Pile Design According to International Practice

8/10/2019 Pile Design According to International Practice

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Dr David Cathie

Cathie Associates

Offshore pile design:

International practice



Offshore pile design : International practice

Outline

Pile design in the offshore industry

International standards and methods

Pile resistance (capacity) methods – API, CPT

Pile driveability

Pile driving monitoring

Piled tripods for wind converters – key issues

Conclusions

Oil and gas industry has a lot of experience in developing both small and large

offshore projects.Practical solutions are available today for safe design of tripod structures




Tallest offshore piled structure

Bullwinkle, GOM

Bullwinkle piles:

28 x 84”OD, 165m long

Bullwinkle platform:

529m high

412m water depth




Offshore oil & gas industry

10,000+ platforms worldwide

~99% piled jacket structures

Location Ground conditions

Gulf of MexicoOffshore Brazil

West Africa

Normally consolidated clay

Middle East

Australia

Carbonate soils, sands, calcarenite

Far East Loose to medium dense sands, soft clays

North Sea Medium dense to very dense sands, verysoft to hard clays




Pile sizes – piling hammers

Typical pile OD: 1.2 – 2.4m (1.8 – 2.4m in N.Sea)

Typical length: 40 – 100m

Pile hammers:

90-150 kJ hydraulic hammers for typical “small” piles

600kJ or more for large piles

Put in table with rated energyIHC range

Menck MHU range

is similar




Pile design in the offshore industry

Industry is risk adverse, and highly cost conscious

Consequences of structural failure leading to shutdown arevery high, and unacceptable

Consequences of installation delays are very high, and

unacceptable (production delayed, cost overrun)

Innovation is seen as risky and must have very high cost-benefit

Reliability of pile design is very high (no failures reported).Belief that methods are conservative to very conservative. No

account taken of ageing effects.




International Standards

API RP2A – WSD 29th edition, 2000

API RP2A – LRFD, 1st edition 1993

DNV classification notes No. 30.4, 1992(based on API 1987)

ISO 19902:2007 Fixed steel offshorestructures (based on API)

API RP2A – WSD 29th edition, 2000, errata

and supplement 3, 2007, provides theCommentary on CPT-based methods forpile capacity (C6.4.3c)




API pile design approach

Pile capacity/resistance methods

Main text API 93 method

CPT methods in commentary in 2007 edition

Axial/lateral response

Cyclic loads




API Main text method

Shaft resistance

f = K σv’ tanδ

f <= flim




API 2007 - CPT methods

Motivation

Research programs

Key features

Database

Industry acceptance




API 2007 CPT-based pile resistance

Motivation

Improve reliability and reduce conservatism

Based on more fundamental understanding of pile behaviour

Practical method capturing basic mechanics of driven pile

Direct use of CPT results in silica sand

Research Programs

Euripides (started 1995) in Eemshaven, Netherlands: dense to

very dense sands

Pile load tests in Dunkirk (dense to very dense marine sands) –

CLAROM site

Pile load test in Labenne (loose to medium dense sand), LCPC

site.




Key features of CPT methods

Direct use of cone resistance (qc) todetermine radial stress (σ’rc)

Effect of distance from pile tip

“friction fatigue” or degradation

during driving as pile progresses

Unit shaft resistance based on

residual soil-pile friction angle




CPT methods – database

ICP database: 20 open-ended tubular piles in sand

Length: 2m to 47m

Diameter: 0.07m to 2.0m (average 0.65m)

Range of Dr at tip: 57-96%

UWA database: 32 open-ended tubular piles in sand

Length: 5.3m to 79.1m

Diameter: 0.36m to 2.0m (average 0.73m)

Range of Dr at tip: 15-100% (average 68%)

Range of Dr along shaft: 23-100% (average 74%)




CPT method – application by Shell

0

20

40

60

80

0 4 8 12 16

P e n e t r a t i o n B e l o w

S e a f l o o r [ m ]

Dynamic SRD [MN]

ICP

API

0

5

10

15

20

25

30

35

0 4 8 12 16 20

P e n e t r a t i o n B e l o w

S e a f l o o r [ m ]

Dynamic SRD [MN]

ICP

API

Overy (2007) The Use of ICP Design Methods for theFoundations of Nine Platforms installed in the UK North Sea, Int.Offshore Site Investigation and Geotechnics Conference




CPT Method – industry acceptance

Adopted by Shell UK in 1996 for requalification of a number ofNorth Sea platforms (Overy, 2007)

Pile length: 26m to 87m

Diameter: 0.66m to 2.13m

Variable soil conditions

Adopted by API, 2007 as a “recent and more reliable method...considered fundamentally better..”

