In the early stint of the previous century, most of the existing offshore technologies like rigs, platforms, oilwells, floating drydocks, drilling rigs,etc. were directly towed across the seas in what was known as "wet tow". The mobilization was long, tedious,cumbersome and often posed risks of on-voyage damage. By the end of the 1960s,the towage companies realized that if they could manage a large barge-like vessel that would carry on themselves all these bulk items in what was conceptualized as the " dry tow", things could be easier. There was this ideal of carrying a floating cargo as a cargo on another floating object! All these had a bottom reaction" having a capability of a submergence stern resting on the seabed. In 1976, a renaissance was sparked off when the first heavy-lifter semi-submersible barge 'Ocean Servant I' entered the heavy lifting market. the induction of the buoyancy casings at the four corners enabled it to remain afloat and submerged without the need of bottom reaction. It had two 500-hp omni-directional propellers and was still self-towed. In 1979,the 1st self-propelled semi-submersible mighty barge 'Super Servant' came into the limelight.
Heavy lift ships to be precise are hefty and ginormous special-purpose vessels that are about 3000 times the capacity of a blue whale! Sounds quirky, isn't it? They miraculously have a lifting capacity of over 10000 tons. These ships have a large, wide and longish carriage deck which is mainly allowed to submerge below the water line and large bodies like oil rigs or whole ships are allowed to float on it or mounted on a dry dock. The alternation in freeboard and buoyancy is carried out by suitable ballasting and de-ballasting practices. The deck, henceforth comes out or remains submerged in water in such cases. Probably, you may wonder out of curiosity as to how the ship still remains unsinkable even after carrying such brawny loads. Well, we do give a quick glance at one of the most quintessential aspects naval architecture namely, draught, floatation and buoyancy .
A brief recall of Buoyancy and Displacement
We all since our good old school days are not alien to a fundamental concept of Archimedes' Principle. According to the statement, it is said that : "When a body is completely submerged in a fluid, or floating so that it is partially submerged, the body is subject to an upwardly acting buoyant force, which in turn is equal to the weight of the fluid displaced by the body."
For high stability and to prevent heeling of the vessel sideways, stability is a quintessential aspect in such heavy lifting operations about which will be discussed shortly. However for a simple piece of reasoning that a ship does not lose its trim and tilt at one end, there should be a uniform weight distribution. Hence for a Heavy Lift Ship, the load is generally kept on the larger span of the weather deck which on many occassions remain submerged beforehand to handle larger displacements(and hence larger load capacities).
Fig. 3 Floating of a ship and its loss of floatation (Image Courtesy:www.schoology.com)
Fig.4 Heavy Lift Ship carrying a smaller vessel.Has a submerged main
deck (Image Courtesy:www.amusingplanet.com)
Stability factors in Heavy Lifting Vessels
Loading and heavylifting operations in vessels are not only a matter of how much load a ship can carry but how much does it survive under this load. If you are simply allowed to troll on a walkway with a medium sized brick you may do that easily. But what if the load is being increased persistently and if you are asked to carry a tree trunk ? The probable answer from the majority is that can't.But if momentarily you imagine yourself to be a giant and somehow manage to lift it up, what will be your status? Will you manage to keep yourself upright? And even if so, till how long? Suppose, someone gives you a slight push. Will you be there like before or tumble over? Well same is the case of ships. Stability is an inherent factor for a small boat as well as a heavy lift vessel. In a completely undisturbed condition, or in an equilibrium case, a a given displacement the buoyant force balances the weight acting vertically downwards. Hence in such a case , the centre of gravity(C.G.), the centre of buoyancy(C.B.) and the geometric centroid( which in almost all the normal cases coincides with the centre of gravity) lie on the same line. In such a case, the ship is said to be stable. Considering some external disturbances like wind or some high waves, the stability is said to be lost. Although, by property the C.G. remains intact the position of the C.B. changes. The point of intersection of the original line passing through the C.G and the initial C.B. with that of the line passing through the final position of C.B is termed as the metacentre M.
Fig.5 The position of the various points in an unstable condition (Courtesy: www.marineinsight.com)
In all such heavy lifting errands, like in heavy lift ships, there is a rigorous change in C.G due to loads and also due to operations like lifting through cranes or derricks. This uneven distribution of weight in all such ProjectCargoes often cause the C.G. to be on either side of the centreline. Hence the ship has a high probability to heel towards that side. On normal day-to-day ferries or other ships, the stability mainly gets lost due to two reasons; either shift of C.G due to uneven loads or direct and abrupt change of C.B. due to external conditions leading to heeling. However, in heavy lifting operations, mainly, the culprit is the biased CG.
