Tie-in between a pipeline and a riser is an important operation in the offshore oil and gas development. Normally, divers need to carry out the tie-in operation on the seabed by welding or using flanges in order to connect the pre-laid pipeline end to the jacket riser. However, working on the seabed can have leakage or cost problems. As one of solutions, the surface-tie-in method so called stalk-on method is used in the shallow water. In this method, the riser is welded to the pipeline which is lifted over the sea surface, and the welded pipeline-riser is lowered to the target position.
The main purpose of this paper is to demonstrate the safety evaluation procedure of the stalk-on operation and to share the experience learnt from actual field application. For the sake of the safety evaluation, the stalk-on operation is divided into 3 phases which are the phases of lifting a pipeline by davit cranes, holding a lifted-pipeline and lowering the weld-connected pipeline and the riser. Each phase is numerically simulated in time-domain analysis. The several lesson learnt from actual application are summarized including the consideration of discrepancy between time domain simulation result and the measured one during the installation.