But qualified by offering 4 alternative methods

Should be used only by qualified engineers




API pile design approach

Axial/lateral response

T-Z/Q-Z and P-Y standardised approach

Mainly based on research in 1980’s

Cyclic loading

Axial

Axial and lateral effects uncoupled

Long (=flexible) piles can experience capacity degradation in

clay soils (due to strain softening)

Wave loading rate effect may compensate for degradation.

Lateral Cyclic effects included by softening P-Y response near seabed

and reducing peak lateral pressures

Methods proposed are guidelines only (but everyone uses

them)




Pile driveability - SRD

Soil resistance during driving (SRD)

Alm and Hamre (2001) method

Database 18 installations, 1.83 – 2.74m OD, up to 90m

penetration, MHU 1000-3000, IHC S-400, S-2300

Key feature: degradation of shaft resistance as pile passes,

calibrated to database




Pile driveability – wave equation

Wave equation (SRD v Blow count)

GRL WEAP – same quake, damping soil model as used by Alm

& Hamre

Blow count v depth

Pile acceptance criteria




Pile driveability prediction

SRD v Depth SRD v Blow count Blow count v Depth

0

10

20

30

40

0 50 100 150 200 250

P I L E P E N E T R A T I O N B E L O W

M U D L I N E ( M E T R E S )

BLOWS PER 0.25 METRE

MHU 800S (Eff. = 80%),

Best Estimate SRD

MHU 800S (Eff. = 80%),

High Estimate SRD

MHU 800S (Eff. = 95%),

Best Estimate SRD

MHU 800S (Eff. = 95%),

High Estimate SRD

0

25

50

75

100

0 50 100 150 200 250

S O I L R E S I S T A N C E T O D R I V I N G ( M N )

BLOWS PER 0.25 METRE

MHU 500T (Eff. = 80%), 40m penetration




0

10

20

30

40

0 25 50 75 100

P I L E P E N E T R A T I O N B E L O W

M U D L I N E ( M E T R E S )

SOIL RESISTANCE TO DRIVING (MN)

Best Estimate SRD

High Estimate SRD





Purposes

Confirming pile resistance duringdriving, or after set-up, SRD

Correlate SRD with calculated staticpile resistance for the site.

Establish reliable pile acceptancecriteria (blow count) based on acalibrated wave equation & SRDmodel

Monitor the stresses at pile top andto correlate with risks of tip buckling(when driving in rock)





Operationally, theoffshore environment is

extremely challenging.

It requires specificexperience and extremeprecautions for data ofgood quality

Many attempts haveresulted in failure

Instrumentation

Pile instrumentation consists in installing straingauges and accelerometers at pile top




Pile driving monitoring - underwater

Risks of mechanical damageand electrical instabilityrequire specific operational

procedures

Under-water monitoring requires specific equipment




Pile driving monitoring – signal matching

Signal matching (CAPWAP/TNOWAVE)

Iteratively modifying a numerical soil model until the

calculated reflective wave matches the measured wave

15 45

-8000.0

-2666.7

2666.7

8000.0

Blow No. 1623

ms

kN

6 L/c

W up Msd

W up Cpt

Measured upward and

downward waves

Measured andcalculated

upward waves




LS+P2 with D100

45h set-up

LS+P2+P3 with MHU

600

47h set-up

change to MHU1000

4h set-up

LS+P1+P2+P3+P4

with MHU1000

37.5h set-up

LS+P2+P3+P4+P5with MHU1000

27h set-up

Re-Strike on P5 with

MHU1000

25h set-up

0

20

40

60

80

100

120

140

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

SRD (kN)

d e p t h ( m )

after set-up (static capacity as per API86)

