structural design of drill ships

Structural design of drill ships

Challenges and requirements

© Det Norske Veritas AS. All rights reserved.


AGENDA 09:00 Welcome and introduction

09:30 Sesam for offshore floaters

10:00 Challenges and requirements

10:30 Coffee break

10:45 Hydrodynamic analysis

11:15 Finite element modelling and analysis

12:15 Lunch

13:30 Yield and buckling strength checks

14:00 Fatigue analysis methods

14:30 Coffee break

14:45 Simplified fatigue analysis

15:15 Spectral fatigue analysis

16:00 Closing remarks

2



Typical arrangement

4

Derrick

Drill floor Riser stack

Heli-deck

Moonpool

Gantry cranes

Thrusters



Hull strength requirements

5

Derrick

Drill floor Riser stack

Heli-deck

Moonpool

Cranes

Thrusters



Challenges and high focus areas

6

Drill floor support

Moonpool corners

Crane foundation

Structural discontinuities

Presenter

Presentation Notes

Derrick and support structure Drill floor and support structure Moonpools Global strength Stress concentration and fatigue Loads Riser rack storage Deck openings Structural continuity Discontinues decks Efficiency of longitudinal members Stress concentration, local yield and fatigue Crane pedestals Foundations Computation demand Hydrodynamic analysis (direct load approach) Extensive FE analysis Spectral fatigue calculations



Hull and derrick interface

7

Effect of hull deformations



Rules and regulations for structural design of drill ships IMO MODU code

DNV-OS-C102 Structural design of offshore ships

ABS: Guide for Building and Classing of Drillships – Hull Structural Design and Analysis

8

Required analysis

• Wave load analysis • Cargo hold FE analysis • Local FE analysis for ultimate

strength and fatigue • Simplified fatigue calculations

Optional approach

• Global FE analysis • Direct load application from

wave load analysis • Spectral fatigue calculations



Analysis options and related software from DNV Software

Analysis type DNV ship rules and offshore standards

Other class (ABS, LR, …)

Rule based calculations Nauticus Hull not supported Direct load calculations Sesam HydroD Direct strength calculations, FEA Sesam GeniE Plate code check Sesam GeniE Spectral fatigue calculations Sesam HydroD + GeniE

9



Design conditions and loads – DNV-OS-C102

Design condition Load cases Load basis Wave data

Heading profile Load probability

Transit Ship rules Ship rules Direct for topside acc.

IACS North Atlantic All headings

Rule pressures 10-4 Accelerations 20 years

Drilling Max draught Min draught Direct calculations Max Hs for drilling

Specified heading profile 3 hrs short term

Survival Max draught Min draught Direct calculations North Atlantic or design limit

Specified heading profile 100 years

10

Fatigue design criteria - Minimum 20 years - World wide scatter diagram for transit condition - Site specific scatter diagram for operation (world wide for unrestricted service) - Load probability 10-4

- 80 % operation (unless specified) - 20 % transit (unless specified)



Scope of direct strength calculations – ultimate strength Hull strength

- Cargo hold analysis - Optional: Full ship analysis

Local analysis - Toe of girder bracket at typical transverse web frame - Toe and heel of horizontal stringer in way of transverse bulkhead - Opening on main deck, bottom and inner bottom, e.g. moonpool corner. - Drill floor and support structure - Topside support structure - Crane pedestal foundation and support structure - Foundations for heavy equipment such as BOP, XMAS, mud pumps, etc - …

11



Scope of direct strength calculations – fatigue strength Hull

- Openings on main deck, bottom and inner bottom structure including deck penetrations - Longitudinal stiffener end connections to transverse web frame and bulkhead - Shell plate connection to longitudinal stiffener and transverse frames with special

consideration in the splash zone. - Hopper knuckles and other relevant discontinuities - Attachments, foundations, supports etc. to main deck and bottom structure openings and

penetrations in longitudinal members.

