Direct strength analysis of ship hull

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:


• Target Vertical bending moment (MV_Target) was determined by rule formula to obtain the distribution as:


0 10 20 30 40 50 60 70

-160000000000

-140000000000

-120000000000

-100000000000

-80000000000

-60000000000

-40000000000

-20000000000

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

0 10 20 30 40 50 60 70

-160000000000

-140000000000

-120000000000

-100000000000

-80000000000

-60000000000

-40000000000

-20000000000

0

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

0 10 20 30 40 50 60 70

-80000000000

-60000000000

-40000000000

-20000000000

0

20000000000

40000000000

60000000000

Incremental BM

Comparison of all three BMs

0 10 20 30 40 50 60 70

-200000000000

-150000000000

-100000000000

-50000000000

0

50000000000

100000000000

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)

0 10 20 30 40 50 60 70

-160000000000

-140000000000

-120000000000

-100000000000

-80000000000

-60000000000

-40000000000

-20000000000

0

20000000000

Target Local

0 10 20 30 40 50 60 70

-160000000000

-140000000000

-120000000000

-100000000000

-80000000000

-60000000000

-40000000000

-20000000000

0

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)


Similarly moments were applied on all the frames:(observe that net moment is zero on the hull)


How the moment were applied on main deck:


Results: Axial Stress on the outerhull


Axial Stress on Hull midship:


Stress development on main deck


An enlarged view of deck stress


• 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 -


How the axial force were applied at each frame (taken at frame no: 114)


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]


How the bending forces were applied on main deck:


Results:Axial Stress on the outerhull


Axial Stress on Hull midship:


Stress development on main deck


An enlarged view of deck stress


• 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

0 10 20 30 40 50 60 70

-160000000000

-140000000000

-120000000000

-100000000000

-80000000000

-60000000000

-40000000000

-20000000000

0Local Target Validate

0 10 20 30 40 50 60 70

-160000000000

-140000000000

-120000000000

-100000000000

-80000000000

-60000000000

-40000000000

-20000000000

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

Direct strength analysis of ship hull

Engineering

Transcript of Direct strength analysis of ship hull