Monitoring the status of the anchorage is a task of utmost importance, as the safety of the ship depends on timely and adequate responses to potential risks and hazards during its stay.
Common hazards associated with anchorage include nearby traffic, adverse wind and wave conditions, high tide and strong current conditions, and risks arising from combined operations during anchorage, such as ship-to-ship (STS) transfers. Security concerns, particularly in areas with reported incidents, are another critical hazard.
It is important to note that addressing security issues requires a thorough Security Risk Assessment for the voyage and adherence to the ship’s specific Ship Security Plan. However, measures implemented for monitoring nearby traffic can also enhance security vigilance and contribute to mitigating these risks.
During the execution phase, accurately plotting the anchor’s position at the time of deployment is crucial for defining all necessary safety margins. All anchored ships exhibit specific movement patterns, typically aligning upwind unless current forces exceed the influence of the wind.
The Safety Swinging Circle
When the wind changes, the ship will turn to face upwind again, with the anchor chain acting as a radius that holds the ship within a specific circle. The most effective way to monitor the ship’s position during anchorage is by using the Safety Swinging Circle (SSC). This circle represents the maximum limit that the ship’s stern can reach if the anchor chain is fully extended.
Calculation of Safety Swinging Circle
SSC = Length of Anchor Cable + Length of Ship + Safety margin
Key Points for Calculation:
- Length of Anchor Cable: The length of the anchor chain used during anchoring.
- Length of Ship: The length overall (LOA) as stated in the ship’s particulars.
- Safety Margin: A value specified in the ship’s Safety Management System (SMS) that provides a buffer or “warning zone” in case of potential anchor dragging.
When the Safety Swinging Circle is drawn, it must not cross the Safety Contour or any manually drawn “no-go” or limit lines on navigational charts.
Example Calculation:
For a ship with a length overall of 180 meters anchored in 25 meters of water, using 6 cables of anchor chain (1 cable = 27.5 m), and an SMS-defined safety margin of 100 meters:
SSC = (6 cables × 27.5 m) + 180 m + 100 m = 460 meters
Additional Considerations
The Safety Swinging Circle is calculated conservatively, incorporating parameters that err on the side of caution:
- Ship length vs. bridge-to-bow distance: The calculation uses the full ship length, but GNSS position fixes typically refer to the bridge position, making the actual distance slightly less.
- Anchor chain length: The calculation uses the total anchor chain length deployed, but the actual radius is slightly shorter since the chain lies along the seabed rather than forming a perfect straight line.
While these adjustments may slightly affect the theoretical circle, the differences are minimal in practice. The key takeaway is that as long as the ship swings within the Safety Swinging Circle, there are no navigational safety concerns.
ECDIS Plotting
Modern ECDIS systems include an “Anchoring Watch” feature that allows users to set the Anchor Drop Point and define the Safety Swinging Circle, either in nautical miles or meters, depending on the system. Once set, the display visually represents these parameters through relevant graphics.
During the anchorage period, the ship’s movement should remain within the defined circle. The Officer of the Watch (OOW) is responsible for verifying this in accordance with operational procedures outlined in the SMS. If the ship approaches the circle’s limits, it serves as a warning trigger for the OOW to take further action.
In addition to monitoring the ship’s position, the OOW must continuously observe wind force, current conditions, and the vessel’s motion. A recommended best practice is to activate the past track feature on the ECDIS. Any deviation or unusual changes in the ship’s movement pattern, compared to the past track, should be treated as a warning trend and addressed promptly.
Paper Chart Plotting
The anchorage watch on paper charts follows a similar process to ECDIS plotting but requires manual input. The OOW must plot the anchor drop position and manually calculate the Safety Swinging Circle. However, paper chart monitoring introduces additional considerations.
On a paper chart, two circles should be drawn:
- The Bridge Swinging Safety Circle: This represents the area where the ship’s bridge must remain to ensure safety.
- The Overall Swinging Safety Circle: This represents the maximum extent of the ship, including the stern, if the anchor chain is fully extended.
The reason for the dual circles is that safety monitoring parameters, such as bearings or radar distances, are measured from the bridge. If the bridge remains within the Bridge Swinging Safety Circle, the ship is considered safe. However, if the bridge approaches or exceeds the circle’s limits, it indicates the anchor chain is fully extended, serving as a warning trigger for the OOW.
Manual Monitoring Techniques
For effective monitoring, the OOW should:
- Identify two or more distinct and easily visible points (day and night) on shore or radar, ensuring they form a significant vertical angle.
- Draw safety bearings, safety distances, or both from these points. Bearings should be clearly marked.
Position Fixing and Monitoring
The OOW has a higher workload when using paper charts since positions must be fixed manually and verified through position lines. GNSS data can be used to assist but must also be cross-checked with other methods.
- Fix positions using a light pencil and mark the time at each fix to create a record of the ship’s past positions (at least 3-4 previous fixes).
- Continuously monitor the Safety Lines for indications of any deviation or anchor dragging.
Supportive Tools
Some GNSS systems offer an “Anchoring Watch” mode. During anchor let-go, the OOW can activate the relevant function to automatically define the Safety Swinging Circle based on the ship’s position. The system then monitors this area and raises an alarm if the ship moves outside the pre-set circle.
While GNSS anchoring features provide useful support, they should not replace visual monitoring or manual checks. The OOW must remain vigilant and take action well before alarms are triggered to ensure the ship’s safety.
Watch during anchorage
The Officer of the Watch (OOW) plays a critical role in ensuring the safety of the ship during anchorage, with responsibilities that go beyond position monitoring to include traffic, weather, and operational oversight:
#1 Traffic monitoring
The OOW must continuously monitor nearby traffic conditions using both radar and ECDIS displays. Radar is particularly crucial for detecting small vessels, boats, or other craft operating without AIS, which are common near coastal areas. Active communication with passing ships is essential, and the OOW should consistently monitor designated traffic channels (anchorage/port) or VTS channels when in a Vessel Traffic Services area.
#2 Weather and environmental monitoring
Keeping an eye on weather and environmental conditions is vital for anchorage safety. Rapid changes to adverse weather may leave insufficient time to respond effectively. The OOW must remain alert, closely follow weather forecasts (including updates from local radio warning stations), and promptly inform the Master if prevailing conditions differ significantly from those at the time of anchoring.
#3 Operational oversight during anchorage activities
When the ship is engaged in other operations during anchorage, such as bunkering or ship-to-ship (STS) transfers, the Bridge OOW must not be assigned additional duties unrelated to watchkeeping. Past incidents during anchorage have often been linked to the OOW being absent from the bridge or distracted by other responsibilities.
#4 Equivalence to transit watch
An anchor watch is as crucial as a watch during transit. The OOW’s duties are similar in both cases, with the main difference being the absence of a passage plan during anchorage, which is replaced by an anchoring safety procedure. Masters’ standing orders should provide clear guidance on when the OOW should notify the Master during anchorage, just as they do during transit.
#5 Incorporation in company SMS
The company’s Safety Management System (SMS) should explicitly reference anchoring watch procedures and include a dedicated Bridge Watch Condition for anchorage. This ensures consistency in watchkeeping standards and reinforces the importance of proper oversight during this critical phase.
Visual bearings during anchor watch:
Choosing a pair of Lights/fixed objects in line to check vessel position is a quick and efficient way to determine if there is any sign of dragging. Ideally the lights/ objects in line should be on the vessel’s beam.