Topside supporting structure - Attachments, foundations, supports etc. to main deck and hull - Hull connections including substructure for drill floor - Topside stool and supporting structures - Crane pedestal foundation and supporting structures.

12



13

My drillship



Main dimensions and design conditions Main dimensions

- Rule length 240 m - Breadth 43 m - Scantling draught 15 m - Block coefficient 0.89

Load conditions - Transit T=10 m - Drilling and survival T=12m

Hull girder limits - Stillwater sagging Ms -2330500 kNm - Stillwater hogging Ms 1923560 kNm

Unrestricted service - Fatigue world wide - Survival North Atlantic

Max sea state for drilling operation - Hs = t m

Heading profile - 60 % head sea - 30 % ± 15 degrees - 10 % ± 30 degrees

14



My tools – Sesam HydroD for wave load analysis

15



My tools – Nauticus Hull for rule strength calculations

16



My tools – Sesam GeniE for direct strength calculations

17



Safeguarding life, property and the environment

www.dnv.com

18

Structural design of drill ship

Hydrodynamic analysis






10:30 Coffee break



12:15 Lunch



14:30 Coffee break




2



Design conditions and loads – DNV-OS-C102

4

Fatigue - World wide scatter diagram (for unrestricted service) - Load probability 10-4

- 80 % operation - 20 % transit

Design condition Load cases Load basis Wave data

Heading profile Load probability

Transit Ship rules Ship rules Direct for topside acc.

IACS North Atlantic All headings

Rule pressures 10-4 Accelerations 20 years

Drilling Max draught Min draught Direct calculations Max Hs for drilling

Specified heading profile 3 hours short term

Survival Max draught Min draught Direct calculations North Atlantic or design limit

Specified heading profile 100 years



Scope of hydrodynamic analysis

5

Transit Drilling Survival

Scatter diagram ULS: North Atlantic Fatigue: World wide

Max specified Hs Site specific Unrestricted: North Atlantic

Wave spreading Short-crested cos2 Short-crested cos2 Long-crested

Heading profile All headings 60 % head sea 30 % ± 15 degrees 10 % ± 30 degrees

60 % head sea 30 % ± 15 degrees 10 % ± 30 degrees

Calculation scope Topside accelerations

Topside accelerations Wave bending moment

Topside accelerations Bending moment Pressures

Probability level ULS: 20 years Fatigue: 10-4

3 hrs short term Fatigue: 10-4

100 years Fatigue: 10-4



6


Sesam HydroD



7

HydroD Key features

- Hydrostatics and stability calculations - Linear and non linear hydrodynamics

Benefits - Handling of multiple loading conditions and models through one user interface and

database - Sharing models with structural analysis - Direct transfer of static and dynamic loads to structural model


FPSO Full Ship Analysis

8

Hydrodynamic Analysis

Hull shape as real ship

Correct draft and trim

Weight and buoyancy distribution according to loading manual

Mass and buoyancy in balance

Obtain correct weight and mass distribution

Balance of loading conditions

Challenges Model requirements



HydroD models Environment

- Air and water properties - Water depth - Wave directions - Wave frequencies

Hull geometry - Panel model - Morrison model

Mass distribution - Compartments - Mass model

Structural model - For load transfer

9



10

Panel model



Panel model guidelines

Mesh size - In general depending on wave length (length < L/5)

- At least 30-40 panels along the ship length - Wave period = 4s wave length = 25m panel length = 5m

- Mesh size finer - Towards still water level - Towards large transitions in shape

- Not too coarse in curved areas, in order to compute correct volume

If shallow water - Use ½ or even ¼ panel length. Test convergence!

