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Design Requirements forFixed Steel Structures in API

and ISO

Moises A. Abraham, Chevron

December 2012

Platong II Gulf of Thailand

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Topics of Discussion

Evolution of API RP 2A-WSD 21 st Edition into the 22 nd Edition.

Changes in the 22 nd Edition of RP2A-WSD.

New Tubular Joint Strength Provisions in the 22 nd Edition.

Coexistence of API RP 2A-WSD 22 nd Edition and API RP 2A-LRFD 2 nd

Edition.

Alignment of API Offshore Structures Standards with ISO 19900 Series. Adopting ISO 19902 for RP2A-LRFD 2 nd Edition.

Code Check Comparison between ISO 19902 and API RP 2A-WSD 21 stEdition.

Review of Calibration Methodology in the 1980s.

Topics of Discussion

References

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Evolution of API RP 2A-WSD 21 st Edition into the 22 nd

Edition.

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API RP 2A-WSD21 st Edition

API RP 2GEN

API RP 2MET

API RP GEO

API RP 2A-WSD22 nd Edition

API RP 2EQ

API RP 2SIM

API RP 2TOP

API RP 2MOP

Published

Will be publishedin 2013

Being Developed

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Changes in the 22 nd Edition of RP 2A-WSD

Balloted in 2011 and approved with 96% of the votes cast.

22 nd Edition final table of contents to include 3 new sections i.e. Scope,Normative References and Terms, Definitions and Acronyms. Threesections were removed i.e. Section 14 Surveys, Section 17

Assessment of Existing Platforms and Section 18 Fire, Blast and Accidental Loadings.

The 1989 edition of AISC Specification for Structural Steel Buildings(ASD) is included as a normative reference. LRFD in later editions of this

AISC specification are based on calibration with building design practicesand may not be applicable to offshore platforms.

Section 4.7 contains an updated guidance to determine ExposureCategory used in selecting required level of design for platforms.

Users are referred to API 2MET for wind, wave and current environmentaldata previously included in 2A. The detailed steps to follow in applying thedata from API 2MET remain in 2A.

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Changes in the 22 nd Edition of RP 2A-WSD

The newly required robustness assessment for new platforms is a checkfor platform survival in a lower probability extreme event.

The elevation of the underside of the deck for new L-1 and L-2 platformsmust be no lower than the 1000-year return period max crest elevationprovided in API 2MET.

While the 22 nd edition no longer recommends a minimum of 1.5m (5 ft) airgap, the user is reminded to allow for any known or predicted seafloorsubsidence, water depth uncertainty, platform rotation, etc.

Extreme Level Earthquake (ELE) and Abnormal Level Earthquake (ALE)are defined in 2EQ. The ELE was the Strength Level Earthquake (SLE)and the ALE was the Ductility Level Earthquake (DLE) in earlier editionsof RP 2A.

Simplified Fatigue was removed from the commentary to be consistentwith API 2MET. Wave conditions for which the simplified approach wascalibrated are no longer in 2MET. All new and reused structures are nowrequired to have a detailed fatigue analysis.

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Changes in the 22 nd Edition of RP 2A-WSD

Revised Section 7.2.3 on Minimum Capacity requirements for tubular joints (50% effective strength check). For the purposes of this

requirement, the chord capacity shall be determined using Equation 7.1(Pa) with a factor of safety (FS) equal to 1.0.

API 2MOP Recommended Practice for Marine Operations identical toISO 19901-6 was issued in 2010 and is now a normative reference in the22 nd edition.

Section on in-place surveys has been removed and expanded on in thenew API 2SIM.

Section 17 on assessment of existing platforms and its commentary havebeen moved to and expanded on in the new API 2SIM.

Any reused (not the same as change of use) platform must meet therequirements of a new platform with special considerations provide toaccount for fatigue damage experienced at the original platform location.

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The new tubular joint strength equations are based on screened testdatabases, augmented by an extensive new series of validated nonlinearFE simulations.

Additional experimental information available of the effect of additionalchord loads on joint capacity was incorporated in the new formulation.

The increased reliability (reduced scatter) provided by the new staticstrength formulation justified the reduction in load factor of safety to 1.6from the previous value of 1.7.

Joint classification is unchanged from the 21 st edition.

A new brace load interaction equation is adopted. This new interactionequation provides a better fit to the available test data than does the arcsine expression in the 21 st edition.

New Tubular Joint Strength Provisions in API RP 2A-WSD 22 nd Edition by Pecknold et al.

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0.12

opbaipbaa M

M M M

PP

IR

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New Tubular Joint Strength Provisions in API RP 2A-

WSD 22 nd Edition by Pecknold et al.

The punching shear design formulation has been fully eliminated.

