3.8.9 Alarm parameters
The OOW should make full use of the various tools incorporated into the system on board. The ECDIS system is fitted with added route-monitoring tools to assist the OOW including:
Safety Contour
TheSafety Contour provides a visible boundary with respect to the depth of available water. It is to be set at the maximum dynamic draft plus 50% of the static draft in open seas and at max draft plus 10% of the static draft in coastal/restricted waters in accordance with the company’s UKC Policy (Ref FOP 3.2.8b). However, if the ENC does not have a depth contour for the selected water depth (e.g. 12m), the displayed contour will automatically default to the next deepest option (e.g. 15m).
It may, therefore, be permissible for a vessel to cross this contour during transit. To ensure the mariner is fully aware of the depth of surrounding water the display must be populated with spot soundings and isolated dangers. ‘No Go Areas’ can also be annotated on the screen using the Add Info function.
The Safety Contour will provide an alarm to the navigator when reached.
Please refer to the Publication NP232 “Guide to ECDIS Implementation, Policy and Procedures”, section 12.13 Navigating across the Safety Contour for further details regarding methodology and risks.
It is recommended that when using method 1 or method 2, the Bridge team be fully aware of the risks and the same be discussed in the pre-voyage bridge team meeting.
Safety Depth
Highlights a spot sounding, the depth of which is insufficient for a vessel to safely pass over. TheSafety Depth is to be set at the same value as the Safety contour. Using the example above, it is possible to set the Safety Depth to 12m. All spot soundings 12m and shallower will then be prominently highlighted on the chart.
The Safety Depth will provide an alarm to the navigator when reached.
Shallow Contour
The area between the 0m contour and shallow contour is coloured allowing the gradient of the seabed to be graphically displayed. It should not be set at the same value as the safety contour value as the navigator will lose the benefit of being able to distinguish the depth gradient of the waterway. It should be set at the closest available contour, even if this contour is less than the vessel draft.
In the below example it is easy to understand the above recommendation:
The action is taking place on board a VLCC, 21 meters draft, which if we were adjusting the safety contour to 24 meters, safety depth to 24 meters, shallow contour to 24 meters, Deep contour to 42 meters. This VLCC has received the order to drop anchor in the Suddong Special Purpose anchorage, which is located at Singapore and is suitable to VLCCs. Company UKC Policy is complied with. This anchorage is very busy, and submitted to strong currents. A shallow patch lies south of this anchorage (15 meters).
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With the above settings, we cannot distinguish the shallow patch that is lying south of the anchorage, as it is shown on the below picture.
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By changing the setting of the shallow contour to 20 meters the shallow patch is now clearly visible to the navigator.
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This picture is clearly better than the first one, but the navigator must pay attention when sailing in the light blue area which is the gradient between 20 and 30 meters in this case. With a draft of 21 meters, he must pay attention to the soundings which are displayed in bold (the Safety depth Value is responsible for the bolding of these spot soundings, from 0 to the value that the navigator has selected – 24 meters in this example).
This is where the 4 colours mode is important to select as in 2 colours mode, the light blue will become dark blue and the grey will become white. Therefore in 2 colours mode, the navigator would not be able to see the shallow patch even he has selected the proper values for the safety depth, safety contour, shallow contour and deep contour.
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Here the same settings as above, (Safety contour to 24 meters safety depth to 24 meters, shallow contour to 20 meters, Deep contour to 42) but with 2 colours option:
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