11



Hull modelling in GeniE

Model from scratch

Import DXF

Import from Rhino – plug-in available with GeniE 6.3

12


Convert model to GeniE format

6 June 2012

Import DXF – a typical tanker

13


Convert model to GeniE format

6 June 2012

Import lines from Rhino

14

Rhino model GeniE lines

GeniE mesh GeniE surface



15

Mass model



Mass model alternatives

Alternatives - FE model (beam/shell/solid) - Point mass model - Structure model

Requirements - Vertical and transverse centre of gravity - Roll radius of gyration - Longitudinal mass distribution

Alternatives - Direct input of global mass data - Direct input of mass matrix

Requirements - Vertical and transverse centre of gravity - Transverse centre of gravity - Roll radius of gyration and inertia - Pitch radius of gyration and inertia

16

With sectional loads: No sectional loads:



Example of mass models

17

Beams with varying density Mass points

Structural model and compartments

Direct input



Verification of still water loads The mass and buoyancy forces may be verified by computing the still water forces

and moments - HydroD stability analysis (requires a license extension for stability)

When the environment, models and loading conditions are defined, a stability analysis may be run

?

18



19

Environment



Wave headings Typically 15-30 degrees interval

Head sea = 180 degrees

Short crested sea requires main headings ±90 degrees - Transit 0-360 degrees - Operation and survival 180 ± 120 degrees (120=30+90)

20



Wave frequencies Define 25-30 periods, say from 4 – 40 s

Ensure good representation of relevant responses, including peak values

21



22

Roll damping



About roll damping Roll damping is non-linear and must be linearized for a frequency domain analysis

Linearization according to probability level of design value - 20 years for transit - 100 years for survival - 10-4 for fatigue

Long and short term statistics sensitive to roll if eigenperiod if there is significant wave energy in the range of the eigen period

23

0,00

2,00

4,00

6,00

8,00

10,00

12,00

0 5 10 15 20 25 30 35 40

No damp

5 %

10 %



Roll damping options Use an external damping matrix

- General or critical

Use the roll damping model in Wadam - Requires an iteration since maximum roll angle is a parameter

- If maximum roll angle is from short term statistics, automatic iteration can be performed - If maximum roll angle is from long term statistics, manual iterations must be performed

Use the quadratic roll-damping coefficient - Typically obtained from model tests - Requires short term stochastic iteration

Use Morison elements - Tune drag coefficient to obtain correct damping

Only option 4 allows for load transfer of the roll-damping force

24



25

Load cross sections



Sectional loads

Calculating of global shear forces and bending moment distribution along vessel - Stillwater loads - Wave loads

Z-coordinate = Neutral axis of structure, not waterline (or any other position) - Sectional loads include horizontal pressure components sensitive to location of z-

coordinate

26



27

Postprocessing



Basic highlights – Postresp Plotting of response variables – RAO (HW(ω))2

Combinations of response variables

Calculating short-term response

Calculating long-term statistics

28


Transfer function Seastate Short term Response

Postresp short term

Postresp long term

Scatter diagram Long term Response



29

Statistical computations Short term statistics

- For a given duration of a sea state - Compute most probable largest response - Compute probability of exceedance - No. of zero up-crossings

- For a given response level - Compute probability of exceedance

- For a given probability of exceedance - Compute corresponding response level

- For a given duration and probability level - Compute response level - Compute probability of exceedance

Long term statistics - Assign probability to each direction - Select scatter diagram - Select spreading function - Create long-term response



30

Demo of HydroD



Topics Panel model

Mass model

Balancing


Post processing

31




www.dnv.com

32


Finite element modelling and analysis






10:30 Coffee break



12:15 Lunch



14:30 Coffee break




2



Cargo hold analysis

4

Derrick

Drill floor Riser rack

Heli-deck

Moonpool

Gantry cranes

Thrusters

Minimum extent = moonpool + one hold fwd and aft - Longer often needed due to non-regular structure

Mesh size: stiffener spacing



Local FE models Mesh size

- Local yield: 50x50, 100x100 or 200x200 - Fatigue: t x t

5

Derrick

Drill floor foundation

Moonpool corners

Crane foundation

Deck openings



Hull and derrick interface

6

Fx Fy

Fz

Fx Fy

Fz

Derrick design



Derrick loads and accelerations

Design condition

Static loads [t] Topside acceleration

Mass Hook load Riser tension av at al

Transit 2000 1.70 4.42 2.70

Drilling 2100 1500 1250 0.64 0.77 1.11

Survival 2100 1.52 2.62 2.10

7

Riser tension

Hook load (drilling string)

