Section
6
Hull Girder Deflections
Hull deflection measurements can be conducted by investigating the bearing offset change from one vessel condition to another. For such a task, a strain gauge measurement combined with either the crankshaft deflection measurements or the M/E bearing reaction measurements should be applied. It would also be possible to consider M/E bedplate deflection measurements combined with the strain gauges if the accuracy of the readings can be trusted. Strain gauge method is convenient because of its consistent accuracy, and the error initially introduced will be constant throughout the repeated measurements. This is important information as the primary interest is normally in investigating the change in hull deflection from one state to another (dry dock condition vs. different afloat condition – Section 6, Figure 8), and by doing so, the constant error will be eliminated. Other methods like jack-up, optical, laser and piano wire do not have this advantage of error control.
FIGURE 8 Vessel Deflections Change with Loading Condition
Dry dock deflections
Ballast – still water deflections
Laden – still water deflections
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n o i t u 2 l E o L S B l A a m T i t p O
7
s n o i t u l o s y r o t c a f s i t a s 0 1 f o l o o p a m o r f s n o i t u l o s d e t c e l e s : s t l u s e r n o i t a z i m i t p O
Alignment Optimization
m h t i r o g l A c i t e n e G h t i w n o i t a z i m i t p O
e n . ] i g m 0 0 0 0 4 8 0 0 8 4 0 g a m 0 0 0 0 0 0 1 1 0 0 0 n S [ 0 0 0 0 0 0 0 0 0 0 0 E . . . . . . . . . . . 0 0 0 0 0 0 0 0 0 0 0 - - - - - l | | | | | | | | | | | a t m e ] 0 0 0 0 0 0 0 0 0 0 0 r s m 0 0 0 5 5 5 5 5 5 5 5 e f m 0 0 0 1 1 1 1 1 1 1 1 h f [ - . . . . . . . . . . . T O 0 0 0 0 0 0 0 0 0 0 0 l . | | | | | | | | | | | l t u c ] 0 0 0 0 0 0 0 0 0 0 0 H e m 0 5 7 2 0 8 6 4 2 1 0 l m 0 0 0 1 1 0 0 0 0 0 0 n f [ - . . . . . . . . . . . i e 0 0 0 0 0 0 0 0 0 0 0 M D - - - - - - - - l . | | | | | | | | | | | l t u c 0 0 0 0 0 0 0 0 0 0 0 H e ] 0 0 0 0 0 0 0 0 0 0 0 l m 0 5 7 2 0 8 6 4 2 1 0 x f m - . . . . . . . . . . . a e [ 0 0 0 1 1 0 0 0 0 0 0 M D | | | | | | | | | | | d e 0 9 3 4 3 2 2 2 2 2 8 n ] 0 7 9 8 0 1 2 3 4 5 5 A i y m 0 4 1 8 0 0 0 0 0 0 0 G f d m - . . . . . . . . . . . e [ 0 3 6 6 7 7 7 7 7 7 7 d | | | | | | | | | | | t l e . ] 0 9 3 4 9 4 2 2 4 8 8 a s n m 0 2 2 1 4 7 0 3 6 8 0 t f i m 0 4 1 9 0 0 1 1 1 1 2 o f M [ - . . . . . . . . . . . T O 0 3 6 6 7 7 7 7 7 7 7 | | | | | | | | | | | t l e . ] 0 9 3 4 9 4 2 2 4 8 8 a s x m 0 7 9 3 4 5 6 7 8 9 0 t f a m 0 9 8 2 1 9 7 5 3 2 2 o f M [ - . . . . . . . . . . . T O 0 3 6 8 8 7 7 7 7 7 7 | | | | | | | | | | | | | | | 1 7 3 5 1 3 4 6 8 5 2 1 2 7 8 9 9 0 8 8 0 2 ) ] 4 9 8 7 6 8 7 9 1 5 8 y y N - . . . . . . . . . . . R d k 4 5 7 4 2 3 9 2 4 8 4 ( [ 4 4 2 2 7 5 9 7 6 2 9 5 1 2 1 2 2 2 3 ) s 6 2 0 3 2 3 3 0 5 6 6 f 9 7 8 3 2 2 6 6 9 0 3 f ] 9 0 7 9 5 9 2 9 9 7 6 0 y O N - . . . . . . . . . . . 0 R . k 4 6 4 2 7 2 1 5 4 2 5 0 n [ 4 4 2 3 6 5 0 6 7 2 9 0 i 5 1 2 1 3 2 2 3 0 M 1 ( . 