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Seabed Geotechnics to Unlock Deepwater Oil and Gas Resources
a report by
David White and Mark Randolph
Subsea & Pipelines
Centre for Offshore Foundation Systems, The University of Western Australia
New Geotechnical Frontiers unstable scarp and extensive in-field flowlines and long export pipelines to
The exploitation of hydrocarbon reserves is moving further from the shore land. In-field flowlines need design strategies to mitigate thermal and
into deeper waters. The safe and economic development of these pressure-induced expansion, and also require extensions through the
resources needs new research and technology to securely establish water column to floating facilities by means of risers – often in the form
offshore structures and pipelines. The scientific challenge is to create of steel catenary risers (SCRs). In addition to the challenges posed by the
robust models for the near-seabed behaviour, where the infrastructure, scarp onto the continental shelf, export flowlines require appropriate
the ocean and the weak seabed sediments are mobile and interact. A new protection measures in moderate to shallow water depths in order to
frontier of geotechnical research is the shallowest few metres of the stabilise them on a seabed that is itself potentially mobile under extreme
seabed, where the sediment is weak and mobile, hydrodynamic forces can storm events.
be significant and existing characterisation techniques provide only a
crude quantification of the mechanical behaviour. The design methodologies to tackle each of these challenges are in their
infancy and beyond the scope of conventional geotechnical engineering
The Centre for Offshore Foundation Systems (COFS) at the University of analysis. The principal reasons for this lie in the mobility of the
Western Australia is home to more than 40 academics, researchers and infrastructure and the seabed, the low soil strengths and effective stresses
technical staff, focused principally on offshore geotechnics. Our research in the critical upper metre of the seabed and the interaction with strong
group has been further invigorated over the past few years by the new ocean currents at the seabed.
challenges presented by deepwater developments and the associated
ultralong pipelines and subsea infrastructures. Conventional offshore foundation design aims for stability and minimal
movement. Piles and shallow foundations supporting platforms move by
An additional local challenge is presented by the carbonate soils found no more than a few per cent of their size under the action of the design
offshore Australia. The mechanical behaviour of these soils contrasts storm, and suffer negligible cumulative movement or settlement. By
significantly with ‘conventional’ geotechnical behaviour. Compared with contrast, the deepwater infrastructure of pipelines, risers and anchor
terrigenous clays and sands, carbonate soils are extremely contractile and piles and the geohazard of submarine slides are dynamic. The pipelines
liquefiable, but also have high frictional strength. Design approaches that must move significant distances – several diameters – to relieve load
have emerged from experience in ‘conventional’ soils are often from thermal and pressure cycles,
1,2
but must not accumulate excessive
inappropriate for our local materials. axial movement (‘walk’).
3
The catenary risers must survive movements
imposed by the moored facility, which is tethered by anchor piles that
The changing infrastructure required to develop a hydrocarbon field as can severely weaken the surrounding soil through large lateral
activity moves to deeper water, located further from shore, is illustrated in movements.
4
Similarly, the on-bottom pipelines extending to shore pass
Figure 1. This is representative of current trends off the coast of Australia through water depths in which storm action causes scour and
and in most other major petroleum-producing regions. New projects are liquefaction of the seabed sediments, potentially causing significant
facing challenges associated with characterisation of the softer deepwater movement or self-burial of the pipeline. Current research activity is
sediments (which offshore Australia are usually of carbonate mineralogy, tackling many of these challenges, and project-specific model
presenting particular challenges), crossing the steep and potentially testing using the COFS geotechnical centrifuge facility is providing
support for designs where current analysis methods are inadequate or
require validation.
David White is an Australian Research Council Future Fellow at the Centre for Offshore
Foundations at the University of Western Australia (UWA). His principal research areas are
pipeline geotechnics, foundation engineering and soil mechanics. He has authored more Characterisation of Soft Deepwater Sediments
than 100 technical papers, five of which have won awards. He acts as a consultant to
The shallowest one metre of the seabed has historically been
industry through the Perth-based firm Advanced Geomechanics. Professor White was
awarded a PhD in 2002 from the University of Cambridge, where he held a lectureship
overlooked in geotechnical site investigations, where the aim has
until 2007.
been to establish the ground conditions relevant to foundation and
anchor design. However, attention is increasingly focused on this
Mark Randolph is an Australian Research Council Federation Fellow at the Centre for
shallowest zone of soil, which must be characterised for pipeline and
Offshore Foundation Systems at the University of Western Australia (UWA). His research
interests include many aspects of the performance of piled foundations, but are currently
SCR design. In deepwater, the seabed sediments are generally lightly
focused on offshore developments in deepwater, especially soil characterisation and the overconsolidated, with strength intercepts at the mudline of 0–15kPa
estimation of limiting loads for foundation, pipeline and anchoring systems through a
and gradients in the range of 1–2kPa/m. Therefore, it is extremely
combination of centrifuge model tests and numerical analysis.
difficult to obtain high-quality samples – especially considering the
E:
randolph@civil.uwa.edu.au
water depths of up to 2,000m – and to carry out accurate laboratory
tests at such very low effective stresses.
© TOUCH BRIEFINGS 2009
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