THE DECISION TO SORTIE OR REMAIN IN PORT
In general, the decision on whether to sortie or remain in port is largely based on the answer to a single question: Would the vessel in question be better off attempting an evasion at sea or remaining in port? In the case of Darwin, however, remaining in port is not an option because local harbor authorities state that the Port of Darwin does not have safe berths or anchorages under tropical cyclone conditions. Consequently, it is the clear policy of the Port of Darwin authorities to instruct all ships to leave the harbor prior to the passage of cyclone.
Evasion rationale should include consideration of the following general factors:
- Vessel characteristics
- Berth and anchorage conditions
- Most recent tropical cyclone warning advisory
- Tropical cyclone climatology
Individual vessel characteristics and berth/anchorage conditions are best determined by those responsible for each vessel and local port authorities. Tropical cyclone warnings are issued by the Naval Pacific Meteorology and Oceanography Center/Joint Typhoon Warning Center Pearl Harbor for the U. S. Government and by the Australian Bureau of Meteorology Darwin Tropical Warning Center. The interpretation of tropical cyclone climatology is addressed in the following sections.
Under current arrangements, the Australian Bureau of Meteorology issues a Tropical Cyclone Watch when a tropical low or a cyclone is located within the Northern Territories forecasting district, but not expected to bring gale force (34-47 kt) winds to the Darwin city area within the next 24 hours. Radio, coast radio, and television stations disseminate this watch. In addition, the Port Authority will provide written notification to all vessels in the port area. A continuous VHF radio watch on channel 10 or 16 will be maintained during this period and various procedures will be followed including but not limited to the following:
- All vessels berthed at wharves must be fully manned and secured for sea
- Double up mooring lines
- All ships outboard of other ships will be required to move to an open wharf or proceed to an anchorage
The Australian Bureau of Meteorology will issue a Tropical Cyclone Warning if a cyclone is expected to cause gale force winds in the Darwin area within the next 24 hours. Any significant changes or developments will be broadcast as "Flash" warnings and updated warnings will be issued every 3 hours and repeatedly broadcast. When a tropical cyclone warning is issued, all vessels exceeding 200 gross tons, whether alongside a wharf or at anchor, are directed to proceed to sea beyond the port limits and maneuver until the port is reopened by the Harbor Master. Vessels up to 200 gross tons must follow detailed procedures as outlined in the Darwin Port Authority Cyclone Procedures. During a tropical cyclone warning, the vessel maximum length acceptable within the mooring basin is 114.8 ft (35 m).
It is recommended that Commanding Officers obtain the latest version of the Darwin Port Authority Cyclone Procedures upon entering port and coordinate their actions with the Port Authority in the event of an approaching tropical cyclone.
Evasion at Sea
It is the recommendation of this evaluation that all U.S. Navy ships sortie from the Port of Darwin at the first indication that Darwin will be threatened by an approaching tropical cyclone. It is important to note that tropical cyclones in the Darwin area tend to be slow moving and quite variable in their heading.
Commanding officers and ship masters with access to tropical cyclone warnings and advisories coupled with OTSR services are most capable of making the safest and most prudent decision for successful storm evasion and avoidance. In all sortie situations, OTSR services from Naval Pacific Meteorology and Oceanography Center (NPMOC), Yokosuka should be requested and utilized. The best sortie route in a specific tropical cyclone scenario is largely dependent on the location of the tropical cyclone and the forecast track. During any tropical cyclone threat there is normally one to three options available to the commanding officer and ship master. Selection of the best option is the objective of all concerned!
For Darwin, the following guidelines are offered:
- Storms forming poleward of 11° S and east of about 136° E are not normally considered a threat. The storms tend to move off to the southeast or if they move west they pass overland and become general "rain depressions."
- Tropical cyclones that form to the east and move west southwest, passing over some land areas or over islands to the northeast of Darwin, are considered threats, but are generally weakened somewhat by interaction over land. An evasion route initially to the west with options of then running to the southwest, west, or north around Melville Island is recommended.
- Tropical cyclones approaching from the northwest, generally recurvers from tracks north of Melville, are the biggest threats. An early sortie could be to the west or southwest with the intent to outrun the tropical storm. Following seas should be experienced on a southwest run. Note that if the tropical cyclone continues west-southwest a vessel would be in the dangerous semicircle. This would be a reason for going early! For a late sortie after the tropical storm recurves from westerly to a southeasterly heading, evasion options to be considered include proceeding to the east through Clarence Strait or a speed run to the west and southwest.
In all cases, the timing of the evasion is affected by:
- The time required to make preparations to get underway
- The time required to reach open water and gain sea room
- The forward speed of the tropical cyclone. The speed of advance (SOA) is generally on the order of 6-8 kt in the 125-135° longitude band around Darwin with increasing SOA to 9-10 kt west of 125° and further increasing to 10-15 kt along the western coast of Australia.
- The radius of hazardous winds and seas that can adversely impact a vessel's ability to reach open water.
Remaining in Port
The Port of Darwin is not considered to be a safe place for a ship to be berthed during a tropical cyclone passage. The Darwin Port Authority states that ships would be ordered to evacuate from the port whenever a tropical cyclone threatens the port, so remaining in port is not an option. It is the recommendation of this evaluation that all U.S. Navy ships sortie from the Port in advance of an approaching tropical cyclone and not wait for the order to put to sea.