RUDDER TRUNK MISMATCHA Case Study

BY:APOORV DHAWAN200737TP138INTERNSHIP DONE AT L&T HAZIRA.

•Plate storage yard•Covered fabrication shop•2 Erection bays of 160 m x 30 m•Slipway of 160 m x 35 m for launching of vessels, launching draft up to 4.5 meters•Outfitting jetty of 160 m•Adequate Lift & Shift facilities

SHIPYARD PARTICULARS

VESSEL PARTICULARS•OWNER: Biglift•TYPE: Multi-purpose Heavy-lift type•CLASS: Lloyd’s Register•DEADWEIGHT: 18680 DWT•LOA: 154.8 m•BREADTH: 26.5 m•DESIGN DRAUGHT: 7.0 m

IMPORTANCE OF RUDDER TRUNK

• Transfers the axial forces and radial forces acting on the rudder to the hull.

• Carries the weight of the rudder.

• Ensures water-tightness of hull.

• Provides a housing for the rudder stock.

• Houses the bearings for the rudder stock to rotate.

DRAWING OF RUDDER & TRUNK

DRAWING OF RUDDER TRUNK

PROBLEM FACED

• During a QC inspection to verify the perpendicularity and intersection of M/E shaft centre axis and rudder stock centre axis, a problem was observed.

• Mismatch between rudder trunk and hull observed.

DRAWING OF RUDDER TRUNK

DETAIL DIMENSIONS OF RUDDER TRUNK SKEG

DRAWING ILLUSTRATING THE MISMATCH

PICTURE OF THE SKEG

IMPLICATIONS OF A MISMATCH BETWEEN RUDDER TRUNK AND HULL

• Greater prop wash, which increases chances of fatigue failure.

• Uneven distribution of stresses in region of mismatch.

• Might damage water-tight integrity between rudder trunk and hull.

PROCESS

• Detail dimensions of rudder trunk from OEM obtained.

DETAIL DIMENSIONS OF RUDDER TRUNK SKEG

• Forwarded to Navtec for developing structural drawings.

• Post receipt of the skeg drawings from Navtec, the offsets of the skeg (Block 1012C) at various frames had been compared with the corresponding dimensions of the trunking and found correct.

• Skeg drawing issued for production.

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OBSERVATION

1. Deviation in skeg plating :

Ranged from (+5) mm to (-23) mm.

2. Deviation in side plating of trunk:

Ranged from (+32) mm to (-3) mm.

DRAWING ILLUSTRATING THE MISMATCH

DRAWING OF RUDDER TRUNK

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• Distortion in welding of hull.

• Proper machining clearances (to account for welding) might not have been taken by the OEM.

Accuracy of dimension in the skeg structure and the rudder trunk were not maintained to the desired level.

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SOLUTIONS TO THE CAUSES

• Modifying the hull

• Modifying the rudder trunk

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MODIFYING THE HULL

• Skeg extends from 6300 mm ABL to 9525 mm ABL.

• Involves cutting of the stiffeners and attached panels to the skeg.

3D MODEL OF HULL FROM FR. 0 TO FR. 5

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MODIFYING THE RUDDER TRUNK

• IF HOT WORK WAS DONE IN THE RUDDER TRUNK ABOVE 60◦C, THE WARRANTY OF THE TRUNK WOULD VOID.

• So, 3 different proposals were prepared.

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PROPOSAL 1

• Provision of a weld joint between the skegand the rudder trunk.

DRAWING ILLUSTRATING PROPOSAL 1

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PROPOSAL 2

• Heating and bending the side plates of the trunk so as to match with the contour of the respective skeg plates.

DRAWING ILLUSTRATING PROPOSAL 2

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PROPOSAL 3

• Replacing the side plates of the trunk so as to match with the contour of the respective skeg plates.

DRAWING ILLUSTRATING PROPOSAL 3

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RESPONSE OF OEM (Vander Velden)

• FAVORITE: Proposal 1

• AGAINST: Proposal 2 & 3– Amount of heat input due to additional

welding.

– Possible bending of trunk structure as this influences the high stress zone between lower trunk shell pipe and bottom plate of skeg.

– Heat tension can lead to structural deformation of machined surfaces.

– Can destroy the synthetical neck bearing bush.

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RESPONSE OF CLASS (Lloyd’s Register)

• FAVORITE: Proposal 3

• AGAINST: Proposal 1

– High stress concentration in the area where the weld joint had to provided.

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RESPONSE OF SHIP OWNER (Biglift)

• FAVORITE: Any proposal accepted by class

• AGAINST: Proposal 1

– No compromise on any disturbance of flow on the streamlined body.

– Shell plating on the streamlined body should be perfectly aligned with the trunk section before welding.

– Thickness compensation would be rejected.

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FINAL SOLUTION SELECTED

• Neither of the 3 suggested solutions were adopted.

• Rudder trunk sent back to OEM (based in Germany) for repair.

• Warranty would not void.

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SYSTEM IMPROVEMENT

• After completion of the skeg structure the offsets from centerline should have been checked before clearing for erection with the adjoining blocks.

• Before clearing the rudder trunk for installation, the dimension of the trunk should have been checked with the same of skeg at a level of 6750 mm from baseline.

MY LEARNING

HOW TO SELECT A SOLUTION

• There are two aspects I differently analyzed,

– Cost aspect

– Engineering aspect

• COST ASPECT

– Solution 1 are the most cost-effective solutions as it involves only welding of 2 plates.

– Modification in the hull would prove to be the most expensive.

MY LEARNING

• ENGINEERING ASPECT

– Solution adopted is the most favorable

– OEM exactly knows the machining tolerances the rudder trunk was manufactured with.

– Would be able to do the hot work by insulating properly the required parts of the rudder trunk.

MY LEARNING