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Ormen Lange Pipelines – Geotechnical Challenges
a report by
Gudmund Eiksund,
1
Harald Brennodden,
1
Gunnar Paulsen
2
and Stig-Arne Witsø
3
Subsea & Pipelines
1. Senior Engineer, GeoPartner Marin AS Trondheim; 2. Reinertsen AS Trondheim; 3. StatoilHydro Oslo
Ormen Lange is a major gas field located 120km west of mid-Norway. an area where the pipeline routes pass a narrow canyon, all seven
The selected development scenario is a subsea tie-back to an onshore lines are installed in a common corridor that is 17m wide.
terminal located at Nyhamna on the island Gossen. The project
Ormen Lange consists of two 30-inch multiphase pipelines, two In the offshore shallow water area, the seabed is relatively flat. The
6-inch monoethylene glycol (MEG) pipelines and two umbilicals soil conditions are generally sand of varying thickness overlaying soft
connecting the onshore terminal to the two templates at the clay with undrained shear strength in the range of 5–10kPa.
deepwater field. The first 33km of Langeled, a 42-inch gas export
pipeline to UK, is also included in the project as the nearshore part is The Storegga escarpment is the remaining back wall after a large
installed in the same corridor as the Ormen Lange field pipelines. landslide incident some 8,000 years ago.
1
At Storegga, the water
depth increases from 260 to 550m over 3km with, local inclinations
StatoilHydro has been the operator for the development phase, up to 30º. An area overview with all pipelines and a vertical profile
whereas Shell is the operator for the operational phase. Reinertsen along the MEG-A pipeline route is shown in Figure 2. The vertical
AS has been responsible for the detail engineering of the pipelines profile illustrates the steep back wall from the landslide.
with GeoPartner Marin as subcontractor responsible for the
geotechnical engineering. In the final 15km from the foot of the Storegga escarpment to the
template area, the seabed depth increases gradually from 550 to
In total, 961 rock supports (including both pipe supports and rock 850m. The soil conditions vary between hard clay in slide blocks and
covers) and 206 counterfills have been installed on the seabed. The infill of soft clay between the blocks. Shallow seismic data along all
rock installation was performed by VanOord. The total design
required more than 1.4 million m
3
of crushed rock, of which the
counterfills required about 440,000m
3
. Including rock settlement and
installation tolerances, 4.5 million tonnes of rock were used.
The requirements for seabed
intervention are optimised
Soil Conditions
based on an interaction
The seabed soil conditions show large variation along the 120km-long
pipeline route. The pipeline route can be divided into three main areas: between pipeline design and
nearshore, offshore shallow water and offshore deepwater, as shown
geotechnical analysis.
in Figure 1.
From the Nyhamna landfall, the seabed depth increases rapidly over the
first kilometres down to a depth of 200m, with the steepest inclination pipeline routes provided valuable information about the local
of 25º. In this area the seabed consists of sand over pockets of soft clay. distribution of soft and hard clay; however, a top layer of sand in the
Several supplementary soil investigations, including cone penetration back wall area masks the soft clay at many locations. The soft clay
tests and soil sampling, revealed local variations in the stratigraphy, was therefore the basic assumption for evaluation of rock support
particularly where a top layer of sand masked the underlying soft clay. stability. The strength of the soft clay decreases gradually from 5kPa
at the foot of the slide escarpment to 1kPa at the template area. The
The seabed in the nearshore area (Bjørnsundet) varies between thickness of the soft clay layer ranges from 0 to 20m.
exposed bedrock and soft clay infilled between the rock outcrops. In
Methodology for Stability Analysis of Rock Supports
Gudmund Eiksund is a Senior Engineer at GeoPartner
Marin as in Trondheim, Norway. He was previously
General
Senior Scientist at SINTEF. He has 15 years of The requirements for seabed intervention are optimised based on
experience in offshore geotechnical engineering
an interaction between pipeline design and geotechnical analysis.
involving seabed subsidence caused by oil and gas
extraction, soil structure interaction for steel jacket
This inter-discipline interaction is important for a large and
platforms, design of suction anchors for semi-
complicated project such as Ormen Lange. For many of the
submersible platforms, rock supports for seabed
installations and pipelines. Mr Eiksund graduated in
pipeline supports several iterations were required, involving pipeline
civil engineering from the University of Trondheim. analysis, geotechnical stability analysis and counterfill design. The
E:
gudmund.eiksund@geopartner.no
interaction between pipeline and geotechnical analyses can be
summarised as follows.
© TOUCH BRIEFINGS 2009
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