GUIDELINES ON SHAFTING ALIGNMENT phần 4 pps

Prediction of Hull Deflection by FEM 7.1 Objective In order to determine the effect of hull deflection on shafting alignment, it is important to predict the deflection of the engine room

Frequency (Hz)

Fig 6.6 Frequency-response function of the laser displacement sensor at a response time of 500 ms

Laser sensor Output：

1.0V/10.0mm

DC Power

Digital data recorder Input range：±5V

Tightened steel strip

Fig 6.7 Diagram of the new hull deflection measurement system

Laser beam

Fig 6.8 Sensor calibration using dial gauge

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Displacement (mm)

Fig 6.9 Sensor calibration results (a) Output voltage as the result of increasing and decreasing displacement; (b) Relationship between output voltage and displacement

(a) (b)

6.3 Example of Measurement Results

The measured results of deflections of the top plate of the double bottom in the subject VLCC are shown in Fig

6.10 and Fig 6.11 Figure 10 shows the data from shafting portion of the top plate while Fig 11 shows the

result along the engine side It becomes evident from the results that hogging will be caused when the ship

condition changes from light to deep draught These results are in very good agreement with the results

obtained by FEM analysis both quantitatively and qualitatively In addition, these relative sagging deflections

are further confirmed by the results obtained from using another measurement method shown in Fig 6.5(e)

where a laser beam is used as the reference line

Measured deflection at shafting portion

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Measuring location (mm)

Full (19.5 m) - Light (9.0 m) Full (19.5 m) - Ballast (14.0 m) Full (19.5 m) - Ballast (16.0 m) Full (19.5 m) - Light (9.0 m) Full (19.5 m) - Ballast (14.0 m) Full (19.5 m) - Ballast (16.0 m)

Measured deflection at shafting portion of double bottom tank top plate

Fig 6.10 Measured deflection of the double bottom top plate beneath the shafting

Measured deflection at engine portion

Measured deflection at engine portion of double bottom tank top plate

-0.1

0

0.1

0.2

0.3

0.4

0.5

Measuring location (mm)

Full (19.5 m) - Ballast (14.0 m) Full (19.5 m) - Ballast (16.5 m) Full (19.5 m) - Ballast (14.0 m) Full (19.5 m) - Ballast (16.5 m)

Fig 6.11 Measured deflection of the double bottom tank top plate on which the engine is seated

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7 Prediction of Hull Deflection by FEM

7.1 Objective

In order to determine the effect of hull deflection on shafting alignment, it is important to predict the deflection

of the engine room double bottom tank top due to changes in draught Therefore, the onboard measurement of hull deflection as described in Chapter 6 is necessary However, if the FE analysis can be proven to be a viable substitute for onboard measurement in predicting hull deflection, then such expensive and time-consuming onboard measurements can be avoided In addition, it will enable the shafting alignment designs to take into account hull deflections due to changes in draught at the design stage However, due consideration should also

be given to the modelling range, loads and boundary conditions, and other factors involved, because there are few examples of FE analyses conducted of the aft-hull structure including the engine room using detailed models for recently designed ships

The ClassNK Research Institute has conducted FE analysis on a VLCC oil carrier using a detailed aft-hull structure model including a detailed engine room model

7.2 FE Model

7.2.1 The Subject Ship and Modelling Range

The subject ship was a VLCC oil carrier (300,000 DWT) powered by a two-stroke, seven cylinder main diesel engine The general arrangement and double bottom plan for the engine room are shown in Fig 7.1 Two FE models, Model A and Model B, were constructed to investigate an optimum modelling range Model A comprised the aft-hull structure from the engine room foreside bulkhead Model B comprised the aft-hull structure from the foreside bulkhead (Frame (Fr.) 64) of the No 5 cargo tank The main diesel engine, accommodation space, rudder, and other equipment (auxiliary machinery and boilers, etc.) were not included

E/R

Fr 64

Fr 53

Fr 16

Fr -7

No.5 C.O.T

FRAME S.P 900 mm

48

43

39

29

34 25

20 16

Fig 7.1 General arrangement and double bottom plan of the engine room

7.2.2 FE Model

Model A and Model B were presented in Figure 7.3 The hull structure was modelled using shell elements, and the strength members (e.g longitudinal frame, longitudinal beam, transverse frame, deck beam, etc.) in the aft-hull structure from Fr 53 onwards were modelled using beam elements to reproduce their stiffness

