VLCCs, because of their inertia require great caution while anchoring. They can suffer equipment failure if attempting to anchor whilst moving at speeds as low as half a knot over the ground. Hence, the vessel must be nearly stopped not only in the linear direction but axial too, meaning the bow should not be swinging much either while anchoring. The depth at which the vessel can safely anchor is about 110m or less, beyond which the windlass may have extreme difficulty in recovering the anchor.
VLCC in deep water : Anchoring into “Deep water” which is defined depth of water is beyond 50 meter, must be carried out with “Walk-back Style, however, on the VLCC operation there exists such a big inertia, Master can treat with Walking-back style even in the anchorage where water depth less than 50m, if following conditions are to be forecasted.
a) Strong wind / current effect.
b) High density anchorage or anchorage with restricted area;
c) Indistinct depth of water or bottom material.
If the area where water is depth beyond 50m, master should be considered he is going to anchor into the “deep water”, all chains must be drawn out by “Walk-back Style”.
Ship’s heading & speed before commencing Walk-back chains
Ship’s heading on final approaching should be faced into the Wind and Current which can provide good advantage to control good ship’s pose (heading) when Anchor just Folding and also for Minimizing of Swinging rate to windward or current-ward.
Ship’s speed during Walk-back chain must be controlled as “0” with speed over the ground by GPS and/or Doppler Sonar, or less than speed of Windlass Walk-back Capacity (Speed).
Having established most likely position for the anchor: confirm safety of actual anchorage location (length of cable + length of vessel) consider navigation hazards, tidal factors, other vessels, sea state and swell plot bridge (or radar) swinging circles create visual clearing bearings/ radar clearing ranges brief the anchor watch.
Although, on R-FPP (Right turn Fixed Pitch Propeller) vessels, when ship is propelling with astern engine, ship’s head turns to right as on theoretical, however, it’s to be affected greatly by the primary inertia.
Swing-way shall be estimated totally with following factors :
a) Character of movement while using astern engine on FPR vessels.
b) Character that ships heading turns to windward.
This factor is remarkable as in ballast condition.
c) Drifting value of wind and/or current to lee side.
This factor is remarkable while ship’s proceeding with low speed.
d) Trend of Wind / Current direction which ship’s heading will follow.
e) Since in each case, Final inertia -way prior to anchoring is most affected to movement during Walk-back chains, this inertia can be managed occasionally with One Kick of Ahead Engine with Hard Angle of Rudder.
To plan as a gradual decrease as to secure remaining speed over the ground “0” by GPS speed when commencing to “Walking back Chains” by means of operating main engine to ahead and astern.
Including anchoring at strong current area, it’s required to confirm if the ship has been stationary with O.G. speed by GPS and/or Doppler Sonar, also by all means of followings.
For the sake to be sure if ship has stopped moving completely, the master should check with following signs totally, even if no landmarks and signals are available to use.
a) Reports by Bow anchor watchmen with observing the condition of the cutwater on stem (and/or the anchor chain).
b) Same as above, it’s a good practice to use Hand-lead to check the fine movement.
c) GPS speed monitor on ARPA showing slight positive speed with the Owned cursor directing towards almost vertically with ship’s heading.
d) Astern propelled wave has reached to forward of living quarter.
Walk Back speed
On the standard, the weighing chain speed is 15cm / second (= one shackle 27.5 meter / 3 minutes). Its corresponding to 0.3 kts of ship’s speed. It means ships speed with combining turning rate and drifting to wind and current lee side should be controlled less than 0.3 kts against direction anchor chain is going.
Do not use propelling beyond maneuvering Half Ahead / Astern Engine, except in emergency cases, once walk back is commenced.
Communication between commander in wheel house and bow anchor watchmen
For the good image as macro-wise to actual ship’s pose and condition of anchoring progress, Bow watchmen must report following to the Commander on the Bridge.
a) Approaching speed should follow a gradual decrease plan.
Before arriving at anchorage, Eye estimation speed by bow watchmen should be verified with GPS or Doppler log.
b) Reducing speed for anchoring.
Please note ship’s speed is corresponded as follows
1.0 kts = aprx. 50cm / sec
0.5 kts = aprx. 25cm / sec
0.2 kts = aprx. 10cm / sec
c) Anchoring speed (Going walk-back).
If ship is stationary just prior to anchoring with observing cutwater at stem and anchor chains in water
d) Status chain number have been drawn on each shackle interval.
It is preferable to report as “XX shackles in the water” in order for the commander to easily visualize Length of chains being in the water and can estimate timing of touching anchor with bottom. both commander and bow watchmen must acknowledge the length of anchor shank when taking account length of cable.
e) Anchor chain direction and tightness after bottom touch.
Report shall be combination of following factor
For Horizontal direction:
“clockwise” and “straight down” (or upright)
For Vertical direction :
“as Long hand (of the clock)”
“as Short hand (of the clock) ”
For Tightness :
“Strong tight”, “Moderate tight, “Easy tight” and “Slack”
Ex. Chain – 2 o’clock – as short hand – with moderate tight
Arriving at the anchoring position :
Observe the other anchored vessels. This will give a good indication of the appropriate heading. Before dropping anchor, ensure that the charted depth corresponds to to the depth shown on the echo sounder after adjustment for draft and height of tide. The echo sounder can provide a good indication if set correctly to take into consideration the draft correction.
The anchor position :
Take a position when the anchor is dropped. This will give a good idea where the anchor is. Use more than one method of fixing the position. Visual and electronic methods should be used. Verify the vessel’s position when she has brought up and settled down. Draw the estimated swinging circle on the chart. Check the position frequently by visual and electronic means. Set any alarms fitted to electronic equipment. If you think the vessel is dragging call the Master and engineroom. Inform other vessels.
The status of brought-up anchor should be confirmed as holding chains by windlass brake after drawn out required length of chains.
Securing the cable when at anchor :
After the vessel is brought up, place the guillotine bar or other chain stopper device into the locked position, tighten the windlass brake and disengage the gear. The chain stopper must be properly located between links and secured with the safety pin. The chain stopper is designed to take the weight of the cable. The weight of the cable must not be on the windlass brake alone.