This document summarizes a webinar on integrated hydrodynamic and structural analysis using the Sesam software. It discusses: 1) An overview of Sesam and its capabilities for hydrostatic, hydrodynamic, and structural analysis of floating structures. 2) A case study analyzing the loads and stresses on an FPSO at different loading conditions, comparing results from the quasi-static and full dynamic solvers. The dynamic solver produced lower pressures, loads, and stresses. 3) Techniques for ensuring accurate load transfer between the hydrodynamic and structural models, including pressure scaling near the waterline and checking the global load balance.

1. DNV GL © 2014 SAFER, SMARTER, GREENERDNV GL © 2014
29 April 2014
Torgeir Vada
SOFTWARE
Integrated hydrodynamic and structural analysis
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DNV GL Tech Talk webinar

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About the presenter
 Name: Torgeir Vada
 Position: Product Manager for floating structures
 Background:
– PhD in Applied mathematics/Hydrodynamics from University of Oslo, 1985
– Worked in DNV since 1985, with Sesam since 1997
– Worked as developer and in various line management roles
– Member of technology leadership committee for hydrodynamics in DNV GL
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Agenda
 Introduction to the tool used in the case study: Sesam for Floaters
 Internal dynamics
o Quasi-static approach
o Full handling of internal fluid dynamics
 Case study: Analysis of an FPSO
o Loads around the waterline
o Checking load transfer quality
o Submodelling and fatigue
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Introduction to the tool used in
the case study:
Sesam for Floaters

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FE analysis
4. Global stress and
deflection & fatigue
screening
Sesam – a fully integrated analysis system
1. Stability and wave load
analysis
Wave
scatter diagram
2. Pressure loads and
accelerations
Loadtransfer
3. Structural model loads
(internal + external pressure)
Local FE analysis
5. Local stress and
deflection & fatigue

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The Sesam floating structure package
 Linear structural analysis of unlimited size
 Hydrostatic analysis including stability code checking
 Hydrodynamic analysis
 Buckling code check of plates and beams
 Global to sub-model boundary conditions
 Fatigue analysis of plates and beams
 Coupled analysis, mooring and riser design
 Marine operations

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The Sesam floating structure package – main tools
 Sesam GeniE for modelling and structural analysis
 Sesam HydroD for hydrostatics and hydrodynamics
 Sesam Manager for managing the analysis workflow
 Sesam DeepC for umbilicals, mooring and riser analysis
 Sesam Marine for marine operations
 Sesam CAESES for parametric modelling and optimization
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What can you do with Sesam HydroD?
 Model environment and prepare input data for hydrostatic and hydrodynamic
analysis
 Perform hydrostatics and stability computations (including free surface)
 Calculate still water shear and bending moment distribution
 Perform hydrodynamic computations on fixed and floating rigid bodies, with and
without forward speed
 Calculate wave load statistics and determine design loads
 Transfer hydrostatic and hydrodynamic loads to structural analysis
HydroDD1.3-04 Date: 31 May2005 15:01:34
0 50 100 150
-2-101234
GZ-Curve
HeelAngle[deg]
GZ[m]

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Why Sesam HydroD?
 Advanced modeling features
– Anchor and TLP elements simulation
– Multi-body analysis – hydrodynamic, stiffness
and damping coupling are included
 Second order motions and forces
– Mean drift force
– Quadratic transfer function (QTFs) for motions
and forces
 Non-linear time domain analysis
– Hydrostatic and Froude-Krylov pressures to
instantaneous free surface
– Exact handling of gravity and inertia according
to vessel motions
– Morison drag force considered in time domain
– 5th order Stokes wave for shallow water
– Quadratic damping coefficients

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Internal dynamics
Quasi-static approach
Full handling of internal fluid dynamics

