Saturday, 26 April 2014

MV Sewol - What Possibly Happened?

The case of the sinking of the South Korean Ferry MV Sewol surely has a lot of untold mysteries hidden underneath. She started heeling rapidly to her port side sometime before 08:58 AM (Korea Standard Time), then distress calls were made to Jindo Vessel Traffic Services Center (VTS). Not less than two hours later, the first ship had approached for rescue operations. But the first question that must be pinging your mind now is "Where were the lifeboats? Liferafts? Why didn't the Captain of the ship give an evacuation order with ample time in hand? "

According to the possibilities that have been taken into consideration by our team of Naval Architecture students, we have discussed possible scenarios that may have led to the tragedy which has been reported to take away 185 lives as to today (25 / 04 / 2014)

Rather than directly answering to the questions (which would be vague at this stage of the investigation), lets make this much easier with a little bit of background. The picture below shows the standard route followed by Sweol in its voyages from Incheon to Cheju do island. 

Route of MV Sewol from Incheon to Cheju do and the accident site (Courtsey: Google Earth)

If you notice carefully in and around the area of the accident site, the colour of the sea water shown changes to blackish. It is because the region is full of underwater rocks. So you must be thinking she hit a rock and got grounded. As reported by a state broadcaster, she was off her usual course on the day of accident. But possibility of grounding can still be easily ruled out. Wondering why? Understand the figures below:


Weight, Buoyancy and Vertical Reaction acting on a ship's hull when grounded.

Clear from the above diagram, the side of the hull hitting the rock will eventually tend to emerge from the water and the other side will tend to sink below. That is, the ship heels by the side which has not faced grounding impact. If one considers that Sewol's hull hit an underwater rock, then going by the capsizing pattern (she capsized by the Port Side), she must have hit the rock by her starboard side. At higher angles of heel as shown below, the opening created by the rock on her steel hull would be clearly visible. Her starboard side was intact. Grounding ruled out!

Starboard side of MV Sewol's hull didnot have any marks of grounding impact.

MV Sewol was "Ferry Naminoue" before it was renovated in 2013. What happened during the renovations is actually a question raising issue. In 2012, extra passenger cabins were added to her third, fourth and fifth decks, increasing the passanger capacity by 181. This caused a rise in the center of gravity of the ship by 0.51 meters. Passanger ships generally operate in moderate GM (metacentric height). A high GM would make the ship too stiff and uncomfortable for passengers. On the other side, an unsafely low GM is also not preferred so as to maintain the required stability margin. But the rise in center of gravity of the ship eventually caused a decrease in its GM after the renovation. It is even unknown if the owner had made any other changes on the ship after regulatory approvals were completed. 

Not only this, the ship carried more than three times of cargo weight on the day of accident. She was designed to carry cargo load of 987 tons but she had 3608 tons of cargo that day. Obviously, another cause supporting the rise of center of gravity and even decreased metacentric height.  If a ship with marginal metacentric height turns at high speed, it is likely to heel by significant angle, and in worst cases even capsize.
This scenario matches pretty well with MV Sewol's case. The Captain was possibly aware of the marginal metacentric height and this is why he may not have ordered the passangers to move to the upper decks during the first one hour, expecting that the ship's heel could be controlled, if weight of the passangers (64.2 tons approximately, if an average passanger has weight of 65 kg) were limited to the lower decks thus preventing further rise in center of gravity.

The third mate Park Han-Geyol reportedly ordered the helmsman to make a 5 degree turn, which was a part of the ship's course. But tracking data show that she made a turn of 45 degrees at a speed of around 18 knots (4 knots below its design speed). Whether this was a careless move, or a failure of the steering mechanism, is unknown as of yet. But two things can be easily inferred:
  • If it was not a failure of the steering mechanism, the human error involving carelessness of the helmsman and the navigation officers was the prime reason behind such a steep turn.
  • But a ship is not designed to heel over and capsize even when it turns steeply at its design speed. In this case, the speed was 4 knots lower. There comes in the problem of stability which Sewol surely had. She was operating at a marginal metacentric height (already explained 2 minutes ago!). The more the metacentric height of the ship, greater is the uprighting moment of a ship when it heels to either side. A reduced GM must have caused the righting lever GZ to fall below what it should have been for safe operation of the ship. And as the ship took a steep turn of 45 degrees at 18 knots (which is not much below its maximum design speed of 22 knots), the centripetal force acting on the ship caused it to heel to an angle at which the righting lever reduced to zero, causing her to capsize. Sewol obviously made this turn to her starboard, as she capsized by heeling towards port side (apply Laws of Physics and understand the figure below or watch the video below).

