Contents
Introduction to DSA Calculation of Target values of BMs BM achieved due to local loads (MV_FEM) Calculation of incremental moments at each frame Comparison of all three BMs Present method of BM correction Application of Incremental Moment at each frame Method 1: Incremental moment applied as equally distributed Moment (My) Method 2: Incremental moment applied as bending force across ship section (Fx) Validation of Incremental moment applied Comparison of two validation check Conclusion
Introduction to DSA
Direct Strength Assessment consists of:
Creation of 3D FE model
Computation and application of local loads to FE model
Correction of global BMs
Analyses
Yielding and buckling check against various criteria
Modifications of scantlings if necessary
Internal and external local loads (for sagging) applied on the Midpart of NOPV
Introduction to DSA
Fr. 37 Fr. 152
Mv (Target) is obtained as:
Mv = Msw + LCF.Mwv
While Msw is given by client, vertical wave bending moment (Mwv) is calculated
by Rule formula.
Calculation of Target values of BMs
At remaining places Kwm was obtained by the linear interpolation according to the graph:
• Cw is calculated by:
Calculation of Target values of BMs
• Target Vertical bending moment (MV_Target) was determined by rule formula to obtain the distribution as:
Calculation of Target values of BMs
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0
Target BM
• First mid part of hull was being solved after applying all internal and external local loads.
• Then Local Vertical bending moment (MV_FEM) at each frames was obtained by running macro
• The plot of MV_FEM is shown:
VBM achieved due to local loads
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Local BM
• Incremental correction (MV_T_INCR = δM) is to be applied at each web frame. • These corrections may be equally distributed on the nodes on the longitudinal members of that section. This incremental moment can be obtained by: ΔMi = Mi (target)- Mi (local/FEM) , for i= 2 to n-1 δMi = ΔMi+1 – ΔMi
(Ref: Naval Rules (Part 6,ch. 1, section 2.6))
Calculation of incremental moments at each frame
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Incremental BM
Comparison of all three BMs
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M_Local M_Target m_incremental
• At present the moment correction is done by taking the difference of target and local BM at midship and then balance moment is applied at the ends of the model
• Though midship is subjected to maximum load and is prone to failure but still moments achieved at others frames are very much different from target values (as shown in figures)
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Target Local
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Local target
Present method of BM correction
• As observed in the earlier case, there was much difference in Local Bending Moment (BM) and target BM at frames other than midship.• This can be overcome by the application of incremental moment at each frames as was discussed earlier.There are two methods of applying this incremental bending moment Method 1: Incremental moment applied as equally distributed Moment
(My) [ According to Naval Rule] Method 2: Incremental moment applied as bending force across ship
section (Fx) [ According to HSCR]
Application of Incremental Moment at each frame
Method 1: Incremental moment applied as equally distributed Moment (My)
• Only midpart was considered for the application of the moment.
• At ends (at Fr. 37 and Fr. 152) moments was applied at the master node• For rest of the frames, moments were equally applied on all the nodes of the
longitudinal member of that frame through a Macro.• Main inputs in the macro were: Incremental Moments to be applied at each
frames and numbers of total frames in between.
How the moment were applied at each frame (taken at frame no: 114)
Method 1: Incremental moment applied as equally distributed Moment (My)
Similarly moments were applied on all the frames:(observe that net moment is zero on the hull)
Method 1: Incremental moment applied as equally distributed Moment (My)
How the moment were applied on main deck:
Method 1: Incremental moment applied as equally distributed Moment (My)
Results: Axial Stress on the outerhull
Method 1: Incremental moment applied as equally distributed Moment (My)
Axial Stress on Hull midship:
Method 1: Incremental moment applied as equally distributed Moment (My)
Stress development on main deck
Method 1: Incremental moment applied as equally distributed Moment (My)
An enlarged view of deck stress
Method 1: Incremental moment applied as equally distributed Moment (My)
• Only midpart is considered• Moment obtained was used to get axial force at each frame by some
empirical formula.• Then loads were applied as axial force on all the longitudinal members
attached to the nodes of that frame through a Macro• Axial force at the ends (aft & fwd) was applied just like other frames (i.e at
all nodes and attached element of longitudinal members at that frame) • Inputs were: Incremental Bending Moment at each frame Moment of area ( Iyy ) of each frame Neutral Axis (Zna) of each frame
Method 2: Incremental moment applied as bending force across ship section (Fx)
Values of Iyy and Zna are given at following locations:
For current calculation, Iyy and NA was obtained by interpolation of the valuesgiven at few frames (total of 7) as shown above
FramesIyy(mm4) Zna(mm) Mincremental
(N-mm)
Fr no 19 3.31E+12 6338 -
Fr no 46 3.77E+12 4932 -3197459478
Fr no 76 5.7E+12 5099 -1332301997
Fr no 98 6.33E+12 4911.3 1196537146
Fr no 116 4.83E+12 5020.6 4874403948
Fr no 140 4.03E+12 5256 2397567373
Fr no 160 2.98E+12 5780 -
Method 2: Incremental moment applied as bending force across ship section (Fx)
How the axial force were applied at each frame (taken at frame no: 114)
Method 2: Incremental moment applied as bending force across ship section (Fx)
Similarly bending forces were applied on all the frames:
[Note that axial force at aft and fwd ends has very high magnitude as compared to other frames. Its due to large incremental moments at the ends]
Method 2: Incremental moment applied as bending force across ship section (Fx)
How the bending forces were applied on main deck:
Method 2: Incremental moment applied as bending force across ship section (Fx)
Results:Axial Stress on the outerhull
Method 2: Incremental moment applied as bending force across ship section (Fx)
Axial Stress on Hull midship:
Method 2: Incremental moment applied as bending force across ship section (Fx)
Stress development on main deck
Method 2: Incremental moment applied as bending force across ship section (Fx)
An enlarged view of deck stress
Method 2: Incremental moment applied as bending force across ship section (Fx)
• Validation check were carried out at five locations along the midpart (at frame no. 60,84,104,120,148)
• To carry out the validation, simple support was removed from aft end (frame no. 37) and reaction obtained from earlier analysis were applied.
• Rigid plane and master node was created at the required frame position and it was constrained to all degree of freedom like shown
Validation of Incremental moment applied
• Then it was solved to get the reaction moment (My) at master node created and then compared it with the target moment to be achieved at that frame.
• An acceptable result with both method was obtained as moment were within the error limit.
Validation of Incremental moment applied
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0Local Target Validate
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0target local validation
Incremental BM as MyIncremental BM as Fx
The comparison of two analysis and its validation is shown below
Comparison of two validation check done
Validation done on
Target Moment
Incremental Moment as My Error %
Incremental Moment as Fx Error %
Frame 60 -1.00574E+11 -9.66E+10 3.98 -9.73E+10 3.25
Frame 84 -1.34496E+11 -1.31E+11 2.79 -1.26E+11 6.32
Frame 104 -1.35381E+11 -1.35E+11 0.11 -1.32E+11 2.50
Frame 120 -1.34962E+11 -1.35E+11 0.13 -1.37E+11 -1.51
Frame 148 -84943368462 -8.41E+10 0.95 -9.09E+10 -7.05
• Methodology for BM correction at each and every frame during DSA of naval vessels is developed and validated.
• Two methods of application of incremental moments (as per IRS naval rule and HCSR) has been studied and results are compared
Conclusion
Thank you
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