API 4F Design Study: ASD‐89 to LRFD‐05

Mike Effenberger, P.ESathish Ramamoorthy, Ph.D., P.E.June, 2013;   PN 1101213

Preliminary subject to QA check

Document No. SES1101213‐PPT‐002,Rev.0

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Post Presentation Note

• This presentation was presented at the API Conference in Washington DC in the summer of 2013. Shortly before the presentation it was discovered that the SAP2000 conversion program had random errors in the conversion of the effective length values in the converted SAP2000 file. Due to the effective length errors, comparison results might have errors and would provide results that could result in incorrect conclusions. Therefore numerical results have been removed from the presentation. The prior reported errors (2012) in the element numbering system and loads applied to the models were corrected. Also all models have been converted from StruCAD to SAP2000.

• Presentation provides the format that the results will be reported.• Changes or updates to the 2012 presentation are provided in red

font. (to show why the presentation has red color for certain items)

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Contents

• API 4F Work • Background• Objectives• Design Study‐Methodology• Model Details• AISC‐05 Analysis Specifications• DnV Load Factors (New)• Rig Design Study Updated

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API 4F 

• API Funded Study – Check Effect of Load Factors for Drilling Structures• Paper – Code Conversion Issues Going From ASD to LRFD – Draft Provided 

– Comments Due June 24, 2012• First Study Meeting – 2‐22‐2012

API 4F Work Group– Mark Trevithick (T&T Engineering) – Chair– Anthony Mannering (Precision Drilling)– Marcus McCoo (NOV)– Paul Landis (Lee C Moore)

Load Factors – DnV Load Factors (Replaced ASCE7 Factors (6/2013) Models Received (last): 03/05/2012 Finish Work Date: _______

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Background

• API 4F, Specification for Drilling and Well Servicing Structures states requirements and gives recommendations for suitable steel 

structures for drilling and well‐servicing operations  API 4F specifies the steel structures to be designed in accordance with 

Allowable Stress Design/Elastic design per AISC 335‐89

• API 4F committee is interested in going from the Allowable Stress Design to the latest strength design Allowable Strength Design (ASD)/Load and Resistance Factor Design (LRFD) provision in AISC 360‐05 specification.

• Latest AISC specification is AISC 360‐10. The 2010 edition supersedes and is an update of the 2005 edition. The 2010 edition is yet to be supported by Computer Programs.  It is in draft mode in latest Beta Version.

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Background

• In the LRFD design methodology, design results are affected by the load factors selected for the individual loads (self weight, hook load, and environmental loads) For building design, load factors and load combinations are 

specified in ASCE 7, Minimum Design Loads for Buildings and Other Structures

• The load factors are generally estimated using the statistical data for the individual loads and reliability concepts Statistical data for Hook load is not readily available. Therefore, 

the load factors given in ASCE 7‐05 was initially used for the design study (first case evaluation).  The committee changed the load factors to factors based on DnV‐OS‐C101.

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Objectives

• Paper documenting the experience of other codes/specification while changing from ASD89 to ASD05 or LRFD05 methodology

• Perform design study on six structures (workover, mast, derrick & substructure) designed using the ASD and LRFD design provisions.

• Hook load was initially considered a live load with a load factor of 1.6 (ASCE 7, First Case Evaluation) Decided by the API 4F work group on 02/22/2012

• Load Factors changed by committee on 6/5/2013 using factors based on DnV Load Factors. Hook, Rotary, and Setback changed to Load Factor similar to Dead Load of 1.3.

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Methodology

• Design performed using the Allowable Stress Design/Elastic Design provisions in AISC 335‐89 (ASD89) will be the base case.

• The structures will be evaluated according to ASD89 and ASD05 / LRFD05 design provisions per AISC 360‐05 specification.

• The design results between the Elastic design (89) and Strength Design (05) will be compared for evaluation.

