STRESSES IN SHIP

STRESSES IN SHIP

STRESSES

I) TENSILE , II) COMPRESSIVE, III) SHEAR

GLOBAL STRESSES AFFECT THE WHOLE SHIP

LOCAL STRESSES AFFECTING PARTICULAR PART OF SHIP

• A SHIP AT SEA IS SUBJECTED TO A NUMBER OF FORCES CAUSING THE STRUCTURE TO DISTORT

1. STATIC FORCES: SHIP FLOATING AT REST IN STILL WATER WILL HAVE ITS WEIGHT ACTING VERTICALLY DOWNWARD AND WATER PRESSURE ACTING PERPENDICULAR TO OUTSIDE SURFACE OF HULL.
2. DYNAMIC FORCES: DUE TO MOTION OF SHIP THAT IS HAMMERING OF WATER ON SHIP ,PASSAGE OF WAVE ALONG THE SHIP, MOVING MACHINERY PART.

• STRUCTURAL STRESSES CAUSED BY ABOVE FORCES TO WHICH SHIP STRUCTURE  IS SUBJECTED

1. LONGITUDINAL STRESSES ( HOGGING AND SAGGING)
2. TRANSVERSE STRESSES (RAKING AND EFFECT OF WATER PRESSURE)
3. LOCAL DYNAMIC STRESSES (PANTING AND POUNDING)
4. OTHER STRESSES ARE CAUSED BY DRY DOCKING, LOCAL WEIGHT AND VIBRATION.

• LONGITUDINAL STRESSES: SHIP MAY BE REGARDED AS A NON UNIFORM BEAM, CARRYING NON UNIFORMLY DISTRIBUTED WEIGHTS AND HAVING VARYING DEGREE OF SUPPORT ALONG ITS LENGTH.

• LONGITUDINAL STRESSES IN STILL WATER

1. HOGGING- IF SHIP IS LOADED HEAVILY AT FORWARD AND AFT COMPARED TO MID SHIP . HERE BUOYANCY AT MIDSHIP EXEEDS WEIGHT OF SHIP. EFFECT COULD BE WORST IF WAVE CREST AT MID SHIP WITH WAVE LENGTH EQUAL TO SHIP LENGTH. IN THIS CASE UPPER DECK IS UNDER TENSION AND BOTTOM SHELL EXPERIENCE COMPRESSION.
2. SAGGING- CONSIDER A SHIP LOADED HEAVILY AT MIDSHIP COMPARED TO STEM AND STERN , HERE WEIGHT OF MIDSHIP WILL EXEEDS THE BUOYANCY AND SHIP WILL SAG. THIS IS WORST WHEN WAVE REST ARE AT THE END OF SHIP WITH WAVE LENGTH SIMILAR TO SHIP LENGTH

• LONGITUDINAL  STRESSES IN SEA WAY CAUSING HOGGING AND SAGGING

1. WHEN A SHIP PASSES THROUGH WAVES , ALTERATION IN THE DISTRIBUTION OF BUOYANCY AMIDSHIP IS GREATER WHILE AT THE END IT IS SMALLER. THIS TENDS TO CAUSE THE SHIP TO HOG. A FEW SECOND LATER THE WAVE THROUGH LIES AMIDSHIP . THE BUOYANCY AMID SHIP IS REDUCED WHILE AT THE END IT IS INCREASED CAUSING VESSEL TO SAG.
2. THE EFFECT OF THIS FLUCTUATION IN STRESSES IN EXTREME CASES COMPLETE REVERSAL OF STRESS EVERY FEW SECOND. FORTUNATELY SUCH REVERSAL ARE NOT SUFFICIENTLY NUMEROUS TO CAUSE FATIGUE, BUT WILL CAUSE DAMAGE TO ANY FAULTY PART OF STRUCTURE.

• EFFECT OF HOGGING SAGGING:

FLUCTUATION OF STRESSES MAY DAMAGE BOTTOM SHELL IN SAGGING CONDITION OR REVERSAL OF STRESSES IN FEW SECOND MAY DAMAGE SHIP DECK DUE TO HOGGING. IT MAY ALSO CAUSE SHIP TO BREAK.