Forces on a ship when it turns to Starboard Side.



Sewol was not only designed to carry passengers. It had additional capacity to carry cars and containers. Reportedly, many passangers heard loud explosions after the sudden heeling of the ship. That was possibly due to shift of unproperly lashed containers and cars in the ship's hull. This might have not only gave rise to explosions, but caused the ship to heel further to the port side due to a shift in transverse center of gravity.

Also, unlike other ships, ROPAX vessels like MV Sewol are not designed with bulkheads that divide the ship into watertight compartments. This possibly caused the entire car decks to get flooded once the ship started taking in water due to large angles of heel. The problem with ships taking in water is actually something that results in exponential rise in risk of capsizing. Why? If a ship starts taking in water, the free surface generated by the water in the ship creates a free surface effect which raises the center of gravity thus rendering the ship more unstable and prone to taking in more water. Since ROPAX ship's donot have bulkheads, this problem cannot be limited to a compartment, and the entire ship floods eventually.

What can be seen from all these evidences by the help of Naval Architectural Principles is that, the prime cause of the ship's capsizing must have been the reduced stability, coupled with the human error of the crew involved in not carrying out effective evacuation procedures at the correct time.



Official investigations are on the way and actions have already been taken against the Captain and officers. Not only this, the authenticity of Korean Register of Shipping is also under investigation as there can be every possibility of illegal approval of many such similar ship designs that are already sailing even now with hundreds of lives in danger!





Article By: Soumya Chakraborty



Author's Note: This was a report of an investigation done by the team of students who own the LSD blog. Conclusions in this article have been drawn on the basis of informations obtained from News and by application of Naval Architectural principles to estimate what might have actually caused MV Sewol to sink the way it had, on 16th April, 2014. The video used in this doesnot belong to LSD and full credit goes to the owner. Thank You for reading. In case of any queries and doubts please comment or write to learnshipdesign@gmail.com

Wednesday, 23 April 2014

Autonomous Underwater Vehicles-Their Design and Functioning

INTRODUCTION


An Autonomous Underwater Vehicle (AUV), also called  unmanned underwater vehicles, are robots that perform underwater survey missions such as detection and mapping of obstructions, rocks, submerged wrecks( like that of ships) without the need for input from an operator.The first ever AUV was the SPURV (Special Purpose Underwater Research Vehicle), developed at the Applied Physics Laboratory at the University of Washington as early as 1957.
The design of these vehicles is influenced by the purpose of their operation. How the individual components function as a whole is also determined largely by the circuital pathways and of course, it is necessary to factor into account, the energy management. AUV's are sold by around 10 major manufacturers on the international market, including prominent ones like Kongsberg Maritime, Bluefin Robotics, and International Submarine Engineering (ISE) Ltd.

Collections of propelled AUVs and gliding AUVs (also called gliders) are now often used for mapping and oceanographic research, for military reconnaissance and harbour protection, or for deep-sea oil-well maintenance and emergency response. Today, fleets of up to 20 such AUVs have been deployed, but in the coming years far larger fleets could come into service.

DESIGN


National Institute of Oceanography (N.I.O), Goa,India, has developed an AUV ‘Maya’ as a tool for scientific and commercial applications. 

Equipment:

AUVs operate independently of the ship and have no connecting cables. AUVs can be equipped with a wide variety of oceanographic sensors or sonar systems. NOAA’s hydrographic survey AUVs are typically equipped with side scan sonar, Conductivity-Temperature-Depth (CTD) sensors, GPS-aided Inertial Navigation Systems (INS), and an Acoustic Doppler Current Profiler (ADCP).Primarily oceanographic tools, AUVs carry sensors to navigate autonomously and map features of the ocean. Typical sensors include compasses, depth sensors, sidescan and other sonars, magnetometers, thermistors and conductivity probes. 