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Design Basis

• AISC‐1989 Specification Allowable Stress Design (ASD‐89)

• AISC‐2005 Specification Allowable Strength Design (ASD‐05) Load and Resistance Factor Design (LRFD‐05)

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Rig Analyses Being Performed

ASD-89 (StruCAD)– Without 1/3 Stress Allowable Increase in Allowable– With 1/3 Stress Allowable Increase in Allowable

ASD-89 (SAP)– Without 1/3 Stress Allowable Increase in Allowable– With 1/3 Stress Allowable Increase in Allowable

ASD-05- P-Delta – AISC – ASCE 7(1.6)

ASD-05- P-Delta – DnV (1.3)

LRFD-05 (DnV 1.3)

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Derrick and Mast Models Provided to SES for Design Study

Model Name Type

Hook Load(kip)

No of Lines

SetBack(kip)

Height (ft)

Reference Wind Speed (knots)

Elevation from Water 

Line(ft)Operating Unexpected Expected

Rig 1 Mast 441 10 xxx 92 42 70 93 102.5

Rig 2 Mast & Substructure 750 12 500 160 32 71.7 95.6 Land Rig

Rig 3 Single Derrick 1500 14 xxx 195 50 100 115 70

Rig 4 Dual Derrick25001500(Aux)

1614 

(Aux)1750 242 65.1 xxx 100 83.7

Rig 5** Workover Mast 250 6 xxx 104 25 60 75 Land Rig

Notes:Analytical models for Rig1 to Rig 4 were developed in StruCADFor Rig 5, Analytical model was developed in STAAD

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Rig 1441 kip Hook Load Mast

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Rig 2750 kip Hook Load Mast & Substructure

Y

Z

X

Z

X Y

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Rig 31500 kip Hook Load Single Derrick

X

Z

Y

Z

X Y

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Rig 42500 kip Hook Load Dual Derrick

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Rig 5250 kip Hook Load Workover Mast

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Software Details

• SAP2000 Ultimate 16 (2I (Eye) Beta) from Computers and Structures, Inc (CSI) was used for the latest design study. 

• SAP is a integrated software for structural analysis and design P‐Δ & P‐ Δ with large displacements Buckling analysis Design Codes

– ASD‐89– ASD‐05– LRFD‐05– LRFD‐10 (Later)

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StruCAD to SAP2000 Conversion

• StruCAD models were imported in SAP. The imported models were compared with the StruCAD models.

• In general, most of the model features are imported without any issues. However, there are few StruCADcommands/features that are not imported in SAP. These are manually edited in the imported model or using the SPD StruCAD option.

• SAP technical support and development team were notified about the import issues.

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StruCAD to SAP 2000 ConversionOutstanding issues

• Effective Length Factors not imported Effective length specified/used in the StruCAD model is entered 

as “Design Overwrites” in SAP model.  Conversion not fully corrected.

• Wind Loads/Area Cards are not imported Wind loads are converted to Member and Joint Loads in 

StruCAD (Using SPD file) and imported in SAP• Acceleration Loads Discrepancy in the base reactions for the acceleration loads in 

the StruCAD and SAP models. Therefore, the acceleration loads are converted to Member/joint loads in StruCAD (Using SPD file) and then imported in SAP

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StruCAD to SAP 2000 ConversionDesign issues

• Cm factor  Difference in calculating the Cm factor between StruCAD and SAP (Beta version has special provision to Ignore lateral loads)

• Brace Spacing StruCAD has option to input the brace spacing for estimating the effective length. SAP does not have an equivalent parameter to modify.

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API 4F Design Study‐Status6‐24‐2013

• Paper – Issued as Draft (Close 6-24-2012) Issue to API End of June• Rig Evaluation

All 5 Models are ready for Evaluation (StruCAD)– Converted Rig #5 to StruCAD– Removed duplicate elements– Removed plate and shell elements

Conversion Issue– Minor Axis Moment Error Issue (Fixed)– Load Card Format Issue (Fixed)– Loads generally compare between program results, still possible issues

• Section property round off• Other

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API 4F Design Study‐Status (Cont)6‐24‐2013

Analysis Issues– K Factors– Cm Issue– Brace Spacing– All 5 Rig analyses were performed

• Results under review

• DnV Load Factors (Factors selected by Committee 6-5-13)

• Next Load Factors (Pending completion of DnV Results)

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Design BasisAllowable Stress Design‐89

• The imported models were run in SAP and compared with the StruCAD results In general, the member forces are comparable to the StruCAD member forces. For some members, there are discrepancies.

