A method of using a subsea vessel for buoyancy, comprising: floating the vessel in water, wherein the vessel includes a shell arranged around at least one inner enclosure containing gas, with concrete material poured to fill between the shell and the enclosure; submerging the vessel until supported by a seabed; characterised in that a subsea platform transporter is provided, the transporter comprising: four said vessels, each comprising a column member having one or more internal cavities filled with buoyant material; a plurality of thrusters; at least four pontoon members (71, 72) forming a rectangular shape; a subsea platform and a platform release mechanism; a controller; the method further comprising: towing the subsea platform transporter by boat to a position above a support structure comprising supports on the seabed; releasing the subsea platform transporter from the boat; using the controller to control the buoyancy of the column members by filling the column members with sea water to facilitate a gradual descent of the subsea platform transporter; activating the thrusters to achieve a desired alignment for resting on the supports; using the release mechanism to disengage the subsea platform transporter from the subsea platform such that the transporter ascends to the surface. floating the vessel in water, wherein the vessel includes a shell arranged around at least one inner enclosure containing gas, with concrete material poured to fill between the shell and the enclosure; submerging the vessel until supported by a seabed; characterised in that a subsea platform transporter is provided, the transporter comprising: four said vessels, each comprising a column member having one or more internal cavities filled with buoyant material; a plurality of thrusters; at least four pontoon members (71, 72) forming a rectangular shape; a subsea platform and a platform release mechanism; a controller; the method further comprising: towing the subsea platform transporter by boat to a position above a support structure comprising supports on the seabed; releasing the subsea platform transporter from the boat; using the controller to control the buoyancy of the column members by filling the column members with sea water to facilitate a gradual descent of the subsea platform transporter; activating the thrusters to achieve a desired alignment for resting on the supports; using the release mechanism to disengage the subsea platform transporter from the subsea platform such that the transporter ascends to the surface. four said vessels, each comprising a column member having one or more internal cavities filled with buoyant material; a plurality of thrusters; at least four pontoon members (71, 72) forming a rectangular shape; a subsea platform and a platform release mechanism; a controller; the method further comprising: towing the subsea platform transporter by boat to a position above a support structure comprising supports on the seabed; releasing the subsea platform transporter from the boat; using the controller to control the buoyancy of the column members by filling the column members with sea water to facilitate a gradual descent of the subsea platform transporter; activating the thrusters to achieve a desired alignment for resting on the supports; using the release mechanism to disengage the subsea platform transporter from the subsea platform such that the transporter ascends to the surface. towing the subsea platform transporter by boat to a position above a support structure comprising supports on the seabed; releasing the subsea platform transporter from the boat; using the controller to control the buoyancy of the column members by filling the column members with sea water to facilitate a gradual descent of the subsea platform transporter; activating the thrusters to achieve a desired alignment for resting on the supports; using the release mechanism to disengage the subsea platform transporter from the subsea platform such that the transporter ascends to the surface.