To improve the reliability of large hot tap pipeline operations, Shell Expro commissioned a detailed non-linear Finite Element study to investigate the structural and mechanical response of such systems, in particular during pressure testing of the branch welds. The study included load steps simulating welding of branch to pipeline, hydrostatic pressure testing of the welds, coupon cutting and removal, and normal pipeline operation. A limit state analysis was used to determine the ultimate collapse load of the coupon during external hydrostatic pressure testing, and was verified by a small number of scale model tests and then used to complete a full parametric study covering all branch sizes and configurations of interest. In some cases the buckling pressure was found to be quite low and the effect of welding stiffeners to the coupon was quantified. Pipeline spring back deflections were also determined and assessed as to their effect on coupon cutting and removal operations. The results of the study were applied to a large hot tap connection on the FLAGS gas pipeline during the summer of 1990 and ensured a high level of safety at all stages of the operation.