• Allowable Stress Design (ASD‐89)– Static linear analysis– With 1/3rd increase in allowable stress for expected and 

unexpected load case

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Design BasisAllowable Strength Design‐05

• Allowable Strength Design (ASD‐05)– Effective length method– Second order analysis method (includes p‐Δ and p‐δ)– Without 1/3rd increase in allowable for wind load combinations– API 4F load combinations

Case Design Loading Dead Load Hook Load Rotary Load Setback Load Environmental LoadsCondition (%) (%) (%) (%) (%)

1a Operating 100 100 0 100 1001b Operating 100 TE*100% 100 100 1002 Expected 100 TE*100% 100 0 1003a Unexpected 100 TE*100% 100 100 1003b* Unexpected 100 ‐ ‐ ‐ ‐4 Erection 100 100 0 0 1005 Transportation 100 100 0 100 100

Note: Load combinations 3b, 4, & 5 for Earthquake loads, Erection, and Transportation, respectively, were not included in 

this study

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Design BasisLoad and Resistance Factor Design ‐05

• Load and Resistance Factor Design (LRFD‐05)– Effective length method– General second order analysis method (includes p‐Δ and p‐δ)– Without 1/3rd increase in allowable for wind load combinations– Hook, Rotary, & Setback loads are considered as permanent 

equipment loads with 1.3 load factors similar to DnV Load Factors

Case Design Loading Dead Load Hook Load Rotary Load Setback Load Environmental LoadsCondition (%) (%) (%) (%) (%)

1a Operating 130 130 0 130 1001b Operating 130 TE*130% 130 130 1002 Expected 130 TE*130% 130 0 1303a Unexpected 130 TE*130% 130 130 1303b Unexpected 130 ‐ ‐ ‐ ‐4 Erection 130 130 0 0 1005 Transportation 130 130 0 130 100

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Comparison of Analysis MethodsAISC‐05 Specification

Direct Analysis Method

Effective Length Method

First‐Order Analysis Method

Specification Reference Appendix 7 Section C.2.2a Section C.2.2b

Limits on Applicability No Yes Yes

Type of analysis Second‐Order Second‐Order First‐Order

Member stiffness Reduced EI and EA Nominal EI and EA Nominal EI and EA

Notional lateral load Yes Yes Yes

Column effective length K=1 Sidesway buckling analysis

K=1

Note: In AISC‐10 specification, Direct Analysis is moved to Section C. Effective length and First‐Order Analysis Methods are moved to Appendix 7

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Comparison of Analysis Methods

Direct Analysis Method

Effective Length Method

First‐Order Analysis Method

Specification Reference Appendix 7 Section C.2.2a Section C.2.2b

Limits on Applicability No Yes Yes

Type of analysis Second‐Order Second‐Order First‐Order

Member stiffness Reduced EI and EA Nominal EI and EA Nominal EI and EA

Notional lateral load Yes Yes Yes

Column effective length K=1 Sidesway buckling analysis

K=1

Not considered in this study

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Effective Length Method Details

Effective Length Method

Amplified First Order

Linear Load combinationNo P‐DeltaK=1Force effects are amplified by B1 and B2 factor to account for P‐Delta**

General Second Order

Non‐linear load combinationP‐deltaEffective length from StruCAD model (side sway buckling analysis was not performed)Notional lateral load for gravity load combinations is not included

** B1 and B2 factors account for the P‐δ and P‐Δ. For the current API 4F study, to study the effect of changes in the specification for member strength, the B1 and B2 factor of 1.0 was used. Comparison of design results between the ASD‐05 (First order) and ASD‐89 will show the effect of changes in specifications.

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StruCAD vs SAP2000Load Comparison

• Using RIG #4 for comparison Reactions

– Reactions (X,Y,Z) are generally within 1%

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StruCAD vs SAP2000 ComparisonASD 89 (Green Book)

Rig ## Elem.

RIG #1363

RIG #2 924

RIG #3 1545

RIG #4 1149

RIG #5 467

StruCADW/O 1/3SAP 2000W/O 1/3StruCADW/ 1/3SAP 2000W/ 1/3StruCADALL LDCOMB

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COMPARISON OF ANALYSIS RESULTSDnv Based Load Factors (Committee 6/5/13)

SAP2000 Analyses

RIG #1Bootstrap

Mast

RIG #2Mast with

Substructure

RIG #3Derrick

RIG #4Dual Derrick

RIG #5Guyed

Workover

ASD ‘89W/O 1/3ASD ‘89W/ 1/3ASD 05PΔ Factor 1.6

ASD 05PΔ Factor 1.3

LRFD 05

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Stability Analysis Methods Comparison

From AISC Webinar by Louis Geschwinder,“Design for Stability using the 2010 Specification”

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Contact InformationMichael (Mike) Effenberger, mike.effenberger@stress.com

Sathish Ramamoorthy, sathish.ramamoorthy@stress.com

Stress Engineering Services13800 Westfair East DriveHouston, Texas 77041

Phone: (281) 955‐2900Fax: (281) 955‐2638www.stress.com