Physical Modelling of the Upheaval Resistance of Buried Offshore ...

Gravel
berm
Loose
sand
Pipe
(a) (b)
Figure 12. Soil deformation: loose sand with gravel berm
(C8) at different stages of the test.
rather than the pipe diameter/embedment. The
mobilised displacement ratios to peak load, δ/H ≈
0.5 % and these agree well with previous workers
(eg. Matyas and Davis, 1983; Trautmann et al.,
1985).
As shown earlier, the uplift force for loose sand
may be predicted conservatively using the vertical
slip surface model (Majer; 1955) and using φ RQ WKH
slip surface, and Ko = 1 – sin(φ WKH DW UHVW ODWHUDO
earth pressure coefficient (Figure 4) despite the
different post-failure mechanism observed. The
load-displacement response for the dense sand and
gravel shows a peak uplift resistance before the
uplift resistance reduces to a residual value similar to
the values for a loose sand. The dense soil sample is
dilating, but then post-peak strain softening reduces
the mobilised angle of friction to φ’ as for the loose
sand tests.
5 SUMMARY AND CONCLUSIONS
Tests have been carried out to study the uplift
behaviour of pipelines in the laboratory and the
geotechnical centrifuge. The pipes were of prototype
diameter 32 mm, 48 mm, 240 mm and 250 mm and
soil conditions consisted of dry, loose or dense sand
or gravel with or without a berm.
Uplift factors are presented for the tests. Loose
sand gave fd ≈ 0.5. Dense sand and gravel gave a
peak uplift factor, fd > 1 on initial loading, but this
reduced to a residual value, fd ≈ 0.6 after a
displacement that may be proportional to the
diameter of the soil particles. For design, it is
therefore important to either know the density of the
seabed or otherwise use a conservative uplift factor.
The load-displacement behaviour of the pipes
shows the displacement required to mobilise full soil
resistance, δ/H ≈ 0.4 % for all loose sand tests. Peak
load is mobilised more quickly in dense soil (δ/H ≈
0.1 %).
Soil deformation mechanisms were examined by
taking digital photographs/video of the front face of
the test box. Photographs are presented which
indicate soil displacement mechanisms after failure
for the different soil types. These vary with initial
soil density, but all included gap formation behind
the pipe while the peak uplift force is being
mobilised and soil flow around the pipe after failure.
Results from centrifuge model tests with gravel
berms showed a significant increase in uplift
capacity (fd ≈ 0.8) but a less significant increase was
found for sand berms. It is believed that dilation of
the berm during initial pipe displacement together
with the weight of the berm increases the uplift
capacity.
ACKNOWLEDGEMENTS
The work described in this paper was supported by
funds from Stolt Offshore and this support is
gratefully acknowledged. The authors would like to
thank Mr. Colin Fyfe and Dr. Shiping Yao for their
hard work during the testing programme and to the
technical staff of the Department of Civil
Engineering, University of Dundee for all of their
help.
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