The article discusses the efficiency of applying a 12RPE-4,4/1,75 rotary-piston engine as an expansion machine for a promising energy technology plant for the production of hydrogen sulfide from the depths of the Black Sea. In this installation, high-pressure hydrogen sulfide is used in the expansion machine to produce mechanical energy, which can ensure the operation of pumps and other equipment. 12RPE -4,4/1,75 is a 12-cylinder short-stroke engine with an S/D ratio of 0.4. A feature of the engine is that for the distribution of the working fluid are used inlet and outlet openings, the overlap of which is carried out by the central rotor. Thus, the central rotor performs the function of a slide valve and a housing in which the pistons move. The use of the spool distribution provides a simple and compact design. The pressure of hydrogen sulfide at the inlet to the rotary-piston expansion machine is determined in the range of pipeline immersion depths of 0...1000 m, at a temperature of 280 ... 285 K and gas content of hydrogen sulfide in seawater 0.0...10.0 m3/m3. The definition of the main parameters of the working fluid and the working process is performed for a daily production of seawater of 100 m3/day. It is calculated that for a gas content of 2.5 m3/m3 with a depth of immersion of the lifting pipeline H = 250...1000 m, the power of the expansion machine is 0.20...0.61 kW, for gas content 5 m3/m3 – 0.65...0.86 kW for gas content 7.5 m3/m3 – 1.20...1.87 kW, for gas content 10 m3/m3 – 1.65...2.55 kW. Indicator diagrams of the expansion machine operation depending on the gas content of hydrogen sulfide in seawater and the depth of immersion of the pipeline were obtained. It was determined that with an increase in pressure at the inlet, the effective efficiency of the hydrogen sulfide expansion machine lies within 0.21...0.49. Also, with an increase in inlet pressure, the specific effective consumption decreases from 51.5 to 25.17 kg/kWh. Low values of specific effective consumption of hydrogen sulfide and high values of effective efficiency are due to the design features of the rotary-piston expansion machine, which combines the advantages of the piston and rotary engines.