Vessel Interaction: Have You Considered the Possibilities?

One radio call you do not want to hear from the gangway watch during cargo operations is, "The ship is moving!" Unfortunately, this is exactly what was heard during a recent port call. Shortly after berthing and starting cargo operations on a container vessel with multiple gantry cranes, the vessel started to move ahead. The first warning was by an alert seaman at the gangway.

When vessels move through restricted waterways, whether it be depth of water under keel, width of the waterway or combination of these and the block coefficient of the vessel, the water has to go somewhere. The water moving manifests itself as wake, wash and forces acting on other vessels and fixed and floating objects (FFO). The effect that these have on other vessels (or FFO) must be taken into account.

^{Courtesy of West of England P&I}

As can be seen in the above graphic, the initial motion as the underway vessel approaches is for the berthed vessel to slowly surge away from the approaching vessel. As the underway vessel closes, the berthed vessel will surge back towards the approaching vessel with that end of the berthed vessel moving off the berth. As the underway vessel passes abeam, the berthed vessel will be drawn bodily off the berth. Finally, as the underway vessel passes, the berthed vessel again surges in the direction of the passing one with that end of the berthed vessel moving off the berth.

The amount of force that is imparted to the berthed vessel in terms of surge and motion on or off the berth is dependent on a number of factors. They include:

•  The speed of the passing vessel, with the hydrodynamic force imparted being approximately proportional to the square of the vessel’s speed through the water.

•  The passing distance, with reduced distance leading to greater hydrodynamic forces.

•  The under keel clearance and variations in depth. The smaller the under keel clearance the greater the hydrodynamic force.

•  The relative displacement of both vessels.

•  The orientation of the berth to the passing vessel.

•  Whether the berth is open to water on all sides such as at a sea island, or only on one side, for example at a river berth.

The two that have the greatest effect are the speed of the passing vessel and how close it passes to the berthed vessel. These are also, ironically, the ones that can be controlled by the crew of the passing vessel.

These forces can result in damage to vessel's accommodation ladders and mooring lines, as well as dock structures such as unloading arms for tankers, cranes or fendering. As the vessel surges up and down the dock, the vessel's accommodation ladder may come in contact with mooring bollards or other structures on the pier damaging both. In terms of mooring bollards, the accommodations ladder and associated structure such as turntables normally come out on the losing end.

In the event your vessel starts to surge, the gangway watch has two options. If the gangway is set up to do so, the accommodations ladder can be quickly heaved off the dock. Personnel on the ladder may prohibit doing so immediately, so if there is a delay and the vessel is already moving bodily off the dock, the best option may be to pay out the hoisting wire so as to not pull it off the dock and into the water. The best option would be the former, however, as mooring lines may part and there may become no option of keeping the accommodations ladder on the pier.

Mooring lines will stretch - to a point. When they part, the force that is acting on them is considerable, likely over 100 tons, which is then transferred into the line, causing snapback injuries and damage. In the event the mooring lines are set on constant tension winches, the winches and brakes will be set to render at a force less than the breaking strain of the lines.

When a line is renders, the lines acting in the opposite direction will then start heaving to maintain their own constant tension. For example, in the above graphic, as the berthed vessel initially surges ahead, the stern lines and headsprings will likely render with the headlines and backsprings taking up the slack. The vessel will appear to "walk" down the dock. It is for this reason that ports with frequent passing traffic, such as Antwerp, Belgium, have port regulations that prohibit all mooring lines being in constant tension at the same time.

If your vessel is berthed and affected by a passing vessel or if your vessel is accused of affecting berthed vessels or FFO as it passes, there are a number of things to do:

•  Backup the VDR/ECDIS data.

•  Notify the terminal or port authority.

•  Obtain a list of all vessels that passed or were passed at that time.

•  Contact vessel's technical management and P&I club.

Maintaining proper situational awareness is key to both preventing interaction with other vessels or FFO when your vessel is in motion or anticipating when your berthed vessel may be at risk. As master, pilot or deck officer on a vessel, watching the effect your vessel is having on the surroundings is critical. Whether it be a large wake that may affect recreational vessels or beachgoers or the subtler (but quite dangerous) interaction with berthed vessels, one must perceive what is happening, comprehend the effect it may have and then project the same into the future.

Likewise, the deck watch officer or seaman of a berthed vessel cannot turn a blind eye to what is happening in the water around them. Collisions happen. Interactions with over vessels happen. Knowing what is moving (or supposed to be moving) in the water around you will help you anticipate problems developing in time to mitigate the outcome. Proper tending of mooring lines, ensuring they do not become slack will minimize the effect of surge. Newton's first law of motion states that an objection in motion will tend to stay in motion until acted upon by an equal and opposite force. Keeping your berthed vessel from getting in motion (properly tended mooring lines) is a big start.

Lessons Learnt

•  Passage planning in confined waters should take into account both safe speed and compliance with regulatory speed limits. The speed of the vessel should be closely monitored and adjusted as required.

•  Areas where speed limits are critical should be identified and discussed during the Master/Pilot information exchange.

•  Good bridge resource management would require any potential non-compliance with speed limits to be brought to the attention of the master and/or pilot.

•  Necessary reductions in speed should be made gradually and in good time as circumstances allow.

•  Master, officers and pilots should be fully aware of the effects of interaction on draft, maneuverability and wave generation in restricted waters.

•  Vessels berthed alongside river/channel berths should ensure that moorings are frequently tended and not allowed to become slack.

Let's be safe out there!