Progressive assembly and welding: In the progressive assembly method, the bottom plates are assembled and welded first. Thereafter the shell plates are erected, held in place, tacked, and completely welded. This shall be done course by course, working upwards to the top curb angle. No course shall be added as long as the previous course has not been entirely welded. The erection and completion of the roof framing and roof plates then follow.
Complete assembly followed by welding of horizontal seams: In the complete assembly method, the bottom plates are assembled and welded first. Thereafter the shell plates are erected, held in place, and tacked, and only the vertical seams are completely welded, leaving the horizontal seams un-welded. This shall be done course by course, working upwards to the top curb angle. No course shall be added as long as the vertical seams of the previous course have not been entirely welded. The erection and completion of the roof framing and roof plates then follow. Finally, the horizontal seams are welded, working upwards from the bottom course or downwards from the top curb angle.
Jacking-up method: Some contractors employ a system of erection in which the bottom plates are completed, The top course is erected on the bottom plates, the roof framing and sheeting are completed and a number of jacks are then assembled around the structure. By means of these jacks, the completed top course together with the roof framing and sheeting is lifted to a height sufficient to insert the next lower course. The jacking method and the supporting of the partly erected shell shall have no adverse effect on the roundness of the shell. The welding is completed at each stage of lift until all courses of the shell plates have been inserted and the finished height is reached. The final operation is the welding of the bottom course to the bottom plates.
Flotation method: The flotation method is used for floating roof tanks. After the completion of the bottom plating and erection and welding of the two lower courses of the tank, the floating roof is assembled on the tank bottom and completed. The tank is then filled with water and, using the floating roof as a working platform, the third and subsequent courses are erected and welded, water being pumped in as each course is completed. Regular checks on the vertical alignment and roundness are required. This method may only be used at locations where soil settlement is very limited and with the agreement of the Principal. The predicted soil settlements of the soil investigation report shall be taken into account. A small crane is usually erected on the floating roof for hoisting the shell plates into position.
