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Study on applicability of currently used soil-pipe interaction equations for segmented buried pipelines subjected to fault movement

Mohammad Hossein Erami1, Masakatsu Miyajima2 and Shougo Kaneko3

1PhD Candidate, Earthquake Engineering Dept., Kanazawa University

Kanazawa 920-1192, Japan, [email protected] 2Professor, Earthquake Engineering Dept., Kanazawa University

Kanazawa 920-1192, Japan, [email protected] 3Senior Researcher, Ductile Iron Pipe R&D Dept., Kubota Corporation

Amagasaki 660-0095, Japan, [email protected]

ABSTRACT This study investigates the applicability of force-displacement equations suggested in currently used design codes, based on “Guidelines for the seismic design of oil and gas pipeline systems” of ASCE (1984)[1] as their main reference, to introduce the soil-pipe interaction for segmented type of pipeline systems. Hence, results of finite element method (FEM) analyses are verified by full-scale experiments on a segmented ductile iron pipeline with 93mm diameter and 15m length. Pipeline is installed at a 60cm depth from the ground surface in two types of sandy soil with different values of sub-grade reaction. Adopted fault is a reverse type which has an intersection angle of 60 degrees with pipeline and moves in three same steps to reach its total movement of 35cm. Findings reveal that the aforesaid interaction equations are basically developed for continuous pipelines and the effect of connection joints on the integrated structural behavior of segmented pipelines is not considered in them. Hence, suggesting them by currently used guidelines for seismic design of fault crossing segmented pipelines leads to overestimation of soil resistance against relative downward movement of pipeline in surrounding soil continuum. Key words: Soil-Pipe Interaction, Soil-Equivalent Springs, Experimental Test, FEM Analysis, Segmented Buried Pipeline, Ductile Iron Pipe 1. INTRODUCTION One of the most important seismic hazards on buried pipelines is movement of faults crossed by them. During fault movement phenomenon, surrounding soil acts as both support and load transmission continuum for buried pipes. Hence, in FEM studies, definition of soil-pipe interaction has undoubted effect on the exactitude of modeling and accuracy of analysis results. One of the often used series of soil-pipe interaction

1256Pipelines 2012: Innovations in Design, Construction, Operations, and Maintenance–—Doing More with Less © ASCE 2012

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equations are those suggested in “Guidelines for the seismic design of oil and gas pipeline systems” by ASCE. This guideline is the main currently used specification for seismic design of pipelines and major reference for later codes such as those issued by American Lifeline Alliance (ALA)[2] and Indian national information center of earthquake engineering[3]. The series of equations recommended in ASCE guideline for 2D soil-pipe interaction in vertical plane includes three distinct equations, one of them for interaction in axial direction and two more for vertical transverse of downward and upward directions. In these equations, frictional force in soil-pipe interface and the weight of soil layer over the pipe are respectively bases of soil resistance against relative movement of the pipe in surrounding soil in axial and upward vertical transverse directions. Apparently, these origins of soil resistance are logically valid for both of continuous and segmented pipeline systems. However, recommended equation for vertical transverse downward relative movement of pipe and consequently for estimating the stiffness of corresponding soil-equivalent springs used in FEM analysis has some uncertainties. That is, this equation is basically derived for continuous pipeline assuming it as a beam with no stiffness change in whole length and rested on elastic foundation. And this study investigates applicability of this equation for pipeline systems which have joints with much different rotational resistance compared to bending stiffness of pipe body. Appropriate performance of segmented pipelines during previous seismic events, in addition to aforesaid uncertainties, makes it necessary to do more detailed studies on accurate analysis methods for this type of pipeline systems. In this way, results of computer-aided pipeline analyses are compared to actual behavior of pipeline, observed in full-scale experimental tests, conducted by using test facilities in Kubota Corporation, Japan.

2. MODEL DESCRIPTION

In this study, a segmented ductile iron pipeline, with nominal size of 75, external diameter of 93mm and 7.5mm wall thickness in total length of 15m composed of nine 1m length segments between two 3m length ones, at both ends, is considered. Table 1, indicates summary of the parameters for all three pairs of experimental tests, including the unit weight and sub-grade reaction of soil, pipe burial depth and fault displacements in vertical and horizontal directions.

Table 1. Summary of experimental tests parameters

Test Number Unit Weight

of Soil (kN/m3)

Value ofSub-grade Reaction

(kN/m3)

Burial Depthof Pipe (cm)

Fault movement inhorizontal direction

(cm)

Fault movement in vertical direction

(cm) D-10 17.7 40800 60 5.75 10 S-10 16.4 13140 60 5.75 10 D-20 17.7 40800 60 11.5 20 S-20 16.4 13140 60 11.5 20 D-30 17.7 40800 60 17.25 30 S-30 16.4 13140 60 17.25 30

Note that, as shown in table 1, models indicated by ”S” relate to soil with the lower sub-grade reaction value of 13140(kN/m3) while “D” series are relevant to soil with higher sub-grade reaction value of 40800(kN/m3). Modeling details of both computer aided simulations and experimental tests are illustrated as follows.


