structural design of drill ships

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

Challenges and requirements

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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

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Typical arrangement

Derrick

Drill floor Riser stack

Heli-deck

Moonpool Gantry cranes

Thrusters

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Hull strength requirements

Derrick

Drill floor Riser stack

Heli-deck

Moonpool

Cranes

Thrusters

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Challenges and high focus areas

Drill floor support

Moonpool corners

Crane foundation Structural

discontinuities

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Hull and derrick interface

Effect of hull deformations

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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

• Wave load analysis

• Cargo hold FE analysis

• Local FE analysis for ultimate

strength and fatigue

• Simplified fatigue calculations

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Analysis options and related software from DNV Software

offshore standards

Other class (ABS, LR, …)

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Design conditions and loads – DNV-OS-C102

Design

Wave data

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

North Atlantic or design limit Specified heading profile 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)

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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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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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My drillship

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Main dimensions and design conditions

- 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

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My tools – Sesam HydroD for wave load analysis

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My tools – Nauticus Hull for rule strength calculations

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My tools – Sesam GeniE for direct strength calculations

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Safeguarding life, property and the environment

www.dnv.com

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Structural design of drill ship

Hydrodynamic analysis

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AGENDA

 12:15 Lunch

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Design conditions and loads – DNV-OS-C102

Transit Ship rules Ship rules

Direct for topside acc

IACS North Atlantic All headings

Rule pressures 10 -4

Accelerations 20 years

Drilling Max draught

Max Hs for drilling Specified heading profile 3 hours short term

Survival Max draught

North Atlantic or design limit Specified heading profile 100 years

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Scope of hydrodynamic analysis

Scatter diagram ULS: North Atlantic

Fatigue: World wide

Max specified Hs Site specific

Unrestricted: North Atlantic Wave spreading Short-crested cos 2 Short-crested cos 2 Long-crested

Heading profile All headings 60 % head sea

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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Sesam HydroD

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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

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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Panel model

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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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Hull modelling in GeniE

 Model from scratch

 Import DXF

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

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Import DXF – a typical tanker

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Convert model to GeniE format

6 June 2012

Import lines from Rhino

GeniE mesh

GeniE surface

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Mass model

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Mass model alternatives

- 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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Example of mass models

Beams with varying density Mass points

Structural model and compartments Direct input

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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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Environment

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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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Wave frequencies

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

 Ensure good representation of relevant responses, including peak values

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Roll damping

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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

0,00 2,00 4,00 6,00 8,00 10,00 12,00

No damp

5 %

10 %

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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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Load cross sections

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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 coordinate

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z-Postprocessing

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Basic highlights – Postresp

 Plotting of response variables – RAO ( H W (ω)) 2

 Combinations of response variables

 Calculating short-term response

 Calculating long-term statistics

Transfer function

Postresp short term

Postresp long term

Scatter diagram Long term Response

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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

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Demo of HydroD

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Safeguarding life, property and the environment

www.dnv.com

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Finite element modelling and analysis

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AGENDA

 12:15 Lunch

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Cargo hold analysis

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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Moonpool corners

Crane foundation Deck

openings

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Hull and derrick interface

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Derrick loads and accelerations

Design

condition

Riser tension Hook load (drilling string)

Inertia loads

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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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Load cases – hull strength

Design

condition

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

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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Load cases for topsides – Transit

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Topside interface loads – Transit

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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Load cases for topsides – Drilling and survival

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Topside interface loads – Drilling and survival

Drilling Hull girder loads Topside loads

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Combination of topside loads – Drilling and survival

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Final load cases for topside supports

2323 -1619 -2323 1619 -2323 -1619

4406 -5508 -4406 5508 -4406 -5508

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Application of loads and boundary conditions

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

Applied VBM = Target VBM ÷ VBM pressures ÷ VBM forces

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Cargo hold analysis

Nauticus Hull Sesam GeniE

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 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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- Wave load analysis for slender structures

- Pile and soil analysis

- Code checks

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Cargo hold analysis workflow

Nauticus Hull:

GeniE:

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GeniE Concept Model

Concept Model

Compartments

Corrosion Addition

Structure Type

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GeniE Concept Model

Concept Model

Local pressure loads

Hull Girder loads (Slicer) GeniE

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GeniE Concept Model

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Local modelling Sesam GeniE

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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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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

analyse

analyse Submod

global model

sub-model

prescribed b.c

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