Inertia loads



Overview of load cases Hull strength, transverse structure

- Ship rules (transit conditions)

Hull girder longitudinal strength - Drilling: Longitudinal structure (head seas, direct) - Survival: Longitudinal structure (head seas, direct)

Topside and support structure in transit (all headings) - Head sea - Beam sea - Oblique sea

Topside and support structure in drilling and survival (heading profile) - Max longitudinal acceleration - Max transverse acceleration - Max vertical acceleration

8



Load cases – hull strength Design condition

Load basis Load case Global loads Pressure Derrick and topside

Transit Rule Rules Rules Rules Vertical forces

Drilling Direct, max Hs Max draught Min draught

Max sagging Max hogging

Static - dynamic Static + dynamic Vertical forces

Survival Direct North Atlantic

Max draught Min draught

Max sagging Max hogging

Static - dynamic Static + dynamic Vertical forces

9

Design condition

Load basis Load case Global loads Pressure (bilge)

Derrick force

Transit Rule Drilling Transit

Sag: -6 780 383 Hog: 6 221 616

180 130 Fz = 23 012

Drilling Direct, max Hs Max draught Min draught

Sag: -4 539 500 Hog: 4 132 560

90 160

Fz = 50 696 (incl. hook and riser)

Survival Direct North Atlantic

Max draught Min draught

Sag: -8 342 000 Hog: 6 842 060

60 190 Fz = 23 787

My drillship:



Load cases for topsides – Transit

Load case Max response Hull girder loads Topside loads av at al Wind

Head sea Hull deflection Sagging Ms + Mw 0.5 0.0 -r 1 Hogging Ms + Mw -0.5 0.0 +r 1

Beam sea Transverse acceleration Hogging Ms + a * Mw 1.0 1.0 -c 1 Hogging Ms + a * Mw 1.0 -1.0 -c 1

Oblique sea Longitudinal acceleration Hogging Ms + h * Mw +j 0.4 1.0 1

Transverse acceleration Sagging Ms + k * Mw +m 1.0 0.9 1 Sagging Ms + k * Mw +m -1.0 0.9 1

10

L < 100 100 < L < 200 L > 200

a 0.9 = -0.004 L + 1.3 0.5

h 0.7 = 0.002 L + 0 .5 0.9

k 0.4 = -0.003 L + 0.7 0.1

c 0.4 = -0.003 L + 0.7 0.1

j 0.2 = -0.002 L + 0.4 0

m 0.7 = -0.004 L + 1.1 0.3

r 1 = -0.004 L + 1.4 0.6



Topside interface loads – Transit

Heading Max response Hull girder loads Topside loads

Fx Fy Fz

Head sea Hull deflection Sagging -6 780 383 -3235 0 21316

Hogging 6 221 616 3235 0 17924

Beam sea Transverse acceleration Hogging 4 072 588 -539 8840 23012

Hogging 4 072 588 -539 -8840 23012

Oblique sea

Longitudinal acceleration Hogging 5 791 810 5392 3536 19620

Transverse acceleration Sagging -2 775 488 4853 8840 20638

Sagging -2 775 488 4853 -8840 20638

11



Load cases for topsides – Drilling and survival

Max response Hull girder loads Topside loads av at al Wind

Longitudinal acceleration Sagging Ms + Mw -b -c 1.0 1 Transverse acceleration Hogging Ms + Mw 0.8 1.0 -e 1 Vertical acceleration Hogging Ms + Mw 1.0 +f -g 1

