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03501322
Foundation of offshore structure
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���������� �� �������� ������������ ����������� Design practice practice in offshore geotechnical geotechnical engineering engineering grew out out of onshore practice, practice, but the two application areas have tended to diverge over the 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 of deep skirts, transferring transferring the effective foundation depth to the level of the skirt tips, or by forcing forcing footings footings to penetra penetrate te several several diameters diameters into into the seabed; seabed; underw underwater ater installation has has allowed allowed the use of ‘suction’ ‘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 differe differences nces have led led to the develop development ment of of separate separate design codes codes for offshore 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
Typeof Offshore Production Concepts
Fixedplatform
Semi-submersible
Onshoreplatform Jackup rig
Tension leg platform (TLP) Drill ship
Seven Typesof Offshore Deepwater Systems
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http://www.naturalgas.org/naturalgas/extraction_offshore.asp
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Pile foundation
Shallow foundation
Spudcan
Dead load 1. Gravity (Mass of structure) Live load 1. Vibration 2. Barge 3. Erecting 4. Installation 5. Buoyancy Environmental load 1. Wind 2. Current wave 3. Seismic (Earthquake) 4. Ice
Pile Foundation
Pile foundation
Typical pile-structure-soil interaction.
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.
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 30–40 m) depend on the bending resistance of the pile interacting with the passive resistance of the soil in the near-surface 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
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foundation
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Ground
Ground
Soft soil
Soft soil
Firmed soil /Rock
Firmed soil /Rock
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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 diameter, 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 directly in terms of the cone resistance, while for fine-grained sediments parameters are based either on the undrained shear strength or the in situ vertical effective stress together with an overconsolidation (or yield stress) ratio.
Pile foundation
Schematical chart of pile foundation design
Pile foundation
Pile foundation
Pile foundation
Pile foundation
Low frequency, high amplitude give low resolution into the earth High frequency, low amplitude give high resolution in the shallow depth
Soil strata
Pile foundation
Pile foundation
Boring log
Pile foundation
Boring log
Pile foundation
(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
Density of typical soil constituents
foundation
Typical soil constituents for carbonate soil Typical soil constituents for clay Typical soil constituents for sand
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Pile foundation
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
Pile foundation
(Rocks)
Pile foundation
Pile foundation
Pile foundation
Pile foundation
Qp1
Pile foundation
Lateral load
Z o (No skin friction)
Pile foundation
Pile foundation
annular = ring-shape
Pile foundation
Pile foundation
Pile foundation ′
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Pile foundation
Pile foundation
Pile foundation
Pile foundation
Pile foundation
Pile foundation
Pile foundation
Pile foundation
Pile foundation
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
Pile foundation
Pile foundation
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Pile foundation
Pile foundation
Pile foundation
Pile foundation
Pile foundation
Pile foundation
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Pile foundation
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Pile
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foundation
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Main pile shallow water
Pile
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foundation
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Main pile connection above water-level
Pile
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foundation
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Skirt pile with free-riding underwater hammer
Pile
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foundation
Barge =
Pile foundation
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Shallow Foundation
Shallow foundation
Shallow foundation
Shallow foundation
Shallow foundation
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Shallow foundation
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Shallow foundation
Shallow foundation
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*ISO = International Standards Office, British Standards Institute, London.
Shallow foundation
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Spudcan
Spudcan
Spudcan
Spudcan
Spigot =
Spudcan
Ballast = ,
Spudcan
Spudcan
Spudcan
Spudcan
Spudcan
Anchoring system
Anchoring system
Anchoring system
Mooring = fix firmly
Anchoring system
Footprint =
Anchoring system
Anchoring system
Anchoring system
Anchoring system
Anchoring system
taut = encroach =
Anchoring system
shank = , fluke =
Anchoring system
Anchoring system
North sea