How to do crude oil washing on ships in 10 easy steps

COW or Crude Oil Washing cleans cargo tanks with jets of high pressure oil when the ship is unloaded. The crude oil is pumped through the washing machines to the jets and serves as a cleaning agent which is then pumped to land with the load/Product Discharge. It has been found that crude oil is more reliable washing medium than water. The washing / dissolving effect results in heavy, waxy and asphaltic deposits on the sides and bottom of the Holding tank to convert back into the liquid so that they are easily discharge To the shore. The Crude Oil Washing (COW) techniques has several advantages over washing with water, using Cow as the washing medium reduces the Sludge deposition, thereby reduces the discharge (Stripping) time and increasing the load/cargo flow. There low deposition of mud at the bottom of the tank which reduces Ship constant and so increase its dead weight. On the other hand washing with water takes more time due to more deposition of mud at the bottom. They not only cause delay in washing but also release hydrocarbon gases which becomes a challenge to gas free the tank. There are three essential prerequisites for the washing of petroleum product:

A fully functional inert gas system for maintaining the tank atmosphere in an inert state during the whole wash cycle.
Fixed Piping Lines for Tank washing system
Means to ensure that the bottom of the tank is clean and dry at the end of the operation.

The vessel must be equipped with an inert gas system which can function properly and produce Inert Gas with a maximum oxygen content of 5%. This should be maintained during the COW to ensure that the oxygen content does not exceed this value. No COW operation to be performed if the Inert Gas installation of the ship is not working properly and the oxygen content is not within the desired limit. The oxygen content must not exceed 8% by volume. The IG pressure in the tank must not be less than 200 mm. wg. The vessel to be washed with oil must be equipped with a fixed tank wash system connected by permanent lines to the main load venting system or by separate laundry lines of the loading system. The crude oil washing of discharging tanks takes place in a single step or in several stages during the discharge of the charge.

One Step Washing:

As soon as the tank is almost emptied , Washing starts and the tank is dried under pressure during the last washing step using stripping. The machine adjusts to the vertical angle from 0 to 140 degrees. Since single-stage washing is performed only during the last tank discharge stage, it is necessary to use the ship's instructor to remove the pump or reduce the speed of the charge pump so that the vacuum pump is effective. This method is generally useful between discharges at two terminals or between ignition operations.

Multi-stage washing

This involves washing the interior areas of the tank in stages as the load is pumped out of the tank. The revolutions of the charge pump are not reduced. Depending on the back pressure in the distributor of the vessel, this process results in a very small time loss during the bulk discharge of the tank. The wax and sediment collected in the spacers and other structural elements above the bottom of the tank are removed and pumped with the load to the ground. Any wax or sediment on the underside of the tank remains as long as the soil is not washed and stripped as a separate step. This increases the total discharge time, but is required for sludge control.


The machines are pre-set in the first stage at a vertical angle of 120 ° to 60 °. In the second step, when the loading level is about two thirds of the tank depth, it is set at a vertical angle of 70 ° to 30 ° and during the third stage when it is set to about 1 meter of the residual charge in the tank to 40 ° to 0 ° is. Each step is superimposed by approximately 10 ° in the preceding step, and the cycle is normally 11/2, ie, in the first step, for example, the nozzle moves in the horizontal plane by slowly changing its angle from top to bottom or from 120 * to 60 * from top to bottom, or from 60 * to 120 *, and from top to bottom, or from 120 * to 60 *

The regulation requires that the cow jets cover at least 90% of the horizontal surface area of the floors, rods and structural elements and at least 85% of the vertical direct zone. Shaded areas should be covered with splashes or deformations.

List of oil washing equipment controls
In the case of unloading of raw materials, the master must notify the competent authority and the terminal (or any other ship in transit to the ship), at least 24 hours in advance or at the time of compliance with the regulations. The washing of crude oil should only be approved once each step of the COW operation must be specified in the oil wash operation plan. There must be an operational checklist for the use of the crew at each port of discharge. This includes the control and calibration of all instruments to be used during the operation of the cow.

Pre-Arrival Checks at Discharge Ports

1. Notification to the terminal of the operation of the cow.
2. Is the oxygen analyzer successfully tested and operated?
3. Is the water heater and the flushing system of the motor compartment insulated?
4. All Valves Marked washing is isolated from that of engine room pipe line.
5. Are all V/vs closed for stationary tank washing machines?
6. Are the tanks cleaning lines under pressure and tested for leakage?
7. Do you make portable drives for the M/C quick tank sink that will be tested?
8. Have gauges on the top line relief manifold has to be verified.
9. Should the trimming system control equipment cheked?
10. Has the communication system been reviewed and tested?
11. Has the organizational plan been set up and defined the duties and responsibilities of crew?

 Checklist before cow Operation

1. All checks and conditions are in order prior to arrival.
2. Unloading/operating the raw oil wash has been discussed with ship and shore staff and is readily available the agreed plan for a simple reference?
3. Was the communication link between the control and control stations and the monitoring/shore station repaired and functioning properly?
5. Has the fixed and portable oxygen analyzer been verified and calibrated?
6. Is the inert gas generator functioning properly and the oxygen content of the inert gas is delivered below 5% of the volume?
7. Is the oxygen of the tank washed about below 8% in volume?
8. Is the gas pressure is positive for the entire cargo tank?
9. Keep person responsible for checking all the lines for leaks as soon as the operation starts?
10. Are stationary machines for the necessary washing and portable motor units, if installed, mounted and repaired?
11. Have valves and lines both in pump-room and deck been checked?