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Quasi-static method
 The internal fluid is regarded as rigid body, no internal waves or relative motion wrt.
hull structure
 Internal free surface is accounted for with additional restoring matrix.
 Tank fluid Mass added to the total hull mass, to be balanced with buoyancy force.
 Filling fraction is defined in pre-processing.
 Reference points for each tanks shall be pre-calculated.
– “Acceleration point” (CoG of the tank fluid)
– “Zero level point” (geometry center of the internal free surface, or roof center for
a full tank)
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Full dynamic method, introduction
 The internal radiation is solved for each tank.
_ _
_ _ _
 The acceleration point is not needed anymore for calculating local pressure.
 More accurate.
 Sloshing mode to be captured (Linear).
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Only known as a global load
Computed from a distributed load

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Comparison with Molin’s experiment
 Two rectangular tanks next to each other with the same geometry.
 The fluid level are set as 19cm for both tank in case1
 The fluid level are set as 19cm for one tank and 39cm for the other in case2
 Roll motion to be investigated.
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Experiment layout
Panel model in HydroD
(filling height 19cm & 39cm)

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Comparison with Molin’s experiment, continue
 The 1st peak corresponds to the eigen period of the hull in water
 The 2nd & 3rd peaks relate to the sloshing modes of the tanks
 Smaller filling fraction, smaller sloshing frequency
 Sloshing modes captured very well. Linear effects only.
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Case 1 19cm in both tanks Case 2 19cm & 39cm

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LNG carrier study
 Dynamic pressure in compartments
Compartments included in Panel
model to calculate internal
hydrodynamics

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LNG carrier study
 Dynamic pressure in compartments
25 compartments
with 4 for liquid
cargo tanks
balancing
Automatic
balancing

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LNG carrier study
 Dynamic pressure in compartments
– Surge, heave and pitch not so affected
– Sway, yaw and roll affected both for full and half tank
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Case study: Analysis of an FPSO

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FPSO ULS and FLS analysis modelled in Sesam Manager
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Wave load computation
+ Structural analysis

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The load transfer workflow
 This is the core workflow in both ULS and FLS analysis
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Compute hydrodynamic loads
Transfer loads to FEM model
Load transfer verification
Structural analysis
HydroD
Sestra
Cutres (global model only)

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Loads transferred from HydroD to Sestra
 Hydrodynamic pressure on the outer hull
 Hydrodynamic pressure from internal fluid
 Inertia and gravity loads
 Line loads on beams (Morison’s equation)
 Nodal loads
– Anchor and TLP elements
– Pressure area elements => axial loads on beams
 Ma = F => sum of all transferred loads should (ideally) be zero
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The FPSO used in this study
Length 165.7 m
Beam 43 m
Full load condition: All cargo compartments full
All ballast compartments
empty
Mass 111,180 tonne
COG (78.6m, 0, 12.35m)
Radii of gyration (19.6m, 95m, 95m)
Draft 15.5 m
Half load condition: All compartments half filled
Mass 77,047 tonne
COG (78.6m, 0, 7.5m)
Radii of gyration (19.7m, 95m, 95m)
Draft 10.8 m
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FEM model
Compartment model

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Half filled compartments – rigid body motions
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Blue: Dynamic Red: Quasi-static
Surge Sway Heave
Roll
Pitch Yaw
Wave heading: 135°

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Full compartments – rigid body motions
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Blue: Dynamic Red: Quasi-static
Surge Sway Heave
Roll Pitch Yaw
Wave heading: 135°

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Half-filled compartments – pressure distribution
on foremost bulkhead
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Wave period = 10s
Wave heading: 135°
• Significantly lower
pressures in dynamic
solution
• Zero pressure at
waterline in dynamic
solution

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Full compartments – pressure distribution on
foremost bulkhead
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Wave period = 10s
Wave heading: 135°
• Up to 10% lower
pressures in dynamic
solution
• In general quite similar

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Half-filled – MVonMises stress distribution on
foremost bulkhead
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Wave period = 10s
Wave heading: 135°
• 20% lower maximum
stresses in dynamic solver
• Lower stress level in most of
the bulkhead in dynamic
solver

