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Calculation Method of the Pull-Back Force for Cable Laying of the TrenchlessCompleted Power PipelineAuthor(s): Chuan Wu, Guojun Wen, Xiaoming Wu, Lei Han, Jie Xu, Wenqiao Wang, Hui Xie, and RuiLiSource: Journal of Coastal Research, 73(sp1):681-686.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/SI73-117.1URL: http://www.bioone.org/doi/full/10.2112/SI73-117.1

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Journal of Coastal Research SI 73 681-686 Coconut Creek, Florida WINTER 2015

Calculation Method of the Pull-Back Force for Cable Laying of the Trenchless Completed Power Pipeline

Chuan Wu†, Guojun Wen‡*, Xiaoming Wu†, Lei Han‡, Jie Xu†, Wenqiao Wang‡, Hui Xie‡, and Rui Li§

ABSTRACT Wu, C.; Wen, G.; Wu, X.; Han, L.; Xu, J.; Wang, W.; Xie, H., and Li, R., 2015. Calculation method of the pull-back force for cable laying of the trenchless completed power pipeline. In: Mi, W.; Lee, L.H.; Hirasawa, K., and Li, W.(eds.), Recent Developments on Port and Ocean Engineering. Journal of Coastal Research, Special Issue, No. 73, pp. 681-686. Coconut Creek (Florida), ISSN 0749-0208. Typically, trenchless drilling rig is used in cable laying in power system. Then through the windlass; the cable is pulled into the pipeline. Although the size of the pull-back force is very important to calculate the pull-back force in cable laying, the existing calculation equation for pull-back force is targeted at pull-back force in pipeline laying. This article analyzes the frequently-used calculation equations and its service condition in pipeline laying. And a pull-back force test instrument in cable laying is designed, by which a large number of pull-back force of cable laying in practical engineering project is collected. Through the comparison of the measured values and calculation values by the equation for pull-back force, it is founded that the equation for pull-back force in pipeline laying is suitable for the calculation of pull-back force in cable laying in case that it multiplied by a certain coefficient. ADDITIONAL INDEX WORDS: HDD, pull-back force in pipeline laying, pull-back force in cable laying, test instrument for pull-back force.

________________________________________________________________________________________

INTRODUCTION Trenchless technology (HDD) is an underground pipeline

laying technology without excavating surface. As Figure 1 shows, it is a construction diagram of pipeline laying by trenchless drilling rig, which can be divided into the following three stages:

Figure 1. The diagram of construction process with trenchless rig.

1) Drill the pilot hole underground by trenchless drilling rig. 2) Enlarge diameter of the pilot hole by reaming drill bit, so

that the pipeline can smoothly pass through.

3) Pull the pipeline into the hole to complete the construction after reaming.

In the process of dragging pipeline, it needs to select suitable bearing capacity of the pipeline and the size of the corresponding trenchless drilling rig according to the pull-back force. So the drag force must be calculated before construction (Polak and Chu, 2005; Royal et al., 2009; Zwierzchowska, 2006).

Figure 2. The diagram for power cable laying of completed trenchless pipeline.

For electric power system, after the trenchless pipeline is laid,

power cable will be dragged into the hoist cable through the trenchless drilling rig. As Figure 2 shows, it is the power cable laying diagram of completed trenchless pipeline, by using windlass dragging the power cable into the pipe through

†School of Engineering China University of Geosciences Wuhan 430074, China

‡School of Mechanical & Electrical Information China University of Geosciences Wuhan 430074, China

§School of Resource China University of Geosciences Wuhan 430074, China

____________________ DOI: 10.2112/SI73-117.1 received 1 August 2014; accepted in revision 1 November 2014. *Corresponding author: wenguojun@cug.edu.cn © Coastal Education & Research Foundation , Inc. 2015

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wirerope. Here the drag force is related to the size of the bearing capacity of wirerope and the power of windlass, so it is needed to calculate the pull-back force before construction. However, there is no related research on the calculation method of pull-back force in cable laying yet. Because the previous construction of the pull-back force is estimated by experienced workers, the fracture is easy to happen for the bearing capacity of wirerope is less than the cable-laying drag force (Marcaccio, Spagnoli and Frascari, 2003). Thus, the calculation of the pull-back force in cable laying is particularly important.

There exists many calculation equations for pull-back force in pipeline laying, in which mud property, friction between the pipeline and soil as well as the influence of different soil on the laying drag force in the process of pipeline laying should be considered. But in the process of calculating pull-back force in cable laying, these effects are not considered. As Figure 3 shows, it is actual pipeline path of a trenchless cable laying construction.

