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Structural Design of Drill Ships
- 2. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 2
- 3. Typical arrangement
Derrick
Heli-deck
Gantry cranes
Drill floor
Riser stack
Moonpool
Thrusters
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 4
- 4. Hull strength requirements
Derrick
Heli-deck
Cranes
Drill floor
Riser stack
Moonpool
Thrusters
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 5
- 5. Challenges and high focus areas
Drill floor
support
Crane
foundation
Structural
discontinuities
Moonpool
corners
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 6
- 6. Hull and derrick interface
Effect of hull deformations
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 7
- 7. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 8
- 8. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 9
- 9. 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
Min draught Specified heading profile
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)
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 10
- 10. 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
- …
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 11
- 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.
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 12
- 13. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 14
- 14. My tools – Sesam HydroD for wave load analysis
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 15
- 15. My tools – Nauticus Hull for rule strength calculations
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 16
- 16. My tools – Sesam GeniE for direct strength calculations
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 17
- 17. Safeguarding life, property
and the environment
www.dnv.com
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 18
- 19. 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
11:30 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 2
- 20. 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
Min draught Specified heading profile
Max draught North Atlantic or design limit
Survival Direct calculations 100 years
Min draught Specified heading profile
Fatigue
- World wide scatter diagram (for unrestricted service)
- Load probability 10-4
- 80 % operation
- 20 % transit
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 4
- 21. 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 5
- 22. Hydrodynamic analysis
Sesam HydroD
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 6
- 23. 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 7
- 24. 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
© Det Norske Veritas AS. All rights reserved. 8
- 25. 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 9
- 27. 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!
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 11
- 28. Hull modelling in GeniE
Model from scratch
Import DXF
Import from Rhino – plug-in available with GeniE 6.3
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 12
- 29. Import DXF – a typical tanker
Convert model to GeniE format
6 June 2012
© Det Norske Veritas AS. All rights reserved. 13
- 30. Import lines from Rhino
Rhino model GeniE lines
GeniE mesh
GeniE surface
Convert model to GeniE format
6 June 2012
© Det Norske Veritas AS. All rights reserved. 14
- 32. 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 16
- 33. Example of mass models
Direct input Beams with varying density Mass points
Structural model and compartments
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 17
- 34. 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
?
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 18
- 36. 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)
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 20
- 37. Wave frequencies
Define 25-30 periods, say from 4 – 40 s
Ensure good representation of relevant
responses, including peak values
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 21
- 39. 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 23
- 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 24
- 42. 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 26
- 44. Basic highlights – Postresp
Plotting of response variables – RAO (HW(ω))2
Combinations of response variables
Calculating short-term response
Calculating long-term statistics
Hydrodynamic analysis
Seastate Transfer function Short term Response
Postresp short term
Long term Response Scatter diagram
Postresp long term
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 28
- 45. 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
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 29
- 47. Topics
Panel model
Mass model
Balancing
Hydrodynamic analysis
Post processing
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 31
- 48. Safeguarding life, property
and the environment
www.dnv.com
Structural design of drill ship
© Det Norske Veritas AS. All rights reserved. 32
- 50. 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
11:30 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 2
- 51. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 4
- 52. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 5
- 53. Hull and derrick interface
Derrick design
Fy
Fx
Fz
Fy
Fx
Fz
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 6
- 54. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 7
- 55. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 8
- 56. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 9
- 57. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 10
- 58. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 11
- 59. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 12
- 60. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 13
- 61. Combination of topside loads – Drilling and survival
Topside loads
Hull girder loads
Fx Fy Fz Local loads
+ + -
+ - -
Hogging
- + -
- - - Tank pressure
+ + - Sea pressure
+ - -
Sagging
- + -
- - -
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 14
- 62. Final load cases for topside supports
Topside loads
Drilling
Fx Fy Fz Local loads
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
Fx Fy Fz Local loads
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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 15
- 63. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 16
- 64. Cargo hold analysis
Nauticus Hull
Sesam GeniE
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 17
- 65. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 18
- 66. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 19
- 67. Cargo hold analysis workflow
Cross section Rule loads
Nauticus Hull:
Extruded section Concept model
GeniE:
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 20
- 68. GeniE Concept Model
Compartments
Concept Model
Corrosion Addition
Structure Type
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 21
- 69. GeniE Concept Model
GeniE
Local pressure loads
Hull Girder loads (Slicer)
Concept Model
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 22
- 70. GeniE Concept Model
GeniE
Mesh
Linear analysis
Concept Model
Capacity model for
buckling analysis
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 23
- 71. Local modelling
Sesam GeniE
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 24
- 72. Submodelling in GeniE
Define a sub-set
Add local details
Change mesh density
Apply prescribed displacement as
boundary conditions
Run Submod
Run analysis
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 25
- 73. 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
Structural design of drill ships
Slide 27
© Det Norske Veritas AS. All rights reserved.
