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

Stolt Offshore sponsored the University of
Dundee to undertake a research project to improve
understanding of the failure mechanisms associated
with pipeline uplift and the monotonic uplift
resistance. This paper reports some of the model
tests carried out for drained loading conditions.
2 EXPERIMENTAL METHOD
2.1 Test geometry
Model testing was carried out both in the laboratory
(at 1g) and in a 5g acceleration field in the Dundee
Geotechnical centrifuge. Both series of tests
consisted of the extraction of a 500 mm length of
pipe from a box of soil with continuous
measurement of force and pipe displacement.
Because both geometries modelled plane strain
conditions and both pipes were rigid; results are
presented later per unit length of pipe. The front face
of the soil boxes were perspex and the pipe was
positioned perpendicular to the perspex front face,
allowing the soil (and pipe) to be observed as pullout
progressed.
Pipes of 32 mm and 48 mm outside diameter and
length of 495 mm were tested in the laboratory,
whilst pipes of diameter 48 mm and length 498 mm
were tested at 5g (and 5.2g) in the geotechnical
centrifuge. Scaling laws (as reported by Schofield,
1980) ensured that the 48 mm pipe at 5g would
behave as a prototype 5 times larger (i.e.
D=5x48mm= 240 mm).
2.2 Laboratory and centrifuge testing
methodology
The preparation procedures were the same for both
types of tests. Sand was placed to a depth of 30-50
mm in the base of the box and the pipe was
positioned on the surface of the soil. The pipe was
located with its ends almost in contact with the front
and back perspex faces of the box (Figure 2a) and
grease was pushed into the gap to prevent soil
entering the gap between the pipe and the front face.
Additional sand was then added until the required
pipe embedment depth was achieved. Whilst the
sand sample was being placed above the pipe, it was
important that the pipe was free to settle vertically so
that a net vertical load was not applied to the pipe
before the pull-out test commenced. If this were not
done, the mobilisation distance to peak load would
be underestimated. During a number of the tests, a
grid of black markers at approximately 50 mm
centres were placed in the soil touching the perspex
front face while the soil was being prepared. This
allowed later photographic measurement of soil
movements through the perspex front face (Figure
2a).
Figure 2. Testing apparatus - (a) the laboratory test
apparatus (b) centrifuge model package
The pipe was then pulled out of the soil using a
rigid hanger arrangement to ensure that the pipe
moved with uniform displacement and vertically
(Figure 2a, b). Load was measured using a load cell
and displacement using a potentiometer or LVDT.
This was done using an Instron device in the
laboratory or a specially designed actuator in the
centrifuge. For tests with the soil marker grids, a
series of digital photographs or analogue video was
taken of the front face of the box as the pipe moved
towards the soil surface.
Soil preparation varied depending on the required
density and whether the soil was dry or saturated.
High sand and gravel dry densities (close to ρmax)
were achieved by layered compaction. Loose, dry
sand was prepared by quick sand pouring. Loose,
saturated sand samples were prepared by pluviating
sand through air into 200 mm of water. A sand or
gravel berm was constructed once the strongbox was
placed on the centrifuge arm by pouring through a
funnel onto the soil surface. This would have created
an intermediate density berm.
2.3 Programme of tests
The tests reported in this paper were conducted on
either silica sand or gravel. The sand was uniformly
graded with d50 = 0.3 mm, Gs = 2.65 and the
particles were subrounded. The angle of repose of
the loose soil was measured to be 32 o and this is
believed to be close to the critical state angle of
friction, φcrit. The gravel was subangular with d50 = 5
mm and had an angle of repose of 35 o .
Model tests were conducted in loose sand in the
centrifuge and the laboratory. Other variables
investigated were the diameter of the pipe, its
embedment ratio (H/D), whether the soil was dry or
saturated and the presence of a berm. The series of
tests is summarised in Table 1.