Discussion on some problems of horizontal directio

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Discussion on some problems of horizontal directional drilling of PE pipes for gas use

pe pipes are widely used in gas engineering because of their inherent advantages. The traditional laying method of PE pipes is excavation, which has adverse effects on urban traffic and environment. The development of PE pipe horizontal directional drilling technology can solve this problem well. Compared with the steel pipe directional drilling construction technology, this technology started late, and the relevant standards are not perfect at present. Therefore, the discussion on this technology continues. This paper puts forward some measures to improve the horizontal directional drilling technology of PE pipe from the point of view of stress analysis of pull back PE pipe, and lists the existing problems of this technology

1 total pullback force in the process of directional crossing pullback

in the process of PE pipe pullback, the pipe bears friction drag resistance (caused by friction drag between pipe and drilling or mud, friction drag on the ground), winch force generated by bending along the drill path, fluid resistance generated by hydrodynamic drag force, etc. According to the PPI data of American plastic pipe Association, the stress calculation formula is as follows:

fp=fwbl (1)

where FP - friction drag resistance, n

f - friction coefficient between pipe and drilling mud (generally taken as 0.25) or friction coefficient between pipe and ground (generally taken as 0.40)

wb - resultant force in the vertical direction of unit length pipe, n/m

l - pipe length, m

fc=ef β Fwbl

where FC - winch force, n

e - base of natural logarithm, e=2.71828

β—— Bending angle of pipe, rad

where FHK - fluid resistance, n

p - fluid pressure, pa

dh - drilling diameter, m

dod - pipe outer diameter, m

ft=fp+fc+fhk (4)

where FT - Total pullback force, n

2 axial tensile stress and torsional stress

① axial tensile stress

when considering problems, the maximum axial tension shall be ensured.The stress shall not exceed the safe tensile strength of the pipe. The maximum axial tensile stress is equal to the sum of the tensile stress in the pipe caused by the pull back friction drag resistance, the fluid resistance and the bending tensile stress caused by the bending of the pipe. According to the PPI data of the American plastic pipe Association, the formula for calculating the tensile stress in the pipe wall during the pull back process is as follows:

where σ T -- axial tensile stress, pa

δ—— Pipe wall thickness, m

et - the prospect of paper packaging testing instruments can be said to be quite optimistic - tensile modulus varying with time, pa

r - bending radius of pipe in drilling, m

② torsional stress [1]

in the stage of reaming and pulling back, a transfer device (or rotary joint) can be used to separate the rotating reaming bit from the pulled pipe to reduce the torsional stress. For thick wall high-density PE pipes with SDR value equal to 7 ~ 17, due to the use of transfer gears, the torsional stress on the pipes is very small, and detailed engineering analysis is not required

3 optimization measures that should be taken in PE pipe directional drilling construction

① reduce f

reduce the friction coefficient plant, that is, reduce the friction between the pipe and the ground and the drilling wall mud during construction. The methods adopted are: a. the pipe on the ground is dragged on the roller, which can not only prevent the pipe wall damage, but also reduce the friction resistance. b. The drilling line shall be as straight as possible. c. Try to adopt continuous pullback (only need to overcome dynamic friction) and reduce intermittent pullback (need to overcome maximum static friction). d. Pay attention to maintaining stable mud circulation during pulling back. e. Ensure that the pulling back task is completed before the mud solidifies. f. The slurry shall be reasonably selected to ensure that the drilling hole does not collapse, so as to reduce the friction resistance on the pipeline during back dragging. The preparation of mud should have different emphasis in each stage. In the stage of reaming and pulling back, the mud should have good wall protection, carrying capacity, and good lubrication capacity to reduce friction resistance and torque

② reduce wb

WB mainly refers to the gravity of the pipe itself and the resultant force of its buoyancy in the borehole when there is no collapse and no soil column load in the borehole. As the density of high-density PE pipe is similar to that of water, if the closed pipe pulling head is used for "dry" pipe laying, the pipe string will float above the borehole. Due to the buoyancy and friction between the wettable clay and the pipe, the pipe string will be subject to the action of lateral load and friction resistance. If the open pulling head is used for "wet" pipe laying, the pipe string will be filled with water (drilling mud is better), the resultant force in the vertical direction will be sharply reduced, and the total friction force can be reduced to 10% lower than the weight of PE pipe string [1]. Therefore, in order to reduce WB, wet drilling is more reasonable. However, as for PE pipes for gas transmission, generally, the pipe diameter is small, so the buoyancy received in the drilling is not very large, and the interior of the pipes needs to be treated after the completion of wet drilling. Therefore, it is recommended to use dry method to pull the pipes as far as possible under the condition that the total tensile stress is less than the safe tensile strength of the pipes

③ decrease β (i.e. increase R)

decrease β, In engineering practice, it is mainly manifested in the opposite entry and excavation angle α 1、 α 2 control, α 1、 α 2 the smaller, β Smaller. According to the scheme shown in Figure 1, it is suggested to reasonably select the location of the work pit if the site permits, and try to use the lower pipe work pit with a long excavation distance to replace the second curve section (PE pipe curvature radius ≥ 125 times of the pipe outer diameter), so that α 2 is approximately equal to zero, and the tensile stress of the pipe in the excavated work pit can be reduced to the minimum. Only the first curve section affects the tensile stress of the pipe. The penetration angle depends on the curvature radius of the drill pipe (≥ 75m) [2], which is calculated according to the curvature radius of 75m α 1 is about 20 °, so it is recommended that α 1 the value should be 0 ° ~ 20 ° [3]