November 15,
Submod
- 74. Safeguarding life, property
and the environment
www.dnv.com
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 28
- 76. 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
11:30 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 2
- 77. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 4
- 78. Plate code check
Sesam GeniE
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 5
- 79. Plate code check in GeniE
Fully integrated with the FE model and result
Automatic idealization of buckling panels
Concept Model Capacity Model
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 6
- 80. Buckling results
Colour code presentation of Utilization Factors (UF)
Worse case – colour code presentation of the maximum UF from all load cases.
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 7
- 82. Safeguarding life, property
and the environment
www.dnv.com
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 9
- 84. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 2
- 85. 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”
Structural design of drill ships
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- 86. Fatigue calculation methods
Simplified
Deterministic
Spectral
Time domain
Structural design of drill ships
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- 87. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 6
- 88. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 7
- 89. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 8
- 90. 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 Γ1 + ∑ 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 9
- 91. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 10
- 92. DNV Software’s fatigue calculators
Simplified Deterministic Spectral Time domain
Nauticus Hull
Framework
Postresp
Stofat
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 11
- 93. Critical details and calculation
options
Structural design of drill ships
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- 94. 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
• Loads RAOs: HydroD
• Stress RAOs: GeniE
• Fatigue: Stofat
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 13
- 95. Top stiffener and web frame
Simplified
• Loads: Nauticus Hull
• Stress: Nauticus Hull, GeniE
• Fatigue: Nauticus Hull
Full stochastic
• Loads RAOs: HydroD
• Stress RAOs: GeniE
• Fatigue: Stofat
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 14
- 96. Side shell plating
Simplified
• Loads: Nauticus Hull
• Stress: CN 30.7
• Fatigue: Nauticus Hull
Component stochastic
• Loads RAOs: HydroD
• Stress: CN 30.7
• Fatigue: Postresp
Full stochastic
• Loads RAOs: HydroD
• Stress RAOs: GeniE
• Fatigue: Stofat
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 15
- 97. Deck openings and penetrations
Simplified
• Loads: Nauticus Hull
• Stress: CN 30.7 (Nauticus Hull)
• Fatigue: Nauticus Hull
Component stochastic
• Loads RAOs: HydroD
• Stress: CN 30.7
• Fatigue: Postresp
Full stochastic
• Loads RAOs: HydroD
• Stress RAOs: GeniE
• Fatigue: Stofat
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 16
- 98. Topside support
Simplified
• Loads: Nauticus Hull
• Stress: CN 30.7 (Nauticus Hull)
• Fatigue: Nauticus Hull
Component stochastic
• Loads RAOs: HydroD
• Stress: CN 30.7
• Fatigue: Postresp
Full stochastic
• Loads RAOs: HydroD
• Stress RAOs: GeniE
• Fatigue: Stofat
Structural design of drill ships
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- 99. Hopper knuckle
Simplified
• Loads: Nauticus Hull
• Stress: GeniE
• Fatigue: Nauticus Hull
Full stochastic
• Loads RAOs: HydroD
• Stress RAOs: GeniE
• Fatigue: Stofat
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 18
- 101. Site specific conditions
Scatter diagram Wave spectrum
Heading profile
Direction Probability
Head sea 60%
±15 degrees 30%
±30 degrees 10%
Structural design of drill ships
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- 102. 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
Structural design of drill ships
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- 103. 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
Structural design of drill ships
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- 104. Safeguarding life, property
and the environment
www.dnv.com
Structural design of drill ships
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- 106. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 2
- 107. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 4
- 108. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 5
- 109. Safeguarding life, property
and the environment
www.dnv.com
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 6
- 111. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 2
- 112. AGENDA
09:00 Welcome and introduction
09:30 Basic characteristics of drill ships
10:00 Sesam for offshore floaters
10:30 Coffee break
10:45 Challenges and requirements
11:15 Hydrodynamic analysis
12:15 Lunch
13:30 Finite element modelling and analysis
14:00 Yield and buckling strength checks
14:30 Coffee break
14:45 Fatigue analysis methods
15:15 Simplified fatigue analysis
15:45 Coffee break
16:00 Spectral fatigue analysis
16:30 Closing remarks
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 3
- 113. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 4
- 114. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 5
- 115. 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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 6
- 116. 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]
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 7
- 117. 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.
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 8
- 118. Spectral fatigue analysis
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
Structural design of drill ships
© Det Norske Veritas AS. All rights reserved. 9
- 119. Spectral fatigue analysis
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
Structural design of drill ships
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- 120. 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
Structural design of drill ships
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- 121. Safeguarding life, property
and the environment
www.dnv.com
Structural design of drill ships
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