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Half-filled – stresses on all bulkheads
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Wave period = 10s
Wave heading: 135°
• 30% lower maximum
stresses in dynamic
solver
• Lower stress level in
most of the bulkheads
in dynamic solver

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Full load – MVonMises stress distribution on
foremost bulkhead
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Wave period = 10s
Wave heading: 135°
• 10% lower maximum
stresses in dynamic solver
• Difference within a few per
cent on most of the
bulkhead
• Much smaller difference on
stresses than on pressures

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Full load – stresses on all bulkheads
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Wave period = 10s
Wave heading: 135°
• In general very small
differences

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Loads around the waterline

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Pressure scaling for waterline elements
 Retain correct pressure, but get incorrect force
– Constant pressure centroid
Or
 Scale pressure at waterline elements to get correct force
– Area adjusted
 padjust = A1/A2 x poriginal
– where A1 is the wet area of the element
and A2 is the total area of the element
– This is applied whether or not the centroid is below the free surface
A2
A1

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Pressure reduction zone
 Modify pressure in the area +/- A around the free surface
level to account for the presence of water above the mean
waterline and absence of water below. Default: A=0.0 (i.e.
no change)
 DNV class note 30.7:
A
zAz
r wl
p
2

Reduction factor when |z-zwl| < A

34. Example
Pressure reduction zone

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User defined pressure reduction region
User defined wall-sided part
 Apply a user defined pressure reduction region on a selected part of the vessel
– The method is only recommended on the part of the vessel which is wall-sided
and should thus be controlled by the user

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Checking load transfer quality

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Check report from hydrodynamic analysis
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This number should ideally be 0

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Reaction forces – global load balance
 These are quite small in all four analyses indicating good global balance in the load
transfer
– Example below is for half filled condition with dynamic solver
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Half-filled – sectional load comparison – load distribution
consistency – internal dynamics
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Fx Fy Fz
Mx My Mz
Blue: HydroD - load integration Red: Cutres – stress integration

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Recommendations for load transfer to FEM model
 Avoid loads with unknown distribution
– E.g. additional damping or restoring matrices
 Use FEM model as mass model for the hydrodynamic analysis
– To obtain identical mass matrices
 Avoid putting the fixed nodes close to “interesting” parts of the structure
– There will always be some imbalance which will create artifical reaction forces
at these nodes
 Convergence of local loads may require a finer mesh than convergence of global
responses
– Use fine mesh in areas with large curvature
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Submodelling and fatigue

42. Fatigue analysis workflow and Submod properties
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Global model and sub-model
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Sub-modelling procedure
 Do first the global analysis in Sesam Sestra
 Then create the sub-model in e.g. Sesam GeniE
– With prescribed displacement boundary
conditions where geometry is cut
 Submod:
– 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 the structural analysis for the sub-model,
this is a standard Sesam Sestra analysis
 It is important to perform load transfer from
Sesam HydroD to the local model since the loads
must be the same as on the global model.
Slide 53
analyseanalyse
analyseanalyse
SubmodSubmod
global model
sub-model
prescribed b.c.

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Sesam HydroD to compute local wave loads
 Rerunning Sesam HydroD for the sub-
model is easy:
– Panel and mass models are typically
the same as for the global model
– Wave periods and directions must be
the same as for the global model
– The basic hydrodynamic results from
the global analysis can be reused so
the local analysis is much faster
– Structural model in Sesam HydroD:
Simply replace global model with sub-
model
– Pressure loads for panels outside the
sub-model are discarded
– Also needed if there are no wet
surfaces since the inertia and gravity
loads will still apply
Slide 54
Global model
Sub-model
Panel model

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Typical workflow – Local analysis
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Local structural
analysis
Stress
extrapolation
Stress distribution for
each 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 40.0
Wave 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 at
hot spot (Hot spot stress)
Notch stress
Nominal stress
Scatter diagram
SN data

47. DNV GL © 2014
SAFER, SMARTER, GREENER
www.dnvgl.com
Thank you!
software@dnvgl.com
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