Propulsion:
One of the largest design considerations for autonomous underwater vehicles (AUV’s) that have specific mission scenarios is the propulsive efficiency. The propulsive efficiency affects the amount of power storage required to achieve a specific mission. As the efficiency increases the volume of energy being stored decreases. The decrease in volume allows for a smaller vehicle, which results in a vehicle that requires less thrust to attain a specific speed.
Propeller based thrusters or Kort nozzles are the most common among AUVs.These thrusters are usually powered by electric motors and corrosion of motor internals are avoided. One consideration which impacts this process of waterproofing is the decision to use brushed motors or brush-less motors which also impacts reliability, efficiency, and cost. Propellers are usually designed with a complex geometry that changes along the blades radius. The process of selecting an efficient propulsive system becomes an iterative process between motor, propeller, and battery storage. 

A Kort Nozzle is a propeller fitted with a non-rotating nozzle.

Navigation

AUVs navigate using an underwater acoustic positioning systemWhen some reference such as a support ship is available, baseline  positioning is used to calculate where the sub sea vehicle is relative to the known (GPS) position of the surface craft by means of acoustic range and bearing measurements which is some sort of a via mechanism.Orientation (including heading) is determined by in an alignment process where the Kalman filter uses gyros and accelerometers to determine local gravity vector and the Earths Rotation which are essential in navigation.Alignment is done both statically and while in motion.
Power
AUVs use many of the existing rechargeable battery systems in existence today, lithium ion(the one in an average mobile phone), lithium polymer, nickel metal hydride among others , and are having some sort of system for battery management. Some vehicles use primary batteries . Some larger vehicles are powered by the extremely efficient semi-fuel cells, but these require proper handling and up keep,also they are not cheap and require disposal of wastes. An emerging trend is to combine different battery and power systems with supercapacitors. High density energy sources are what designers look for today.

The battery compartment showing the 50 kWh Al/HP semi fuel cell. 


AUV BLUEFIN 21 : MH 370

Bluefin-21 AUV is currently completing mission ten in the underwater search area. Bluefin-21 has now completed more than 80 per cent of the focused underwater search area. No contacts of interest have been found to date.

                                                

As per the reports of the indianexpress ,underwater search for the crucial flight recorders of the crashed Malaysian jet can be completed , provided the weather is favorable for the AUV. The focused underwater search area is defined as a circle of 10 km-radius around the second Towed Pinger Locator detection which occurred on April 8.Don't forget to watch the video below.



Finding the black box and the wreckage are crucial to know why the Beijing-bound plane veered off from its route and mysteriously vanished after taking off from Kuala Lumpur. The mystery of the missing plane has continued to baffle aviation and security authorities who have so far failed to trace the aircraft despite deploying hi-tech radar and other gadgets.

THE CHOICE

There comes a choice in the basic design form which influences the hydrodynamic properties of flow around it and which in turn influences the performance and effiecieny. Something which helps in the analysis of the flow are computational flow techniques (CFD), now according to this approach, the design form (B) on the right has a more streamlined form and has better flow charateristics around it, reduces power consumption. 




However the design (A) on the left has better storage and larger equipment carrying capacity. The choice of design is now largely influenced by the needs of the owner or organisation. Both designs have been used, but an AUV equipped with the most routine equipments and basic power storage for a medium endurance mission generally edges in favour of the cylindrical body shape. Having said that, there are underwater semi-submersibles and robots which come in a range of shapes and sizes including the design (B). Factors worth considering before making the choice would be power requirements, equipment capacity, manoeuvring characteristics and cost. Hope this helps in understanding why most AUVs are cylindrical although there are ways to make them more streamlined. LSD

Article By: Sudripto Khasnabis

Author's Note: This article is intended to familiarize the reader about Autonomous Underwater Vehicles in light of the recent MH 370 Flight incident and how AUV's are proving to be useful in this search.The videos and the figure do not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts,do not forget to write to me at learnshipdesign@gmail.com

Saturday, 19 April 2014

The Physics Behind Parametric Rolling

Parametric Rolling is a problem related to hullforms that experience considerable change in submerged volume when a wave passes longitudinally along the ship. This is significantly seen mostly in hulls having large bow and stern flares, stern overhang, and fine underwater hull form, that is mostly in container ships, fishing vessels and in some cases, passanger ships too. The same problem was not encountered in full form hulls like oil tankers and bulk carriers. Why? 

