Saturday, May 10, 2008

Guidelines For Avoiding Hurricanes At Sea

Regarding Hurricanes at sea, Bowditch warns: “Even the largest and most seaworthy vessels become virtually unmanageable,and may sustain heavy damage. Less sturdy vessels do not survive.” Adm. Nimitz commented: “The time for taking all measures for a ship’s safety is while still able to do so.”

The U.S. National Weather Service has guidelines for avoiding hurricanes in the North Atlantic Basin, the web page is here. From that site:
Hurricanes have been the cause of many maritime disasters and unfortunately, there is no single rule of thumb that can be used by mariners to ensure safe separation from a hurricane at sea. Instead, constant monitoring of hurricane potential & continual risk analysis when used with some fundamental guidelines become the basic tools to minimize a hurricane's impact to vessels at sea or in port. Today, even as our understanding of hurricanes increases, there is still much error inherent in forecasting the movement & intensity of these systems. Through the use of a recurring risk analysis, mariners can minimize potential impacts of a hurricane encounter.
The core of the guidelines is the 34 kt rule and the 1-2-3 Rule. From the NWS:
"The 34-knot wind radius rule states that ships should stay outside the area of a hurricane where winds of 34 knots or greater are analyzed or predicted."
According to the NWS:
"The 1-2-3 Rule is the single most important aid in accounting for hurricane forecast track errors (FTE). Understanding and use of this technique should be mandatory for any vessel operating near a hurricane."
The 1-2-3 Rule uses:
1. the 24 hr forecast + radius of 34 kts winds + 100 mile error radius
2. the 48 hr forecast + radius of 34 kts winds + 200 mile error radius
3. the 72 hr forecast + radius of 34 kts winds + 300 mile error radius

The result is the diagram above- also here (pdf)

NWS warns:
The 1-2-3 rule establishes a minimum recommended distance to maintain from a hurricane in the Atlantic. Larger buffer zones should be established in situations with higher forecast uncertainty, limited crew experience, decreased vessel handling, or other factors set by the vessel master. The rule does not account for sudden & rapid intensification of hurricanes that could result in an outward expansion of the 34 KT wind field. Also, the rule does not account for the typical expansion of the wind field as a system transitions from hurricane to extratropical gale/storm.

There is more detail in this publication on pages 45-50: Mariner’s Guide For Hurricane Awareness In The North Atlantic Basin (its a 60+ page pdf file)

The original Mariners Weather Log Aug 1999 article "Hurricane Avoidance Using the “34-Knot Wind Radius” and “1-2-3” Rules by Michael Carr,George Burkley and Lee Chesneau is here.

In avoiding heavy weather, the rule is safety first of course but, being mindful of commercial pressure, avoidance actions that are both effective and efficient is optimum. Achieving this requires methods that are appropriate and systematic.

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