12

L < 100 100 < L < 200 L > 200

b 0.5 = 0.003 L + 0.2 0.8

c 0.6 = -0.002 L + 0 .8 0.4

e 0.6 = 0.004 L + 0.2 1,0

f 0.8 = -0.005 L + 1.3 0.3

g 0.6 = 0.004 L + 0.2 1.0



Topside interface loads – Drilling and survival

Drilling Hull girder loads Topside loads

Hogging Sagging Fx Fy Fz Longitudinal acceleration

4 132 560 -4 539 500 2323 647 46499

Transverse acceleration 2323 1619 48658 Vertical acceleration 2323 486 48928

13

Survival Hull girder loads Topside loads

Hogging Sagging Fx Fy Fz Longitudinal acceleration

6 842 060 -8 342 000 4406 2203 18052

Transverse acceleration 4406 5508 23150 Vertical acceleration 4406 1652 23787



Combination of topside loads – Drilling and survival

Hull girder loads Topside loads Local loads Fx Fy Fz

Hogging

+ + -

Tank pressure Sea pressure

+ - - - + - - - -

Sagging

+ + - + - - - + - - - -

14



Final load cases for topside supports

Drilling Topside loads Local loads Fx Fy Fz

Hogging 4 132 560

2323 1619

-48928 Tank

pressure Sea pressure

2323 -1619 -2323 1619 -2323 -1619

Sagging -4 539 500

2323 1619 2323 -1619 -2323 1619 -2323 -1619

15

Survival Topside loads Local loads Fx Fy Fz

Hogging 6 842 060

4406 5508

-23287 Tank

pressure Sea pressure

4406 -5508 -4406 5508 -4406 -5508

Sagging -8 342 000

4406 5508 4406 -5508 -4406 5508 -4406 -5508



Application of loads and boundary conditions

16

Note! Target bending moment to be adjusted for applied VBM from other loads

Applied VBM = Target VBM ÷ VBM pressures ÷ VBM forces

cog

Riser tension

Hook load

Inertia loads

Global bending

Pressures



17

Cargo hold analysis

Nauticus Hull Sesam GeniE



18

Nauticus Hull Hull strength calculations according to DNV

rules and IACS common structural rules

Section Scantlings - Global and local strength rule check and

scantling calculations - Fatigue calculations of longitudinals

Rule Check XL - Suite of Excel based analysis programs for

various rule check calculations

FEA interface to Sesam GeniE - Transfer and extruding cross sections - Generation of rule loads, boundary conditions,

sets and corrosion additions to cargo hold models



Sesam GeniE Finite element program purpose-made for ship

and offshore structures - Modelling with beams and/or plates - Load application - Structural analysis - Eigenvalue analysis - Wave load analysis for slender structures - Pile and soil analysis - Code checks

19



20

Cargo hold analysis workflow

Nauticus Hull:

GeniE:

Cross section Rule loads

Extruded section Concept model



21

GeniE Concept Model

Concept Model

Compartments

Corrosion Addition

Structure Type



22

GeniE Concept Model

Concept Model

Local pressure loads

Hull Girder loads (Slicer)

GeniE



23

GeniE Concept Model

Concept Model

Mesh

Linear analysis

Capacity model for buckling analysis

GeniE



24

Local modelling

Sesam GeniE



Submodelling in GeniE Define a sub-set

Add local details

Change mesh density

Apply prescribed displacement as boundary conditions

Run Submod

Run analysis

25



Sub-modelling procedure Do first the global analysis

Then create the sub-model - With prescribed boundary conditions where geometry

is cut

Submod module: - Reads the sub-model - Reads the global analysis results file - Compares the two models and fetches displacements

from global analysis - Imposes these as prescribed displacements on the

sub-model boundaries with prescribed b.c.

Perform sub-model analysis

Check results

Submod Slide 27 November 15,

analyse

analyse

Submod

global model

sub-model

prescribed b.c.




