After a comprehensive gas turbine overhaul, excessive noise levels were detected on the Main Generator A gas turbine exhaust duct on Statoil’s Åsgard B platform. The levels exceeded HSE recommendations and operation of Main Generator A had to be avoided. As a stopgap measure, Main Generator B was regulated to manage full operation. However as Generator B had a larger NOx emission, this led to recurring added penalties. To avert these penalties, Statoil needed to get Generator A back up and running. But first, they needed to mitigate the noise. Lloyd's Register Consulting was brought on to investigate the noise source and solve the problem.
A portion of the flow was passing below the tip of the lower vane. As unstable supersonic flows were present in this region (higher than Mach 3) with marked fluctuations in the frequency range around the tonal noise frequency, we concluded that this was the noise source.
By mounting a bleed flow distributor can, we were able to guide the bleed flow away from the bottom of the exhaust collector. The noise problem was thereby eliminated.
In summer 2011, excessive noise levels suddenly surfaced on Statoil’s Åsgard B platform from the Main Generator A gas turbine exhaust duct. Prior to this occurrence, the Main Generator A gas turbine had been through a thorough overhaul. After reinstallation, the performance of the gas turbine had significantly increased.
Initial noise investigation
For clear identification of the main noise contributor, we first performed a number of noise measurements. We found that the main cause for the increased noise level was a sharp tonal frequency component close to 800 Hz. As this component only appeared for certain openings of the CDP bleed valve, we established that the noise was linked to the bleed flow. At certain part-load operating conditions, LM2500+ DLE gas turbines use continuous bleed-off flow after the last compressor stage. On the Main Generator A exhaust collector, the bleed flow was introduced into the exhaust duct inside the exhaust collector in vicinity of the lower flow enhancer guide vane.
Because the Main Generator A gas turbine had been upgraded, it was suspected that the bleed flow rates had now increased beyond a threshold level, which had triggered the excessive noise levels. We therefore developed the hypothesis that the noise was caused by a screech-like unstable interaction inside the exhaust collector.
Simulating the flow
To describe the flow inside the gas turbine exhaust duct, we initiated a flow analysis. Based on computational fluid dynamics (CFD), we simulated the bleed flow and its interaction with the exhaust collector flow. The simulation revealed that problematic flow features were present near the lower tip of the flow enhancer guide vane. A portion of the flow was passing below the tip of the lower vane; a shape similar to the tip of a whistle or flute. We found that unstable supersonic flows were present in this region (higher than Mach 3) with marked fluctuations in the frequency range around the tonal noise frequency. From experience, we know that the encounter between very high Mach numbers and a sharp edge is very likely to generate excessive noise – and we concluded that this was the source of the problem.
To solve the problem, we needed to guide the bleed flow away from the bottom of the exhaust collector. To that end, we decided to adapt a bleed distributor concept we had recently developed in collaboration with Mjørud for a somewhat similar project. The conceptual design was customised to solve the problem at hand in close cooperation with Statoil. The bleed distributor was based on four layers of perforated plates. These would break up the flow into thousands of small jet flows and separate the flow domains. After installation of the bleed distributor, the flow was fundamentally changed. We had thereby averted the interaction of the supersonic flow out of the bleed pipe with the lower flow enhancer guide vane. The excessive noise levels had been relieved and Main Generator A gas turbine was back up and running.