The assembly of large pipe columns from pipe sections less than fifty feet long is a well-established practice that is time consuming and costly. The installation time is inherently costly because massive hoisting gear, sometimes a drilling rig, is involved. If the work is being done off shore, floating platforms can add substantially to the time related cost.
The activity is permeated with massive machines and materials. Typically, the dimensions involved may include pipe diameter of two feet to ten feet with pipe wall thickness as much as three inches and an assembled length in the range of five hundred to fifteen hundred feet. Cost is already minimized in the design phase. The remaining cost reduction effort is directed to reduction of time involved in the assembly of the column. Four welders, or more, may be simultaneously involved in one joining weld. Each weld may be ultra sonically inspected in detail at least once on each joint. If flaws are found by testing, repair work disrupts the smoothest of routines.
Time saving efforts encounter some natural barriers to further improvement. Quite often, joining activities are suspended to allow dissipation of heat evolved from welding. If more welders are assigned to simultaneously work on a joint, the temperature of adjacent metal rises more rapidly. If certain temperature limits are reached, welding must be stopped to allow local material to cool and any potential time gain may be lost. Because ultra sonic testing for welding flaws must be done, the cool down period can be somewhat salvaged by testing during that otherwise wasted interval. Eliminating the cool down period appears to add the inevitable testing time to other parts of the time schedule.
The heat imposed by the welding process causes the temperature to rise in adjacent material at a rate somewhat inversely proportional to the weight of metal in the immediate vicinity and to the heat dissipating surface area near the welding activity. Those factors seem fixed and altering them is a challenge this invention addresses.
The weight of deposited weld metal is time related and the amount required is influenced by residual stress found in the outer peripheral surface of every weld. The last weld metal applied gets hotter than the first applied and the last applied shrinks more than the first applied. The resulting residual stress imbalance places tension stress on the outside surface of pipe being considered and places compression stress on the inside surface. Residual tension stress reduces the amount of imposed tension stress column bending can be allowed to add to a joint of specific physical dimension. Reducing the residual stress, hence, reduces the weight of weld metal required for a given joint strength and saves welding time.
It is therefore an object of this invention to generally double the heat dissipation surface of pipe adjacent to welding activity by separating the weld starting and weld completing activities by one pipe section length to avoid cool down delays.
It is another object of this invention to provide ultra sonic testing activity without delaying welding activity by placing testers between welders progressing peripherally around pipe being simultaneously welded and tested.
It is still another object of this invention to provide process steps to reduce the residual stress caused by welding to reduce the amount of weld metal required to produce a joint having specific strength.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached claims and appended drawings.