Experimental Study On Removing Subsea Wellhead Using Abrasive Water Jet Cutting Technique

The cutting of subsea abandoned wellhead is the most important and expensive job during the removal operation of offshore oil platform. Abrasive water jet cutting technique is safe and environmentally friendly approach for cutting of oil wellhead with multi-layer casing. However, during the process of cutting subsea wellhead with abrasive water jet technique, the gradual increase of jet standoff distance lowers the cutting efficiency. Aiming to improve the working performance of cutting tool, the relevant theoretical and experimental studies are carried out.Experiments are firstly carried out to study the effect of various parameters on the cutting efficiency. Experimental study shows that the depth of cutting of casing increases with the increase of pump pressure, nozzle diameter, number of nozzles and duration of cutting, while decreases with the increase of standoff distance. The efficiency of cutting of casing initially increases with the increase of rotation speed of cutting tool and then decreases after the rotation speed reaches the optimal value of 7.895r/min. The same rule is found on the effect of concentration of abrasive on cutting efficiency with the optimal concentration of 26.10%. The maximum cutting efficiency is obtained when using the abrasive of iron sand, which is followed by silicon carbide, garnet and quartz sand in turn.Since many parameters in conventional models for calculating depth of cutting of casing are difficult to be determined, the universality of these models is compromised. Based on the analysis of experimental data, a new model for predicting depth of cutting is developed with the combination of Support Vector Machine technique and Particle Swarm Optimization algorithm (PSO-SVM). Numerical experiment indicates that PSO-SVM model can predicts cutting depth of casing under certain work conditions with satisfying accuracy of average percentage error 3.20%. Support Vector Machine is proven to be a novel approach to model the cutting depth of abrasive water jet.In order to reduce the standoff distance in the process of cutting of subsea wellhead, a brand-new cutting tool with flexible nozzles are proposed and designed. The indoor experiment of flexible nozzles shows that the flexible nozzle can extend out, work stably when pressure keeps constant and be retrieved successfully. Therefore, the design proposal of the flexible nozzle is feasible and is also an important innovation of conventional jet nozzle.The comparison of cutting efficiency between new cutting tool and normal cutting tool is made, which shows that the newly designed cutting tool can greatly improve the efficiency of cutting of multi-layer wellhead.