STABILITY AND STRESSES ON CHEMICAL TANKERS – CAPT AJIT VADAKAYIL
The ships class approved “ trim and stability manual” and the “ procedures and arrangements manual “ must be read in conjunction with what follows.
The chief officer is responsible to the master for stability, SF and BM and trim/ draft of the vessel. The vessel must maintain minimum bow height and propeller submersion so that there is no slamming or propeller racing and overtrip in rough seas.
The loadicator must be calibrated and the printout filed once a month. . A good way to stow the ships constant weights on the loadicator is to read off the draft and trim as soon as the ship floats off the drydock blocks, when all volumes inside bunker and ballast tanks are well calculated using the declivity of blocks as trim.
Ships pre arr load and dischg bar diagrams must have the stresses and trim worked out every hour. The ship must never be loaded deeper than permitted by the relevant loadline.
Double hull ships without centre line bulkheads and wide centre tanks can have huge free surface effect. This can be made worse if the SBT tanks are U shaped. Cumulative list can occur at berth when viscous cargoes like tallow are not heated .
It is not safe to have excessive number of centre tanks slack, and the MOLOO option has to be used to keep ship stable. In all cases the ships stability book must be followed and min safe GM maintained.
Sloshing considerations for part loading is given in ships P@A manual. Sometimes 20% to 80% range is NOT allowed if tank length is >10 mtrs or tank is >10% of LOA, to prevent dynamic slamming damage by high SG free flowing chemicals .
The sloshing effect can even cause high build up of static electricity of accumulator cargoes.
In case of unexplained list of ships without longitudinal bulkhead , having double hull , wide centre tanks and viscous cargoes, while at berth, not attributed to uneven distribution of wing tank cargoes, wing SBT ballast , bunkers or NAABSA grounding do not hesitate to stop the cargo after informing the terminal.
Double check the weight distribution and GM with a risk assessment. Once stability is restored by well-established practices ( do not pump out ballast from listed side ), ensure that the cumulative list situation does not reoccur for the remainder of the cargo operation.
Below : It takes a real idiot - nay- prick-- to break his ship into two -- as you can see , a ship does NOT break easily !
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U shaped SBT tanks are not to be used till departure unless it is safe .
For damage stability ballast conditions are not considered. Damage stability must not be confused with intact stability during CDI and Vetting inspections.
Damage stability assessed under IBC code and the Stability book should contain ships survival capabilities. Master must be aware of the worst damage stability condition in the stability book.
Operators operating manuals must contain procedures for restoring stability in case of unstable conditions developing during cargo operations.
Chief officer must be able to demonstrate intact stability WCS ( worst case scenario ) — all tanks slack with max free surface.
Extra wide cargo tanks and U section of SBT cause loss of intact stability.
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THE NAIVE SAILOR, DAMAGE STABILITY VADAKAYIL
CAPT AJIT VADAKAYIL
30 years in command.