• MINIMIZING EFFECT OF LONGITUDINAL  STRESSES

1. JOINING SHEER STRAKE AND STRINGER PLATE TO FORM A ROUNDED GUNWALE.
2. ALL CONTINUOUS LONGITUDINAL MATERIAL RESIST LONGITUDINAL  BENDING ESPECIALLY THOSE PARTS FARTHEST FROM AXIS OF BENDING CENTRAL  AXIS. EXAMPLE CENTER GIRDER . SIDE GIRDER, INNER BOTTOM, INNER BOTTOM LONGITUDINALS, KEEL AND BOTTOM SHELL, TANK TOP PLATING ETC.
3. AT TOP DECK STRINGER AND SHEER STRAKE  ARE THICKENED TO MAKE STRONG "L" SHAPED JOINT.
4. HULL AT ABOUT HALF DEPTH OF SHIP AND ONE FOURTH LENGTH OF SHIP FROM EACH SIDE IS STRENGTHENED.
5.  FILLING LONGITUDINAL GIRDER AND DECK LONGITUDINALS TO RESIST STRESSES
6. UNEVEN LOAFING OF SHIP IS AVOIDED.
7. IN TANKERS LONGITUDINAL BULKHEAD GIVE GREAT STRENGTH.
8. SPECIAL STEEL FOR HIGH STRESSES ARE NOW USED ESPECIALLY IN LARGE SHIPS.

• SHEARING STRESSES

LONGITUDINAL STRESSES CAUSED BY WEIGHT AND BUOYANCY DISTRIBUTION CAN GIVE RISE TO LONGITUDINAL SHEARING STRESSES . MAXIMUM LONGITUDINAL SHEARING STRESSES OCCURS AT NEUTRAL AXIS AND DECREASE O MINIMUM AT DECK AND KEEL. VERTICAL SHEARING STRESSES ALSO OCCUR  AS A RESULT  OF NON UNIFORM LONGITUDINAL  DISTRIBUTION OF WEIGHT AND BUOYANCY. 

• TRANSVERSE STRESSES: TRANSVERSE STRUCTURE OF SHIP IS SUBJECTED TO

1. FORCE DUE TO WEIGHT OF SHIP STRUCTURE, MACHINERY, FUEL, WATER AND CARGO.
2. WATER PRESSURE
3. FORCE CREATED BY LONGITUDINAL BENDING.

• MINIMIZING EFFECT OF FORCE DUE TO WEIGHT OF SHIP STRUCTURE, MACHINERY, FUEL WATER, CARGO.

1. DECK MUST BE DESIGNED TO SUPPORT THE WEIGHT OF ACCOMODATION, WINCHES, AND CARGO WHILE EXPOSED DECK MAY HAVE TO WITHSTAND A TREMENDOUS WEIGHT OF WATER SHIPPED IN HEAVY WEATHER
2. DECK PLATING IS CONNECTED TO BEAM WHICH TRANSMIT LOADS TO LONGITUDINAL GIRDER AND SIDE FRAME.
3. IN WAY OF HEAVY LOCAL LOAD SUCH AS WINCHES ADDITIONAL STIFFING IS ARRANGED.
4. SHELL PLATING AND FRAMES FROM PILLARS WHICH SUPPORT WEIGHTS FROM DECK
5. TANK TOP TO BE ABLE TO CARRY WEIGHT OF THE HOLD CARGO OR UPTHRUST BY LIQUID IN TANKS.
6. MACHINERY SEAT EXTREMELY WELL SUPPORTED  TO PREVENT ANY MOVEMENT OF MACHINERY 
7. ADDITIONAL GIRDER IN DOUBLE BOTTOM AND THICKNESS OF TANK TOP OF ENGINE TO BE GREATER.

• FORCES DUE TO WATER PRESSURE:

1. FORCES EXERTED ON BOTTOM AND SIDE SHELL BY WATER SURROUNDING THE SHIP
2. WATER PRESSURE ACTS PERPENDICULAR TO SHELL OF SHIP AND INCREASES WITH INCREASE IN DEPTH.
3. EFFECT IS TO PUSH THE SHIP SIDE IN AND BOTTOM UP.
4. DOUBLE BOTTOM FORCE AND SIDE FRAME ARE DESIGNED TO WITHSTAND TO WITHSTAND THESE FORCES . WHILE SHELL PLATING THICKNESS IS GREATER TO PREVENT BUCKLING BETWEEN FLOOR AND FRAMES. BOTTOM SHELL THICKER THAN SIDE SHELL.

• RACKING

1. WHEN A SHIP ROLL THERE IS A TENDENCY FOR SHIP TO DISTORT TRANSVERSELY IN A SIMILAR WAY TO THAT IN WHICH A PICTURE FRAME MAY COLLAPSE.THIS IS CALLED RACKING.
2. THIS IS PREVENTED BY BEAM KNEE AND TANK SIDE BRAKET CONNECTION TOGETHER WITH TRANSVERSE BULKHEAD.
3. ITS GREATEST AFFECT IS UNDER LIGHT WEIGHT CONDITION.