The formulas for have been completely revised

The format of the basic capacity equations remain unchanged from the 21 st edition:

The 0.8d multiplier in the 21 st edition for has been eliminated and absorbed bythe coefficient.

The new and formulations more accurately reflect the influence of jointgeometry in particularly chord diameter-to-thickness ratio ( ) and chord loads on

joint capacity and are a significant improvement over previous practice (21 st

edition). The strength load factor depends only on joint geometry ( and ). isexcluded because it has only a minor effect.

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sin

sin2

2

FS

d T F QQ M

FS

T F QQP

y f ua

y f ua

uQ f Q

uQ

f gu QQQQ ,,,

uQ a M

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Interaction Curve for Tubular Joints Under Combined

Axial and Uni-directional Moment Loading

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Pa =Qu Qf Fyc T2 /(FS sin )M

a=Q

uQ

f F

ycT2 d/(FS sin )

Joint Strength CheckIR =|P/Pa| + |M/Ma|ipb2 + |M/Ma|opb

API WSD

API LRFD

ISO 19902Puj =Qu Qf Fy T2 /(sin )

Muj =Qu Qf Fy T2 d/(sin )

Joint Strength CheckUj=|PB/PD| + |MB/MD|ipb2 + |MB/MD|opb

Puj =Qu Qf Fy T2 /(sin )Muj =Qu Qf Fy T2 (0.8d) / (sin )

Joint Strength CheckIR=1-cos[( /2)(PD/ jPuj)] + [(MD/ jMuj)ipb2 + (MD/ jMuj)opb2]0.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.0 0.2 0.4 0.6 0.8 1.0

P / P

m a x

M/M max

Interaction Curve for P + M

API LRFD (P+Mipb or Mopb)ISO (P+Mipb)ISO (P+Mopb)

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Coexistence of API RP 2A-WSD 22nd Edition and API

RP 2A-LRFD 2nd Edition.

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API RP 2A-WSD21 st Edition

API RP 2GEN

API RP 2MET

API RP GEO

API RP 2A-WSD22 nd Edition

API RP 2EQ

API RP 2SIM

API RP 2TOP (LRFD) ?

API RP 2MOP

API RP 2A-LRFD1 st Edition

(Withdrawn)

API RP 2A-LRFD2 nd Edition

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Alignment of API Offshore Structures Standards with

ISO 19900 Series.

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API RP 2GEN

API RP 2MET

API RP GEO

API RP 2A-WSD

22 nd Edition

API RP 2EQ

API RP 2SIM

API RP 2TOP LRFD

API RP 2MOP ?

API RP 2A-LRFD

2 nd Edition

ISO 19900

ISO 19901-1

ISO 19901-4

ISO 19902

ISO 19901-2

?

ISO 19901-3

ISO 19901-6

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Adopting ISO 19902 for RP2A-LRFD 2 nd Edition

API RP 2A LRFD has been withdrawn and a modified version of ISO19902 will be adopted for RP 2A-LRFD 2 nd Edition.

Task Group 19 composed of 22 members started the work in 2009.

Activities completed:

Task group has completed the review of 25 sections of ISO 19902.

Written comments on the DNV Report Comparison of API, ISO, and NORSOKOffshore Structural Standards were submitted by task group members.

A code check comparison has been performed between API WSD and ISO19902.

API will fund analytical studies (platform UC check comparisons). The projectwill start in early 2013 and last for two years.

3 contractors perform 3 platform analyses.

Chevron will run one additional platform analysis.

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Code Check Comparison between ISO 19902 and API

RP 2A-WSD 21st Edition

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Jacket dead load = 2000 kipsDeck dead load = 3000 kips

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Code Check Comparison between ISO 19902 and API

RP 2A-WSD 21st Edition

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Pile Members Unity Check ___ ISO 19902 ___ API 21 st Edition

0.540.60

0.850.90

1.111.13

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Code Check Comparison between ISO 19902 and API

RP 2A-WSD 21st Edition

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Row 2 Members Unity Check ___ ISO 19902

___ API 21st

Edition

0.930.760.55

0.59

0.350.34

0.660.73

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Code Check Comparison between ISO 19902 and API

RP 2A-WSD 21st Edition

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Joint Unity Check

___ ISO 19902 ___ API 21 st Edition

0.890.97

0.961.02

0.891.21

0.831.40

0.210.24

0.220.24

0.290.36

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Pile UCs above the mudline are similar in API and ISO.

When hydrostatics is included in the analysis, API and ISO yield different resultsdue to treatment of capped-end forces. The table below shows maximum UCs for

two water depths.

Hydrostatic pressure will dominate deep water jackets and compliant towers inLRFD.

ISO equation 14.3-13 controls the design of critical joints. The intent of theequation is to make critical joints stronger than braces, but the effect may be toosevere.