In fine form hullforms with large flares and overhangs, the profile of waterlines when the ship experiences head seas changes rapidly as shown below:


Dotted Line: Waterline in still water
Continuous Line: Waterline when wave troughs at midships


Dotted Line: Waterline in still water
Continuous Line: Waterline when wave crest at midships

If you compare the figures above, you'll note that

  • When wave trough is at midships, the water plane width is more than that in case of still water, resulting in increased stability (GM) than still water condition.
  • When wave crest is  at midships, the water plane width is less than that in case of still water, making the stability (GM) less than that of still water condition.
This results in periodical increase and decrease in metacentric height of the ship. In one complete passage of a wave along the ship's length the GM increases and decreases once; that is, stability variation occurs twice in once wave period.

How a parametric roll devolops due to this phenomenon, is a very interesting case to study. When a container ship is facing head seas and slight roll motions,

  • In the first quarter (T = 0 to T = 0.25), when the midship experiences a wave trough, the GM increases (obviously, periodically varying according to the wave profile). So the roll angle decreases (initial roll degree was present due to small rolling motions in the ship). Had the ship been in still water, its roll angle at the end of the first quarter would have been zero. But a higher righting lever now actually causes the ship to end up with a slight roll angle to the other side! (just concentrate in the first quarter in the figure below)





  • Curse inertia, because of which the ship begins its roll to the other side. Don't forget, you've entered the second quarter (T=0.25 to T=0.50) i.e the midship now experiencing a wave crest. You're right! Decreased stability. And that means reduced righting lever compared to still water condition. By the end of this quarter, your ship's roll angle to the other side is more than what it would have been in still water. (refer to the second quarter in the above diagram)
In short, your ship is in trouble, as this phenomenon will only keep increasing untill it achieves a resonating condition. Goodbye to containers on the deck!


A container ship after experiencing parametric roll


Watch this video of a simulated parametric roll during a tank test:




PREVENTION and REDUCTION IN PARAMTERIC ROLLING:


  • Having Bilge Keels to damp roll motions





  • Having Antirolling Tanks (often U- Type passive antiroll tanks are preferred)




  • Stabilizer Fins (used in cruises and liners)



  • Well designed and properly checked container lashings




  • Course change from head or following seas to oblique heading when wave periods and heights can cause parametric roll motions. LSD




Article By: Soumya Chakraborty

Author's Note: This one was written to throw light upon the physical phenomena behind parametric rolls. Students in initial undergraduate level find it initially tough to understand the reason behind this action. This article was written in simple and interactive terms to help students get along with the physics behind parametric rolls. Case studies and numerical models of parametric rolling and dampening methods have been done by maritime organizations and researchers. So there was no point including them here. Images and videos used in the article donot belong to LSD and full credit goes to their respective owners. Thank You for reading. In case of any doubts and queries please comment or write to learnshipdesign@gmail.com

Sunday, 13 April 2014

Model Testing- Resistance & Propulsion

For a few minutes, take the privilege of seeing yourself as the owner of one of the world's biggest shipping companies. And adding to your imagination, lets say you have set an order for a ship the kind of which had never sailed before (it may be cruise ship, container ship, yacht, whatever you choose). The amount of money you would have planned for this project would be in the order of thousands of billions (may be more!). You hire a design company, get your design ready, find the world's most sophisticated shipyard to build your ship and launch it at the sea. Obviously you don't let your ship sail just like that! You carry out sea trials to analyse her performance, and Bam! Your ship's design speed was 24 knots and she could practically reach only 12 knots speed! Why do you think this happened? 

Correct! Your ship was of a new kind, and never before a prototype of your ship had been tested in a small scale for its performance evaluation. Before the A380 was designed and built, a lot of work had gone into model testing the aircraft for all possible conditions and circumstances that it would face up there. New ship designs are also model tested at towing tanks for evaluating their performance so that any ship owner wouldn't have to face what you just did a minute ago. 

This article is about how the model of a ship is tested for estimating its resistance and propulsion requirements, which finally gives the hull designer a platform to optimize the hull form for lesser resistance, choose the correct engine for the ship and  allows the propeller designer to design the perfect propeller for the ship.

Following presentation will give a clear theoretical explanation to procedure involved in carrying out model tests for Resistance calculation. Please wait till the prezi is loaded. It is recommended to view in fullscreen mode.




