Structural Design of Drill Ships

Structural design of drill ships
Challenges and requirements

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
 14:45 Simplified fatigue analysis
 15:15 Spectral fatigue analysis
 16:00 Closing remarks


© Det Norske Veritas AS. All rights reserved. 2

Typical arrangement

Derrick

Heli-deck
Gantry cranes

Drill floor
Riser stack

Moonpool
Thrusters



Hull strength requirements

Derrick

Heli-deck
Cranes

Drill floor
Riser stack

Moonpool
Thrusters



Challenges and high focus areas

Drill floor
support
Crane
foundation
Structural
discontinuities
Moonpool
corners



Hull and derrick interface

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

Required analysis Optional approach

• Wave load analysis • Global FE analysis
• Cargo hold FE analysis • Direct load application from
• Local FE analysis for ultimate wave load analysis
strength and fatigue • Spectral fatigue calculations
• Simplified fatigue calculations



Analysis options and related software from DNV Software
Analysis type DNV ship rules and Other class
offshore standards (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



Design conditions and loads – DNV-OS-C102

Design Wave data
Load cases Load basis Load probability
condition Heading profile
Ship rules IACS North Atlantic Rule pressures 10-4
Transit Ship rules
Direct for topside acc. All headings Accelerations 20 years
Max draught Max Hs for drilling
Drilling Direct calculations 3 hrs short term
Min draught Specified heading profile
Max draught North Atlantic or design limit
Survival Direct calculations 100 years

 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
- …



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.



My drillship



Main dimensions and design conditions
 Main dimensions  Unrestricted service
- Rule length 240 m - Fatigue world wide
- Breadth 43 m - Survival North Atlantic
- Scantling draught 15 m
 Max sea state for drilling operation
- Block coefficient 0.89
- Hs = t m
 Load conditions
 Heading profile
- Transit T=10 m
- 60 % head sea
- Drilling and survival T=12m
- 30 % ± 15 degrees
 Hull girder limits - 10 % ± 30 degrees
- Stillwater sagging Ms -2330500 kNm
- Stillwater hogging Ms 1923560 kNm



My tools – Sesam HydroD for wave load analysis



My tools – Nauticus Hull for rule strength calculations



My tools – Sesam GeniE for direct strength calculations



Safeguarding life, property
and the environment

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Structural design of drill ship
Hydrodynamic analysis

AGENDA
 12:15 Lunch



Design conditions and loads – DNV-OS-C102

Design Wave data
Load cases Load basis Load probability
condition Heading profile
Ship rules IACS North Atlantic Rule pressures 10-4
Transit Ship rules
Direct for topside acc. All headings Accelerations 20 years
Max draught Max Hs for drilling
Drilling Direct calculations 3 hours short term
Max draught North Atlantic or design limit
Survival Direct calculations 100 years

 Fatigue
- World wide scatter diagram (for unrestricted service)
- Load probability 10-4
- 80 % operation
- 20 % transit



Scope of hydrodynamic analysis
Transit Drilling Survival
Scatter diagram ULS: North Atlantic Max specified Hs Site specific
Fatigue: World wide Unrestricted: North Atlantic
Wave spreading Short-crested cos2 Short-crested cos2 Long-crested
Heading profile All headings 60 % head sea 60 % head sea
30 % ± 15 degrees 30 % ± 15 degrees
10 % ± 30 degrees 10 % ± 30 degrees
Calculation scope Topside accelerations Topside accelerations Topside accelerations
Wave bending moment Bending moment
Pressures
Probability level ULS: 20 years 3 hrs short term 100 years
Fatigue: 10-4 Fatigue: 10-4 Fatigue: 10-4




Sesam HydroD



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



Hydrodynamic Analysis
Model requirements Challenges

 Hull shape as real ship  Obtain correct weight and mass
distribution
 Correct draft and trim
 Balance of loading conditions
 Weight and buoyancy distribution
according to loading manual
 Mass and buoyancy in balance

FPSO Full Ship Analysis


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



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!