Figure 3. The actual pipeline path of a trenchless cable laying construction.

When calculating the pull-back force in cable laying for the

actual pipeline path in Figure 3, the results vary widely by using different existing equations of pull-back force in pipeline laying. Therefore, whether the equation for pull-back force in pipeline laying can be used to calculate pull-back force in cable laying? If the calculation equation for pull-back force in pipeline laying is used, which equation is more suitable?

According to the problems above, taking the existing equation for pull-back force in pipeline laying as the enter point, this article analyzes the frequently-used calculation equations and its service condition for pull-back force in pipeline laying. According to the experimental data of practical engineering cases, the conclusion of the above questions is gotten.

THE EXISTING CALCULATION EQUATIONS FOR PULL-BACK FORCE IN PIPELINE LAYING

The existing calculating methods for the pull-back force in pipeline laying can be broadly divided into two kinds. Kind one, the influence of pipe deformation on pull-back force is not

considered. This kind of method is commonly simpler, easier to calculate, such as GB50424-2007 “Code for constructing oil and gas transmission pipeline crossing engineering”, GB50268-2008 “Code for construction and acceptance of water and sewerage pipeline works”, net buoyancy calculation method, relieving arch earth pressure calculation method and driscopipe calculation method, etc. Kind two, the influence of pipe deformation on pull-back force is considered. This kind of algorithm is more complex and generally takes the working condition of the whole into consideration, such as algorithm of gas pipeline research institute in the United States (AGA), ASTM method of American Society for Testing Materials, drill path algorithm, Meria Anna Palk algorithm, etc. The two methods all have their special talents and applicable conditions. The following will introduce the representative algorithms of this two kind methods (An, 2008; Hu et al., 2012; Ma and Zhang, 2006; Yang et al., 2011). The Pull-back Force Equation Ignoring the Deformation of the Pipeline

1) GB50424-2007 “Code for constructing oil and gas transmission pipeline crossing engineering” (hereinafter referred to as GB57 for short)

( )2

7.854

DF Lfg D DLkgπ γ δ δ π

= − − +

Where F is the pull-back force (KN) of pipeline laying. L is length crossing the pipeline (m). f is friction coefficient (the general value angel is 0.1 to 0.3). g is acceleration of gravity (generally takes 9.81m2/s). D is the external diameter (m) of pipeline. γ density (t/m3) of mud. δ is wall thickness (m) of pipeline. k is coefficient of viscosity (the general value angel is 0.01 to 0.03). It doesn’t take the bending deformation of the pipeline into consideration in this calculation equation. According to different materials of the pipeline, the equation needs to do some corresponding adjustments. Because the equation is simple, it has a certain application scope.

The relieving arch earth pressure calculation method is as follows:

( )20 01 t 45 1 t 45 + 1

2 2e e a

ekp

D D g g k p

F f Lf

φ φγ

λ

+ ° − + ° + + =

Where F is the pull-back force (KN) of pipeline laying. ef

is the friction coefficient between pipe wall and the hole wall. L is length (m) of the pipe. γe is the bulk density (KN/m3) of crossing soil stratum. D0 is the external diameter (m) of the crossing pipeline. φ is the internal friction angle of crossing soil stratum. ka is the pressure coefficient of active earth (take the value 0.3). P0 is the weight of per unit length (KN/m) of the crossing pipeline. fkp is the solid coefficient of soil. λ is the stability coefficient of the hole wall, which generally takes 30 to

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40 according to the experience. The bending deformation of the pipeline is not considered in this equation and the calculation is more complicated. The conditions used in this equation are harsh, and the soil types and soil layer thickness are limited, but the calculation results are more accurate.

3) GB50268-2008 “Code for construction and acceptance of water and sewerage pipeline works” (hereinafter referred to as GB58 for short)

2

4g k aF DLf D Rππ= +

Where F is the pull-back force (KN) of pipeline laying. D is the external diameter (m) of the pipeline. L is the length (m) of crossing. gf is the resistance (m) in per unit area of the outer

wall of pipeline. Dk is the external diameter (m) of the reaming drill bit. Ra is the extrusion force (KPa) of head-on soil

The bending deformation of the pipeline is not considered in this equation and the calculation is simple. But the using condition is relatively single and calculation results are volatile. The Pull-back Force Equation Considering the Deformation of the Pipeline

1) Algorithm of gas pipeline research institute in the United States (hereinafter referred to as AGA for short)

The algorithms take bending deformation of the pipeline into consideration and the calculation process is divided into a straight line and a curved line segment for calculating respectively. The final result is the summation of all segments. The calculation is more complex, but the accuracy of the result is high.

