Repair and coating technologies today can do more to extend gas turbine service intervals and component life aboard offshore production floaters and platforms. While fuel economy may not be as crucial to offshore oil operators as it is to electrical utilities, repair and overhaul savings can be significant.
Repairing combustor inner casings, for example, can yield significant savings in replacement costs. Advanced turbine repairs also provide new opportunities to "rescue" damaged parts previously thrown away, and they can pay for themselves in reduced overhaul costs. To reap the benefits, operators need an experienced repair contractor with special insight into gas turbine damage mechanisms, and a range of up-to-date turbine inspection tools and fixes.
Modern repair technologies go way beyond patching cracks and bringing machines as close to new as possible. They include lost metal replacement, restoration of airfoil shapes, and component redesign. Advanced coatings can now improve surface smoothness on gas path surfaces as well as protect against erosion and corrosion.
Redesigns of frequent failure parts can actually make a machine better than new. However, effective repairs or improvements depend on the kind of damage encountered in offshore service.
How it occurs
The nature and speed of gas turbine damage mechanisms vary with the duty cycle and the environment. Whatever the service conditions, turbine vanes, blades, combustion liner baskets, and other hot parts are subject to oxidation. The hot metal parts form oxide layers that spall off to expose new metal for more oxidation. Gas turbine performance falls off due to loss of critical part dimensions, or as weakened parts fail altogether.
Offshore operators often run turbines continuously as baseloaded power generators or full-time gas compressors. In continuous duty, with metal temperatures of 1,600-1,800° F, the loss of material continues at a relatively constant rate. Wear and damage from cyclic operations can be severe. Cyclic heating and cooling accelerates oxidation into thermal erosion as oxides spill more frequently. While sustained temperature differentials sometimes crack parts in continuous duty, cyclic thermal transients and the stresses they cause, can rip turbomachinery apart.
Corrosion is rarely a major problem for land-based gas turbines. However, the turbomachinery on offshore installations can seriously corrode due to the sodium in salt spray and the sulfur in natural gas or liquid fuel. Accelerated oxidation or "hot corrosion" becomes more likely at the 1,450-1,650° F firing temperatures typical for such applications. Furthermore, the maritime environment contributes to turbine damage.
Inlet icing for instance, can cause particle erosion on early-stage compressor blades. Like land-based turbomachinery, offshore gas turbines also are vulnerable to impact damage, often caused by foreign objects being ingested.
Finding a fix
Original equipment manufacturers set recommended teardown intervals for gas turbines, but corrosive environments and impact damage may shorten inspection intervals dramatically. Turbine inspection and repairs should not be do-it-yourself operations anymore. While in-house mechanics can spot cracks and severe damage, few operators have the measurement tools to gauge the size, shape, and microfinish of airfoils accurately.
When power-generating gas turbines need work, a full-service inspection and repair organization generally saves operators downtime and money. A single contractor can cut administrative costs and provide a single point of responsibility for a multi-step process. A one-stop turbine repair contractor can cut blade inspection and repair costs 5-10%, and shorten time off-line 10-20%.
Even with one service vendor, good business practice dictates inspection and repair be done under separate contracts to avoid expensive surprises. Look for a service provider with the full spectrum of inspection and repair technologies at its disposal. Precision measurement capabilities are essential to identify what component dimensions and coatings need to be restored.
Borescoping can find visible damage to components without opening the turbine case. Done during routine shutdowns, it can monitor wear, help schedule future maintenance, and enable operators to order parts in advance. Ultrasonic, eddy current, x-ray, and fluorescent penetrant inspections are necessary to gauge more subtle failures.
Even with a Level III certified inspector performing non-destructive testing, turbine blade test methods should be calibrated by sacrificing one blade to measure wall thicknesses. Creep rupture testing and other advanced assessment techniques provide a clear picture of the condition of turbine parts, and the conditions under which they operate.
Rebuilding
Once damaged components have been accurately measured and characterized, a range of repair technologies are available to restore them to original equipment standards. Restoring original dimensions recaptures lost efficiency.
Skillful repairs can make gas turbine parts like new, but they rarely extend service life beyond original manufacturer specifications. Fully experienced, full-service turbine repair contractor can redesign components to make them better than new.
When it comes to restoring today's complex, internally-cooled turbine blades and vanes, coating technologies are as important as welding and brazing. Coatings protect stationary and rotating turbine components against corrosion, erosion and fouling, and often impart a smoother-than-new surface finish for higher efficiency. However, old coatings must be stripped away carefully to protect thin airfoil walls and preserve the precise dimensions of film cooling holes and passages.
Abrasive blasting removes the coatings without damaging the underlying metal. New coating application technology also protects turbine parts more effectively at lower cost. High Velocity Oxy Fuel (HVOF) technology now applies basecoats and topcoats faster than low pressure plasma spray, and finished coatings last as long or longer as well. It can inject aluminum topcoats into cooling passages to protect against high operating temperatures.
Turbine coatings applied by original equipment manufacturers are formulated to withstand fleet-average service conditions. Custom coatings formulated for specific turbines and applications can extend service life and improve performance quite a bit further.
Case studies
One coating on a North Sea platform examined during overhaul, after 20,000 hours of service, and the coating was still protecting the gas path compressor components, with no evidence of pitting.
A small gas turbine widely used for offshore power generation routinely suffered severe hot section erosion. Engineers traced the cause back to coking inherent in the combustor design and the fuels typically used in those applications. As heavy coke deposits built a thick film, fragments flaked off and blew back through the hot section where they quickly eroded parts. The remedy was a self-priming coating containing polytetrafluoroethane (PTFE) which provides a non-stick surface that keeps significant amounts of coke from accumulating. Erosion was dramatically reduced, and hot section life restored by the custom coating.
Another gas turbine in a Norwegian coastal pumping installation burned oil-well off-gas and suffered hot corrosion. Turbine blades showed severe pitting and loss of metal. The surrounding blade shrouds rotted out after just 12,000 operating hours.
Analysis showed the sulfur-rich natural gas and the temperature distribution between the hot blades and relatively cool shroud promoted corrosion. A coating containing chromium sillicide proved highly resistant to high-sulfur environments and ultimately tripled the life of the parts.
Smooth flow gains
Advanced cold-section coatings can significantly improve aerodynamic efficiency of power generating turbine compressors. New compressor blade topcoats, for example, provide dense surfaces down to 25mm smoothness. Airfoil surface smoothness is the key to laminar flow, and smooth gas path surfaces in the fan and compressor help deliver more air to the combustor. The payoffs can be lower fuel consumption or longer-life hot-section components thanks to lower exhaust gas temperatures.For offshore operators faced with expensive repairs, advanced inspection and repair techniques, component redesigns and optimized coatings all promise to save money.
The key is to find a repair contractor with the latest technology and the expertise to use it in both inspection and repair. The gains in service intervals, equipment life and operating efficiency all make the search well worthwhile.