The following videos will give you a clear idea as to how Resistance Tests are carried out. The first video gives an insight about the even keel test carried out as a part of resistance calculation. 

video


The following video shows how trim test is carried out. The model is made to trim about the trim post and the readings of its motion and flow around it, which is finally evaluated.

video
                                 
The following video gives us a proper perception as to how models are run using a carriage to calculate the  resistance from the values obtained from the Dynamo meter.

video.
LSD





Article By: Tanumoy Sinha

Author's Note: The following article explained about the steps you should carry out if you were to test a model in a towing tank. The article is written keeping in mind the reader has got some basic idea about the concepts of resistance. The next part of this series of articles will focus on the different types and procedures carried out in the propulsion tests. I have written this article based on my experience and technical knowledge on the same as a Naval Architect. The picture and the video used above does not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts, write to me at learnshipdesign@gmail.com

                                        

Sunday, 6 April 2014

Ship Of The Week - BALTIKA (Icebreaker)



Icebreakers have been used in the marine industry to clear navigational channels which remain perpetually or seasonally covered with ice. If you have ever watched an ice breaking ship assisting a merchant vessel behind it, you'll note hat the icebreaker being used will generally have a beam higher than that of the ship being assisted by it. So what if a ship with a beam more than that of the available fleet of icebreakers needs assistance? In that case, either once icebreaker cuts a wider channel by making to and fro movements (if you are wondering that this is time consuming, you are right!) or a combination of two icebreakers is used to cut a channel of width that would allow a vessel of larger beam to pass through (again a probem of higher costs!) If you haven't seen such a operation, here is one, below.






Aker Arctic, a Finnish engineering company had solved the problem long back and based on their patented concept in 1999, recently delivered the first ship based on the concept design Aker 100 HD (where HD signifies Heavy Duty). This ship (named BALTIKA) can actually clear channels of width greater than its own width!


THE ASYMMETRICAL HULLFORM



  • BALTIKA has an asymmetrical hullform which helps it achieve a greater icebreaking width by changing its original waterline width to a virtual waterline width. The difference this creates, from that of traditional icebreaking is depicted below. 


  • The direction of movement of BALTIKA in open sea conditions is just like another ship.

    BALTIKA, like other icebreakers can cut channels of its own width.

    That is how she can afford to cut channels of width greater than her original beam.


    • In short, the innovative design of her hull shape allows her to cut ice along two directions of motion. One, the traditional way. But when she needs to assist vessels with beam larger than her own, BALTIKA uses its flexible propulsion system to steer around and move obliquely astern. This will also give you the answer to why she has an obliquely positioned navigational bridge! The oblique end of its hull has the same contours as that needed for icebreaking action (waterlines shown in figure below). 



    BALTIKA in oblique icebreaking mode. [Ref: US5996520]


    • The contour of the waterlines close to the oblique icebreaking end of the hull (marked by 14 above) show the asymmetry in the hullform which becomes even clear when compared to the contour at the sections opposite to the icebreaking end (marked by 12).

    • If you have asked yourself about the resistance of the ship in this orientation, you are on the right path! Infact, the resistance in this mode is higher, but an intelligently positioned set of propulsion units overcome this effect to certain extent. Wondering why a third propeller has been used away from the keel line?



    THE PROPULSION SYSTEM

    I'm not going to bore you with the power and speed specifications, in this section. That you would find in every other maritime updates website. What is interesting, is the position of the propellers and their contributions to the ship's operational requirements. It is obvious that a vessel with such demands of maneuverability would definetely use azimuth propulsion units.


    • In the above figure, note the three propellers (21, 22 and 23). The orientation shown in the figure corresponds to the propeller directions that are maintained during oblique icebreaking process. If you apply simple laws of vector addition, you will notice, that the net force is oriented along the direction of movement (marked A). The contribution of the third propeller (22) to this effect is significant. Infact, it has other secrets too!

    • To decrease the resistance of the ship due to the interaction of the ice that has been immideately broken, the ice must be washed away from the waterline as efficiently as possible. For the propellers 21 and 22, the components of their turbulances perpendicular to the direction of motion serves this purpose. 

    • Not only this, the turbulence created by propellers 21 and 22 enable efficientl ice breaking by creating a turbulence under the unbroken ice and drawing water from the unbroken ice (Yes! the propellers used in BALTIKA are pulling propellers).