(1) Calculation equation of pull-back force in pipeline laying in straight line segment

cos sins p pT W L DRAG W Lμ α α= + ±

Where Ts is the pull-back force (KN) of pipeline laying in straight line segment. μ is the friction coefficient between the pipe wall and the hole wall. Wp is the net weight of the pipeline per unit length (KN/m) after considering drilling fluid buoyancy. α is the inclination angle (deg)of the pipeline. DRAG is the resistance (KN) of fluid in the hole. L is the length (m) of the pipeline in straight line segment.

(2) Calculation equation of pull-back force in pipeline laying in curved line segment

2 cos sinc p p arcT W L DRAG W Lμ α α= + ±

Where Tc is the the pull-back force (KN) of pipeline laying in curved line segment. Larc is the length (m) of the pipeline in curved line segment

(3) The total pull-back force in pipeline laying

1 1

n n

tol s cs c

T T T= =

= +

2) ASTM method of American Society for Testing Materials (hereinafter referred to as ASTM for short)

Figure 4. The borehole trajectory diagram for trenchless pipeline laying.

As shown in Figure 4, this algorithm simplifies the before

hole trajectory of trenchless pipeline laying. The entry point A, exit point D as well as the turning point B and C of straight line and bending line is regarded as the main points of the model by default. The pull-back force in pipeline laying TA, TB, TC and TD of the four points are calculated separately. And take the maximum value of the results as the final pull-back force.

( )1 2 3 4a

A a aT e w L L L Lμ α μ= + + +

( )2 2b a

B A HK b b b a aT e T T w L w H w L eμ α μ αμ μ= + + + −

( )3 3b a

C B HK b b a aT T T w L e w L eμ α μ αμ μ= + + −

( )4 4b b b

D C HK b b b a aT e T T w L w H e w L eμ β μ α μ αμ μ = + + − −

where THK is the fluid resistance (KN),

and ( )2 2=8HK BHT q D Dπ − . q is the fluid pressure (KPa). DBH is

the diameter (m) of borehole. μa is the friction coefficient between pipeline and the ground.. μb is the friction coefficient between pipeline and the borehole wall. wa is gravity (KN/m) per meter of pipeline. α is the angle (deg) of the entry point of pipeline. β is the angle (deg) of the exit point of pipeline.

The bending deformation of the pipeline is considered in this algorithm, but the borehole trajectory of trenchless pipeline laying is simplified into a smooth curve with only two bending points, which are not consistent with actual. So calculation results will have certain errors.

VERIFICATION OF THE ENGINEERING EXAMPLES The test of pull-back force in cable laying is designed for the

needs of the actual pull-back force test for cable laying of the completed power pipeline, which can collect real-time pull-back force in cable laying. Through the comparison of the actual pull-back force collected by the instrument with the calculated pull-back force by the equation, the best calculation equation for pull-back force in cable laying can be acquired.

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Figure 5. The diagram process for pull-back force in cable laying.

The Test Instrument for Pull-back Force in Cable Laying

The diagram of process for pull-back force in cable laying is shown in Figure 5. One end of the instrument is connected with the cable and the other end is connected with the wirerope of windlass. Then the instrument is pulled with the cable from head of the pipeline to the end, in the process of which the pull-back force in the whole construction process is recorded and stored. After completing the construction, the stored data in the instrument is read out for further analysis and processing by using PC software. Finally the pull-back force in the whole construction process is obtained.

Figure 6. Three-dimensional structure of the instrument.

The three-dimensional structure of the instrument is shown

in Figure 6. The windlass is respectively linked to the cable with one pull-tab. In the process of construction, the collected pull-back force by the sensors is transmitted to PCB, by which the pull-back force in cable laying is stored in SD card (Klemas, 2011; Wu et al., 2013). And the entire instrument is powered by battery. Figure 7 is the final completed equipment.

Engineering Project After completing the instrument, a lot of pull-back force data of cable laying in the engineering examples is acquired by using the instrument. Some typical basic data in the engineering cases is shown in Table 1.

Figure 8 is the comparison of the theoretical calculation value of each project by the equations mentioned above with the actual measured value. The following conclusions are gotten from Figure 8:

Figure 7. The physical figure of test instrument for the pull-back force in cable laying.

