Making up ground

Both Rolls-Royce and GE have taken engines from the wing and adapted them for use in the marine industry. And both companies agree that gas turbines have some ground to make up on diesel engines, with there being a considerable challenge ahead in converting the industry to the idea of different ship propulsion concepts. But beyond this, the companies have some differing views on their product strengths and how best to operate their gas

turbines.

Rolls-Royce believes it can break the 40% thermal efficiency barrier with its MT30 gas turbine. “This takes it into the realms of high-speed diesels,” boasts Geraint Lloyd, business manager for commercial marine gas turbines at Rolls-Royce.

It compares with the 39% thermal efficiency GE says it achieves with its LM2500+. GE says the thermal efficiency of the LM2500+ increases to 43% with the heat recovery ability of a steam turbine in a COGES installation.

But Lloyd believes the advantages of a gas turbine are eroded by using a steam turbine.

“The COGES plant is dead,” he says. “As soon as you start coming down in power you lose the regenerative power of the steam turbine and fuel cost increases dramatically.”

To confirm (or refute) this would require looking at the operational statistics available for the relatively small number of commercial ships operating with COGES installations. Unfortunately Royal Caribbean, the operator of these few COGES ships (based on two GE LM2500+ gas turbines), does not disclose such data. And even if it did, it would only be possible to benchmark the performance against the performance of diesel engines.

GE says the relationship between exhaust temperature and power is complex. It says that as power decreases several engine parameters change including mass flow and core speed. It admits that COGES provides the greatest efficiency advantage at high power and says that the advantage at part power, while not as large, is still “significant”. The table shows GE?s estimated performance characteristics for a gas turbine at various power demands.

What has been released about the operational performance of COGES ships is GE?s assertion that the gas turbines onboard Millennium are operating “with strong availability and reliability” and “have not caused a single arrival or departure delay”. GE is predicting a hot section repair interval of 30 months or longer (13,250 hours and above), which coincides with scheduled drydock intervals.

GE recommends burning MGO (ISO8217-grade F/DMA) in the LM2500+. Lloyd says he would expect the MT30 to burn MDO (ISO8217-grade DMA) in regular service. It has been reported that Sweden?s Stena Line has operated the high-speed Stena Carisma?s two Alstom (formerly ABB Stal) GT 35 gas turbines ? an industrial not aeroderivative model ? on HFO for short periods, and Alstom has said those relatively low-temperature engines can run at substantial savings on IF30, a mix of 30% marine gasoil and 70% IF180.

However GE says running its engines on lower grades, while possible, increases the rate of engine deterioration and results in higher maintenance costs; fuel burnt should have less than 1% sulphur content and contain no heavy metal contaminants.

The temptation is always to use the cheapest fuel available. However if this involves an unacceptable compromise on engine component wear rates, what is a marginal short term gain can very quickly turn into a catastrophic engine failure.