What Factors Affect The Efficiency Of Small Rotary Drilling Rigs

The drilling efficiency of a small rotary drilling rig refers to the ratio of the total depth of the drilled pile hole to the total time spent during normal operation. The factors that affect the drilling efficiency of rotary drilling rigs include hole depth, hole diameter, geological conditions, selection of drilling tools, and drill rod type.

The influence of hole depth on the drilling efficiency of rotary drilling rig

During the construction of rotary drilling rig, every time the drill bit is filled with soil, it is necessary to lift and unload the soil, and then lower the drilling tool to the bottom of the hole. Obviously, the smaller the hole depth, the higher the efficiency. Drilling efficiency V=H1/(t1+t2+2H/V1) (m/h), where H is the amount per drilling foot, m;t1, The time required to drill a full bucket, h; t2 is the unloading time, h; H is the depth of the hole, m;V, To increase and decrease drilling speed, m/h. From the above equation, it can be seen that when other parameters remain constant, the larger H, the smaller V. When the hole depth exceeds 100m, the drilling efficiency sharply decreases. When the hole depth exceeds 120m, the drilling efficiency will be lower than that of other construction methods, and the accident rate will significantly increase. Therefore, it is recommended to use other construction methods such as forward and reverse circulation as much as possible when carrying out ultra deep hole construction.

2. The influence of aperture on the drilling efficiency of rotary excavators

At present, the commonly used rotary drilling rig is the rotary drilling bucket, which has a hollow barrel body and two entry bucket doors at the bottom. During drilling, the drilling debris is cut by the drill teeth and enters the barrel through the bucket door. Under the thrust of the lower drilling debris, the upper debris gradually rises into the barrel until it is filled. There are two main factors that affect drilling efficiency, one is the speed at which the debris enters the bucket door, and the other is the speed at which the debris rises inside the bucket. Firstly, the speed of soil entering the bucket door is related to the area of soil entering the bucket door. The larger the drilling diameter, the larger the area of soil entering the bucket door, and the faster the soil entering speed;

Secondly, the rising speed of debris in the bucket body is related to many factors, mainly including:

(1) The weight of the soil debris;

(2) The adhesion force between the bucket wall and the debris soil.

Obviously, the larger the diameter of the borehole, the heavier the soil, but the adhesion force between the bucket wall and the soil will decrease; When the diameter of the drill bit decreases, the weight of the debris decreases, but the adhesion force between the bucket wall and the debris increases.

Of course, there are also other factors such as the shape of the drill bucket, the friction coefficient of the drill bucket surface, and the moisture content of the slag soil, which can affect the rising speed of the slag soil in the bucket body. Based on the above situation, the impact of drilling diameter on drilling efficiency follows the pattern shown in Figure 2. It can be seen from the figure that drilling efficiency decreases when the drilling diameter is less than 1m and greater than 2m.

The impact of geological formations on drilling efficiency is mainly manifested in the difficulty of drilling (drillability of geological formations) and the speed of soil unloading.

3.1 Drillability of Strata

The drillability of geological formations is a complex indicator. Geological drilling experts at home and abroad classify formations according to the difficulty of drilling. China follows the grading method of the former Soviet Union and divides formations into 12 levels, with higher levels indicating poorer drillability. Due to the lack of classification of drillability in the construction of large-diameter cast-in-place pile holes, there is no unified standard and specification for drilling techniques corresponding to different formations. The mechanism of rock fragmentation and powder carrying during pile hole construction with rotary drilling rig is different from geological drilling. Rotary drilling rig mainly divides the drilled layer into large rock fragments by cutting and then installs them into the drill bit body, while geological drilling mainly grinds the formation into small particles by flushing and carries them to the ground through flushing fluid. So the drillability index of the construction stratum by rotary drilling rig is mainly the compressive strength of the stratum. As long as the drilling pressure applied by the drilling rig is greater than the compressive strength of the stratum, the drill teeth can press the stratum and cut it into blocks. The pressure acting on the drill teeth must be greater than the compressive strength of the stratum, that is, F/S>G, where F - pressure; S - the area of the bottom of the drill tooth contact hole; G - compressive strength of the formation.

If the above conditions are met and other factors are optimized, the drilling efficiency will be greatly improved, such as adjusting the rotation speed and drilling tooth angle. As can be seen from the above, the harder the formation, the lower the drilling efficiency, because the required drilling pressure increases, and the time for the formation to fracture increases when the pressure is constant.

3.2 Impact of soil unloading factors on drilling efficiency

Due to differences in the composition, structure, and moisture content of the drilled strata, the viscosity of the drilling debris varies. When the drilling debris is loaded into the bucket and removed from the ground for unloading, the higher its viscosity, the longer the time required for unloading.

4. The influence of drilling tool selection on drilling efficiency

There are many types of drill bits used in small rotary excavators, with diverse structures. Different drill bits should be selected for different equipment models, construction methods, and geological formations. At present, there are six commonly used types of drill bits for rotary drilling rigs: spiral drill bits, rotary drilling buckets, cylindrical coring drill bits, expanded bottom drill bits, impact drill bits, and impact cone drill bits, each of which has many varieties. The selection of drilling tools has a significant impact on the drilling efficiency of rotary drilling rigs, and suitable drill bits should be selected according to the formation being drilled to maximize drilling efficiency. Rotary drilling rigs can be equipped with different drill bits according to the geological conditions, and long drill barrels can be used in cohesive soil layers to accelerate drilling speed; For formations with a high content of sand and gravel, short drill barrels can be used with mud protection to control drilling speed; For formations containing isolated rocks, boulders, and harder rocks, long and short spiral drill bits can be used for treatment. After loosening, the drill barrel can be replaced to continue drilling. Compared with conventional drilling rigs, rotary drilling rigs have a larger rotational torque and can automatically adjust according to the geological conditions.

5. The influence of drill pipe selection on drilling efficiency

The commonly used drill rods for rotary drilling rigs are divided into two categories: friction type and machine lock type. The friction type drill rod mainly relies on the frictional force between the drill rod keys to generate drilling pressure. The characteristics of this type of drilling rig are: increasing pressure while drilling, no need to find a pressure point, no need to unlock when lifting the drill rod, easy operation, high speed of lifting and lowering the drilling tool, but low pressure, and high efficiency in drilling in softer formations; Machine locked drill pipe is a type of drill pipe that uses pressure points between each layer of the drill pipe to connect and transfer the pressure from the power head to the drilling tool at the bottom of the hole to form drilling pressure. The characteristic of this type of drill pipe is that it can only generate high drilling pressure when the pressure points between each layer are connected during drilling. When lifting the drill pipe, it needs to be unlocked first, and the lifting and lowering speed is slow, which is suitable for harder formations.