• FORCE CREATED BY LONGITUDINAL BENDING

EFFICIENCY OF SHIP STRUCTURE IN WITHSTAND LONGITUDINAL BENDING DEPENDS TO A LARGE LARGE EXTENT O THE ABILITY  OF TRANSVERSE STRUCTURE TO PREVENT COLLAPSE OF THE SHELL PLATING AND DECK.

LOCAL DYNAMIC STRESSES

PANTING:

1. AS THE WAVES PASSES ALONG THE SHIP THEY CAUSES FLUCTUATION IN WATER PRESSURE WHICH TENDS TO CREATE AN IN AND OUT MOVEMENT OF SHELL PLATING SUCH A MOVEMENT IS TERMED AS PANTING.
2. EFFECT OF THIS IS TO BE FOUND GREATEST AT THE ENDS OF SHIP ESPECIALLY AT THE FORE END WHERE THE SHELL IS RELATIVELY PLAT 
3. IF UNRESTRICTED COULD EVENTUALLY LEAD TO FATIGUE OF MATERIAL.
4. PITCHING MOTION OF SHIP PRODUCES ADDITIONAL VARIATION IN WATER PRESSURE PARTICULARLY AT THE LOW AND STERN WHICH COULD CAUSE PAINTING

• PANTING ARRANGEMENT IN FORE PEAK 

1. FORWARD OF COLLISION BULKHEAD PAINTING STRINGERS ARE FITTED NOT MOTE THAN 2 METER APART VERTICALLY.
2. STRINGERS ARE BRACKETED TO SHELL FRAMES AND PANTING BEAM IS FIXED ON ALTERNATE FRAME UNDER EACH PANTING STRINGER
3. PILLARS ARE FITTED ON SHIP CENTER LINE (USUALLY TO WASH BULK HEAD TO TIE THE PAINTING BEAM TOGETHER)
4. TO STIFFEN THE JOINT BETWEEN EACH BEAM AND INNER EDGE OF STRINGER , THE PLATE MAY BE SHAPED OR GUSSETS FITTED.
5. AT FORE END STRINGER ARE JOINED BY FLAT PLATES CALLED BREAST HOOK.

• PANTING ARRANGEMENTS AT AFT OF THE COLLISION BULKHEAD

1. DEEP FRAMING: 20% STRONGER THAN NORMAL, MUST BE FITTED BELOW THE LOWEST DECK . BETWEEN COLLISION BULKHEAD AND A POINT 15% OF THE SHIP LENGTH 
2. FRAMES AND TANK SIDE BRACKET CONNECTION MUST HAVE STRONGER WELDING THAN NORMAL.
3. SIDE STRINGER IN THE LINE WITH WITH PANTING STRINGER MUST BE FITTED THROUGHOUT THE DEEP FRAMING REGION OR THE SHELL PLATING THICKENED.

• COLLISION BULK HEAD

STIFFENED BY VERTICAL BULB ANGLE SPACED ABOUT 600 MM APART INSIDE THE PEAK.

• PANTING ARRANGEMENT IN AFT PEAK

1. SAME AS FORE PEAK STRINGER AND BEAM MAY BE FITTED 2.5 METER APART
2. FRAME ABOVE THE STERN TUBE MUST BE STIFFENED BY FLANGED TIE PLATES TO REDUCE THE POSSIBILITY OF VIBRATION OR FLOOR SHOULD EXTEND ABOVE THE STERN TUBE.

• POUNDING

1. WHEN A SHIP MEETS HEAVY WEATHER AND COMMENCES HEAVING AND PITCHING THE RISE OF THE SHIP OCCASIONALLY SYNCHRONISES WITH THE TROUGH OF A WAVE THE FORE END THEN EMERGES FROM THE WATER AND RE ENTERS WITH TREMENDOUS SLAMMING EFFECT KNOWN AS POUNDING.
2. DOES NOT OCCUR WITH GREAT REGULARITY.
3. CAUSE DAMAGE TO BOTTOM OF SHIP FORWARD.