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28


Yield and buckling strength checks






10:30 Coffee break



12:15 Lunch



14:30 Coffee break




2



Acceptance criteria

Normal stress

Shear Yield (VonMises)

Buckling

Transit, hull transverse structure 160 f1

90 f1 (one plate flange) 100 f1 (two plate flanges) 180 f1

0.85 (linear buckling)

Transit, topside support Drilling Survival

0.8 0.8 (ultimate capacity)

4

Nominal stress:

Peak stress: Mesh size Yield

(VonMises)

Transit 50x50

100x100 200 x 200

1.53 1.33 1.13

Operation and survival 50x50

100x100 200 x 200

1.70 1.48 1.25

f1 = 1 for normal steel, 1.28 for NV-32 steel, 1.39 for NV-36 steel



5

Plate code check

Sesam GeniE



6

Plate code check in GeniE

Concept Model Capacity Model

Fully integrated with the FE model and result

Automatic idealization of buckling panels



7

Buckling results Colour code presentation of Utilization Factors (UF)

Worse case – colour code presentation of the maximum UF from all load cases.



8

Generate report




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9


Fatigue analysis methods






10:30 Coffee break



12:15 Lunch



14:30 Coffee break




2



Sources for fatigue calculation methods DNV

- OS-C102 “Structural Design of Offshore Ships” - RP-C102 “Structural Design of Offshore Ships” - RP-C203 “Fatigue Strength Analysis of Offshore Steel

Structures” - RP-C206 “Fatigue Methodology of Offshore Ships” - CN 30.7 “Fatigue Assessment of Ship Structures”

ABS - “Guide for Building and Classing Floating Production

Installations” - “Guide for Fatigue Assessment for Offshore Structures” - “Guide for Spectral-Based Fatigue Analysis for Floating

Production, Storage and Offloading (FPSO) Installations” - “Guide for the Fatigue Assessment of Ship-type

Installations”

LR - “Rules and Regulations for the Classification of Offshore

Installation at a Fixed Location” - “Floating Offshore Installations Assessment of Structures” - “Fatigue Design Assessment Level 1” - “Fatigue Design Assessment Level 3”

4



Fatigue calculation methods

Simplified

Deterministic

Spectral

Time domain

5



6

Fatigue loads and stress components Global wave bending moments Hull girder stress Stress in topside supports due to global hull

deflections Stress in turret and moonpool areas due to hull

deflections

Wave pressure Shell plate local bending stress Local stiffener bending stress Secondary stiffener bending due to deflection

of main girder system Local peak stresses in knuckles due to

deflection of main girder system

Vessel motions (accelerations) Liquid pressure in tanks Stress in topside support from inertia forces Mooring and riser fastenings



Simplified fatigue

Pros - Computation demand

Cons - Handling of combined load effects

7

Weibull long term load distribution

Load cycle at a given probability level

Stress by rule formulas or FE analysis

Fatigue damage from Weibull distribution



Deterministic fatigue calculations

Pros - Non-linear load effects can be included

Cons - Uncertainties selection of representative

waves

8

H

Hi

log N Ni

Selected deterministic waves

Wave height probability distribution

FE analysis Fatigue damage by summation of part damage from each load case



Spectral fatigue calculations – full stochastic and component stochastic

Pros - “All” linear load effects and statistics

preserved through the analysis

Cons - No non-linear effects - Computation demand - Assumes narrow banded process

9

Unit waves for “all” wave headings and frequencies

FE analysis or stress component approach

Stress RAOs

Wave scatter diagram and spectrum

∑ ∑= ==

+Γ=

loadN

n

seastatesheadings

ji

mijnijnn mrpm

anD

1 1,10

0 )22(2

1

Fatigue damage by summation of part damage from each cell in the scatter diagram



Time domain fatigue calculations

Pros - Broad banded processes - Non-linear load effects

Cons - Selection of sea states - Computation demand

10

Wave statistics

Fatigue damage by rainflow counting

Time series simulation of selected sea states

FE analysis



DNV Software’s fatigue calculators

Simplified Deterministic Spectral Time domain Nauticus Hull Framework Postresp Stofat

11



12

Critical details and calculation options

Presenter

Presentation Notes

Read the text as written above.