On one side that causes the vessel to list henceforth. As depicted in the above figure, there is a normal GZ, from the initial C.B to the line passing through the final C.B and is called the righting arm. This can cause a righting moment to the ship and may attain back its position. A negative GM is always uncongenial for the ship which may lead to its capsizing of the ships( which is a pertinent disaster !). Also by the virtue of a reverting counter buoyancy, the ship may, in certain chances attain back its stability,i.e, become upright. This tendency of a negative inherent stability is measured in terms of a certain Angle of Loll. It is the state of the ship which is unstable when upright. It has a dramatic property property to lurch to another side on the action of external forces producing the same value of Angle of Loll on the opposite side. Remember, do not confuse it with list!! Listing is caused by the sifting of cargo and material grain causing a change in C.G of the body. But this counter event called Loll is just a reaction force in counter to Rolling. In case of a suspicion that the vessel might be subject to Loll, we must do something which may sound quirky! Ballast or do some weight additions to that side which has the loll or the greater inclination downwards. This dramatically depletes the severity of the Angle of Loll, but provides a boon at the end.The draft increases drastically and the freeboard decreases along with the so-called free-surfce effects. The C.G. immerses further and the greater submersion of weight increases the "Buoyant Reaction", which in turn creates a negative Angle of Loll at that condition.
Fig.6: Angle of Loll(Source:Googleimages)
Another simpler implication is to just undo the cause for this loll.Mainly, the majority of these events are caused by heavylifting activities on the superficial weather deck that drastically shifts the Centre of Gravity to the heavy load carrying side. Cranes, Derricks or other facilities carrying out these operations are the other culprits, which along with the weights they are carrying act as moment arms, causing a resultant tilt. Maybe, the following image explains it better!
Fig.7.: Heavylifting operations on a ship and the resultant shift of C.G (Courtesy:www.sphlashmaritime.com)
So, for a heavy lift ship, care must be taken while loading and offloading operations in the deck such that the ship is free of loll. As a Heavy Lifter is designed specifically for loading purposes, it is not as if heavylifting operations should be avoided. But the weight should have even distributions and as it involves weights in the order of 10000 tonnes, things have to be in line! Most of these have modern ballasting technologies,thrusters and Dynamic Positioning Systems. Heavy Lifters, an alternative to the older technology of heavy lifting cranes or projection cargoes straddle the cargo(jackets, rigs, oil wells, semi-submersibles, ships etc.) uniformly on both sides and allow proper suspension of weight in a manner that avoids Loll or list under normal circumstances.
The Cargoes and its Sea keeping moves
The versatility of the Heavy Lifters coupled with its mammoth capabilities enabled it to be the workhorse for several varieties of cargo like the floating plants, drydocks, rigs, Pressure vessels,cranes, jackets just to name a few. Even the navy vessels which were of high precision were often transported by the virtue of these which substantiated their flexibility. The serviceability limit weights by the order of 4000-5500 tons which leapfrogged to 21,000 plus tons for the largest rigs in the modern days. Marine technologies which has contributed significantly in the revolution of these vessels, has further diversified along with the modern day computational software for complex motion and vibration responses. These ships are subjugated to a high ordeal of strength and endurance testing which affirms its competence in their operations. Well, model testings are an indispensable part in any form of engineering and large scale productions. Heavy lifters are not an exception with motion response tests which are objected to mainly test the non-linear behavior of the ships in rolling motion.
Some of the important cargoes which have been transported so far by these ships are:
· Jack-ups ' Norbe 2 and 5 :
In 1988,
these two mighty jack-ups were transited from Salvador, Brazil to
Mumbai( erstwhile Bombay)in India. They had a projection lengths of 4.5 metres
and had spud can diameters of 14 metres. If not for these ships, then the net
transit time would have been a matter of 150 days. The footings posed a great
deal of trouble and in case of direct stowage would have resulted in a drag
speed of 3-4 knots which would have made things cumbersome. These were carried
out by the 'Mighty Servants 3' and made it possible in a matter of a little
over three weeks( almost a month!)The legs were for stability placed on the
cribbling blocks for the ease of dynamic loading. The lower deck was cut off to
accomodate the rigs upto the safe submersion limit and seafastenings were
provided on the aft legs of the rigs.