Hull modelling in GeniE

 Model from scratch
 Import DXF
 Import from Rhino – plug-in available with GeniE 6.3



Import DXF – a typical tanker

Convert model to GeniE format
6 June 2012

Import lines from Rhino

Rhino model GeniE lines

GeniE mesh
GeniE surface

Convert model to GeniE format
6 June 2012

Mass model



Mass model alternatives
With sectional loads: No sectional loads:
 Alternatives  Alternatives
- FE model (beam/shell/solid) - Direct input of global mass data
- Point mass model - Direct input of mass matrix
- Structure model

 Requirements  Requirements
- Vertical and transverse centre of gravity - Vertical and transverse centre of gravity
- Roll radius of gyration - Transverse centre of gravity
- Longitudinal mass distribution - Roll radius of gyration and inertia
- Pitch radius of gyration and inertia



Example of mass models

Direct input Beams with varying density Mass points

Structural model and compartments



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

?



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)



Wave frequencies
 Define 25-30 periods, say from 4 – 40 s
 Ensure good representation of relevant
responses, including peak values



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

12,00

10,00

8,00

No damp
6,00
5%
10 %
4,00

2,00

0,00
0 5 10 15 20 25 30 35 40



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



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



Postprocessing



Basic highlights – Postresp
 Plotting of response variables – RAO (HW(ω))2
 Combinations of response variables
 Calculating short-term response
 Calculating long-term statistics


Seastate Transfer function Short term Response
Postresp short term

Long term Response Scatter diagram
Postresp long term



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

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



Demo of HydroD



Topics
 Panel model
 Mass model
 Balancing
 Hydrodynamic analysis
 Post processing



and the environment

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Finite element modelling and analysis

AGENDA
 12:15 Lunch



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

 Mesh size: stiffener spacing

Derrick

Heli-deck
Gantry cranes

Drill floor
Riser rack

Moonpool
Thrusters



Local FE models
 Mesh size
- Local yield: 50x50, 100x100 or 200x200
- Fatigue: t x t

Derrick

Deck
openings Drill floor
foundation Crane
foundation

Moonpool
corners



Hull and derrick interface
Derrick design

Fy
Fx
Fz

Fy
Fx
Fz



Derrick loads and accelerations

Hook load (drilling string)

Inertia loads

Riser tension

Design Static loads [t] Topside acceleration
condition 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



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



Load cases – hull strength
Design Load basis Load case Global loads Pressure Derrick and topside
condition
Transit Rule Rules Rules Rules Vertical forces
Max draught Max sagging Static - dynamic
Drilling Direct, max Hs Vertical forces
Min draught Max hogging Static + dynamic
Direct Max draught Max sagging Static - dynamic
Survival Vertical forces
North Atlantic Min draught Max hogging Static + dynamic

My drillship:
Design Load basis Load case Global loads Pressure Derrick force
condition (bilge)
Drilling Sag: -6 780 383 180
Transit Rule Fz = 23 012
Transit Hog: 6 221 616 130
Max draught Sag: -4 539 500 90 Fz = 50 696
Drilling Direct, max Hs
Min draught Hog: 4 132 560 160 (incl. hook and riser)
Direct Max draught Sag: -8 342 000 60
Survival Fz = 23 787
North Atlantic Min draught Hog: 6 842 060 190



Load cases for topsides – Transit
Topside loads
Load case Max response Hull girder loads
av at al Wind
Sagging Ms + Mw 0.5 0.0 -r 1
Head sea Hull deflection
Hogging Ms + Mw -0.5 0.0 +r 1
Hogging Ms + a * Mw 1.0 1.0 -c 1
Beam sea Transverse acceleration
Hogging Ms + a * Mw 1.0 -1.0 -c 1
Longitudinal acceleration Hogging Ms + h * Mw +j 0.4 1.0 1
Oblique sea Sagging Ms + k * Mw +m 1.0 0.9 1
Transverse acceleration
Sagging Ms + k * Mw +m -1.0 0.9 1

