Assessment of the Theoretical Net Relief Drilling Rate for Conductor Pipes

The following paper deals with the prediction of the theoretical net drilling time for a conductor pipe (CP) of offshore platforms when it plugs. The CP is driven to prevent the looser surface layer from caving in and obstructing the wellbore, because the shallow section of most wells on- and offshore is drilled in unconsolidated sediment. Due to the small CP diameter, the CP plugs very often. Normally the material is drilled out by using jetting, normal drill rigs and sometimes jack-up rigs. As offshore operations time is very important, this paper tries to assess the theoretical net time needed to excavate within the CP as a relief drilling operation, by means of Nishimatsu and Evans cutting theories. The results are compared with publicly available TBM excavation rates and then theoretical drilling rates for different unconfined compressive strength values for CP diameters of 700, 900 and 1000 mm are suggested. The results are considered for drag bits, however, they are only theoretical assumptions and cannot be applied for commercial projects with the same geotechnical properties.