A change in draught may arise during a voyage due to factors such as cargo and ballast changes as well as sea water density, and dynamic factors such as squat, pitch /roll etc. These factors should be considered in the appraisal and appropriate figures calculated for each leg of the voyage plan. Any significant events affecting draught be managed and documented within the voyage plan and their effects noted.
Dynamic factors dependent on vessel characteristic s or variable factors such as speed and turn radius should be noted and an estimate made of the impact so that they may be accounted for during monitoring.
Bridge team management must ensure that the UKC is calculated for each leg of the voyage as well as for both arrival and departure at each port.
This is especially important for a vessel which loads such that the UKC may be limited due to shallow depths in the channel or approaches on its departure from the berth.
In such cases it may be required to restrict the loaded draft so that the UKC requirements are not breached at any time during its passage.
In cases where voyage orders specify a draught or cargo nomination that will result in a lesser UKC than the ones stated above or if it appears that the vessel may not be able to comply with the minimum UKC requirements during ANY part of the voyage, then :
It is for this reason that any policy needs to establish a procedure where a risk assessment can be carried out for a given voyage, determine where the greatest risk may occur and give an element of safety to the calculated figures ultimately used for the safety contour. The policy should also define under what circumstances safety margins may be reduced, and with whose approval.
My company had this guidelines:-
a) Inform the Office at the earliest opportunity and submit a copy of the UKC calculation for review.
b) Within port limits, obtain the latest sounding information, including nature of the bottom, port UKC requirements and customary practices directly from the local authorities or terminal well before arrival.
c) If alongside, vacate the berth if in any doubt about the risk of grounding.
The Master will then investigate the matter with the charterers / agents and carry out a Risk Assessment.
The company risk assessment should take into account all relevant information in the UKC calculations such as the following:
⦁ Previous knowledge or operations by company vessels in that port/sea area.
⦁ Local knowledge (sailing directions, port survey, harbor or deep sea pilot)
⦁ Geological stability, silting, dredging and construction activity
⦁ Accuracy of tidal data (aguges, meters etc)
If NAABSA (Not Always Afloat But Safely Aground) clause is exercised by the charterers, then each case will be reviewed using all the above factors and Risk Assessment.
The Company will review the Risk Assessment and if satisfied will notify their approval to the Master to agree with the proposed voyage.
This notification will contain certain specific measures to be taken for that period, like speed during transit, etc.; however, it is pertinent to note that each case will be viewed on its own merit.
FACTORS AFFECTING UKC CALCULATION
The following factors shall be taken into account when calculating the controlling depth on any passage:
· The predicted height of the tide and the tidal window for safe passage.
· Changes in the predicted tidal height, which are caused by wind speed and direction and high or low barometric pressure.
· Nature and stability of the bottom – i.e. sand waves, siltation etc.
· Accuracy of hydrographic data, a note as to the reliability of which is often included on charts.
· Change of water density and the increase in draught due to fresh water allowance.
· The vessel’s size and handling characteristics and increase in draught due to heel.
· Wave response allowance, which is the vertical displacement of the hull due to heave, roll and pitch motions.
· The reliability of draft observations and calculations, including estimates of hogging and sagging.
· Reduced depths over pipelines and other obstructions.
Once the available under keel clearance has been calculated taking into account all the applicable factors, including those above, it can then be determined whether any speed reduction is required to counter the effects of squat.