Longitudinal bracket toe and heel

• Loads: Nauticus Hull • Stress: Nauticus Hull, GeniE • Fatigue: Nauticus Hull

Simplified

• Loads RAOs: HydroD • Stress: CN 30.7, GeniE • Fatigue: Postresp

Component stochastic

• Loads RAOs: HydroD • Stress RAOs: GeniE • Fatigue: Stofat

Full stochastic

13



Top stiffener and web frame

14

• Loads: Nauticus Hull • Stress: Nauticus Hull, GeniE • Fatigue: Nauticus Hull

Simplified


Full stochastic



Side shell plating

15

• Loads: Nauticus Hull • Stress: CN 30.7 • Fatigue: Nauticus Hull

Simplified

• Loads RAOs: HydroD • Stress: CN 30.7 • Fatigue: Postresp



Full stochastic



Deck openings and penetrations

16

• Loads: Nauticus Hull • Stress: CN 30.7 (Nauticus Hull) • Fatigue: Nauticus Hull

Simplified




Full stochastic



Topside support

17

• Loads: Nauticus Hull • Stress: CN 30.7 (Nauticus Hull) • Fatigue: Nauticus Hull

Simplified




Full stochastic



Hopper knuckle

18

• Loads: Nauticus Hull • Stress: GeniE • Fatigue: Nauticus Hull

Simplified


Full stochastic



19

Wave statistics

Presenter

Presentation Notes

Read the text as written above.



Site specific conditions

Direction Probability Head sea 60%

±15 degrees 30% ±30 degrees 10%

20

Heading profile

Scatter diagram Wave spectrum

Presenter

Presentation Notes

Scatter diagram Wave spectrum Wave spreading Wave directionality Heading profile



Site specific fe factor – draft DNV-RP-C102

Zone no. Vessel length

300m 200m 100m 1 0.79 0.88 0.92 2 0.64 0.73 0.78 3 0.95 1.00 1.00 …

…

…

…

104 0.88 0.94 0.97

22

fe factor derived as the weighted average by sailing time in each zone



Trade specific scatter diagram Combine scatter diagram by weighted summation of occurrence/probability of each

sea state by sailing time:

23

Hs Tz

5 6 1 10 20 2 30 40

Hs Tz

5 6 1 10 20 2 30 40

+2* Hs Tz

5 6 1 5*10+2*20=70 140 2 210 280

= 5*

Scatter 1

fe factor derived from wave load analysis as the ratio between the long term loads in trade specific and North Atlantic scatter diagrams

Scatter 2 Combined scatter




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24


Simplified fatigue analysis






10:30 Coffee break



12:15 Lunch



14:30 Coffee break




2



4

Simplified fatigue analysis in Nauticus Hull

Fatigue loads

∆+∆⋅∆⋅+∆

=∆lg

lgem a

bff

σσσσ

σ maxDT

ap q m

hd

n nm

nn

Nload

= + ≤=∑ν

η0

1

1Γ( )

or

Combination of global and local stresses

Rule formulation of long term stress distribution

Stress calculation

Fatigue damage calculation

Presenter

Presentation Notes

Simplified fatigue as described in CN30.7: for longitudinal and plating subjected to lateral loads and global bending.



Updates to fatigue calculations in Nauticus Hull Nov 2011 New features

- Specification of past and future operation - User defined loading conditions - Partial filling of tanks - Sailing route and mean stress reduction factor assignment to loading conditions - Re-coated at conversion - Fatigue report module

Benefits - Quick and easy prediction of remaining fatigue life - Improved decision basis inspection and repairs - Document compliance with offshore standards

5




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6


Spectral fatigue analysis






10:30 Coffee break



12:15 Lunch



14:30 Coffee break




2




09:30 Basic characteristics of drill ships


10:30 Coffee break



12:15 Lunch



14:30 Coffee break



15:45 Coffee break



3



4

Why direct load and strength calculations Rule loads are not always the truth Modern

calculation tools give more accurate loads- Ultimate strength loads- Fatigue loads- Phasing and simultaneity of different load effects

Design and strength optimizations based on analysis closer to actual operating conditions

