Assessment of Offshore Structures

8/10/2019 Assessment of Offshore Structures

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ISO 19901-9

Current developments in

assessment of fixed offshore structDr Ramsay Fraser

Aberdeen,

September 2013


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ISO & API code development


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ISO 19900 series

19901-9 SIM

Andrea MangDec 2012


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ISO 19900 and API RP2xx alignment

Andrea MangDec 2012


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way forwardsegmentation of API RP2A

Andrea MangDec 2012


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alignment strategy

19901-9

Andrea MangDec 2012


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technical challenges

Andrea MangDec 2012


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reliability levels in API standards

Andrea MangDec 2012


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19901-9 evaluation, assessment & mitigatio

19902 states...

Prevention and mitigation

measures should be consideredat all stages of the assessment

Process


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assessment triggersa) Changes from the original design or previous assessment basis, including

1) addition of personnel or facilities such that the platform exposure level is changed to a more onerous level,

2) modification to the facilities such that the magnitude or disposition of the permanent, variable or environmental acstructure are more onerous,

3) more onerous environmental conditions and/or criteria,

4) more onerous component or foundation resistance data and/or criteria,

5) physical changes to the structure's design basis, e.g. excessive scour or subsidence, and

6) inadequate deck height, such that waves associated with previous or new criteria will impact the deck, and provide

action was not previously considered.

7) 20 years after installation (unless it has already been performed)

b) Damage or deterioration of a primary structural component: minor structural damage can be asses

- appropriate local analysis without performing a full assessment; however, cumulative effects of m

damage shall be documented and included in a full assessment, where appropriate.

c) Exceedance of design service life, if either

the fatigue life (including safety factors) is less than the required extended service life, or

degradation of the structure due to corrosion is present, or is likely to occur, within the required

extended service life.

(More guidance on rigid and flexible joints is reqd. plus interpretation of fatigue is required)


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assessment methods and

consistent reliability


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ISO 19902assessment methods

1. DLA (Design Level Assessment)Used for design & assessmentuseful to inform how the structure works (and should be recommeChecks extreme and still water (& seismic)

2. RSR (Reserve Strength Ratio)

shall be achieved but needs to be tied to the RP via hazard curveUsed for assessment (can be used for design)Checks extreme (& seismic) but still water check shall also be pe

3. SRA (Structural Reliability Analysis)

can be used for assessment (used in design for calibration)


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1. DLA (Design Level Assessment)Load factors derived on a weighted basis for components in a Gen

jacket

2. RSR (Reserve Strength Ratio)accounts for system failure mechanism of actual structure

3. SRA (Structural Reliability Analysis)accounts for uncertainty in system capacity in actual structure


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1. DLA (Design Level Assessment)Load factors derived on a weighted basis for components in a Gen

jacket

2. RSR (Reserve Strength Ratio)accounts for system failure mechanism of actual structure

3. SRA (Structural Reliability Analysis)accounts for uncertainty in system capacity in actual structure

method is

increasingly

specific to the

actual jacket

being

assessed


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19900 hazard curves


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reliability of a structure


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load pdf and exceedance curves

1 1.5 2 2.5

annualprobability

densityp(E/E100

)

E/E100

NW Shelf

North Sea

limit

0

0.002

0.004

0.006

0.008

0.01

1 1.5 2

annualprobabilityofe

xceedanceQ(E/E100

)

E/E100

NW Shelf

North Sea

limit


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hazard curve slope (resistance uncertainty)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

1 1.2 1.4 1.6 1.8 2 2.2 2.4

annua

lpro

ba

bility

densityp

(E/E

100

)

E/E100

dxxPxpRPPf RL )()(1)(xpL

)(xPR


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0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

1 1.2 1.4 1.6 1.8 2 2.2 2.4

annua

lpro

ba

bility

densityp

(E/E

100

)

E/E100

)(xpL

)(1xPR

dxxPxpRPP

iRLif)()(

1

)(2xPR

)(3xPR


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0

0.0005

0.001

0.0015

0.002

0.0025

0.003

1 1.2 1.4 1.6 1.8 2 2.2 2.4

pL

(E/E

100

)xP

R(E/E

100

)

E/E100


dxxPxpRP

PiRLif

)()(1

)/()/( 100100 1 EEPEEp RL

)/()/( 1002100 EEPEEp RL

)/()/( 1003100 EEPEEp RL


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1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

100 1000 10000 100000

Baseshearnorma

lise

dby

100ye

ar

bases

hear

return period = 1/Pf

Nsea (1 leg) Nsea (4 braces) Nsea (no resistance or load uncertainty)

1

1

fP2

1

fP3

1

fP

hazard curves


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Example system failure by single leg memb


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0.730.760.790.820.850.880.910.940.971.001.031.061.091.12

1.151.181.211.241.271.301.331.361.391.421.451.481.511.54

1.571.601.631.661.691.721.75

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

100 1000 10000 100000

Baseshe

arnorma

lise

dby

100y

ear

bases

hear

return period = 1/PfNsea (1 leg) Nsea (4 braces) Nsea (no resistance or load uncerta

GoM (1 leg) GoM (4 braces) GoM (no resistance or load uncerta

1.92

1.85

RP=6500yrs

Pf=1.

