## 1 Introduction  
\"The 21 st century is the era of ocean development\" has now become a global consensus. For China, aiming at developing the ocean is a far-reaching choice. For the present, it will certainly drive the rapid development of ocean engineering as part of the process of becoming a maritime power. At present, the construction of large-scale marine projects such as the Hong Kong-Zhuhai-Macao Bridge and the subsea tunnel requires large-scale subsea caisson operations, and large-scale marine engineering caisson operations have higher requirements for the flatness of the seabed. Seabed digging operations require large grab dredgers for construction operations [1; 2; 3]. Therefore, seabed digging and leveling operations are important and challenging issues in current large-scale ocean engineering research. The basic structure of a grab dredger consists of the lower sheave block, the upper sheave block, the bucket body, the gear segment, the hoist wires, the closing wires, etc. Under the gravity action of the grab, the digging force is generated through the action of the closing wires. This is a process of digging dynamic balance [4; 5]. As shown in Figure 1a, the grab digging operation is carried out by the following steps: open the bucket, lower the grab down to the material, apply tension to the closing wires, keep the hoist wires static, gradually close the bucket, and start digging with the grab; load the grab dredger to a certain height, while raising the closing wires and hoist wires to achieve full closure of the bucket; then, move the grab to the release site, release the closing wires, keep the hoist wires static, and release the bucket. The operation just described is discontinuous and cyclic [6,7]. Under normal and flat digging conditions with a grab dredger, the digging curve is different, depending upon the conditions. In the normal digging state, the digging curve is steep, while, in the flat digging state, the digging curve is smooth. In order to achieve the effect of flat digging, it is necessary to control the movement of the closing wires and the hoist wires [8, 9, 10, 11]. When lifting the closing wires, while automatically controlling the hoist wires of the grab, the digging plane formed is flat, as shown in Figure 1b. In order to achieve automatic flat digging with a grab dredger, the first step lies in improving the structural design of the bucket through the analysis of digging resistance. Over the years, many useful results have been obtained through experimental and theoretical studies of the mechanical mechanisms of grab dredgers, laying a solid foundation for their extensive application. [PERSON] [12] analyzed and studied a mechanical model of the closing process of the grab dredger during the digging operation and conducted theoretical simulations of--and experimental research on--the digging characteristics of the grab dredger. This is one of the earliest studies of the force characteristics of the grab. [PERSON] [13] analyzed the translation process of the grab and obtained a calculation method and formula for the digging resistance. [PERSON] [14] simulated and calculated the process of grabbing bulk materials and obtained reasonable parameters of grabbing resistance and pushing resistance. [PERSON] [15] conducted static finite element analysis on--and dynamic numerical simulation of--the mechanical characteristics of the grab dredger; and conducted an experimental study on the grab digging process. An analysis of the above literature shows that, in order to study the mechanical mechanism of the grab digging material, the simulation of--and experimental research into--the grab digging process are both very meaningful. In the excavation process of the grab, the interactions between the material and the grab device and the interactions between the materials themselves are more complicated. In this study, the research method is different from the traditional research methods.