The element size of the aft-hull structure from Fr 53 onwards was approximately equivalent to the

longitudinal frame spacing, while in the cargo hold part it was approximately equivalent to the transverse spacing The analysis code used was Visual NASTRAN for Windows 2002

Fr 64

Fr 16

Fr 53

Fr 16

x

y

z

Fig 7.2 FE models

(a) Model A (b) Model B

7.3 Loads and Boundary Conditions

7.3.1 Load Conditions

The deflection of the engine room double bottom due to changes in draught can be obtained by subtracting the deflection observed in the light loading condition from that of the full loading condition Therefore, FE analyses have to be carried out for two different load conditions having a relative difference, i.e the draught condition and fully loaded tank condition The draught and tank conditions during sea trials applied to these analyses are presented in Table 7.1 Since analyses were carried out assuming actual service conditions, the weight of the accommodation space, rudder, propeller, main diesel engine, and major equipment, shown in Table 7.2, were taken into account in the analyses The hydrostatic pressure of seawater which acts on the outer shell

of the ship and the liquid weight of the load which acts on each tank inner plate were applied to each element as pressure loads The weights of equipment were applied to each node as concentrated loads

Table 7.1 Draught and Tank Conditions

Cargo

Ballast Water

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Table 7.2 Load Conditions

7.3.2 Boundary Conditions

To establish an optimum boundary condition for predicting hull deflection, two boundary conditions were applied as the following constraint ① and constraint ② An optimum boundary condition was examined by

comparing the analysis results resulting from each condition

Each boundary condition was applied to all nodes of the front bulkhead of each FE model, as shown in Fig 7.3

- Constraint ①

This is an optimum boundary condition for cargo hold strength analysis All constraints were applied to nodes

in the front bulkhead of each model Details of the boundary condition are as follow

- Symmetry constraint was applied to all nodes in the front surface of the model

- A constraint in the X direction was applied to the node located at the intersection of the transverse bulkhead with the upper-deck centre-line

- A constraint in the Y direction was applied to nodes located at the intersection of the longitudinal bulkhead with the upper-deck, and at the intersection of the side shell with the upper-deck

- Constraint ②

- Absolute constraints were applied to all nodes in the foremost bulkhead of the model

(a) Constraint ① (b) Constraint ②

：Absolute constraint

Front of model

：X direction constraint

：Y direction constraint

：Z direction symmetry constraint

Front of model

X

Y

Ｚ

Fig 7.3 Boundary conditions

7.4 Effects of Analysis Conditions

7.4.1 Effect of Modelling Range and Boundary Conditions

To investigate the effect of the modelling range and boundary conditions, analyses using two models, model A and B, and two boundary conditions, constraints ① and ②, were performed under the light load condition and

the full load condition In addition, a comparison of the relative displacements of deflection at the top of the engine room double bottom tank obtained from each analysis condition was carried out

Deflections of each loading condition were provided based on the reference line which connected each displacement magnitude at the foremost point (Fr 44) and aftmost point (Fr 16) on the red line shown in Fig 7.4 The relative displacements of the deflection due to changes in draught could be obtained by subtracting the deflection observed in the light load condition from the deflection observed in the full load condition The relative displacement resulting from each model and each boundary condition are shown in Fig 7.5 Since the red line measuring the magnitude of the displacement was not a straight line but was a discontinuous line divided into two portions consisting of the main engine installation portion and the intermediate shaft portion, the line in the chart had discontinuous points in the middle position

48 43 39

29 34 25

20

16

FRAME S.P 900 mm

Fig 7.4 Measurement lines

0 1 2 3 4 5

Distance from aftmost bulkhead in engine room （mm）

Model A Constraint (1) Model A Constraint (2) Model B Constraint (1) Model B Constraint (2)

Fig 7.5 Relative displacements on double bottom tank top in engine room

As Fig.7.5 indicates, there is a difference in the relative displacement of up to about 1.5 mm between Constraint ① and Constraint ② in Model A This difference can be expected to have been caused by the

effect of deformation of Fr 53 which applied one of the boundary conditions The deformed shapes of Fr 53 under each boundary constraint in Model A are presented in Fig 7.6

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Fig 7.6 Deformed shape of Frame 53

X

Y

Ｚ

(a) Model A Constraint ① (b) Model A Constraint ②

As can be seen from Fig 7.6, the restrained bulkhead (Fr 53) under Constraint ① was transformed in the X

and Y directions This showed that the deflection of the double bottom might be influenced by the deformation

of Model A caused by deformation of Fr 53 Therefore, applying an absolute constraint like Constraint ② in order to disregard the deformation of Fr 53 should not be assumed to be an optimum boundary condition for Model A