1 S ) N s 3 1 2 4 2 8 5 2 6 7 5 : O f 3 3 7 8 9 4 0 8 3 9 0 S I f ] 5 3 1 9 1 6 2 5 0 1 9 S T y O N - . . . . . . . . . . . E C R . k 8 6 4 5 1 5 5 5 3 9 4 N A x [ 1 0 3 7 8 5 8 4 4 9 8 T E a 5 1 2 1 2 3 1 3 I R M F ( T R O 2 5 1 3 6 2 8 9 2 6 6 2 P 7 0 6 1 7 6 8 0 0 4 9 5 P y ] 8 6 8 5 6 3 7 0 1 1 4 U R N - . . . . . . . . . . . : S l k 6 7 0 8 8 2 6 5 1 3 1 g e [ 5 8 2 0 0 3 2 - n d - 1 2 1 i r t S 3 8 4 8 5 5 6 5 1 9 8 ] 8 7 3 9 1 5 1 9 9 5 1 0 ] 2 6 7 2 0 2 9 9 2 3 3 [ N - . . . . . . . . . . . 9 y k 1 1 8 3 4 6 2 7 5 5 6 R [ 0 4 4 3 6 8 7 7 6 2 9 6 1 1 2 2 2 2 3 : n | | | | | | | | | | | o | | | | i - > > > > > > > > > > > t e 7 4 7 1 6 8 0 2 4 6 8 a d o - 1 2 4 4 4 5 5 5 5 5 r o N e N - < < < < < < < < < < < n e . G p 1 2 3 4 5 6 7 8 9 0 1 u o 1 1 S N
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TABLE 3 Dry Dock – Bearing Reactions for Prescribed Offset Dry dock condition offset and bearing reactions Reactions
Offset 800
Ry (dy) [kN] 1 2 3 4 5 6 7 8 9 10 11
544.411 45.927 127.873 24.785 272.691 153.893 299.704 272.986 264.188 328.505 94.822
GA Define d Dy [mm] 0 3.479 6.193 6.884 7.003 7.012 7.022 7.032 7.042 7.052 7.058
700 600 500 400 300 200 100 0 1
2
3 4 5 [kN] Bearing Reactions
6 Bearing Offset * 100 [mm]
7 8 9 10
] N k [ s n o i t c a e R g n i r a e B
11
] m m [ 0 0 1 * t e s f f O g n i r a e B
TABLE 4 Ballast Vessel Hull Deflections – Bearing Reactions and Total Bearing Offset Ballast vessel offset and bearing reactions Reactions
Offset 800
Ry (dy) [kN] 1 2 3 4 5 6 7 8 9 10 11
544.996 46.072 124.78 32.933 267.522 152.923 301.263 265.96 274.995 322.706 95.636
GA Defined Dy [mm] 0 3.429 6.123 6.914 7.049 7.074 7.102 7.132 7.164 7.188 7.208
700
600
500
400
300
200
100 0 1
2 3 4 Bearing Reactions [kN] 5 Bearing Offset * 100 [mm]
6 7 8 9 10 11
] N k [ s n o i t c a e R g n i r a e B
108
] m m [ 0 0 1 * t e s f f O g n i r a e B
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Section
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Alignment Optimization
TABLE 5 Laden Vessel Hull Deflections – Bearing Reactions and Total Bearing Offset Laden vessel offset and bearing reactions Reactions
Offset 900
Ry (dy) [kN] 1 2 3 4 5 6 7 8 9 10 11
518.533 106.331 34.172 275.984 81.192 155.648 285.205 345.582 143.036 399.197 84.905
GA Defined Dy [mm] 0 3.979 6.893 8.234 8.149 7.954 7.762 7.572 7.384 7.298 7.208
800 700 600 500 400 300 200 100 0 1
2
3
4 5 Bearing Reactions [kN] Bearing Offset * 100 [mm]
6
7 8 9 10 11
] N k [ s n o i t c a e R g n i r a e B
] m m [ 0 0 1 * t e s f f O g n i r a e B
For the estimated hull deflections, the bearing reactions in all three cases, i.e., even keel (dry dock), ballast and laden, are satisfactory. The solution is robust, and if predicted hull deflections are within given limits, no unloaded bearings are to be expected. Another important issue to be investigated is the misalignment slope between the shaft and the tail shaft bearing. The misalignment shall be reduced by slope boring if the shaft exerts exces sive pressure on the bearing shell. ABS shaft alignment software is used in the bearing contact investigation.
