When offshore drilling rig jackets are built, they’re often built on their sides and then dragged onto a barge for transport out to sea where they are positioned, shoved off the barge to float, then gradually flooded as they’re tugged into position to settle on their pre-cast sub sea bases. Skidding a huge structure up to 25,000 tons to load it on a barge required the use of re-positionable hydraulic jacks. Most often the structure is just dragged along a row of wide flange steel I-beams.
The hydraulic jacks are positioned by either a latching mechanism engaging a hole in the beam of a gripper mechanism that uses hydraulic pressure to clamp on the beam flanges. One or more jacks can then exert their massive push forces on the structure to move it ahead some distance, usually on the order of 1 metre. Once the jacks are fully extended, the gripper disengages or the advancing load relaxed in the case of a latch, so that the jack can then retract and draw the gripper or latch ahead to re-position itself for another push. This inchworm sequence: clamp, push, unclamp, advance is repeated until the structure is moved the desired distance.
The same method, using wide flanged beams as skidding surface, is used to move drill towers about on offshore drilling rigs. In shipyards where ships are built in sections, this method can be used to marry sections together, accurately positioning them to be welded together.