An offshore platform used in the production of hydrocarbons is constructed in several phases. Portions of the offshore platform are manufactured onshore and assembled either in close proximity to the place of manufacture or at the location where the offshore platform is to be positioned for production. One portion of an offshore platform that is manufactured onshore and towed to the location where the offshore platform is to be positioned is the jacket. An offshore platform jacket is normally fabricated in a yard adjacent to a slipway. After fabrication, it is skidded onto launch beams, first over the skid beams 14 on dock, then over the skid beams 16 on a launch barge. The barge is then towed out to sea by a tugboat 20. Subsequent to the ocean tow, it is launched into the sea over tilting (rocker) beams 22, located at the end of the skid beams (see FIGS. 1-4).
A great majority of shallow water jackets (those that cannot be lifted off because of weight limitations) and all of deep-water jackets follow this routine before they are finally upended, piled and their deck installed.
During the launching operation, both jacket and barge usually undergo one of the highest stress levels. Approximately ten (10%) percent by weight of additional steel is required in many jackets in order to resist the one-time severe launching stresses. This additional steel, which is not needed except for launching, becomes part of the permanent structure and attracts additional wave and current loads throughout the life of the structure.
Generally, the strength of the launch barge is also tested during launching. The barges are heavily reinforced around the tilting beam supports. One jacket may deliver up to twenty thousand (20,000) tons of load onto each pin of the rocker beams.