    How is it ensured that the ice doesnot hit the propeller turntable shaft before being cut by the ship's hull? The answer lies in the figure below:









    OTHER OPERATIONS

    She has also been optimized for rescue and oil spill recovery operations. For the former, her forward end can be fitted with a heli-deck. For carrying out oil spill recovery (It is a process by which oceanic oill spill is sucked into the ship's tanks), she has a boom on her starboard side (can be seen in the figure above) which is deployed into the water. The oily water is sucked into the skimmer tanks which separate the oil from water. This operation is depicted in the figure below:









    FEW PICTURES OF AKER 100 BALTIKA


    Flooring Construction of BALTIKA


    BALTIKA just after its launch

    General Arrangement of BALTIKA


    BALTIKA during sea trials in 2014.
    LSD



    Article By: Soumya Chakraborty


    Author's Note: This article was written in interest of the unique hull design to meet the operational requirements of efficient ice breaking, so the ship dimensions and main particulars were not mentioned, as they can be easily found in wikipedia. In case of any doubts or queries, please comment or write to me at learnshipdesign@gmail.com




    Thursday, 3 April 2014

    Know A Ship - Container Ships

    INTRODUCTION:

    Container ships are the ones in charge of transporting standardized containers and they are used to transport any kind of cargo all over the world. 

    The standard containers are part of the "Inter-modal freight transport" which allows transporting of (in an easy, fast and efficiency way) the freight between the different modes (ships, trains or trucks) without any handling of the cargo itself when changing modes.

    The 95% of the standardized containers(as will be discussed) are from 20 or 40 feet long.The dimension of the ships depends on the number of containers that it is supposed to carry with.
    Containers of Sea-Land Inc. at the port of Hoboken
    in the 1930s 
    The classification of container ships may be made based on the following points:


    1. Developing generations: based on capacity, determined by the number of TEU

    2. Mode of Cargo Handling: 

    • Box ships-(most common type of container ship, the containers are loaded from the outside with special cranes.)
    • LoLo (See our article on RoRo/LoLo vessels
    • RoRo(Same one as above

    3. Ship generation:

    • Panamax (14,501 TEU
    • Post-Panamax (10-14,500 TEU)
    • Suezmax (5,101-10,00 TEU)
    • Post-Suezmax (3,001-5,100 TEU)
    • Post Malacamax (2,001-3,000 TEU)
    • Others (Feeders,Small Feeders)
    4. Level of specializing: 
    general cargo, semi container, purpose-built, container
    ship.
    5. Service range: feeder ships, mother ships.

    A Modern Day Container Ship





    THE CONTAINER:

    "Inter-modal" indicates that the container can be moved from one mode of transport to another  without unloading and reloading the contents of the container.


    Container capacity is described in twenty-foot equivalent units (TEU ).  An equivalent unit is a measure of containerized cargo capacity equal to one standard 20 ft × 8 ft (i.e; length × width) 

    container.
    A typical 40 ft long shipping container
    As this is an approximate measure, the height of the box is not considered; for example, the 9 ft 6 in high cube and the 4-foot-3-inch half height 20-foot containers are also called one TEU.


    The 45 ft  containers are also commonly designated as two TEU, although they are 45 feet  and not 40 feet. 

    Similarly Two TEU are equivalent to one forty-foot equivalent unit (FEU).


    Huge investments in containerization have paid off and container traffic continues to grow.This growth will continue to grow until the original aim of containerizing every adjoining mode of transport has been  accomplished.







    BEHAVIOR OF CONTAINER SHIPS IN MARINE ENVIRONMENT:

    Three main types of conditions due to cargo loading can be found:


    Static loads:

    Mainly caused by the pressure due to the piling up of the containers that can brake cargo. This pressure depends on the dimension, and number of units piled up.


    Dynamic loads:

    Produced during the load and unload, transportation and moving of the containers. These loads can produce accelerations, vibrations and sometimes shaking due to the movement of water.
     Some examples of dynamic loading and how they affect the container and the cargo.

    Yawing loads:
    Rotation of the ship around its vertical axis. It occurs due to the impossibility of the ship to have a straight direction.



    Vertical oscillations:
    Upper and lower accelerations on the ship beyond its vertical axIs. Only with calmed sea there exist balance on this load. These oscillations affect the containers and its cargo. This can produce the elevation or the sink of the ship due to the movement of the sea.





    Linear movements beyond longitudinal and transverse axis

    The ship is accelerated or decelerated to prow and stern, and from one band to another.These loads can produce important torsion loads.




    Lateral movements:

    Movement of the ship around its longitudinal axis. Can produce angles from the horizontal normally from 10º to 30º but can raise to 45º.