Different conical transition designs requirement between ISO and API.

Code Check Comparison between ISO 19902 and API

RP 2A-WSD 21st Edition

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HydrostaticHead (ft)

API UC ISO UC ISOEquation

276 0.92 0.76 13.2-31

350 1.41 0.96 13.2-31

zj

b

opbd

B

ipbd

B

d

B j

U M M

M M

PP

U

2

MinimumCapacity

check in API

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Review of Calibration Methodology by Fred Moses et al.

to Develop API RP 2A-LRFD in the 1980s

Every designed structural member (beam, column, brace, etc.) has aprobability of failure (P f ). This P f can also be expressed as (reliability index).

Objective: Derive load and resistance factors that provide a level of safetyclose to current practice (WSD 12 th edition) for each component designcheck .

By carefully selecting load and resistance factors it is possible to achieve: An averaged similar to the average WSD

A narrow spread of

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load.theisSandresistancetheisRwhere 22

S R

S

R

V V

LN

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Review of Calibration Methodology by F. Moses to

Develop API RP 2A-LRFD in the 1980s

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Source: OTC 5699

The average for eachof the curves is similar,but the spread of theLRFD curve is smaller.

In the 1980s calibration,was between 2.01 and

2.78 for differentcomponents i.e. yield,

bending, buckling,tubular joints, etc.

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Acceptable probabilities of Failure from F. Moses Work (1988)

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)( f

P Is Guassian probability distribution function

Range of API 2A-LRFD

Range of API 2A-WSD(12 th edition, used inoriginal calibration)

Pf of 3x10 -5Proposed for L1 structures(Permanently Manned)

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Target Probabilities of Failure in ISO 19902 and API Regional

Differences. Partial action factors in ISO 19902 were derived

from F. Moses Work for the GOM. Hamonization in safety levels requireslocation-dependent partial action factors.

A target probability of failure of 3x10 -5 per year has been proposed for new,permanently manned, installations.

Fatigue damage design factors are harmonized in ISO and API

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Environment Partial ActionFactor ( f,E )

Mean RSR

Gulf of Mexico 1.58* 2.16*

Australia 1.59 2.18

North Sea 1.40 1.82

1.25and1.35 D f E f ,,

Failure Critical Inspectable Not Inspectable

No 2 5

Yes 5 10

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Points of discussion

Code check equations have evolved in recent years (i.e. tubular joint checks). Hasthis evolution changed the validity of the load and resistance factors developed byMoses et al in the 1980s? Do we need to recalibrate?

Have the wind and wave probability distributions changed (mean and COV)?Should partial action factors be revised to achieve the same performance levels?

Are code check comparisons between codes enough to validate and harmonize thestandards?

How do we reconcile the tubular joint check differences between ISO and API?

Research work is now in progress to incorporate strength provisions of the new AISC specification into offshore design practices. How do we reconcile the deckdesign approach in API 2TOP and ISO 19901-3?

Target reliabilities for offshore installations that are evacuated or unmanned duringthe design event (loss of life is negligible) have been developed by cost-benefitanalysis (incremental cost of improving safety). These analyses performed in the1980s guided updates to API. Do we need to revisit these analyses and reassesstarget reliabilities?

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References

OTC 5699, 1988, Calibration of the Draft RP2A-LRFD for Fixed Platforms, F. Moses andR.D. Larrabee.

OTC 5882, 1988, Development of a Reliability-Based Alternative to API RP2A, J.R. Lloyd,and D.I. Karsan.

OTC 23443, 2012, Alignment of API Offshore Structures Standards with ISO 19900 Seriesand Usage of the API suite, D. Wisch, A. Mangiavacchi.

OTC 17310, 2005, New API Tubular Joint Strength Design Provisions, D. Pecknold, P.Marshall and J. Bucknell.

OTC 23558, 2012, Insights into Using the 22 nd Edition of API RP 2A Recommended Practicefor Planning, Designing and Constructing Fixed Offshore Platforms Working StressDesign, K. A. Digre and F.J. Zwerneman.

Load factor calibration for ISO 13819 Regional Annex: Component Resistance, OffshoreTechnology Report, MSL Engineering Limited, 2001.

Implications for the Assessment of Existing Fixed Steel Structures of Proposed ISO 13819-2Member Strength Formulations, PAFA Consulting Engineers, August 2000.

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Load and Resistance Factors API LRFD & ISO 19902

Load factors in ISO 19902 are identical to those in API LRFD 1 st edition, except:

1.35 only applies to the GoM (L1 structures), other regions have to determinetheir own coefficient.

1.17 only applies to the GoM (L2 structures 15% loading reduction from L1).

ISO 19902 resistance factors are identical to those in API LRFD.

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