Moreover, when thinking about the deformation of the entire ship, Fr 53 would be constrained in the X and Y directions by the foreside cargo hold structure Therefore, it is unlikely that Constraint ①, which has freedom

in the X and Y directions, would be an optimum boundary condition Although Constraint ① has been used

as an optimum boundary condition for the strength evaluation of the cargo hold structure, it would not necessarily be an optimum boundary condition for the aft-hull structure model having an asymmetric structure in the longitudinal direction

On the other hand, the relative displacements of Constraints ① and ② of Model B corresponded well

This means that the influence of the boundary conditions has been counteracted as a result of extending the modelling range up to the cargo hold structure and taking the distance of the double bottom between the restrained bulkhead and engine room Reproducing the deformed shape of Fr 53 in Model B to Model A's boundary conditions would make it possible to provide a simplification of the analysis model However, it will

be difficult to reproduce the deformed shape of the bulkhead completely as a boundary condition Therefore, when predicting the deflection of the engine room double bottom, it is necessary to construct not only the aft-hull structure but also the aftmost cargo hold structure Further, a reliable result can be obtained by a simplified absolute constraint in the foremost bulkhead of model

7.4.2 Effect of Load Conditions

In the above-mentioned analyses, analyses were carried out under conditions that were as close to actual service conditions as possible Therefore, the weight of the hull structure, liquid in the tanks, the accommodation space, rudder, propeller, main diesel engine, and major equipment had been taken into account in the analyses However, since only the relative amount of deflection is necessary in shafting alignment calculation, it is sufficient just to determine the respective differences in deflection between the ballast and full load conditions as the boundary conditions Therefore, unchanging loads between the light load condition and full load condition (i.e., the weight of the accommodation space, main diesel engine, and major equipment), and the weight of the rudder and the propeller which was thought to have hardly influence at all on the analysis results were omitted Analyses were carried out using only the draught and tank condition shown in Fig 7.1 The analysis results are presented in Fig 7.7 Here, the load condition which was not omitted is referred to as Load A and the load

condition which was omitted is referred to as Load B

0 1 2 3 4 5

Distance from aftmost bulkhead in engine room （mm）

Load A Load B

Fig 7.7 Relative displacements on double bottom tank top in engine room

As Fig 7.7 indicates, the analysis result for Load B, which was the simplified load condition, was almost equal to the analysis result for Load A Therefore, the load conditions shown in Table 7.2 can be omitted, and a reliable analysis result can be obtained by using only the relative differences in the respective load conditions such as the draught and tank condition

7.5 Effects of Added Stiffness of Main Engine on Predicted Deflection

7.5.1 FE Model of Main Diesel Engine

The main engine is expected to be the stiffest structure in the engine room and it is conceivable that its stiffness has some influence on deflection of the double bottom Therefore, in order to estimate the influence of main engine stiffness on double bottom deflection, an FE model of a main engine was constructed and analyses of Model B including the main engine model (hereinafter called Model B+M/E) were carried out The main engine was modelled using elements with the size of the holding down bolt spacing (about 200 mm) so as to take into consideration installation of the engine model on the exact position The bed plates of the main engine bearings were modelled using solid elements, and other structures (e.g., the engine frame, cylinder cover, etc.) were modelled using shell elements The FE model of the main engine is presented in Fig 7.8

Fig 7.8 FE model for M/E structure

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7.5.2 Integration of the Main Engine and the Hull Structure

When the main engine is actually integrated onto the engine seating surface on the double bottom, fluid resin is cast between the main engine and the seating surface The resin layer mutually transfers deformations of the main engine and double bottom while absorbing the respective deformations of each Therefore, in order to reproduce the integrated condition of the main engine accurately, solid elements with the material properties of the resin were applied to the resin layer The appearance of the main engine integration is presented in Fig 7.9

Resin M/E

Fig 7.9 Integration of M/E structure with hull in FE model using solid element for resin layer on seating surface

7.5.3 Effect on the Double Bottom

The relative displacements of Model B and Model B+M/E on the double bottom tank top in the engine room are presented in Fig 7.10 In addition, a detailed view of the relative displacement on the engine seating portion is presented in Fig 7.11

0 1 2 3 4 5

Distance from aftmost bulkhead in engine room （mm）

Model B+M/E

Fig 7.10 Relative displacements on double bottom tank top in engine room