Hydraulically releasable coupling device

A hydraulically releasable coupling device for use as an intermediate piece between a coiled tubing and a subsea tool or accessory formed as an elongate body having a through bore and comprising at least two interconnected, substantially concentric sleeve-shaped parts telescopically disposed, one within the other, to overlap each other along a predetermined longitudinal portion of said coupling device and, within a longitudinally extending area of overlap, said sleeve-shaped parts, which comprise an upstream sleeve-shaped part adapted to connect to said coiled tubing and a downstream part adapted to connect to said tool or accessory are connected and interlocked by means of an unexpanded, radially expandable locking ring having a locking effect that is configured to be brought to an end by causing the locking ring to expand radially and thereby disconnect, from one another, said sleeve-shaped parts of the coupling device together with shear-pin penetrating opposite sleeve walls, wherein a securing sleeve or ring portion is movable upon the occurrence of an intentional release operation enclosing the locking ring at least partially in an unexpanded locking position thereof, said securing ring portion being connected to a piston body having a through passage which is in a downstream direction provided with a restriction means in form of a nozzle or a choke valve or a choked opening, said tool or accessory being adapted to be in a position operating at high fluid pressures when combined with a through-flowing amount of fluid of moderate value passing through per time unit, said piston body having an upstream piston head with an annular upstream reaction surface and an annular downstream reaction surface for pressure fluid in counterflow, the piston head exhibiting a larger diameter than a downstream extension formed as a piston skirt, in which a restriction element is mounted, wherein a longitudinal portion of said piston skirt, next to the piston head, is enclosed by a cylinder chamber of the piston head accommodating a return spring.