(Please read my previous art on grounding calculations)
In my last articles on grounding I dealt with simple formulae in respect of pinnacle grounding and I also talked about refloating with limited resources. Sometimes, it may be felt that refloating by limited means is not possible in near future. The spring tides is far away and vessel is not secured well wrt. coast line, etc. Sometimes, a good salvage company may be readily available and the best choice is to refloat at earliest as the tidal journey has already commenced from springs towards neaps. Salvage experts would also be needed in case of damaged or foundered vessel. A salvge company when approached, may need a variety of information, which would be provided by the ship.
The following information would be important in order to decide upon further action:
- An accurate position of the grounding site, including latitude and longitude, applicable chart numbers, etc. This will give time to salvage company to study the area prior reaching the site.
- Ship’s draft at last port as well as estimated draft at time of grounding. Drafts forward, amidships and aft, immediately following grounding at regular intervals. The comparison will give an idea of present uptrust and location of contact. Location of grounding point if determined and estimated ground reaction.
- Type of sea floor at the grounding site.
- Regular logging down of the state of tide.
- Soundings along the ship from bow to stern, corrected to the datum of the chart area.
- Course and speed at the time of grounding. Ship’s heading after grounding with details of changes. This will give an idea of the magnitude of impact. Extent and type of damage to the ship.
- Liveliness of the ship, to know if the ship is pivoting at the moment.
- Weather conditions to include: wind direction and velocity; current and weather forecasts if any.
- Sea and ocean tidal stream / current conditions, to include, direction and height of seas and swells.
- Status of ship’s machinery.
- Ship’s cargo list or manifest. Amount and location of known hazardous materials.
- Help available at the scene or in the area, such as tugs, large boats, bulldozers, cranes, etc.
The salvage plan defines various works to be done including:
- Ensuring the availability of equipment.
- Fixing up the dates and time for the different events.
- Chalking out, the responsibilities of each member of the team.
- Coordinating with participating unit.
The idea being the dates and time stipulated in the project can be complied with. A good salvage plan should also consider the following:
- The safety of the personnel involved.
- Safety of navigation.
- Accurate, minutes to minutes itinerary of work, considering the timings of tides;
- The resources available / required.
- The changes occurring with time.
- Areas of risk and weaknesses.
- Grounding reaction (static).
- Refloating effort, (vertical / horizontal).
- Stability while grounded and while floating;
- Strength of the hull, damaged areas and points for application of forces and lifting.
- Summary of the technique applied during refloating.
- Hydrographic features.
- Possibility of combining two or more principles to get maximum advantage.
- Working time schedule backwards from the appropriate time of high water.
The vessel cannot be under more than 10.00 meters of water, as building cofferdams for depths greater than this would be difficult and costly, due to pressures imposed to the same.
This method is the fastest for the removal of a small size vessel from the bottom. The organisation of a salvage operation with the use of compressed air requires experience by the person responsible so that it is done successfully.
The use of compressed air in refloating operations requires that any patching or repairs done to the hull of the vessel are complete and waterproof, which is not required for pumping operations. Extensive work is necessary by the divers, whether for patching the hull or stabilising the craft, and this type of work can be expensive, in terms of material, labor and time. Stability calculations required during the de-watering operation using compressed air is an important consideration.
This method requires a lot of seamanship with cables and tackles. The size of the ship to be saved is directly related to the lifting capacity available on the market.
The largest floating crane in operation probably is of a capacity to lift 14,000 tonnes when using it’s booms in yoyo. Handy sizes available are of the order 3000t.
For the success of a rescue operation on the high seas, the following requirements are basic:
- Tugboat, with suitable equipment and crew.
- Crew must be well versed with navigation especially the towed voyage.
Underwater Rapid Attachment
Steel plate patches, wooden patches, security bolts, lifting bolts, spinter boxes, cofferdams, salvage bolting, submarine bolt driving gun, etc. are available.
Submarine gun is an explosively actuated gun, which instantaneously and without previous preparation of the plate drives at high velocity, special steel alloy threaded bolts into steel plate up to 1 inch thick. The bolt is of special 100-tonne tensile steel with shearing strength of 10 tonnes per bolt. The lengthy and costly business of drilling, threading and bolting by normal methods is replaced by this most effective process. Bolts can be immediately and rigidly secured in the structure with strengths considerably in excess of the mild steel bolts usually employed. Bolts are available for wide variety of applications viz. threaded bolts, punch bolt, hollow bolts, etc. These are of heat-treated alloy steel of tensile strength of 90 tonnes per square inch. Bolting strengths are considerably in excess of those obtained with the usual mild steel bolts.
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