Download - Structural Design of Drill Ships
Structural design of drill ships
Challenges and requirements
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Structural design of drill ships
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
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Structural design of drill ships
Typical arrangement
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Derrick
Drill floor Riser stack
Heli-deck
Moonpool
Gantry cranes
Thrusters
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Structural design of drill ships
Hull strength requirements
5
Derrick
Drill floor Riser stack
Heli-deck
Moonpool
Cranes
Thrusters
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Structural design of drill ships
Challenges and high focus areas
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Drill floor support
Moonpool corners
Crane foundation
Structural discontinuities
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Structural design of drill ships
Hull and derrick interface
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Effect of hull deformations
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Structural design of drill ships
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
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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
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Structural design of drill ships
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
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Structural design of drill ships
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
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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)
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Structural design of drill ships
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 - …
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Structural design of drill ships
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.
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Structural design of drill ships
13
My drillship
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Structural design of drill ships
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
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Structural design of drill ships
My tools – Sesam HydroD for wave load analysis
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Structural design of drill ships
My tools – Nauticus Hull for rule strength calculations
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Structural design of drill ships
My tools – Sesam GeniE for direct strength calculations
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Structural design of drill ships
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Structural design of drill ship
Hydrodynamic analysis
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ship
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
2
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Structural design of drill ship
Design conditions and loads – DNV-OS-C102
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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
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Structural design of drill ship
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
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Structural design of drill ship
6
Hydrodynamic analysis
Sesam HydroD
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Structural design of drill ship
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
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FPSO Full Ship Analysis
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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
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Structural design of drill ship
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
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Structural design of drill ship
10
Panel model
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Structural design of drill ship
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!
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Structural design of drill ship
Hull modelling in GeniE
Model from scratch
Import DXF
Import from Rhino – plug-in available with GeniE 6.3
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Convert model to GeniE format
6 June 2012
Import DXF – a typical tanker
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Convert model to GeniE format
6 June 2012
Import lines from Rhino
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Rhino model GeniE lines
GeniE mesh GeniE surface
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Structural design of drill ship
15
Mass model
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Structural design of drill ship
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
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With sectional loads: No sectional loads:
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Structural design of drill ship
Example of mass models
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Beams with varying density Mass points
Structural model and compartments
Direct input
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Structural design of drill ship
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
?
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Structural design of drill ship
19
Environment
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Structural design of drill ship
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)
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Structural design of drill ship
Wave frequencies Define 25-30 periods, say from 4 – 40 s
Ensure good representation of relevant responses, including peak values
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Structural design of drill ship
22
Roll damping
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Structural design of drill ship
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
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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 %
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Structural design of drill ship
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
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Structural design of drill ship
25
Load cross sections
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Structural design of drill ship
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
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Structural design of drill ship
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Postprocessing
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Structural design of drill ship
Basic highlights – Postresp Plotting of response variables – RAO (HW(ω))2
Combinations of response variables
Calculating short-term response
Calculating long-term statistics
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Hydrodynamic analysis
Transfer function Seastate Short term Response
Postresp short term
Postresp long term
Scatter diagram Long term Response
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Structural design of drill ship
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
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Structural design of drill ship
30
Demo of HydroD
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Structural design of drill ship
Topics Panel model
Mass model
Balancing
Hydrodynamic analysis
Post processing
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Structural design of drill ship
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Structural design of drill ships
Finite element modelling and analysis
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
2
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Structural design of drill ships
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
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Structural design of drill ships
Local FE models Mesh size
- Local yield: 50x50, 100x100 or 200x200 - Fatigue: t x t
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Derrick
Drill floor foundation
Moonpool corners
Crane foundation
Deck openings
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Structural design of drill ships
Hull and derrick interface
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Fx Fy
Fz
Fx Fy
Fz
Derrick design
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Structural design of drill ships
