Eiksund_edit.1_Outsourcing_book_temp.qxd 02/11/2009 11:42 Page 108
Ormen Lange Pipelines – Geotechnical Challenges
Figure 4: Large Common Rock Support for Three
system (PLS) loading conditions. The deformation accumulation during
Large-diameter Pipelines
earthquake loading was estimated using the empirical methods
proposed by Wong
2
and Richard and Elms;
3
the calculation procedure
used is presented in more detailed by Eiksund et al.
4
Import and
Stability Analysis
export pipelines
The slope stability of rock supports is mainly calculated using the
slope stability program SLIDE.
5
Design of counterfills is an iterative
400m
process based on experience with trial and error to find the optimal
50,000m
3 length and thickness. The goal is to find a stable solution with
minimum use of rock. The stability is checked for many different
failure surfaces. In case a counterfill is required, the dimensions are
determined on basis of the three failure modes shown in Figure 3.
The local failure mode (1) determines the thickness of the counterfill,
the global failure mode (2) determines the length of the counterfill
and the local failure mode (3) defines the end stability of the
counterfill and maximum edge height.
Figure 5: Example of Complicated Rock Fill Geometry in Bjørnsundet
Examples of Geotechnical Design Tasks
An extensive soil investigation programme provided regional data, and
a selection of site-specific investigations covered many local variations.
Sub-bottom profiling was performed along the entire pipeline route
and combined with cone penetration tests and laboratory tests on soil
samples. On this basis design parameters for the soil properties were
established for the entire routes.
An extensive soil investigation
programme provided regional
data, and a selection of
site-specific investigations
covered many local variations.
In spite of the comprehensive soil investigation programme, additional
Figure 6: A Large Rock Support with Counterfill in the
soil investigations were found to be required in cases where soft clay
Storegga Escarpment under sand was not discovered by seismic investigations.
Nyhamna Landfall
On the relatively steep slope outside Nyhamna, the seabed consists of
sand or very soft clay. The large-diameter pipelines span the lower part
of the slope, extending into horizontal curves. To enable pipe lay in the
curve at the bottom of the landfall slope, a 400m-long continuous rock
support needed to be installed, as shown in Figure 4. The volume of
this support, including counterfill, was approximately 50,000m
3
. An
extensive soil investigation programme was performed to reveal
possible pockets with soft clay under the sand.
Nearshore (Bjørnsundet)
In Bjørnsundet all of the pipelines are placed together in a very narrow
valley. To fulfil the required minimum pipeline spacing for installation
feasibility and global buckling during operation, it was necessary to lift
the pipelines from the seabed onto several rock supports. An example
of this is shown in Figure 5.
EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 2
108
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