Study On Removing Subsea Casing Using Abrasive Water Jet

With the unceasing development of offshore oil and gas exploration, a batch of offshore platforms completed their historical mission in succession, so the decommissioning and removal of oil production platforms are hot point of engineering. The cutting of the oil wellhead and risers is one of the critical operations during the platform removing process; the popular cutting methods include mechanical cutting, diamond wire cutting, shaped charge explosion, and abrasive jet cutting currently. The wellhead is usually composed of eccentric multi-layer casings and risers which annular spaces are cemented. Since it's required that the wellhead must be severed at least 5m below the seabed, both diamond wire method and mechanical cutting method are not applicable. The use of shaped charge explosive methods has been practically minimized due mainly to either safety or environmental concerns. The abrasive jet cutting is found to be the most effective and environmentally friendly method for cutting of wellhead and riser as the use of this method is not restricted by the cutting position and casing eccentricity.The Cavity Expansion Theory has been employed to analyze the physical process of cutting metal materials with abrasive water jet technique. A mathematical model for penetration depth generated by abrasive particle is formulated. The calculation results of this model indicate that higher jet pressure and high-density abrasives can lead to the substantial increase of cutting efficiency, which provides the theoretical basis for optimizing the abrasive in field operation.Experiments are carried out in submerged condition to study the effect of various parameters on the cutting efficiency, including jet pressure, nozzle diameter, number of nozzles, revolution speed of cutting head, duration of cutting, standoff distance, mass concentration of abrasive, and abrasive categories. The results show that higher jet pressure, larger-diameter nozzle, more nozzles, longer working duration, and shorter standoff distance lead to deeper penetration. The threshold pressure for the P110 casing is 9.4 MPa. The optimum rotation speed of the cutting head is found to be 7.8 rpm. The maximum cutting efficiency can be achieved if the mass concentration of abrasive is 0.261. The performance of iron abrasive is the best as the abrasive and the price-performance ratio of garnet is the best. In this paper's experimental condition, the math model for calculating cutting depth with variation of time is built, which provide the theoretical basis for the prediction of casing cutting depth and the confirmation time of cutting off casing.According to various pipes and related equipments, abrasive cutting systems for in-pipe cutting and out-pipe cutting are designed. The abrasive cutting tool for in-pipe cutting is capable of cutting casings or risers with diameter range of 7 in. to 20 in., while the outer-pipe abrasive cutting head is capable of cutting risers with diameter range of 30 in. to 42 in. The rotary dynamic seal problem is properly handled properly in the presence of high concentration sand with the assembly of the sand prevention and seal that is designed specially. Thought the nozzle abrasion testing, it is recommended that nozzles made of ROCTEC500 material are suitable for the long period operation and nozzles made of YG6X material are used for other operations.Based on principle four stage hydraulic cylinder, hydraulic anchor system is designed in order for the cutting tool to be fixed on the outer surface of riser. The crank rocker structure is employed to deal with the wire winding problem that would take place when the outer-pipe cutting tool is radically cutting riser. Thought the full-size simulation experiment in site, the in-pipe cutting tool of abrasive jet has a good performance. The rotary dynamic seal and the sand control installation work reliability and have long operating life. The quality and life span of abrasive jet nozzles meet the requirements of operation in site.A novel cutting head mounted with retractable nozzles is designed to decrease the standoff distance during the cutting operation so as to increase the cutting efficiency. In terms of different stages of the cutting, the dimensionless standoff distance can be decreased to 1/16.7 or 1/27 of the original value with the nozzle ofφ3mm.The Back Propagation Neural Network is introduced to model the cutting depth on casing generated by abrasive water jet. Based on the experimental data, a BP model is established to predict the cutting depth of abrasive jet cutting of casing. The prediction of the BP model is satisfying with the maximum, minimum and average relative error found to be 17.02%, 0.36%, and 4.26%; the results of the prediction can provide direction for engineering design. At the same time, the method of applying the nerual network technique to model for the cutting depth of casing can be popularized to cutting depth modeling of other material with abrasive jet.Thought the theoretical analysis and experimental study, the physical process of cutting casing with abrasive jet technique is analyzed. Cutting tools are developed to realize the cutting purpose and successfully tested both in indoor environment and in field. The results obtained in this study promote and expand the application of abrasive jet technique in the removal of underwater structures.