nitrous oxides (NOx), carbon monoxide (CO) and unburned hydrocarbon (UHC). The NOx and CO/UHC levels are a function of the flame temperature, where increased temperature will produce more NOx and less CO/UHC.