Dry dock condition no slope boring
Dry dock condition with slope boring
Contact pressure 497 MPa
Contact pressure reduced to 139 MPa
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Slope boring requirements for the dry dock condition would satisfy the ballast condition also.
Slope boring requirements for the dry dock condition would satisfy the loaded condition also. The misalignment slope is 0.15 mrad, which is below normal industry requirements for slope change.
The optimization algorithm applied here appears to determine the desired number of acceptable solutions within given constraints. The solution is found in a relatively short time. All of the benefits of conducting the shaft alignment optimization are immediately obvious from the presented example. It is noticed that the original alignment, as defined by taking the conventional approach in conducting alignment, will not result in a satisfactory static loading condition for the estimated hull deflections applied. In the conventional approach, the second aftmost main engine bearing and possibly the intermediate shaft bearing may get unloaded. Unloading of the main engine bearing confirms the very problems currently plaguing the propulsion installations. This all gives even more credibility to the proposed method, which can provide satisfactory solutions to the potentially dangerous problem. Another problem is the accurate prediction of the hull girder deflections. The solution to the problem will obviously be very much dependent on the ability to evaluate hull deflections accurately enough to confidently evaluate the alignment. One possible way of doing so is to establish a generic data base of hull girder deflections for certain categories of the vessels and use the data base when vessels of similar design are evaluated. Data can be obtained either analytically or by measurement. The Bureau has already taken steps in that direction. Relatively accurate hull deflection prediction and optimized alignment would allow alignment designers to confidently design alignment for the dry dock vessel condition. The alignment procedure could then be conducted fully in the dry dock. This would significantly increase the accuracy of the whole process, as verification of analysis by measurement would be possible with very little disturbance affecting the system.
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SECTION
1
2
8
Glossary
Abbreviations ABS
American Bureau of Shipping
Bureau
ABS
Class
Classification society
M/E
Main engine; implies diesel engine if not stated differently
Rules
ABS Rules for Building and Classing Steel Vessels are implied if not stated differently
S/T
Stern tube
TDC
Top dead center – defines position of the piston in the engine cylinder.
Definitions Alignment procedure: An executable part of the alignment process where alignment is performed in accordance with the requirements defined by the alignment designer. Alignment process: Consists of the design and analysis, the alignment procedure and measurements. Bearing offset: Bearing offset is vertical displacement of the contact face of the bearing from the optically established central line of the shafting. Bedplate pre-sagging: Process by which the vertical deformation (catenary curve) is introduced on engine’s bedplate to prevent engine alignment problems. Bore sighting: See sighting-through. Crankshaft deflections: Change in distance between crank webs, measured during one rotation of the crankshaft. Bearing clearance: Radial gap between the shaft and the bearing shell. Horizontal offset: Horizontal bearing offset is normally not desired. Influence coefficients: Values defining relative change in bearing reactions as the offset at particular bearing changes for unit value. Jack-up procedure: Procedure which uses hydraulic jacks to measure bearing reactions. Lifting/lowering line gradient: Angle of the plotted jack-up line measured in mm/kN (or similar displacement vs. force units). Misalignment angle: Angular difference between central line of the shaft and the central line of the respective bearing. Negative offset: Bearing vertical position below the referenced (zero) line.
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Glossary
Prescribed displacements: alignment
Desired bearing offset prescribed by designer to obtain satisfactory
Positive offset: Bearing vertical position above the referenced (zero) line Rule of thumb: A method established, or a procedure derived entirely from practice or experience, without any basis in scientific knowledge; a roughly practical method. Sag and gap: Procedure of verification of the alignment condition prior to shafting assembly. Sighting through: Optical procedure by which bearings are offset to the prescribed values and slope bored/inclined (if required) Slope boring: requirements.
Procedure by which the bearing is machined so to comply with misalignment
Straight alignment shafting: Propulsion shafting supported by the bearings which are positioned so to ensure straight center line of the undeformed shafting. Straight alignment shafting is also called zero offset alignment. Strain-gauge method: Method used to measure strain change in the shafting. Undeformed shafting: Shafting which central line is straight. This assumes that no gravity and external forces or moments are acting on the propulsion shafting system. Vertical offset: See bearing offset. Zero offset alignment: See straight alignment shafting.
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