From prediction to reality Gas transmission pipeline protected with a surface slab
[email protected]
25 Novembre 2005
Introduction Gaz de France transmission pipeline • Operated by GRTgaz (subsidiary of Gaz de France) • 32 000 km • Diameter from 80 mm to 1100 mm • Operating pressure 67.7 bar • Depth > 0.8 m
Introduction Gaz de France Research & Development centre Transmission technologies section • Pipe diagnosis • Defect assessment and repair • Risk management • New materials and technologies
Issue It is necessary to protect buried pipeline against heavy load traffic
Common protective measure : concrete slab or steel plate
Issue How does the system (pipe + soil + slab) reacts to heavy loadings ? What is the maximum stress in the pipe ? What are the minimum dimensions of the slab ? • Small enough to be laid in semi-urban area • Large enough to be fully efficient
Equipment Experimental tank • dimension : 3m×3m×3m
Equipment Hydraulic jack • Maximum load = 100 tons
Equipment Instrumented pipe Strain gauges
• External Diameter = 323.9 mm
S1
• Wall thickness = 7.7 mm S4
Tube
S2
S3 Soil pressure cells on the pipe PT1 PT4
Tube PT3
PT2
Equipment Soil pressure cells
Est
Ouest 120 cm 2
1
20cm 4
3 65 cm
10 cm
40 cm
45cm
65cm
Soils Sandy soil • Purchased from a construction material shop
Clayey soil • Taken from a construction site (south suburb of Paris)
•
Triaxial tests
•
Grains size
•
Atterberg limits
Tank filling The soil is put in place layer by layer (5 cm thick) Compaction of the layers with a manual dam • Control of the density at 9 points • Control of the water content at 9 points • Control of the strength with a scissometer at 9 points
Tank filling with sandy soil Putting in a layer of sandy material
Tank filling with sandy soil Putting in the pipe in the sandy soil
Tank filling with sandy soil Putting in the slab and the hydraulic jack
Tank filling with clayey soil Trench effect in the clayey soil
Tank filling with clayey soil Laying down the pipe in the clayey soil
Tank filling with clayey soil Filling in of the trench with loose clayey soil (no compaction)
Numerical modelling 2D model Geometry and boundaries conditions Ld
Lb
tôle
F
ed 1
4 lois de contact glissement avec frottement de Coulomb
d 2
3
sol encaissant
Substratum rigide (nœuds encastrés)
ht
4
h Lt
Numerical modelling Non linear soil behavior • Drucker Prager with a cap • Taking into account compaction • Calibrated with triaxial tests Steel property (slab and pipe) • Elastic linear
Shear stress (τ) Slipping
τmax
Contact law • Coulomb friction
Sticking
ϕ Contact pressure (σn)
Numerical modelling Sandy soil plastic strain
Numerical modelling Clayey soil plastic strain
Results – Sandy soil Vertical displacement vs loading (sandy soil)
Force exercée sur la dalle (kN) 0
50
100
150
200
250
Enfoncement de la dalle (mm)
0 Essai Eléments finis -5
-10
-15
300
Results – Clayey soil Vertical displacement vs loading (clayey soil) • Elastic return close to experimental results
Force exercée sur la dalle (kN) 0
100
200
300
Enfoncement de la dalle (mm)
0 Essai -50
-100
-150
-200
Eléments finis
400
Results – sandy soil
S1
Hoop stress vs loading (sandy soil)
S4
canalisation
S2
S3
S3 20 0 Contrainte (MPa)
0
50
100
150
200
-20 -40 -60 -80
Essai Eléments finis
-100 Force exercée sur la dalle (kN)
250
300
Results – clayey soil
S1
Hoop stress vs loading (clayey soil)
S4
canalisation
S3
S3 20
Contrainte (MPa)
0 -20
0
100
200
300
-40 -60 -80 -100
Essai Eléments finis
-120 Force exercée sur la dalle (kN)
400
S2
Results Importance of the pipe / soil interaction model
Soft area PT3
PT3
Bedding Full scale tests
Finite Element models
Results – sandy soil Soil pressure on the pipe vs loading (sandy soil) PT4
PT1 canalisation
PT2
PT3 PT3
PT1
700
700
600
Eléments finis
400 300 200
300 200 100
0
0 0
50
100
Eléments finis
400
100
-100
Essai
500 Pression (kPa)
500 Pression (kPa)
600
Essai
150
200
Force exercée sur la dalle (kN)
250
300
-100
0
50
100
150
200
Force exercée sur la dalle (kN)
250
300
Results – clayey soil PT1
Soil pressure vs loading (clayey soil)
canalisation
PT4
PT2
PT3
PT3
PT1
900
300
800 Eléments finis
600
Eléments finis
200 Pression (kPa)
Pression (kPa)
700
Essai
250
Essai
500 400 300 200 100
150 100 50 0
0 0
100
200
300
Force exercée sur la dalle (kN)
400
-50
0
100
200
300
Force exercée sur la dalle (kN)
400
Conclusion The stress in the pipe stay reasonable • Less than 110 MPa in both tests
The vertical displacement of the slab can be important in presence of trench effect • 170 mm
Numerical modelling can reproduce the tests • Importance of the soil behavior law and the value of its parameters • Importance of an accurate modelling of the pipe/soil interaction
Conclusion Field applications • Minimum pipe / slab distance = 300 mm • Minimum width of slabs in function of the pipe diameter Pipe
Slab width
DN 100
1.1 m
DN 200
1.15 m
DN 300
1.25 m
DN 600
1.6 m
DN 900
1.9 m
DN 1200
2.25 m
Results used in a GESIP guide book (GESIP : Groupe d’Étude de Sécurité des Industries Pétrolières et Chimiques)
Perspective Accurate modelling of the pipe / soil interaction Comparison with classic analytical Marston models Simulations for field applications