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
Topside loads
Heading Max response Hull girder loads
Fx Fy Fz
Sagging -6 780 383 -3235 0 21316
Head sea Hull deflection
Hogging 6 221 616 3235 0 17924
Hogging 4 072 588 -539 8840 23012
Beam sea Transverse acceleration
Hogging 4 072 588 -539 -8840 23012
Longitudinal acceleration Hogging 5 791 810 5392 3536 19620
Oblique sea Sagging -2 775 488 4853 8840 20638
Transverse acceleration
Sagging -2 775 488 4853 -8840 20638



Load cases for topsides – Drilling and survival

Topside loads
Max response Hull girder 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

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
Hull girder loads Topside loads
Drilling
Hogging Sagging Fx Fy Fz
Longitudinal acceleration 2323 647 46499
Transverse acceleration 4 132 560 -4 539 500 2323 1619 48658
Vertical acceleration 2323 486 48928

Hull girder loads Topside loads
Survival
Hogging Sagging Fx Fy Fz
Longitudinal acceleration 4406 2203 18052
Transverse acceleration 6 842 060 -8 342 000 4406 5508 23150
Vertical acceleration 4406 1652 23787



Combination of topside loads – Drilling and survival

Topside loads
Hull girder loads
Fx Fy Fz Local loads
+ + -
+ - -
Hogging
- + -
- - - Tank pressure
+ + - Sea pressure
+ - -
Sagging
- + -
- - -



Final load cases for topside supports
Topside loads
Drilling
2323 1619
2323 -1619
Hogging 4 132 560
-2323 1619
Tank
-2323 -1619
-48928 pressure
2323 1619
Sea pressure
2323 -1619
Sagging -4 539 500
-2323 1619
-2323 -1619

Topside loads
Survival
4406 5508
4406 -5508
Hogging 6 842 060
-4406 5508
Tank
-4406 -5508
-23287 pressure
4406 5508
Sea pressure
4406 -5508
Sagging -8 342 000
-4406 5508
-4406 -5508


Application of loads and boundary conditions
Hook load

Inertia loads
cog

Riser tension

Global bending

Pressures

Note! Target bending moment to be adjusted for applied VBM from other loads
Applied VBM = Target VBM ÷ VBM pressures ÷ VBM forces


Cargo hold analysis

Nauticus Hull
Sesam GeniE



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



Cargo hold analysis workflow

Cross section Rule loads

Nauticus Hull:

Extruded section Concept model

GeniE:



GeniE Concept Model

Compartments

Concept Model

Corrosion Addition

Structure Type



GeniE Concept Model
GeniE

Local pressure loads

Hull Girder loads (Slicer)

Concept Model



GeniE Concept Model
GeniE

Mesh

Linear analysis

Concept Model

Capacity model for
buckling analysis



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



Sub-modelling procedure
 Do first the global analysis global model
 Then create the sub-model analyse
- With prescribed boundary conditions where geometry
is cut

 Submod module:
- Reads the sub-model
- Reads the global analysis results file Submod
- Compares the two models and fetches displacements
from global analysis
prescribed b.c.
- Imposes these as prescribed displacements on the
sub-model boundaries with prescribed b.c. sub-model
 Perform sub-model analysis
analyse
 Check results


Slide 27
© Det Norske Veritas AS. All rights reserved.
November 15,
Submod

and the environment

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Yield and buckling strength checks

Acceptance criteria
Nominal stress:
Normal Shear Yield Buckling
stress (VonMises)
Transit, hull transverse 90 f1 (one plate flange) 0.85
160 f1 180 f1
structure 100 f1 (two plate flanges) (linear buckling)
Transit, topside support
0.8
Drilling 0.8
(ultimate capacity)
Survival
f1 = 1 for normal steel, 1.28 for NV-32 steel, 1.39 for NV-36 steel

Peak stress:
Mesh size Yield
(VonMises)
50x50 1.53
Transit 100x100 1.33
200 x 200 1.13

50x50 1.70
Operation and survival 100x100 1.48
200 x 200 1.25



Plate code check

Sesam GeniE



Plate code check in GeniE
 Fully integrated with the FE model and result
 Automatic idealization of buckling panels

Concept Model Capacity Model


Buckling results
 Colour code presentation of Utilization Factors (UF)
 Worse case – colour code presentation of the maximum UF from all load cases.



