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Publication Title | Energy-Efficient Technologies for Reduction of Offshore CO2 Emmissions

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Energy-Efficient Technologies for

Reduction of Offshore CO2 Emmissions

Marit Jagtøyen Mazzetti, Petter Nekså, Harald Taxt Walnum, and Anne Karin T. Hemmingsen, SINTEF Energy Research

Summary

This paper will discuss novel technologies for increasing the energy efficiency of offshore oil and gas platforms. Three case studies are in progress that are based on actual oil-producing platforms—two on the Norwegian Continental Shelf (NCS) and one in the Brazilian basin. The current focus is on developing compact, novel bottoming cycles for recovery of waste heat from the gas turbine and heat re- covery from the compressor train for gas export. The technologies under investigation use steam and alternative working fluids, such as carbon dioxide (CO2) and hydrocarbons. All the fluids investi- gated in this project are natural working fluids; hence, they will not cause any unexpected environmental issues in the future.

A case study was performed that considered an 18-year period of operation on an actual platform and a scenario in which one gas turbine was removed and replaced with a CO2 bottoming cycle by use of the exhaust heat from a different gas turbine. The beauty of this scenario is that it would not increase the weight on the plat- form because the crate containing the gas turbine to be removed was of a weight similar to that of the crate containing the CO2 bot- toming cycle. The substitution would not affect the ability to cover the heat demand on the platform because a waste-heat-recovery unit (WHRU) could be installed on the platform’s other gas turbine.

The case study indicates a significant reduction in CO2 emis- sions of 22% (63 000 t/a), and does not involve adding additional weight or volume to the platform. If operating on the NCS, the an- nual savings in reduced fuel costs and CO2 tax from implementing this scenario would be USD 17 million, although much lower in other territories.

Introduction

Improved energy efficiency is one of the most effective means of protecting and improving the global environment according to a new report from the International Energy Agency called World En- ergy Outlook (IEA 2012). This claim is valid also for the offshore oil-and-gas-producing industry. The key lies in new and compact technologies designed to streamline gas-fired power production. Such technologies are already in use onshore. If they can be widely adapted for use offshore, CO2 emissions from oil installations could be reduced by as much as 25%. If the new technologies are applied on all Norwegian oil installations, this alone will result in CO2-emission reductions large enough to make a real difference in relation to targets set out in the Norwegian government’s white paper on climate policy (Miljøverndepartementet 2007). Full im- plementation, however, is not feasible, but even a smaller share may give important contributions.

Copyright © 2014 Society of Petroleum Engineers

This paper (SPE 169811) was revised for publication from paper OTC 24034, first presented at the Offshore Technology Conference, Houston, 6–9 May 2013. Original manuscript received for review 5 June 2013. Revised manuscript received for review 20 October 2013. Paper peer approved 20 November 2013.

Offshore oil and gas production are highly energy-intensive pro- cesses, and CO2 emissions from offshore installations constitute a little more than one-quarter of all climate gas emissions from Nor- wegian territory. According to the Bækken and Zenker (2007), CO2 emissions per oil equivalent produced on the NCS were reduced by approximately 20% in the period between 1990 and 2005. This was the result of a combination of improved energy efficiency mainly caused by reduced natural-gas flaring and installation of WHRUs. There were 58 WHRUs installed on the NCS in 2004 (NPD et al. 2004). These WHRUs covered approximately 90% of all heat de- mand for operations on the NCS.

Today, approximately 80% of the CO2 emissions from offshore activities are derived from the gas turbines used to generate elec- tricity on the installations. The turbines use natural gas from the reservoirs as their energy source. In these power plants, the nat- ural gas is compressed together with large volumes of air before combustion, which in turn heats the air flow so that it expands and drives a turbine. The gas turbine drives a generator that produces electricity, or drivescompressors or other rotating machinery. How- ever, large amounts of useful energy are lost as heat in the gas-tur- bine exhaust.

The turbine emits exhaust gases that are hot enough to enable this waste heat to be converted to physical work or electricity. In the EFFORT project (SINTEF 2012), funded by some of the major oil and gas companies, and the Research Council of Norway’s PET- ROMAKS (2008) program, the potential for using the waste heat to generate additional electricity by use of an additional turbine “hooked up” to the power plant is investigated. The idea is to re- cover the energy currently lost (Mazzetti and Nekså 2012; Walnum et al. 2013; Nord and Bolland 2012, In Press).

In modern onshore gas-fired power plants, this “repeated” use of the waste heat is achieved with the surplus heat from gas tur- bines heating water in a boiler. The steam produced drives Turbine 2 (a steam turbine). Combined-cycle power plants of this type have been installed on three Norwegian gas fields: Oseberg, Eldfisk, and Snorre (Kloster 1999). However, such plants are heavy and very space demanding.

Platform installations on oil fields are equipped with large and heavy processing plants for separating oil and water. Thus, it is frequently considered totally impractical to use combined-cycle plants on oil platforms because of weight and space considerations. The focus of the EFFORT project is, therefore, to evaluate technol- ogies that are both lighter and more compact (meaning everything must reside on a platform deck), and further, to adapt these technol- ogies to the demanding constraints that must be taken into account on offshore installations.

Statement of Theory and Definitions

The gas-turbine gross electrical-power output is defined as

Ẇ =(Ẇ gt

shaft

• η ), ................................................................. (1) gen

February 2014 • Oil and Gas Facilities 89

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