④ decreasing p and dh

fluid pressure P has a great relationship with mud viscosity and flow. In the directional drilling construction, the mud viscosity is of great significance. The mud viscosity shall be observed every 2h at the construction site. In addition, according to the PPI data of the American plastic pipe Association, it is recommended that the pull back speed is 5.08 ~ 10.16mm/s. Therefore, the delivery capacity of the pump should be reasonably selected, and the pipe should be pulled at a uniform speed to maintain a stable mud circulation, so as to avoid the instantaneous vacuum phenomenon in the hole and the collapse in the hole. The influence of P can be reduced by controlling the mud viscosity and flow according to the above requirements

the larger the reaming diameter DH is, the better. As long as the borehole can accommodate the pipeline to be laid, the smaller the DH is, the better in terms of reducing the flow resistance. The reaming diameter shall also be determined according to the formation conditions and pipe type. For the pull back PE pipe, the reaming diameter should be 1.2 ~ 1.5 times the outer diameter of the laying pipe, or it can be selected according to table 1 [3]

Table 1 Relationship between pipe diameter and drilling diameter

4 control of pull back pipe length and pull back time

the above analysis is aimed at how to reduce the tensile stress of the pipe. Even if the above conditions are met, attention should also be paid to the influence of the length of the pull back pipe and the pull back time on the tensile stress. According to formulas (1) to (4), the total pullback force is approximately a linear function of the pipe length. Therefore, to ensure that the total tensile stress is not greater than the safe tensile strength of the pipe, it is necessary to ensure that the length of the pullback pipe is within a certain range. The above formulas are approximate calculation formulas in theory. In actual construction, most of the pulling force of the drilling rig is applied to the cutting surface of the reaming equipment and tools. It is difficult to predict how much of the total pulling force is transmitted to the pulled back pipeline. Therefore, how to keep the total tensile stress within the required range to ensure the safety of the pulled back pipe is dedicated to the steel strand The relaxation test of galvanized steel wire and ribbed steel bar is a problem that must be solved in engineering practice. This paper recommends the use of flexible joints, that is, a section of flexible joints is connected at the front end of the pull back pipeline, as shown in Figure 1. The preset breaking force of the flexible joint shall be less than the safety load of the pulled pipe. According to the requirements of CJJ technical specification for polyethylene gas pipeline engineering, the corresponding tensile stress limit is 2% of the yield tensile stress of the pipe, which focuses on showing 50% of the latest achievements in the key utilization field of new materials. After adding flexible joints, no matter what happens, it can ensure that the tensile stress of the pipe is within the required range

because PE pipes have creep characteristics, that is, the safe traction load of PE pipes is related to time, and the effect in a short time is a safe load, which may not be suitable for long-time pipe drawing construction. The axial tensile stress increases along the length of the drawn pipe, and the tensile load at the drag head has the longest action time. The stress at the tail of the pipe is zero, and the stress time is also zero. If the pulling back task can be completed within 24h, it can generally ensure that the extension of the pulling head is less and avoid the damage of PE pipeline. The PPI data of American plastic pipe association provides the allowable safe traction load (safe rebound force) of Gas PE pipe. See Table 2 for some data. After the PE pipe is pulled back, its elastic strain will recover quickly, and the viscoelastic elongation will remember the original length of the laid pipe section. After one night, the thermal contraction of elongation recovery and temperature equilibrium may cause the drag head to retreat into the borehole outlet. In order to prevent this phenomenon, the pipe head shall be pulled out for a length after pulling back, which is about 3% of the total length of the pipe. It is also possible to check whether there are scratches on the drawn pipes after cleaning. The inspection standard shall be in accordance with Article 5.1.1 of CJJ technical specification for T-Pass polyethylene gas pipeline. The scratch depth on the pipe surface shall not exceed 10% of the pipe wall thickness. Table 2 problems and suggestions for safe traction load of HDPE pipes (24h)

5 problems

5.1 problems

although the PE pipe directional drilling construction technology has been developed for many years, due to the lack of relevant specifications and standards, at present, the design, construction and quality evaluation of PE pipe directional drilling engineering can only refer to relevant standards. However, some contradictions are found when referring to these relevant specifications in actual work, which are listed as follows:

① according to the requirements of article 2.5.1 of CJJ specification for construction and acceptance of urban gas transmission and distribution T project and article 6.2.6 of CJJ technical specification for polyethylene gas pipeline engineering, tracer lines (tapes) and warning tapes shall be designed along the buried gas pipeline. It is easy to solve the problem of tracer line (belt) in directional drilling construction. As long as the tracer line is bound with tape and pulled back with the pipe, the laying of warning tape cannot be implemented. Although the PE pipes of directional drilling are buried deep (mostly 3 ~ 5m), the current standard does not specify whether the warning tape laying can be cancelled for the gas pipeline with large buried depth

② article 7.1.11 of CJJ code for construction and acceptance of urban gas transmission and distribution engineering and article 7.1.2 of CJJ technical specification for polyethylene gas pipeline engineering require that PE pipes should be tested for strength and tightness of connected pipe sections before pulling back. However, in the actual construction process, due to the impact of the site, it is difficult to pre connect the pull back pipes (unless coil laying is adopted). In the engineering practice, the construction task can only be completed by means of sectional welding and sectional pull back, so the strength and tightness test cannot be carried out in advance

③ article 6.2.7 of CJJ technical specification for polyethylene gas pipeline engineering provides a formula for calculating the allowable pullback force of PE pipes, indicating that the pullback force is only related to the pipe diameter and wall thickness. In fact, the formula ignores the differences of PE materials and the creep characteristics of PE pipes. Therefore, the calculation of this formula is one-sided and needs further correction

5.2 suggestions

there are still problems in the directional drilling construction of PE pipes for gas, such as how to set the condensate tank, how to add casing in the process of segmental welding and pullback construction, and how to install pipes in local sections

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