Generate report



and the environment

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Fatigue analysis methods

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”



Fatigue calculation methods

Simplified

Deterministic

Spectral

Time domain


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

Weibull long term Load cycle at a given Stress by rule formulas Fatigue damage from
load distribution probability level or FE analysis Weibull distribution

 Pros  Cons
- Computation demand - Handling of combined load effects



Deterministic fatigue calculations
Fatigue damage by
Selected Wave height
FE analysis summation of part
deterministic waves probability distribution damage from each load
case

H

Hi
log N
Ni

 Pros  Cons
- Non-linear load effects can be included - Uncertainties selection of representative
waves



Spectral fatigue calculations –
full stochastic and component stochastic
Unit waves for Fatigue damage by
FE analysis Wave scatter
“all” wave summation of part
or stress diagram and
headings and Stress RAOs damage from each cell
component spectrum
frequencies in the scatter diagram
approach

headings
n  m  Nload seastates
D = 0 Γ1 +  ∑ pn ∑ rijn (2 2m0ijn ) m
a  2  n =1 i =1, j =1

 Pros  Cons
- “All” linear load effects and statistics - No non-linear effects
preserved through the analysis - Computation demand
- Assumes narrow banded process


Time domain fatigue calculations
Time series
simulation of
Wave FE analysis Fatigue damage by rainflow counting
selected sea
statistics states

 Pros  Cons
- Broad banded processes - Selection of sea states
- Non-linear load effects - Computation demand



DNV Software’s fatigue calculators

Simplified Deterministic Spectral Time domain
Nauticus Hull 
Framework   
Postresp  
Stofat 



Critical details and calculation
options



Longitudinal bracket toe and heel

Simplified

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

Component stochastic

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

Full stochastic

• Stress RAOs: GeniE
• Fatigue: Stofat



Top stiffener and web frame

Simplified

• Stress: Nauticus Hull, GeniE

Full stochastic

• Fatigue: Stofat



Side shell plating

Simplified

• Stress: CN 30.7


• Stress: CN 30.7

Full stochastic

• Fatigue: Stofat



Deck openings and penetrations

Simplified

• Stress: CN 30.7 (Nauticus Hull)


• Stress: CN 30.7

Full stochastic

• Fatigue: Stofat



Topside support

Simplified

• Stress: CN 30.7 (Nauticus Hull)


• Stress: CN 30.7

Full stochastic

• Fatigue: Stofat



Hopper knuckle

Simplified

• Stress: GeniE

Full stochastic

• Fatigue: Stofat



Wave statistics



Site specific conditions
Scatter diagram Wave spectrum

Heading profile
Direction Probability
Head sea 60%
±15 degrees 30%
±30 degrees 10%



Site specific fe factor – draft DNV-RP-C102

Vessel length
Zone no. 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
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:

Scatter 1 Scatter 2 Combined scatter
Tz Tz Tz
Hs 5 6 Hs 5 6 Hs 5 6
5* 1 10 20 +2* 1 10 20 = 1 5*10+2*20=70 140
2 30 40 2 30 40 2 210 280

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



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Simplified fatigue analysis

Simplified fatigue analysis in Nauticus Hull
Stress calculation
Fatigue loads

or

Fatigue damage Rule formulation of long Combination of global
calculation term stress distribution and local stresses

∆σ g + b ⋅ ∆σ l
ν 0 Td N load
m ∆σ = f m f e max 
D=
a
∑ pn q Γ(1 + h ) ≤ η
n =1
m
n
a ⋅ ∆σ g + ∆σ l
n