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
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Riser tension
Hook load (drilling string)
Inertia loads
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Structural design of drill ships
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
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Structural design of drill ships
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
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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:
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Structural design of drill ships
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
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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
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Structural design of drill ships
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
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Structural design of drill ships
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
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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
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Structural design of drill ships
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
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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
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Structural design of drill ships
Combination of topside loads – Drilling and survival
Hull girder loads Topside loads Local loads Fx Fy Fz
Hogging
+ + -
Tank pressure Sea pressure
+ - - - + - - - -
Sagging
+ + - + - - - + - - - -
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Structural design of drill ships
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
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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
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Structural design of drill ships
Application of loads and boundary conditions
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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
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Structural design of drill ships
17
Cargo hold analysis
Nauticus Hull Sesam GeniE
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Structural design of drill ships
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
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Structural design of drill ships
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
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Structural design of drill ships
20
Cargo hold analysis workflow
Nauticus Hull:
GeniE:
Cross section Rule loads
Extruded section Concept model
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Structural design of drill ships
21
GeniE Concept Model
Concept Model
Compartments
Corrosion Addition
Structure Type
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Structural design of drill ships
22
GeniE Concept Model
Concept Model
Local pressure loads
Hull Girder loads (Slicer)
GeniE
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Structural design of drill ships
23
GeniE Concept Model
Concept Model
Mesh
Linear analysis
Capacity model for buckling analysis
GeniE
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Structural design of drill ships
24
Local modelling
Sesam GeniE
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Structural design of drill ships
Submodelling in GeniE Define a sub-set
Add local details
Change mesh density
Apply prescribed displacement as boundary conditions
Run Submod
Run analysis
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Structural design of drill ships
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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Structural design of drill ships
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Structural design of drill ships
Yield and buckling strength checks
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
2
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Structural design of drill ships
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
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Structural design of drill ships
5
Plate code check
Sesam GeniE
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Structural design of drill ships
6
Plate code check in GeniE
Concept Model Capacity Model
Fully integrated with the FE model and result
Automatic idealization of buckling panels
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Structural design of drill ships
7
Buckling results Colour code presentation of Utilization Factors (UF)
Worse case – colour code presentation of the maximum UF from all load cases.
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Structural design of drill ships
8
Generate report
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Structural design of drill ships
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Structural design of drill ships
Fatigue analysis methods
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
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Structural design of drill ships
Fatigue calculation methods
Simplified
Deterministic
Spectral
Time domain
5
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Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
DNV Software’s fatigue calculators
Simplified Deterministic Spectral Time domain Nauticus Hull Framework Postresp Stofat
11
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
12
Critical details and calculation options
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
Top stiffener and web frame
14
• Loads: Nauticus Hull • Stress: Nauticus Hull, GeniE • Fatigue: Nauticus Hull
Simplified
• Loads RAOs: HydroD • Stress RAOs: GeniE • Fatigue: Stofat
Full stochastic
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
Side shell plating
15
• Loads: Nauticus Hull • Stress: CN 30.7 • Fatigue: Nauticus Hull
Simplified
• Loads RAOs: HydroD • Stress: CN 30.7 • Fatigue: Postresp
Component stochastic
• Loads RAOs: HydroD • Stress RAOs: GeniE • Fatigue: Stofat
Full stochastic
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
Deck openings and penetrations
16
• Loads: Nauticus Hull • Stress: CN 30.7 (Nauticus Hull) • Fatigue: Nauticus Hull
Simplified
• Loads RAOs: HydroD • Stress: CN 30.7 • Fatigue: Postresp
Component stochastic
• Loads RAOs: HydroD • Stress RAOs: GeniE • Fatigue: Stofat
Full stochastic
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
Topside support
17
• Loads: Nauticus Hull • Stress: CN 30.7 (Nauticus Hull) • Fatigue: Nauticus Hull
Simplified
• Loads RAOs: HydroD • Stress: CN 30.7 • Fatigue: Postresp
Component stochastic
• Loads RAOs: HydroD • Stress RAOs: GeniE • Fatigue: Stofat
Full stochastic
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
Hopper knuckle
18
• Loads: Nauticus Hull • Stress: GeniE • Fatigue: Nauticus Hull
Simplified
• Loads RAOs: HydroD • Stress RAOs: GeniE • Fatigue: Stofat
Full stochastic
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
19
Wave statistics
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
Site specific conditions
Direction Probability Head sea 60%
±15 degrees 30% ±30 degrees 10%
20
Heading profile
Scatter diagram Wave spectrum
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
Safeguarding life, property and the environment
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24
Structural design of drill ships
Simplified fatigue analysis
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
Safeguarding life, property and the environment
www.dnv.com
6
Structural design of drill ships
Spectral fatigue analysis
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
3
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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)
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
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
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
9
Hydrodynamic analysis
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
Spectral fatigue analysis
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
10
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
Spectral fatigue analysis
© Det Norske Veritas AS. All rights reserved.
Structural design of drill ships
11
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
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Structural design of drill ships
Safeguarding life, property and the environment
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12