• ARRANGEMENT TO RESIST POUNDING

1. STRUCTURE TO RESIST THE EFFECT OF POUNDING FROM COLLISION BULKHEAD TO 25% OF SHIP LENGTH FROM FORWARD.
2. FLAT BOTTOM SHELL PLATING ADJACENT TO THE KEEL ON EACH SIDE IS INCREASED  TO THICKNESS BY 15% TO 30% DEPENDING UPON SHIP LENGTH.
3. TRANSVERSE FRAME IN POUNDING REGION IS REDUCED TO 700 MM COMPARED TO 750 TO 900 MM AMIDSHIP
4. PLATE FLOORS ARE FITTED AT EVERY FRAME SPACE AND ARE CONNECTED TO OTHER BOTTOM PLATING BY CONTINUOUS WELDS.
5. EXTRA INTERCOASTAL SIDE GIRDER FITTED SO THAT DISTANCE BETWEEN SIDE GIRDER DOES NOT EXEED 2.2 MM.
6. IN LONGITUDINALLY FRAMED  BOTTOM PLATE FLOORS ARE FITTED AT ALTERNATE FRAME.
7. LONGITUDINAL STRONGER THAN NORMAL
8. LONGITUDINAL GIRDER FITTED 2.2 M APART EXTENDING VERTICALLY FROM TANK TOP.
9. INTERMEDIATE HALF HEIGHT GIRDER ARE FITTED TO SHELL,REDUCING THE UNSUPPORTED WIDTH TO 1.1 MM   

OTHER STRESSES

DRY DOCKING STRESSES

1. A SHIP USUALLY ENTER A DRY DOCK WITH SLIGHT TRIM BY AFT. WHEN VESSEL IS ALLOWED TO SETTLE ON THE KEEL BLOCKS, BEFORE SIDE BLOCKS ARE POSITIONED TRANSVERSE STRESSES WILL BE INDUCED AND IT WILL HAVE SOME SAG IN THE BILGES. THESE STRESSES ARE RESISTED BY DOUBLE BOTTOM STRUCTURE, TRANSVERSE WEBS AND BULKHEADS
2. LONGITUDINAL STRESSES ARE CREATED SINCE VESSEL GENERALLY GROUNDS AFT FIRST AND THIS CREATE LONGITUDINAL BENDING. RESISTANCE TO THE STRESSES WILL BE BY ALL LONGITUDINALLY CONTINUOUS MATERIAL.
3. SHIP STRUCTURE IN WAY OF THE KEEL MUST BE STRONG ENOUGH TO WITHSTAND THIS LOAD. IN MOST SHIPS THE NORMAL ARRANGEMENT OF KEEL AND CENTER GIRDER TOGETHER WITH TRANSVERSE FLOOR IS QUITE SUFFICIENT FOR THIS PURPOSE.
4. BILGE BLOCK ARE FITTED TO SUPPORT THE SIDES OF THE SHIP. IN SOME CASES THEY ARE FITTED AFTER THE WATER IS ART OF THE DOCK, WHILE SOME DOCKS HAVE BLOCKS WHICH MAY SLID INTO PLACE WHILE WATER IS STILL IN DOCK.
5. AS SOON AS THE AFT END TOUCHES THE BLOCKS, SHORES ARE INSERTED BETWEEN STERN AND DOCK SIDE, TO CENTRALIZE THE SHIP IN DOCK AND TO PREVENT THE SHIP SLIPPING OF THE BLOCKS. WHEN THE SHIP GROUNDS ALONG ITS WHOLE LENGTH ADDITIONAL SHORES ARE FITTED ON BOTH ON BOTH SIDES, HOLDING THE SHIP IN POSITION AND PREVENTING TIPPING. THESE SHORES ARE KNOWN AS BREAST SHORES AND HAVE SOME SLIGHT EFFECT I PREVENTING THE SIDE SHEEL BULGING. THEY ARE PREFERABLY  PLACED IN WAY OF TRANSVERSE BULKHEADS OR SIDE FRAME.

• LOCALIZED LOADING

1. LOCALIZED WEIGHT EXAMPLE MACHINERY OR LOCALISED  LOADING OF HEAVY CARGO MAY GIVE RISE TO LOCALIZED DISTORTION OF STRUCTURE.
2. DECKS MUST BE DESIGNED TO SUPPORT THESE WEIGHTS.
3. DECK OPENING HOLES CUT IN DECK PLATING THAT IS HATCH WAY ETC, CREATE AREA OF HIGH LOCAL STRESSES DUE TO THE LACK OF  CONTINUITY  CREATED BY OPENING
4. END OF SUGAR STRUCTURE MAY GIVE MAJOR DISCONTINUITIES IN THE SHIP STRUCTURE GIVING RISE TO LOCALIZED STRESSES WHICH MAY RESULT IN CRACKS

• VIBRATION

1. VIBRATION FROM ENGINE, PROPELLERS ETC TENDS TO CAUSE STRESSES IN THE AFT END OF SHIP.
2. THE MACHINERY SEATS MUST BE EXTREMELY WELL SUPPORTED TO PREVENT MOVEMENT OF MACHINERY.
3. ADDITIONAL GIRDER ARE FITTED ON DOUBLE BOTTOM AND THICKNESS OF THE TANK TOP MUST BE GREATER UNDER THE ENGINE.