Experimental Study on Cutting Offshore Multi-Layer Casing–Cement Composite Structures Using a Premixed Ultra-High-Pressure Abrasive Waterjet

Sub-mudline cutting of abandoned offshore wellbores requires complete separation of multi-layer casing–cement structures under conditions of casing eccentricity and varying jet propagation distances. This study used a rotary premixed ultra-high-pressure abrasive waterjet to conduct 19 concentric screening tests, one independent concentric confirmation test, and one eccentric circumferential severance test on full-scale four-layer casing–cement specimens. Under the tested conditions, increasing the combined pressure–flow input or extending the rotation period enhanced cutting capability, whereas abrasive volume fraction and particle size showed non-monotonic effects. A candidate combination was selected directly from the tested conditions: a pump pressure of 210 MPa, an abrasive volume fraction of 10.6%, garnet abrasive with a nominal median particle size (d50) of 0.18 mm, and a rotation period of 6 h/rev. Both concentric tests achieved V2, defined as penetration through all four casing layers and formation of a slit on the inner witness plate located 575 mm from the nozzle outlet. Applied without retuning to one specimen with a specified eccentric geometry, this combination achieved complete 360° severance and direct physical separation of the upper and lower sections after a cumulative effective cutting time of 6 h. The maximum distance from the nozzle outlet to the outer surface of the outermost casing was 346 mm. This study demonstrates severance feasibility for a single specimen with the specified tool configuration and eccentric geometry, providing a basis for estimating effective cutting time and resource requirements.