Improved decision basis for - In-service structural integrity management- Life extension evaluation

0

500000

1000000

1500000

2000000

0 0.2 0.4 0.6 0.8 1

[kNm

]

VBM (linear)

0

50000

100000

150000

0 0.2 0.4 0.6 0.8 1

[kN]

VSF (linear)

Pressure

Rule

Direct

Time

Stre

ss

Vertical BendingMomentSea Pressure

Double Hull Bending

Total Stress



5

Direct calculated loads vs. rule loads Fatigue loads:

0.00

0.20

0.40

0.60

0.80

1.00

1.20

VerticalBending

HorizontalBending

Pressure WL Vert. Acc.

DirectDNV RuleCSR



6

Spectral vs Simplified Fatigue Analysis Comparison of fatigue damage by DNV rules and Common Scantling Rules relative

to spectral fatigue calculations:

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Bottom atB/4

Side atT/2

Side at T TrunkDeck

Comp. Stoch.DNV RuleCSR



7

Expected Fatigue Crack Frequency

0.0

10.0

20.0

30.0

40.0

50.0

60.0

0 20 40 60 80 100

Calculated Average Fatigue Life [Years]

Sim

ulat

ed C

rack

Fre

quen

cy a

fter 2

0 Ye

ars

[%]

Simplified Stochastic (Spectral)



8

Overview of fatigue methods

Wave loads

Stress calculations:

Environment

Long term rule Weibull distribution

Direct calculated loads -3D potential theory

Fatigue damage calculation:

Actual wave scatter diagram and energy spectrum

Rule formulations for accelerations, pressure and moments on 10-4

probability level

Load transfer to FE model. Complete stress transfer function.

Hotspot stress models for SCF

Rule formulations for stresses.

Rule correlations.

Based on expected largest stress among 10^4 cycles of a rule long term Weibull distribution

Based on summation of part damage from each Rayleigh distributed sea state in scatter diagram.

Simplified Spectral fatigue



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Load transfer

RAO’s•External pressure•Rel. wave elevation•Accelerations•Full load / intermediate/ ballast• ->800 complex lc

Global FE-model

Hydrodynamic model

Local model boundary conditions

Global + local FE-model

RAO’s•External pressure•Internal pressure•Accelerations•Adjusted pressure for

intermittent wetted areas

Global structural analysis

Global stress/deflectionRAO’s•Global stress/deflections•Entire global model

Deflection transfer to local model

Global deflections asboundary conditions on local model




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Local structuralanalysis

Stress extrapolation

Stress distribution foreach load case

RAO’s•Local stress/deflections

Local stress/deflections

Input•Hot spot location

Result•RAO•Principal hot spot stress

Principal hotspot stress

Principal stress

0.E+00

1.E+07

2.E+07

3.E+07

4.E+07

5.E+07

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0Wave period [ s]

0

45

90

135

180

Local stress transfer functions

Fatigue calculations

Input•Wave scatter diagram•Wave spectrum•SN-curve•Stress RAO

•=> Fatigue damage

Stress

Hot spot

Geometric stress

Geometric stress athot spot (Hot spot stress)

Notch stress

Nominal stress

Scatter diagram

SN data




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Fatigue Calculation Program - Stofat Performs stochastic (spectral) fatigue

calculation with loads from a hydrodynamic analysis using a frequency domain approach

Structures modelled by 3D shell and solid elements

Assess whether structure is likely to suffer failure due to the action of repeated loading

Assessment made by SN-curve based fatigue approach

Accumulates partial damages weighed over sea states and wave directions

POSTPROCESSING

RE

SULT

S IN

TE

RFA

CE

FIL

E

STR

UC

TUR

AL

RES

ULT

S IN

TER

FAC

E FI

LE

StofatShell/platefatigue

Stofatdatabase




www.dnv.com

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structural design of drill ships

Technology

design conditions

spectral fatigue analysis

simplified fatigue analysis

fatigue analysis methods

analysis optional

hydrodynamic analysis

analysis options

local fe analysis