Systems and methods to operate a dual-shaft gas turbine engine for hydraulic fracturing

A method for pumping a fracturing fluid into a wellhead, the method comprising:

• controlling, via a fracturing unit controller, based at least in part on one or more of a target pressure, a target flow rate, an actual pressure, or an actual flow rate associated with the fracturing fluid pumped into the wellhead by a hydraulic fracturing pump, one or more of:
• a first rotational speed associated with a compressor and a compressor turbine of a gas turbine engine, the compressor positioned to provide compressed air to a combustor section of the gas turbine engine,
• a second rotational speed associated with a power turbine of the gas turbine engine, the power turbine positioned such that heated combustion gas output from the combustor section causes the power turbine to rotate at the second rotational speed, or
• a third rotational speed associated with an output shaft of a transmission of the gas turbine engine, the output shaft connected to a hydraulic fracturing pump positioned to pump the fracturing fluid into the wellhead, and the transmission having an input shaft connected to the power turbine such that the input shaft rotates at the second rotational speed to drive the hydraulic fracturing pump;
• generating, via one or more speed sensors, one or more speed signals indicative of one or more of the first rotational speed, the second rotational speed, or the third rotational speed;
• generating, based on a determination that the compressor turbine is rotating within the range of the target idle speed, one or more brake control signals, the brake control signals configured to at least partially control operation of a brake assembly of the gas turbine engine in response to the determination, the brake assembly configured to at least partially control the second rotational speed of the power turbine independent from the first rotational speed of the compressor turbine.

A hydraulic fracturing unit assembly comprising:

• a chassis;
• a gas turbine engine connected to the chassis, the gas turbine engine including:
• a compressor positioned to compress air,
• a starter assembly connected to the compressor,
• a compressor turbine connected to the compressor such that the compressor turbine and the compressor rotate a first rotational speed, and
• a power turbine positioned such that heated combustion gas causes the power turbine to rotate at a second rotational speed;
• a brake assembly to prevent rotation of the power turbine independent from rotation of the compressor turbine;
• a transmission including:
• an input shaft connected to the power turbine such that the input shaft rotates at the second rotational speed, and
• an output shaft positioned to be driven by the input shaft at a third rotational speed;
• a hydraulic fracturing pump connected to the chassis and positioned to pump fracturing fluid into a wellhead, the hydraulic fracturing pump connected to the output shaft such that the output shaft drives the hydraulic fracturing pump at the third rotational speed;
• and
• a fracturing unit controller configured to:
• generate one or more idle signals to cause the compressor turbine to rotate within a range of a target idle speed, and
• generate one or more brake control signals based on a determination that the compressor turbine is rotating within the range of the target idle speed, the brake control signals configured to at least partially control operation of the brake assembly in response to the determination.