A study of the heave-induced surge and swap pressures when a drillstring is in the slips of a floating vessel

Abstract

This study presents a theory describing surge and swab pressure when a drillstring is hanging in the slips of a heaving vessel. Downhole pressure measurement shows high pressure pulses in these instances, higher than can be explained with simple steady-state calculation. Several factors and parameters affect this pressure, such as annulus geometry, drillstring elasticity, wellbore friction, fluid compressibility, advanced fluid rheology, and fluid inertia. The advanced fluid rheology includes a flow-development phase and a simple fluid thixotropy model. The theoretical model is tested and compared with actual field data recorded at the surface and downhole, both pressure and string acceleration are measured. Key findings from this study are: (1) The annular flow is induced by the axial stick–slip motion of the bottomhole assembly; (2) The surge swab pressure consists of a pressure-loss term and an inertia term, where the latter is the dominating one in this case; (3) The short periods of the axial string motion call for a transient analysis to get an accurate estimate of the pressure amplitude. • Transient surge and swab model. • Stick slip induced pressure pulses. • Fast model for thixotropy and flow pattern development. • High resolution downhole measurements. Both string acceleration and downhole pressure.