Riser Stalk-on : A Comparative Study of Analysis Results Against Real World Observations

Tie-in of a pipeline to a riser is an essential step in offshore oil and gas development. For shallow waters, the surface tie-in method, usually referred to as the “stalk-on” method, is generally employed. The surface tie-in method involves welding the riser-spool assembly to the pipeline above water and lowering it into the desired location. Main stages of the stalk-on operation include lifting the pipeline using portable davit cranes, hoisting the riser-spool assembly using a vessel crane, welding the pipeline and riser-spool assembly together, and lowering the welded assembly to the target seabed location. One of the major challenges offshore during the surface tie-in method is aligning the spool tip angle and the lifted pipe tip angle for the welding operation, as well as accurately estimating the deflection at the top of the riser. The primary objective of this paper is to compare the observed spool tip angle and riser top deflection offshore with the results obtained from analysis. The analysis explores multiple rigging configurations for controlling riser deflection and achieving the desired spool tip angle. The paper also demonstrates the effectiveness of knee braces and sheave type rigging arrangements in controlling the riser top deflection and stress levels during riser lifting. Lessons learned from actual field operations are summarized, addressing discrepancies between analytical estimations and actual field observations.