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



and the environment

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Spectral fatigue analysis

AGENDA
 09:30 Basic characteristics of drill ships
 12:15 Lunch



Why direct load and strength calculations
 Rule loads are not always the truth  Modern 2000000

calculation tools give more accurate loads 1500000

[kNm ]
- Ultimate strength loads 1000000

- Fatigue loads 500000

- Phasing and simultaneity of different load effects 0
0 0.2 0.4 0.6 0.8 1
 Design and strength optimizations based on analysis VBM (linear)
closer to actual operating conditions
150000

 Improved decision basis for 100000

[kN]
- In-service structural integrity management
50000
- Life extension evaluation
0
0 0.2 0.4 0.6 0.8 1
Vertical Bending
Moment VSF (linear)
Sea Pressure

Double Hull Bending

Total Stress


Stress

Rule

Direct 
Pressure

Time



Direct calculated loads vs. rule loads
 Fatigue loads:

1.20

1.00

0.80
Direct
0.60 DNV Rule
CSR
0.40

0.20

0.00
Vertical Horizontal Pressure WL Vert. Acc.
Bending Bending



Spectral vs Simplified Fatigue Analysis
 Comparison of fatigue damage by DNV rules and Common Scantling Rules relative
to spectral fatigue calculations:

1.20

1.00

0.80
Comp. Stoch.
0.60 DNV Rule
CSR
0.40

0.20

0.00
Bottom at Side at Side at T Trunk
B/4 T/2 Deck



Expected Fatigue Crack Frequency

Simplified Stochastic (Spectral)
60.0
Simulated Crack Frequency

50.0
after 20 Years [%]

40.0

30.0

20.0

10.0

0.0
0 20 40 60 80 100
Calculated Average Fatigue Life [Years]



Overview of fatigue methods
Environment Simplified Spectral fatigue
Actual wave scatter
Long term rule Weibull diagram and energy
distribution spectrum

Wave loads
Rule formulations for Direct calculated loads -
accelerations, pressure 3D potential theory
and moments on 10-4
probability level
Stress calculations: Rule formulations for Load transfer to FE model. Complete
stresses. stress transfer function.
Rule correlations. Hotspot stress models for SCF
Based on expected largest Based on summation of part
Fatigue damage stress among 10^4 cycles damage from each Rayleigh
calculation: of a rule long term distributed sea state in scatter
Weibull distribution diagram.


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

Global FE-model

RAO’s
•External pressure
•Internal pressure
Global + Load transfer •Accelerations
local FE-model •Adjusted pressure for
intermittent wetted areas

Global structural RAO’s
Global stress/deflection •Global stress/deflections
analysis •Entire global model

Global deflections as
Deflection transfer boundary conditions on
Local model
boundary conditions to local model local model




Stress distribution for
Local stress/deflections
each load case
Local structural
analysis RAO’s
Principal stress •Local stress/deflections
5.E+07

4.E+07

3.E+07 0
Local stress transfer
2.E+07
45
90
135
functions
1.E+07
180

0.E+00
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Wave per iod [ s]

Notch stress
Input
Stress Geometric stress at
hot spot (Hot spot stress) •Hot spot location
Geometric stress
Stress
Principal hotspot stress Result
Hot spot
Nominal stress
extrapolation •RAO
•Principal hot spot stress

Input
•Wave scatter diagram
Fatigue •Wave spectrum
Scatter diagram •SN-curve
calculations •Stress RAO

•=> Fatigue damage

SN data



Fatigue Calculation Program - Stofat
 Performs stochastic (spectral) fatigue
POSTPROCESSING
calculation with loads from a hydrodynamic
analysis using a frequency domain approach

STRUCTURAL RESULTS INTERFACE FILE
 Structures modelled by 3D shell and solid
elements Stofat
Shell/plate
 Assess whether structure is likely to suffer fatigue
failure due to the action of repeated loading

RESULTS INTERFACE FILE
 Assessment made by SN-curve based
fatigue approach
 Accumulates partial damages weighed over Stofat
sea states and wave directions database



and the environment

www.dnv.com



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