5E-4

RP=10000yrs

Pf=1

E-4

RP=19000yrs

Pf=5E-5

RP=30000yrs

Pf=3

E-5

code implicit reliability - RP, RSR & gE


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consistent reliability across regionsMike Efthymiou 2011

Location Exposure L1 :Manned High Consequence

L2 Category:Not Normally Manned

ERSR E

RSR

Northern NSea 1.40 1 92 1 09 1 50

NWAustralia 1.702 35 1 26 1 72

Gulf of Mexico 1.592 18 1 17 1 60

LocationExposure L1 (GoM) :Manned Evacuated

E RSR

Gulf of Mexico 1.30 1 78

Used only for winter storm

& sudden hurricane criteria

RP 2A 22ndEd achieves this


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0.0

1.0

2.0

3.0

4.0

5.0

'70 '74 '78 '82 '86 '90 '94 '98Time in years

Designstorml

oad

North Sea

Gulf of Mexico

100 year load

wave load recipe

4.2

3.3

b d t


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response based metocean (global consistency)

n year returnperiod base

shear (Xn)

n year return

period wave

height (Hn)

associated wave

period for n year return

period wave (Tn)

n year return

period crest

height

associated curre

n year return pe

wave

wave theory (WT) eg Stokes 5th Determinecurrent to give

Xn when used

with WT, Hnand Tn

h d (RSR f i t i d)


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hazard curves (RSR for given return period)

Most recent pro

hazard curves steeper than p

However, if the

load has reduc

E10000remains

E/

E100

Richard GibsDec 2012


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Wave in deck

e treme ater le el


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extreme water level

EWL = SWL + settlement + tide + surge + wave crest + diffraction

Example of uncertainty in extrapolation of 10 yrs of data to 100 yr & 10,000 yr R

Richard Gibson

SIM conference

Nov 2012

wave in deck guidance (load and approach)


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wave-in-deck guidance (load and approach)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

100 1000 10000 100000

OverturningMomen

tratio|OTM|/|OTM100|

Return Period (Years)

Jacket

Deck

COV_R=10%,COV_J

=8%,COV_D=35%

Target Capacity

Jacket + Deck


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capacity calculation

DLA assessment guidance


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DLA - assessment guidance

Frame Modeling Joint Modeling Foundation Modeling

Primary Framework Joint Eccentricity Structural/Soil Interaction

Secondary Framework Joint Flexibility Pile/Structure Interaction

Deck Structure Grouted Joints P/Y Modifiers for Condu

Pile Connectivity Doubler Plated Joints Pile Failure Simulation

Grouted Piles Cracked Joints

Conductors Ground JointsConductor Connectivity Member Modeling

Conductor Guide Framing Corrosion Allowance

Support Frame/Deck Modelling Grouted Members

Leg Stubs Damaged Members

Buried members

Design Flooded

non linear analysis re assessment guidance


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non-linear analysis re-assessment guidanceexample of embedding local detailed shell model in global beam mode

local plastic buckling (and tearing)


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local plastic buckling (and tearing)

geotechnical re assessment guidance


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geotechnical re-assessment guidance

1. ICP method or NGI method used to determine soil capacity with grea

on physics rather than the empirical relationships previously available(uncertainties) are reduced and reliability analyses becomes feasibleOvery 2007).

2. The above methods use data from ring shear tests and this may requfurther site investigation for older platforms. Also surface roughness oshear apparatus requires careful maintenance and calibration.

3. In addition, the soil capacity should include the effects of soil strengtwith ageing, cyclic degradation due to large storms, pile interaction atcapacity, soil ductility or brittleness (ie pile tip punch through), shallowpresent) and liquefaction (for seismic response).

4. Conductor modellingshall be modelled as structural (with approprmodifier). Potential further assessment with stiffness from internals if



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Group A1

Pin A2

Pin A4

Group A5

Group B1

Pin B2Pin B4

Group B5

-20

0

20

40

60

80

100

120

140

-100 0 100 200 300 400 500

PileHeadForce,MN

Pile Head Displacement, mm

Group A5/B5

Group A1

Group B1

Pin Pile A4,B1 and B2

Pin Pile A2

Extreme (100y, L1 Installation) - Foundation Results

Foundations Curves factored down by required FoS (1.25)

B4

seismic time-history re-assessment guidanc


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seismic time history re assessment guidanc

Example snap shot of seismic time history


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Example snap shot of seismic time history

assessment conditions some questions


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assessment conditions some questions

Extreme storm conditions

omni-directional 100-year or directional 800-year

(design or assessment)? consistent approach to wave-in-deck

(review existing approach & perhaps clarify from API WID JIP?)

Still water (dead load dominated structures)

Is the operating condition required?

ALE seismic

2500-year but attempt to demonstrate 10,000 load or Pf


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ISO 19901-9

Current developments inassessment of fixed offshore struct

QUESTIONS

Assessment of Offshore Structures

Documents