## 2 Analysis of Factors Affecting the Stability of the Skid-and-Mud-Return Line  
### Establishment of an Analysis Model for the Skid-and-Mud-Return Line  
An RMR drilling system typically consists of several main components: the drilling unit, the mud-return system unit, the mud-processing unit, and the power-monitoring unit. The drilling unit includes the exploration vessels, drill pipes, the derrick, downhole tools, casings, risers, and subsea suction modules. It is used for rock breaking and core extraction. The mud-return system includes the seabed anchoring system, the seabed pump assembly, mud-return pipelines, pipeline docking connectors, and a subsea installation platform. It is used to lift the rock cuttings and mud from the seabed into the exploration vessel to circulate the drilling mud. The skid is typically made of wear-resistant and corrosion-resistant materials, such as high-hardness metal or rubber. Its sealing functions guarantee that there will be no leakage of the pumped mud. The mud-return pipeline transports drilling fluid and other return fluids from the wellhead to the surface equipment for processing and treatment. During the drilling process, the mud-return pipeline plays a role. It brings back the used drilling fluid, rock cuttings, the formation water, and other byproducts from the wellbore to the surface. These returned fluids are processed and separated in surface equipment to recover and treat the drilling fluid.  
In mud-pump systems, it is crucial to maintain a seal between the skid-and-mud-return line to prevent mud leakage. Usually, it joins two components together via a threaded connection or a similar rigid connection to form a reliable joint. This connection method can guarantee high sealing performance and pressure resistance. Therefore, it is suitable for working environments involving high-pressure mud pumping. The installation diagram of deepwater RMR equipment is shown in Figure 1. The water depth in the work area is about 2000 m. The lower mud-return pipeline is about 1300 m long, and the upper mud-return pipeline is about 700 m long. This section mainly focuses on the stability of the return pipeline of the mud circulation system.  
Due to the rigid connection of the skid-and-mud-return line, the stability of the skid-and-mud-return line is analyzed as a whole. The mechanical model of the skid-and-mud-return line is shown in Figure 2a. The skid-and-mud-return line is fixed to the subsea, with the uppermost part of the mud-return line connected to the drilling vessel. The drilling vessel is subject to wind and sea currents, which can affect the skid-and-mud-return line. The vibration from the operation of the mud pumps in the skid and the reverse torque of the mud against the mud pumps are generated. These can affect the stability of the  
Figure 1: Installation diagram of deepwater RMR equipment.  
skid-and-mud-return line. In addition, because the skid-and-mud-return line is in seawater, sea currents can also affect the stability of the skid-and-mud-return line.  
Since the ABAQUS/Aqua module in ABAQUS can be used to calculate drag, buoyancy and fluid inertia load can be calculated from steady and wave-induced flows. ABAQUS is widely used in numerical simulation of offshore petroleum engineering. Based on the mechanical model of the skid-and-mud-return line, an analytical model of the skid-and-mud-return line is established using the ABAQUS 2018 software, as shown in Figure 2b. The bottom of the model is fully fixed. Due to the installation of the skid at 1300 m from the seabed, reverse torque and displacement are applied to the model at this position. Similarly, a load is applied to the top of the model to simulate the effects of the drilling vessel on the skid-and-mud-return line. The sea current force is applied by editing keywords in the ABAQUS/Aqua module. Finally, the model is meshed every one meter for a total of 2000 meshes.