SRD upper bound Stevens/PuechSRD lower bound Stevens/Puech

back analysed SRD

Requested Capacity

CAPWAP

Pile driving monitoring – data example

Driving Data

0

20

40

60

80

100

120

140

160

0 100 200 300 400

blow count (blow/m)

d

e p t ( m )

0

20

40

60

80

100

120

140

160

0 200 400 600 800 1000

energy (kJ)

d e p t h ( m )

-

5 000

10 000

15 000

20 000

25 000

30 000

35 000

40 000

45 000

50 000

0 200 400 600 800 1000 1200

blow count bl/m

S R D ( k N

)

Capwap result




Pile driving monitoring – accuracy and limitations

Generally within 10-15% of static tests

Best agreement in sedimentary soils (sand, clays)

Agreement depends on set-up time and failure criteria for

static test

Limitations

Cannot accurately differentiate between tip and shaft

resistance near the base

Cannot define exact distribution of shaft resistance




Piled tripods for wind converters – key issues

Tripod foundation response very different from monopile

Structural dynamics of tripods

Insensitive to lateral stiffness

Axial stiffness related to pile penetration/capacity (for stiff

piles) so natural frequencies insensitive also to detailed pile

design

Cyclic axial loads much more important than cyclic lateral

Allow generously for scour – not a design problem withtripods

Need practical solutions to design foundations today!




Offshore platform/tripod loading

Moment loading at mudline for a monopile is

translated into axial pile loading for a tripod




Tripod and monopile loads

-200000

-150000-100000

-50000

0

50000

100000

150000

200000

250000

300000

350000

0 20 40 60 80 100 120 140

M e m b e r m o m e n t s [ k N m ]

time [s]

Bending moment at mudlevel during 50 year severe sea stateTripod Pile Monopile

-30000

-25000

-20000

-15000

-10000

-5000

0

5000

10000

0 20 40 60 80 100 120 140

M e m b e r f o r c

e s [ k N ]

time [s]

Axial (vertical) force at mudlevel during 50 year severe sea state

Monopile Tripod Pile max compression Tripod max tension




Tripod - pile head deflections

Extract of displacement time history from 50 yr extreme event covering governing ULSpeak load

Extreme deflections: Axial: 5mm

Lateral: 34mm




(Geotechncial) advantages of tripods/quadripods

Not sensitive to uncertain soil parameters (operational soilmodulus)

Not sensitive to scour assumptions

Lateral pile deflections are restrained by structure stiffness

Main cyclic loads transmitted as axial loading

Offshore oil & gas industry has strong preference for multipleleg structures – almost exclusively builds 3, 4 or 8 legged

piled platforms for offshore operations

Other structures require specific conditions to be cost-effective




Research – tripod foundations

Must not delay design and procurement process

Solutions must be adopted today even if research to confirmor improve methods continues in parallel

Solutions are available today

May be conservative but based on oil & gas experience

Use pile driving monitoring to confirm capacity duringinstallation

Use structural monitoring to confirm eigenfrequenciesand foundation stiffness in different conditions

Not essential today but research desirable to provideimproved (less conservative) methods of design i.e. reducedevelopment costs




Research – tripod foundations

Priority Topic Why?1 Axial pile stiffness at working

loads

Key for accurate structural

dynamics

2 Lateral pile behaviour

subject to cyclic loads

(fixed head, many low level

load cycles)

Not critical design issue for tripods

but little is known

3 Lateral pile stiffness at

working loads

2nd order importance for structural

dynamics, and for cyclic axial pile

capacity

4 Axial pile capacity under

low level cyclic loads

Most previous research

concentrated on higher levels ofcyclic axial load

5 Effect of ageing on pile

response

Ageing is known to increase pile

resistance and stiffness but the

mechanisms are not understood




Conclusions

International oil & gas industry has long and successful trackrecord with piled structures

Offshore industry is conservative and risk adverse (high costsinvolved in all marine work)

New CPT methods of pile design have been introducedrecently because of the recognition that the earlier APImethods were over conservative in some circumstances (e.g.dense sand)

Cyclic loading is handled within (API) design methods forwind/wave loads for jacket or tripod structures

Tripod solutions for wave converters are very robust andinsensitive to variations in foundation conditions

Documents

Pile Design According to International Practice