Pile Offshore Powerpoint

Foundation of offshore structure

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Design practice in offshore geotechnical engineering grew out of onshore practice, but the two application areas have tended to diverge over t he last 30 years, driven partly by the scale of the foundation elements used offshore, and partly by fundamental differences in construction (or installation) techniques . Groups of many moderate-sized piles are replaced by a few very large diameter piles; 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 under-pressure) to aid installation of skirted foundations and caissons.

Emphasis in 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.

Challenges of Offshore Geotechnical Engineering by Mark Randolph, Mark Cassidy, Susan Gourvenec. Centre for Offshore Foundation Systems, The University of Western Australia

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Pile foundation

Shallow foundation

Spudcan

Dead load1. Gravity (Mass of structure)

Live load1. Vibration2. Barge3. Erecting4. Installation 5. Buoyancy

Environmental load1. Wind2. Current wave3. Seismic

(Earthquake)4. Ice

Pile Foundation

Piling for marine and offshore structures

must be installed to develop the required

capacities in bearing, uplift, and lateral

resistance . For offshore bridge piers,

minimization of settlements may also

be criteria. Stiffness under lateral loads, as

well as strength, and the ability to accept

overloads in a ductile mode are also

important characteristics.Typical pile-structure-soil interaction.

Pile foundation

1. For resisting axial compression, the pile transfers its load by skin friction along its outside perimeter and by end bearing on its tip, provided that the tip is either closed or plugged in such a way as not to yield in relation to the pile

2. For resisting axial tension, the deadweight of the pile, plus that of the internal plug of soil, plus the skin friction are available.

3. For resisting lateral loads, most offshore structures in deep water (over 3040 m) depend on the bending resistance of the pile interacting with the passive resistance of the soil in the near-surf ace stratum . The pile must have sufficient strength to resist the resultant moments and shears at these levels and to prevent biaxial buckling. The capacity to resist lateral loads can be improved by increasing the stiffness and moment capacity of the pile in the critical zone near and just below the mudline by grouting in an insert pile, by increasing the wall thickness of the steel pile through this zone, or by filling the pile with concrete in this region.

Pile foundation

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Ground

Softsoil

Firmed soil/Rock

Ground

Softsoil

%

Firmed soil/Rock

Pile foundation

In spite of significant advances in understanding the mechanisms that determine the eventual shaft friction and end-bearing capacity of different types of pile, design methods still rely heavily on empirical correlations .

The most challenging aspect of offshore pile design is therefore the need to extrapolate design parameters from an experimental database that is largely limited to piles of less than 1 m in diamet er, the majority of which are solid or closed ended piles, often installed by jacking.

This compares to modern offshore piles with diameters often in excess of 2 m , with relatively low displacement ratios and invariably installed by dynamic driving.

Increasingly, the cone resistance is used as the primary measure of the soil strength from which pile design parameters may then be deduced; unlike in onshore design, standard penetration test data are thankfully avoided! For sands, design parameters can be expressed direc tly in terms of the cone resistance, while for fine-graine d sediments parameters are based either on the undrained shear s trength or the in situ vertical effective stress together with an overconsolidation(or yield stress) ratio .

Pile foundation

Schematicalchart of pile foundation design

Pile foundation

Pile foundation

Pile foundation

High frequency, low amplitude

give high resolution in the

shallowdepth

Low frequency, high amplitudegive low resolution into the earth

Soil strata

Pile foundation

Boring log

Pile foundation

Pile foundation

Boring log

(a) (d) Cohesive soil (Clay)(e) (f) Silt, clayey sand (g) (h) (Dense, loose) sand

Pile foundation

Cohesionless soil(Sand particle)

Cohesive soil(Clay particle)

Pile foundation

Typical properties of engineering soil Pile foundation

Pile foundation

Siliceous =

Pile foundation

Pile foundation

Density of typical soil constituents

Typical soil constituents for carbonate soil

Typical soil constituents for clay

Typical soil constituents for sand

Pile foundation

Pile foundation

Carbonate deposit in deep water

Deposit are sand from lime stone rock

Oolitic sand deposit near shoreline(shallow than 15 to 20 m)

Carbonate muds deposit in calm seas (deep water)

Pile foundation

Pile foundation

(Rocks)

Pile foundation

Pile foundation

Pile foundation

Qp1

Pile foundation Lateral load

(No skin friction)

Zo

Pile foundation

Pile foundation

annular = ring-shape

Pile foundation

W W

Pile foundation

Pile foundation

Limiting lateral resistance in clays

Pile foundation

Limiting lateral resistance in sand

Pile foundation

Limiting lateral resistance in carbonate

sediments

Pile foundation

Pile foundation

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Pile foundation

Main pile shallow water


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Main pile connection above water-level

Pile foundation

Skirt pile with free-riding underwater hammer


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Barge =

Pile foundation

Shallow Foundation

Shallow foundation

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Shallow foundation

Shallow foundation

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Shallow foundation

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*ISO = International Standards Office, British Standards Institute, London.

Shallow foundation

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Shallow foundation

Spudcan

Spudcan

Spigot =

Spudcan

Ballast =

Spudcan

Anchoring system

Anchoring system

Mooring = fix firmly

Anchoring system

Footprint =

Anchoring system

Anchoring system

taut = encroach =

Anchoring system

shank = ! fluke = ! "#$%&

Anchoring system

North sea

End of presentation

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Pile Offshore Powerpoint