Casing-Running Analysis in Riserless Topholes

Abstract

Riserless drilling poses numerous operational challenges that adversely affect the efficiency of the drilling process. These problems include increased torque and drag, buckling, increased vibration, poor hole cleaning, tubular failures, poor cement jobs, and associated problems during tripping operations. Riserless drilling in deep and ultra-deep water, as well as drilling to deeper depths, requires improved models and comprehensive analyses, especially when larger-diameter casings are run and cemented in a deviated or directionally drilled tophole environment. Current methods that lack proper modeling will severely overestimate hookload values when casing strings are run in and underestimate in the pick-up load estimates. A new modeling approach is proposed to calculate appropriate hookload values, as well as torque and drag. This method can model situations in which the drillstring/casing string is in open water or in openhole under various operating conditions. Both soft- and stiff-string models are used in the hookload estimates and in the wellhead side-loading calculations. Hookload calculations and buckling limitations for the scenarios in which casing and inner strings are run with drilling mud inside the inner string, seawater in the outer string, and pad mud in the hole below the mudline, are presented. The study indicates that the results are influenced by various parameters, including depth of the mud line, offset of the wellhead from the rig center, wellbore inclination, curvature, wellbore torsion, and angle of entry into the wellhead, as well as by the complexity arising from wind, wave forces, and ocean currents. This study compares simulated predictions with actual well data from wells from Angola and the North Sea and describes the accuracy and applicability of the model. It also presents several examples of surface casing-string failure.

A Method for Buckling Analysis of a Riserless Drillstring

“…Use of a slickline to perform a jarring operation from a boat to a subsea well was also simulated in the same reference. A soft-string model was used by Samuel et al (2011) to describe the buckling failures of riserless drillpipe or casing strings. The classical catenary equation was adopted to model the deformed shape of the string based on an assumption that the bending stiffness is negligibly small.…”

“…The predicted results were compared with actual well data. Further comparison with real well data from a casing-running job in riserless topholes was performed by Samuel and Gradishar (2011). In this model, a uniform string was considered.…”