Apparatus, system and method for tethering a subsea assembly

An apparatus 38, system 32' and method for tensioning a tether member 36, tethering a subsea blowout preventer 24 to a subsea mount arranged on a sea bed, comprising a first tensioning mechanism (fig.3,382) configured to apply a first tension and a second tensioning mechanism (fig.3,384) configured to apply a second subsequent greater tension to the tether member. Tensioning mechanisms may be mechanical, the first mechanism being a lockable, rotatable drum (fig.4C,40) reel pre-mounted with the tether member and the second tensioning mechanism being a lockable screw jack (fig.6,58) with linear drive, wherein the first tensioning mechanism is linearly displaced by the second tensioning mechanism to apply the second tension. Tensioning mechanisms may act to reduce tension and further include tensioning monitoring means and a tension release clutch. First tensioning mechanism may apply a pre-tension to reduce slack and the second tensioning mechanism applies a final tethering tension.