Raja S. R. Khan

Procurement of process plant equipment involves decisions based not only on an economic agenda but also on long term plant capability, which in turn depends on equipment reliability. As the greater global community raises environmental concerns and pushes for economic reform, a tool is evermore required for specific and critical selection of plant equipment. Risk assessments based on NASA’s Technology Readiness Level (TRL) scale have been employed in many previous risk models to map technology in terms of risk and reliability. The authors envisage a scale for quantifying technical risk. The focus of this paper is the technical risk assessment of gas turbines as mechanical drivers for producing Liquefied Natural Gas (LNG). This risk assessment is a cornerstone of the TERA philosophy, a Technoeconomic and Environmental Risk Analysis developed by Cranfield University’s Department of Power and Propulsion in the UK. Monte Carlo simulations are used in order to compare the risks of introducing new plant equipment against existing and established plant equipment. Three scenarios are investigated using an 87MW single spool, typical industrial machine; a baseline engine followed by an engine with increased firing temperature and finally an engine with a zero staged compressor. The results suggest that if the baseline engine was to be upgraded then the zero staging option would be a better solution than increasing firing temperature since zero staging gives the lower rise in Total Time to Repair (TTTR), or downtime. The authors suggest a scaling system based on NASA’s TRL but with modified definition criteria for the separate technology readiness levels in order to better relate the scale to gas turbine technology. The intention is to link the modified TRL to downtime, since downtime has been identified as a quantitative measure of technical risk. Latest developments of the modelling are looking at integrating risk analysis and a maintenance cost and scheduling model to provide a platform for total risk assessment. This, coupled with emissions modelling, is set to provide the overall TERA tool for LNG technology selection.

This paper concentrates on the emissions module of what is part of a wider project dealing with various aspects of gas turbine usage as drivers for Liquefied Natural Gas (LNG) production. The framework is known as TERA, a Techno-Economic and Environmental Risk Analysis, developed at Cranfield University with the core of the study being the performance module whilst the risk, economics and environmental modules are built around the performance. Whilst TERA exists for aviation and power production no such system is available for assessment of LNG production. With environmental issues high on the public agenda new legislation on emissions can be expected, especially in Europe. This will mean Oil & Gas companies will have to look for ways to reduce their emissions. One way to reduce turbo machinery losses is to replace out dated and/or obsolete machinery having less overall energy efficiency. The selection of turbomachinery involves assessments of risk, both economic and technical, as well as environmental impacts of the new technology. The core to all of this is the performance assessment, the primary basis on which selection is made. An aviation emissions model, developed at Cranfield University, is adapted for industrial applications. Technical performance calculations are made using the inhouse software called Turbomatch. Performance results for three typical days of the year (summer, winter and spring/autumn) are fed into the emissions model to get the levels of NOx , CO2 , H2 O, CO and unburnt hydrocarbon emissions. Later, NOx , CO2 and H2 O emissions levels are fed into the environmental module to estimate the damage the engine causes to the environment over 100 years with respect to global warming. Two hypothetical engine configurations are investigated based on engine data available in the public domain. The first one, an 85MW single spool industrial machine (SSI-85), is used as the baseline to compare against a 100MW triple spool, intercooled aeroderivative (ITSA-100). The results suggest that the ITSA-100 produces more NOx but has less carbon emissions and consequently less global warming effects. This has varied economic impacts depending on which emission is a priority for reduction. CO2 and H2 O emissions are more important than NOx for LNG gas turbine applications. The paper shows how this simple but effective system can be utilised to give a viable comparison between one or more proposed solutions for turbomachinery selection and replacement. The scope of the system is expanded as other modules come together to give a total assessment in terms of technical, economic, environmental and risk perspectives for LNG production.