· Tension-Leg Wellhead Platform :
In
1989, the first ever Tension Leg was transported by the Heavy Lift from the U.S
Gulf. It was carried out by the same 'Mighty Servant I' was a float-on type of
loading mechanism. The truss deck was carried out by four columns each of 12.2
metres in diameter, 46.2 in height and was a space frame structure of spacings
in the order of 42.2 metres. Pontoons, about 7 metres in diameter connected the
columns near the base.
· Challis Field SALRAM :
The Challis
mooring system consisted of a 121 metres riser attached to a 34 by 34 gravity
base structure. This base structure weighing about 4500 tons was put to board
on 'Mighty Servant 2' around mid of August 1989. To facilitate the proper
spacing, the ship's starboard buoyancy casing was shifted forward
to accommodate the entire structure.
· Damaged Tanker 'Imperial Acadia' :
You all must have pondered upon the speculation of the operational ships,
vessels and structures from one place to another as per the convenience.
Have you ever wondered what would happen if a heavy, expensive and
high-precision vessel is stalled amid the sea? Or some parts or crucial
machinery are broken down? Wet Towage across the seas would be a pursuit of
money, time and most importantly a factor of greater risk. If physically some
parts of the structure is seriously damaged, forcefully towing it across the
rough waters would exalt the chance of damaging it further. Many of the naval vessels
or crafts when damaged are carried up by the Heavy Lift Vessels as they are of
indigenous design and mechanisms worth millions. A lot of U.S Navy minesweepers
have been barged up on 'Mighty Servants I, II and III'. The demobilization
of the Canadian giant 'Imperial Acadia' de remains a milestone in the
achievements of Heavy Lifters. With a length of 135 metres, this high capacity
Canadian Tanker was caved 1.5 to 2 metres on the lower starboard side in
the event of a frivolous sea-storm near a french island. Mighty Servant I
performed the tedious task of safely towing it on-board to a shipyard in
Halifax for hull repairs.
· Floating Drydock 'Karish' :
Drydock facilities are developed generally
for still water conditions. Hence forth their mobilization in wavy, gushy
seawaters account the dynamic water loads acting on them. So mobilization
through the Heavy Lifters through 'dry tow' which affixes it above water
eradicates the need of greater steel strengthening or severe damage in case of
accidental loads. The offloading of 142 metres long, 7500 tons heavy the
'Karish' dry-dock at Batangas, Philippines by Transshelf Heavy
Lifter is reckoned as a notable event in the history of Heavy Lift Ships. Today
several such drydocks or floating jetties, ports are shouldered upon by them in
case of their relocation from one place to another.
The Reigning Giant-Dockwise Vanguard
The Dockwise Vanguard is the largest Heavy Lift Ship which is owned and operated by the Dockwise B.V. A sprawling 70 by 275 metres flat upper deck, it can lift upto 110000 tonnes of weight. Its propulsion is by the virtue of 2 giant propellers, 2 retractable azimuth thrusters and a powerful bow thruster. Launched recently on 7th October 2012, the Vanguard has earned its repute by transporting huge offshore rigs,gas facilities and other enormous vessels. It is a semi-submersible vessel that can immerse its principal deck up to 52 feet below the water level in case of deep draught cargoes. It is equipped with four strong buoyancy casings and can handle cargoes at varying depths by their ballasting measures. Some of the structural giants which were pillion on Vanguard were the Chevron's St. Malo Oil Platform, the Goliat FPSO and the Aasta Hasteen Platform, the latter two being born to Hyundai Heavy Industries Ltd.
Fig. Dockland Vanguard (Source: Googleimages)
Next but not the least- MV Blue Marlin
MV Blue Marlin, as you must have heard is the second largest ship owned by the Netherlands Shipping corp Dockwise. It has a length of 224.8 meters, beam of 63.1 meters and a permissible draught of 13.1 meters. It has a deadweight of over 76000 Tons dead weight. Some of the notable cargoes of this giant were the American Destroyer USS Cole, oil platform Thunderhorse and the amphibious Australian warship HMAS Canberra, just to name a few.LSD
Fig. MV Blue Marlin (Source: Googleimages)
Article by: Subhodeep Ghosh
