- Education and Science
Tropical Revolving storm (TRS)
Tropical revolving storm is the standard nautical term used for cyclone.TRS is also termed as typhoon, cyclone, gale, willy-willy & hurricane etc. These names indicate from which ocean regions these originate. Information regarding TRS & its predicted path is very essential for mariners since TRS can heavily damage the Ship’s structures, causes huge cargo claims & it can cause the lives of the crew. Ship’s structure damages include losing anchors, damage to forward anchor windlass, damages to cranes & superstructures, damages to accommodation & sometimes losing the lifeboats etc. Since the vessel pitches badly, the propeller partly submerges increasing the main engine rpm resulting more load on Engine. Regarding losing cargo especially on container vessels lashings can give up & containers can topple into sea, as these containers float as partly submerged & these are not visible to other ships which can cause severe navigational hazards. In bulk carriers these TRS can even break the ship into two pieces. TRS causes rough seas & because of it visibility will be restricted & radars will form clutters. The ships are more prone for collisions. When you come to crew matters during TRS ships roll & pitch heavily. These rolling include synchronous & parametric rolling, because of this crew feel seasickness & day to day work is disturbed. Preparing meals for the crew also becomes difficult task as the ship severely under pitching & rolling. Because of the above reasons knowledge & understanding of TRS is very essential for mariners.
TRS normally forms in the equator region between latitudes 20 degrees north & 20 degrees south. For the TRS to form sea temperature should be at least 26 degree Celsius. The air on top of the sea surface get heated by sun & rises up creating a very low pressure area , to fill this void area the cool air around that region rushes. Because of the coriolis effect of the earth the air bent inwards & spiral upwards with massive energy. The air rotates & accelerates, picking up the surrounding air & starts moving with massive power. Its diameter can extend to about 1000 nautical miles & its effect can be felt even 500 nautical miles away from the TRS. It rotates clockwise in earth’s southern hemisphere & anticlockwise in northern hemisphere.
The movement or the path of the TRS can be predicted, but there is no assurance that TRS will follow the exact predicted path. Normally in northern hemisphere it moves West-northwest direction until 20 degrees latitude then re curves to North-easterly direction. Its initial speed may be 10 to 15 knots & after curving it will drop then increase up to about 25 knots. In southern hemisphere TRS moves in West-south west direction until 20 degree latitudes then re curves to South-easterly direction. Sometimes in both hemispheres TRS continue its direction without re curving or moves in hairpin direction making loops.
The main parts of a TRS are Eye, Eye wall & outer storm area. The eye is also called as vortex. It is a calm, low pressure area extending to about 30 nautical miles. The pressure is about 60mb below normal pressure. The sky is clear or partly clouded & visibility is poor. The eye wall is the area extending to about 50 miles beyond the eyes outer perimeter. The wind is very violent & sea is extremely rough. The sky filled with Nimbostratus clouds & the visibility is very poor. This area is the most dangerous area. The outer storm area is the surrounding area beyond Eye wall. Its width varies accordingly. In the above region wind is violent & gale force seas will be observed. The sky is clouded with cirrus clouds & good visibility.
Other terms associated with TRS are Track, Path, Vertex (also called as Cod), Dangerous semi circle, Navigable semicircle, Dangerous quadrant (also called as advanced quadrant) etc. Track & path are entirely different things. Track is the line connecting the past positions of TRS. Path is the predicted line which the TRS is expected to travel. Vertex is the predicted position where TRS is going to re curve. Dangerous semicircle is the semicircle of the TRS which lies on starboard side on northern hemisphere & port side on southern hemisphere when facing the TRS. Dangerous quadrant is the fwd quadrant, of the dangerous semicircle.
The signs of the approaching TRS at sea are as follows. The pressure change such as diurnal variation is 3mb below the mean for the time of year in that area, and then the TRS is expected. If the pressure is 5mb below mean for the time of year in that area means you are there in the vicinity of the TRS. The clear change in wind direction & force in a long low swell sea etc. The TRS warnings are received by ships via sat c, radio telex, navtex & weather facsimile etc. The ship has to send an obligatory report to nearest coast radio station & to surrounding ships if the TRS is suspected. The obligatory report solas regulation & it must contain at least ships name, MMSI, position, date & time, & the predicted position of the TRS. The actions to avoid TRS vary. Identify the centre of the TRS. Face the wind & as per buys ballot’s theory, TRS’s centre will lie 8 to 12 points on starboard side in northern hemisphere & port side in southern hemisphere. In northern hemisphere wind veers means you are in the dangerous semicircle & backs means you are in the navigable semicircle. In the southern hemisphere the effects are vice versa. If the wind is steady means you are in the path & this applies to both hemispheres. Plot the TRS’s position on the chart. Construct a circle having the same radius of the TRS, construct tangent lines to above circle 40 degrees from the forecast path. Construct quadrant from TRS’s centre equal to 24 & 48 hours of movement of the storm. The 24 hours quadrant is known as imminent danger area & beyond that up to 48 hours quadrant is known as probable danger area.
Now take evasive action. Never allow the ship to get into dangerous semicircle & make evasive manoeuvres to stay safe. In close proximity to land & if the ship is anchored heaving up the anchor & proceeding to sea is the best & right thing to do, because there is enough sea room to make evasive manoeuvres & to escape. If possible take extra ballast to make the ship stable. No ballast tank should be partly filled, because of free surface effect it will increase the rolling of the ship. Slight trim by head can reduce the rolling but in the other hand it may reduce the fwd buoyancy, so it can be done after calculating the buoyancy forces & stability. No crew to go on deck for normal maintenance work & chief officer’s permission is required to go on deck for any other reason. Duty officers must be careful when going onto bridge wings & reduce the vessel’s speed if necessary. Some masters talk about taking the ships into shelter areas, but practically if your shelter area falls in another country’s territorial waters ship should call for port of refugee. If the depth of the shelter area is less, then ship can ground or damage the bottom due to rough weather. Most of the time, this idea is not practical. The good Master will always give more priority for his crew & vessel than the commercial factors.