    Pitching:

    Movements of the ship around its transverse axis. In this movement the ship is picked up from
    the prow and picked down from the stern and vice-versa.



    The total loads suffered by a container ship can thus be summed up with a figure like this:
    (Note how it is having the same degrees off freedom as the ship's motion itself)





    STRUCTURAL ANALYSIS:

    Container ship designer have a very specific architecture to deal with, they have an enormous deck to carry the containers and are sometimes fitted with huge cranes. All of these need to be built around a strong keel in order to obtain stability.On the other hand the hull of container ships is different depending on if they will be used in frozen water or not.

    All of these components are made of steel plates and stiffened plates due to the excellent behavior of this material subjected to variable distributed loads (cargo, self-weight, water loads and wave loads). The vessel is assumed to be a simple rigid and flexible beam in which the waves of the sea act and create stresses that produce bending of the vessel, this is known as hogging and sagging.


    In order to study the strength of the vessel, it is imperative to distinguish different loads
    situation and the behavior of specific parts of the vessel. The study must be done
    focusing on these parts:

    1. Hull girder strength including torsion strength: 

    The check is done in order to know the hull girder bending stress, shear stress and warping stress. The figure below shows the deformation and hull girder stress of a large container ship.



    2. Local strength plating and ordinary stiffeners: 

    The results of this study are: thickness of plating, shear area and section modulus of stiffeners,
    dimensions and scant-lings of brackets, buckling.

    3. Transverse primary members, stringers, floors, girders: The aim is check yielding and buckling.

    4. Structural continuity: Is useful to provide information to make possible modifications of connections design.

    5. Fatigue: Allows knowing the damage ratio or fatigue life of connections.

    One important fact is the behavior against fatigue of the vessel due to water/wave loads. These are every moment changing so the ship is continuously subjected to variable loads. So the weld design of the plates must avoid fatigue induced failure; otherwise the integrity of the ship structure could be in deep trouble.

    The assessment must be done in the following details:

    • Hatch corners.
    • Connection of side longitudinal stiffeners with stiffeners of transverse
       primary supporting members.



    LOADING OF CONTAINER SHIPS:

    Nowadays there is a very wide span of containers types and methods to load the vessel.

    1) On most ships which are specially designed for container traffic, the containers are carried lengthwise:




    This stowage method is sensible with regard to the interplay of stresses in rough seas and the loading capacity of containers. Stresses in rough seas are greater athwartships than fore and aft.


    2)  However, on many ships the containers are stowed in athwartships bays or are transported athwartships for other reasons. This must be taken into consideration when packing containers and securing cargo.




    This stowage method is not sensible with regard to the stresses in rough seas and the loading capacity of containers.


    3).Even unusual stowage methods like this(below), where some of the containers are stowed athwartships and others fore and aft, are used, but they require greater effort during packing and securing operations.


    Also, certain lashing/securing combinations and arrangements need to be taken into account.



    BENEFITS AND FUTURE TREND OF CONTAINER SHIPS:

    As already discussed,container shipping is the most efficient mode of transport for goods. In one year, a single large containership might carry over 200,0 container loads of cargo. While individual ships vary in size and carrying capacity, many container ships can transport up to 8,000 containers of goods and products on a single voyage. Similarly, on a single voyage, some car carrier ships can handle 7,600 cars. It would require hundreds of freight aircraft, many miles of rail cars, and fleets of trucks to carry the goods that can fit on one large liner ship. 

    Despite the changing production and world trade, the maritime transport has continually grown for the last decades. This growth has been suffered specially by container ships. The use of containers growth is shown by the nowadays construction of bigger ships, reaching some of them "16000 TEU"s.

    The international commercial changes and the evolution of the marine transport directly affect the development and expansion of harbors.Therefore, the commercial opportunities that the containers transport gives are a really big challenge to the marine structural engineers. The future of container ship development is promising. LSD






    Article By: Sudripto Khasnabis



    Author's Note: This article discusses about about The Container Ships from the perspective of their uniqueness and mentions the benefits and structural elements while taking into account it's behavior in the marine environment .The next article will be announced later. The section shall proceed regularly as usual with articles about other ships with a common aim of making the reader aware of the general principles on which their design and construction is based and also their utility and characteristics. The videos and the pictures do not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts,do not forget to write to me at learnshipdesign@gmail.com

    Do keep reading for more at: http://lshipdesign.blogspot.com/ .