A method for providing artificial lift to production fluids within a subterranean well, the method comprising: loading an electrical submersible pump assembly into an interior cavity of a pump launcher, the electrical submersible pump assembly having a motor, a pump and being releasably secured to a piston device, the piston device having a controllable outer diameter profile; releasably securing the pump launcher to a wellhead so that the interior cavity is in fluid communication with an inner bore of a production tubing that extends a length into the subterranean well, said subterranean well comprising a tubing packer located in an annular space outside of the production tubing, said annular space being defined between an inner diameter of the subterranean well and an outer diameter of production tubing, said production tubing comprising: a sealable production fluid inlet providing a fluid path between a region of the subterranean well below the tubing packer and the inner bore, and circulation fluid inlets providing a fluid path between the inner bore and the annular space above the tubing packer; closing the sealable production inlet; operating a surface valve system to provide a flow path for a circulating fluid from a surface pump to the inner bore of the production tubing and so that the circulating fluid, when returning while travelling up through the annular space, enters the surface valve system; pressurizing the circulating fluid with the surface pump and moving the circulating fluid through the valve system so that the valve system directs the circulating fluid into the inner bore of the production tubing to act on pressure surfaces (38, 40) of the piston device to move the electrical submersible pump assembly from the pump launcher and into the subterranean well; and communicating with the piston device to change the outer diameter profile of the piston device to control a descent of the electrical submersible pump assembly through the subterranean well. loading an electrical submersible pump assembly into an interior cavity of a pump launcher, the electrical submersible pump assembly having a motor, a pump and being releasably secured to a piston device, the piston device having a controllable outer diameter profile; releasably securing the pump launcher to a wellhead so that the interior cavity is in fluid communication with an inner bore of a production tubing that extends a length into the subterranean well, said subterranean well comprising a tubing packer located in an annular space outside of the production tubing, said annular space being defined between an inner diameter of the subterranean well and an outer diameter of production tubing, said production tubing comprising: a sealable production fluid inlet providing a fluid path between a region of the subterranean well below the tubing packer and the inner bore, and circulation fluid inlets providing a fluid path between the inner bore and the annular space above the tubing packer; a sealable production fluid inlet providing a fluid path between a region of the subterranean well below the tubing packer and the inner bore, and circulation fluid inlets providing a fluid path between the inner bore and the annular space above the tubing packer; closing the sealable production inlet; operating a surface valve system to provide a flow path for a circulating fluid from a surface pump to the inner bore of the production tubing and so that the circulating fluid, when returning while travelling up through the annular space, enters the surface valve system; pressurizing the circulating fluid with the surface pump and moving the circulating fluid through the valve system so that the valve system directs the circulating fluid into the inner bore of the production tubing to act on pressure surfaces (38, 40) of the piston device to move the electrical submersible pump assembly from the pump launcher and into the subterranean well; and communicating with the piston device to change the outer diameter profile of the piston device to control a descent of the electrical submersible pump assembly through the subterranean well.