An apparatus for driving a load, comprising: a multiple-shaft gas turbine comprising: a high pressure compressor and a high pressure turbine drivingly connected to one another by a first gas turbine shaft; a low pressure compressor and a power turbine drivingly connected to one another by a second gas turbine shaft, extending coaxial with said first gas turbine shaft, said high pressure compressor and said high pressure turbine; a load coupling drivingly connecting said power turbine to said load; a dual-speed turning gear comprising a first worm gear and a second worm gear with an output shaft drivingly engageable to and disengageable from said load; wherein said dual-speed turning gear comprises a low speed turning motor and a high speed turning motor, said low speed turning motor and said high speed turning motor being arranged and controlled to selectively drive said load; the dual-speed turning gear being arranged to slow-roll the load, the power turbine and the low pressure compressor when the gas turbine is shut down and rotate the power turbine, the low pressure compressor and the load during offline washing of the gas turbine, and wherein a shaft of said second worm gear is connected on a first end to said slow speed turning motor and at a second end to said high speed turning motor, and wherein a clutch is arranged between said second end of said shaft and an output shaft of said high speed turning motor, such that when the rotary speed of the output shaft of said high speed turning motor exceeds the rotary speed of said shaft of said second gear worm, the high speed turning motor takes control over the second worm gear. a multiple-shaft gas turbine comprising: a high pressure compressor and a high pressure turbine drivingly connected to one another by a first gas turbine shaft; a low pressure compressor and a power turbine drivingly connected to one another by a second gas turbine shaft, extending coaxial with said first gas turbine shaft, said high pressure compressor and said high pressure turbine; a high pressure compressor and a high pressure turbine drivingly connected to one another by a first gas turbine shaft; a low pressure compressor and a power turbine drivingly connected to one another by a second gas turbine shaft, extending coaxial with said first gas turbine shaft, said high pressure compressor and said high pressure turbine; a load coupling drivingly connecting said power turbine to said load; a dual-speed turning gear comprising a first worm gear and a second worm gear with an output shaft drivingly engageable to and disengageable from said load; wherein said dual-speed turning gear comprises a low speed turning motor and a high speed turning motor, said low speed turning motor and said high speed turning motor being arranged and controlled to selectively drive said load; the dual-speed turning gear being arranged to slow-roll the load, the power turbine and the low pressure compressor when the gas turbine is shut down and rotate the power turbine, the low pressure compressor and the load during offline washing of the gas turbine, and wherein a shaft of said second worm gear is connected on a first end to said slow speed turning motor and at a second end to said high speed turning motor, and wherein a clutch is arranged between said second end of said shaft and an output shaft of said high speed turning motor, such that when the rotary speed of the output shaft of said high speed turning motor exceeds the rotary speed of said shaft of said second gear worm, the high speed turning motor takes control over the second worm gear.