1) The value calculated by GB57 algorithm is smaller than

the actual measured values and the deflection is big. This is due to the friction resistance is not considered in this equation when bending deformation of the cable is occurred. But if taking 1.3 times of the calculate results from this equations as the pull-back force in cable laying, this equation can also be satisfied.

2) The value calculated by UAEPM algorithm is smaller than the actual measured values. This is because the equation doesn’t consider the friction resistance when occurring to the bending deformation of the cable.. In order to ensure the safety of the selected equipment, the results should be higher than the actual value in actual applications. But if taking 1.5 times of the calculate results from this equations as the cable laying, this equation can also be satisfied.

3) In GB58 algorithm, the parameters of resistance and extrusion pressure of soil to the reaming drill bit are involved in. These parameters are all related to reaming drill bit, which is not needed in the process of cable laying. If giving up these parameters in the equation, the equation will not be suitable for the calculation of pull-back force in cable laying. It can also be calculated when these parameters are assigned through estimating, but after the assignment, the calculation results are bound to be greater than the actual value. If taking 0.8 times of the calculate results from this equations as the pull-back force in cable laying, it also will be satisfied to the actual application.

4) The value calculated by AGA algorithm is bigger than the actual measured values. This is mainly because the frictional resistance of bending section is all friction resistance of the pipeline. Therefore, when the bending section is longer, the results will be bigger.

5) The calculation results by ASTM algorithm is volatile, therefore it can be used to calculate of pull-back force in cable laying.

CONCLUSIONS

In order to know if the calculation equation for pull-back force in pipeline laying is applicable to calculate pull-back force in cable lying, this paper analyzes the common used calculation equation and its using conditions based on the existing equation for pull-back force in pipeline laying. And a

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Table 1. Some basic data in the engineering cases.

site

cable specification(KV)

Length (m)

average depth (m)

Inlet angle (deg)

Exit angle (deg)

The second section of the 2th industry roadof Wuhan city

10 360 3 10 14

The third section of the 4th industry road ofWuhan city

10 410 3 11 14

Taixing island of Wuhan city 35 1920 40 7 9

Changxing island of Shanghai city 20 600 27 12 7

The north circular road of Beijing city 6 120 6 10 14

Jiefang road of Shaoxing city 15 200 8 8 11

Figure 8. The comparision of theoretical calculation value with the actual measured values in engineering example.

pull-back force test instrument is designed. From the comparison of the measured value with the theoretical calculated value, it is founded that the equation for pull-back force in pipeline laying is suitable for the calculation of pull-back force in cable laying in case that it multiplied by a certain coefficient.

ACKNOWLEDGEMENTS The research program was funded by National Natural

Sciences Foundation of China (41272174), the Fundamental Research Funds for the Central Universities (CUG130412; CUG090107) and the Fundamental Research Founds for National University, China University of Geosciences (Wuhan) (1410491T03).

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Hu, S.L.; Yan, T.N.; Wang, B., and Liu, H., 2012. Model for calculating pull-back force for pipe-laying in directional drilling. Coal Geology & Exploration, 40(3), 66-69.

Klemas, V., 2011. Remote sensing of wetlands: case studies comparing practical techniques. Journal of Coastal Research, 27(3), 418-427.

Ma, B.S. and Zhang, Y.C., 2006. Curved pipe jacking technology and the calculation of jacking loads for curved section. Geotechnical Engineering Technique, 20(5), 229-232.

Marcaccio, M.; Spagnoli, F., and Frascari, F., 2003. Drilling mud as tracers of sedimentation and geochemical processes on continental shelves. Journal of Coastal Research, 19(1), 89-100.

Polak, M.A. and Chu, D., 2005. Pulling loads for polyethylene pipes in horizontal directional drilling: theoretical modeling and parametric study. Journal of Infrastructure System, 11(2), 142-150.

Royal, A.C.D.; Polak, M.A.; Rogers, C.D.F., and Chapman, D.N., 2009. Pull-in force predictions for horizontal directional

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drilling. Proceedings of the Institution of Civil Engineers-Geotechnical Engineering, 163(4), 197-208.

Wu, C.; Wang, W.Q.; Wen, G.J., and Wu, X.M., 2013. Multiple parameter monitoring system for landslide. International Journal on Smart Sensing and Intelligent Systems, 6(3), 1200-1229.

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Analysis and comparison of pullback force calculation equations for pipeline crossing using horizontal directional drilling. Petroleum Engineering Construction, 37(1), 1-5.

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