The Safe and Unsafe Semicircles of a Storm

July 02, 2013

It is that time of year again when hurricanes and typhoons will affect ships and routes.

Vessels may have the speed capacity to out-run storms, but the economic climate and high fuel costs over recent years can make this prohibitive. Mariners are well aware of the erratic behavior of these storms, which, despite huge advances in forecasting technology, can shift courses not anticipated by mid-range forecasts.

Mariners will still refer to the safe (navigable) and unsafe semi-circles of a storm. These are depicted in the following image from the Hong Kong Observatory:

jul13 semicircle large fig1

These images are for the northern hemisphere, and the exact opposite is valid for southern hemisphere storms.

Understanding of the safe and unsafe semicircles can be beneficial in many circumstances as companies look for ways to save on their fuel costs. For example, in the northern hemisphere, a cape-size ship with low hire-rates approaching a typhoon or hurricane from the south can often safely drift as the ship is in the safe semi-circle and thereby save fuel. However, the captain of the same ship approaching the storm from the north needs to be very cognizant of the potential for the storm track to vary somewhat. Since that ship would be in the dangerous semi-circle, even small changes in the actual track can suddenly place a ship in a dangerous situation without the available speed for storm avoidance.

In general, northern hemisphere storms take a westerly to west-northwesterly track while a ridge of high pressure remains to the north. When that ridge disappears, the storm may enter an area where it may drift with no steering, or it may accelerate rapidly to the north in the advance of low-pressure. There are, of course, innumerable variations to the above but when storms do move to the north, they can frequently accelerate at speeds that ships cannot overcome.

In this month's example, which occurred during the last typhoon season, StormGeo worked very closely with a shipping company whose vessel was heading towards Keelung when the voyage came under the influence of Typhoon Jelawat. With 2.5 days of steaming at full speed, the vessel could have cleared to the north of the storm and approached Keelung. However, extremely high waves would have been encountered NE of the storm and the approach towards Keelung would have been in the Beaufort Force 9 conditions prevailing west of the storm.

StormGeo discussed two possible scenarios with the client. They could either order an increase in speed, pass north of the storm in the dangerous semicircle at the expense of about 80 MT fuel oil, or the client could adjust the ship's schedule and bypass Keelung on the westbound voyage, making up for this later. This is not a decision to make lightly owing to the many parties involved. However, based on the strength of the information presented by StormGeo to both the shore-based personnel and to the ship captain, a mutually agreed decision was made for the vessel to pass south of the storm.

In the following image, the full-speed calculated track of the ship can be seen north of the storm, while the actual ECO-speed track of the ship can be seen south of the storm.

jul13 jelawat large fig2

This storm track was almost exactly the same as the one described earlier. The storm was tracking NW, then curved to the north and then northeast while accelerating. 14+ meter waves can be seen in the NE quadrant of the storm and the vessel would have been approaching 11-meter waves at this time.

With StormGeo's help, the client was able to make a decision that was best for all involved and saved significant fuel with just a minor schedule adjustment. Dangerous storms can often require difficult decisions like these, and StormGeo's ship routing advisory service is fully prepared to help consult with captains and ship operators so that safe and economical decisions are made.