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ESULTS OF

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F PHYSIC

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underestima

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the pipelinehows the cor

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tests are larerestimated rn FE modelsfluences the

al Displacement

pipeline jod 7(b) for t appraised ated in FE

Deflection in Joi

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isplacemeng angular de

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r Tests “D-10”

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for both of d it would

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flection in cnsiderable dnt has effectoints of cros

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ases of “D-2n connection

(a) and “S-20”

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10” (b)

20” and “S-n joints.

(b)

ided nt of on in nt.

ctual vely. ssed n of

-20”


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ounds otherundation wh

Figur

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(b) and corpicted in Fig

Fig

Figur

confirm theference betwude of sub-galmost the lue of 60cmr than assuhich ASCE e

re 9. Angular D

lacement ofault in test

rresponding g. 11(a) and

gure 10. Vertica

re 11. Angular D

e findings oween FE mograde reactiosame unit w

m so this diuming pipeequation of

Deflection in Joi

of adopted t cases of “responses

d 11(b), resp

al Displacemen

Deflection in Jo

of previous odel and exon. Furthermweight and ifference reveline as conf soil force-d

ints of Pipeline

pipeline “D-30” and

of pipelinepectively.

nt of Pipeline fo

oints of Pipeline

step of adoxperimental more, as twalso the burveals that sntinuous prdisplacemen

e for Tests “D-2

to ground d “S-30” aree in terms

or Tests “D-30”

e for Tests “D-3

opted fault test is largeo types of srial depth fooil resistanrismatic bent is based o

0” (a) and “S-2

deformatioe shown in of angular

” (a) and “S-30”

30” (a) and “S-3

movement er for soil wsoil used in for all test cnce should heam on elaon.

20” (b)

on caused Fig. 10(a)

r deflection

” (b)

30” (b)

and with this ases have astic

by and are


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These figures confirm the easier displacement of pipeline in response to adopted fault movement in experimental test compared to FE model and underestimation in angular deflection of end joints of crossed segment as well as larger difference between experimental test and FE simulation for soil with lower value of sub-grade reaction. This difference is more comparable for displacement of end joint of crossed segment which is located in fixed wall side of fault (J5, shown in Fig. 1). Where, although movement of joint in test case of “D-10” is 0.1cm and it is 0.2cm less than one-third of its deflection in “S-10”which is 0.9cm but for next steps of adopted fault movement, this joint’s deflection is inversely proportion to sub-grade reaction value of soil. That is, it moves 1.0cm in “D-20” and 2.1cm in “D-30” which are one-third of this joint’s deflection in “S-20” and “S-30”, 3.1cm and 5.9cm, respectively.

6. CONCLUSIONS

This study investigates the applicability of force-displacement equations suggested in currently used codes, based on “Guidelines for the seismic design of oil and gas pipeline systems” of ASCE as their main reference, to introduce the soil-pipe interaction for segmented type of pipeline systems. In this way, response of pipeline in finite element analyses and full-scale experiments are compared in terms of its vertical displacement and angular deflection of connection joints. Findings of this study demonstrate that in segmented type of buried pipeline, much different rotational resistance of connection joints, compared to bending stiffness of pipe body, governs the integrated response of pipeline to ground deformation induced by fault movement. And it reduces the pipeline to a multi-span beam on elastic foundation rather than a continuous one. That is, segmented pipeline accommodates to ground deformation induced forced rotation by concentrated angular deflection in connection joints, before considerable bending deformation in body of pipe segments. Hence, using equations, basically developed for continuous pipelines, leads to incorrect estimation of pipeline responses and consequently its either unsafe or uneconomic design. Furthermore, comparing behavior of pipeline in two almost same surrounding soil conditions but different in value of sub-grade reaction shows meaningful agreement between this value and downward resistance of soil against pipeline relative movement. Therefore, this correlation can be used to derive new equation for interaction of segmented type of pipeline systems with surrounding soil in downward direction. ACKNOWLEDGMENT: The cooperation of Ductile Iron Pipe R&D Department of Kubota Corporation, Japan in providing experimental data for Ductile Iron pipes and joints is highly appreciated. This research would not have been possible without it. REFERENCES 1) Guidelines for the seismic design of oil and gas pipeline systems, ASCE, 1984. 2) Seismic Guidelines for Water Pipelines, American Lifelines Alliance, 2005. 3) Guidelines for Seismic Design of Buried Pipelines, Indian national information

center of earthquake engineering, 2007.


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4) Ivanov R., Takada S.: Assessment of the vulnerability of jointed ductile iron pipeline crossing active faults, Japan Society of Civil Engineering, Journal of Earthquake Engineering, 2003

5) Liu A., Takada S., Hu Y.: A shell model with an equivalent boundary for buried pipelines under the fault movement, Paper No. 613, Thirteenth World Conference on Earthquake Engineering, 2004.

6) Trifonov OV., Cherniy VP.: A semi-analytical approach to a nonlinear stress-strain analysis of buried steel pipelines crossing active faults. Soil Dynamic Earthquake Engineering, 1-11. 2010.

7) DYNA2E User’s Manual, Version 7.2. 9, CRC Solutions. 8) Design manual for joints of ductile iron pipe type NS, S II and S, Japan Ductile

Iron Pipe Association, (In Japanese).


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