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The challenges of offshore geotechnical engineering

ICSMGE Plenary session D

Design practice in offshore geotechnical engineering grew out of onshore practice, but the two areas have diverged over the last 30 years, driven partly by the scale of the foundation elements used offshore, and partly by fundamental differences in construction techniques.

Groups of many moderate sized piles used onshore are replaced by a few very large diameter piles for offshore applications. Similarly excavation of shallow soft sediments is replaced by the use of deep skirts, transferring the effective foundation depth to the level of the skirt tips, or by forcing footings to penetrate several diameters into the seabed; underwater installation has allowed the use of 'suction' (or underpressure) to aid installation of skirted foundations and caissons.

Emphasis in offshore design is focused more on capacity, paying particular attention to the effects of cyclic loading but generally with less concern on deformations compared with onshore design.

These differences have led to the development of separate design codes for offshore structures, which are in most cases more prescriptive than onshore codes but are also more sophisticated in key areas.

The state of the art report was prepared with colleagues Mark Cassidy and Susan Gourvenec from the University of Western Australia and Carl Erbrich from Advanced Geomechanics in Perth. It describes design principles for foundation and anchoring systems, ranging from shallow footings to piles and caissons, highlighting differences between onshore and offshore practice and also the link (or gap) between research and practice.

It outlines the challenges associated with offshore geotechnics, and reviews current design approaches and research trends for a range of foundation and anchor types.

Shallow foundation systems

In contrast to onshore design, offshore shallow foundations generally incorporate steel or concrete skirts that penetrate through surficial soft soils, forcing any failure surface down to skirt tip level.

Pumping, creating underpressure within the skirted compartments, is generally required to help penetrate the skirts. By subsequently sealing the compartments, the foundation is then able to withstand transient tensile loading, or high applied moments.

Wave loading during design storm conditions typically leads to larger horizontal and moment loading than for onshore structures and hence interaction between vertical, horizontal and moment loading is critical. Rather than the traditional approach of bearing factors accounting for inclination and eccentric of the applied load, three-dimensional failure envelopes in V-M-H space are used in offshore design.

Operation of mobile drilling rigs is still the area with the highest frequency of unpredicted foundation response, both during preloading of the spudcan foundations and during storm conditions.

As these rigs are used in increasing water depths, there is a need for innovative analysis techniques to assess spudcan installation; potential punch through into underlying weaker soil; performance during remobilising in areas where footprints from previous operations exist on the sea floor; and response during eccentric and combined V-M-H loading.

Site characterisation and anchoring systems in deep water Within each offshore region, there has been an inevitable progression from shallow to deep water, with recent installations in 2,000m of water in the Gulf of Mexico and deeper fields being planned.

This has necessitated considerable investment in research to validate new foundation and anchoring systems and also to improve methods for characterising the soil.

Facilities have evolved from fixed steel or concrete platforms, to floating facilities ranging from tension leg platforms with vertical tethers anchored to piles, to SPARs and tankers held in position by catenary mooring chains or, more recently, by lightweight 'taut wire' polyester ropes.

In the soft sediments typically found in deep water, novel 'full flow' penetrometers have been developed to obtain more reliable strength measurements. The average T-bar factor for different sites appears to have a smaller coefficient of variation than the average cone factor, reducing the need to calibrate an appropriate T-bar factor for each site from laboratory tests.

Floating facilities for deep water developments are restrained by mooring systems that may range from vertical tethers, in the case of a tension leg platform, to heavy chains in the form of catenaries that apply predominantly horizontal load to the anchors.

More recently, lightweight polyester ropes provide 'taut wire' mooring systems, where the resulting load on the anchor will lie at 35¦ to 45¦ from the horizontal.

For suction caissons, this angle is such that the failure mode will tend to be vertical, but the potential for a crack to form between the trailing edge of the caisson and the soil is an important consideration.

In ultra deep water, simplicity of installation is at a premium, and novel approaches such as the deep penetrating anchor (DPA) or the simple torpedo anchor become attractive.

Such anchors, typically 10m to 15m long, are allowed to free fall from more than 50m above the seabed and achieve terminal penetration velocities in the vicinity of 30m/s.

This allows the anchor to penetrate to two or three times the length of the anchor, achieving capacities of several MN per anchor. Although such anchors are relatively inefficient in terms of the ratio of holding capacity to weight, this is compensated for by their ease of installation.

Professor Mark Randolph is director of the ARC-funded Special Research Centre for Offshore Foundation Systems at the University of Western Australia. His research includes many aspects of pile design, but is currently focused on offshore developments in deep water, especially soil characterisation and the estimation of limiting loads for foundation and anchoring systems.

Randolph interacts closely with the offshore industry, particularly through his role as a director of specialist geotechnical consultant, Advanced Geomechanics. He is a fellow of the Australian Academy of Science and of the UK Royal Academy of Engineering and was the 2003 Rankine Lecturer for the British Geotechnical Association. He has recently been awarded a five-year Federation Fellowship by the Australian Research Council.

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