Checklist-During the operation of the cow

1. Is the quality of the inert gases supplied often controlled and recorded?
2. Are all deck and machine lines often tested for leaks?
3. Is the current tank only available for COW?
4. Is the pressure on the appropriate tank wash line indicated?
5. Is the processing time of the tank disk indicated?
6. Is the washing machine, which works with the propeller groups, often properly controlled and working?
7. is a responsible person constantly stationed on the bridge?
8. Will trim be satisfactory when bottom washing is in progress as specified in the COW manual?
9. Is the level in the reservoir for tank washing often checked to avoid overflow?
The cow should be abandoned immediately if:There is an error in the inert gas system or oxygen exceeds the permissible limit, or The pressure in the tank falls below the air pressure or a minimum required (usually 200 mm water gauge).

D Checklist-After cow operation

1. Are all the valves between the discharge and the washing line of the tank closed?
2. Has the laundry been drained of oil from the crude tank?
3. Are all valves closed to the washing machine?
4. Are the cargo pumps, tanks and pipes properly drained as indicated?

Sludge and sediment control

Under certain circumstances, substantial sludge may also form in the containers, even in the case of crude oils which are not normally associated with such accumulations. Studies of incidents of large sludge accumulation show that there are specific critical temperatures at which crude oil begins to precipitate hydrocarbon species that form mud. This temperature, cloud point, is the temperature at which the crude waxes change from their liquid phase to suspended, nearly solid particles with associated oil components. These particles separated by phases are installed at the bottom of the reservoir and form mud. Sludge from paraffinic crude oil, once formed, is extremely difficult to return to a liquid phase by heating alone. Even an effective COVER program, which eliminates them from boats, has only managed to solve the problem in shore-based containers.

Electrostatic risks
The oil used for COWing must be free of suspended water to minimize the formation of electrostatic charges by high pressure jets. To ensure that the oil supply to the VAE is dry, the cargo tank to be used must be discharged to the ground before the start of the COW. At least one meter of load must be discharged. This removes all bottoms from the tank before the mold starts. VACA procedures that remove the flushing fluid from the discharge current or use a sliding container that is then filled with clean oil will avoid this problem.

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Author ARPIT SINGH and Amit

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How inert gas is produced on board ship?

The Inert gas is produced in the IGG by the combustion of fuel with air. Marine diesel oil (MDO) or MGO is drawn from the storage tank and pumped through a filter to the main burners. A blower delivers the air necessary for combustion.
Oil and air is mixed in the correct proportion in an air-atomizing burner, which is ignited by the pilot burner. The fuel-air ratio and the oxygen content of the IG can be regulated by a valve in the fuel supply line to the main burner. Purple-blue to rose-purple flame indicates good combustion whereas yellow flame means too much fuel oil or too low delivery air pressure causing low atomizing pressure and risk of soot formation.

The inert gas thus generated consists of mainly Nitrogen and carbon dioxide. It is first cooled in the combustion chamber by means of a sea water jacket, which surrounds the combustion chamber. The gas then comes in direct contact with a water spray. The resultant IG comprises of:

Oxygen  :            2% by volume
Carbon dioxide: 13% by volume
Nitrogen   :        85% by volume

Due to perfect combustion, there is a complete absence of soot. The temperature of the gas leaving the generator is about 5-10* above the cooling water temperature.
During the operation, should there be a reduction in demand for the IG, the excess gas is vented to the atmosphere. The operation is continuously monitored for flame, water or control air failure and excessive cooling water level. Should any emergency condition arise, the IGG automatically shuts off and sounds an audible alarm.
This is a costly installation, which is usually found in product or chemical tanker. With some clean cargo tainting could be a problem, if flue gas from boiler is used.

Chemical tankers & Product tankers are normally FRAMO tankers hence IGG is fitted on these ships, secondly the quality of IG plays a very important role in maintaining the quality of chemicals being transported.
Conventional tankers with turbine driven COP will always have a large water tube boiler and thus Inert gas on these ships will be produced from boiler flue gas. Boiler will be used continuously during cargo discharge for running the cargo oil pump turbines (COPT).
On Framo tankers, the cargo discharge system consists of each cargo tank having a fixed submersible cargo pump, this pump is coupled to a hydraulic motor. The hydraulic motor is driven by high pressure hydraulic oil (220 bar), this high pressure hydraulic oil is delivered by hydraulic power-packs located in the engine room, these power packs are normally driven by diesel engines. Thus these ships use an IGG for production of inert gas. They do not have large boilers as no turbines are present, hence IG from boiler uptake is not possible.
Carbon Molecular Sieve type Nitrogen Generator

The molecular sieve nitrogen generating plant is usually installed on ships such as chemical carriers where oxygen critical cargoes may be carried and purity of the inert gas is of utmost importance.
The system uses the pressure swing absorption method using a carbon molecular sieve which has the unique property of absorbing oxygen in preference to and at a faster rate than nitrogen. When air passes through the molecular sieve, oxygen is absorbed. Nitrogen gets across and is collected. However the sieve gets saturated with oxygen after sometime and needs to be changed over.
The system uses two vessels packed with the carbon molecular sieve. The vessels are alternatively on line or in the process of regeneration. As one reaches saturation, the system switches to the second. Change over time is appox. 1 minute.

The oxygen from the saturated sieve is removed by applying a reduced pressure to the vessel and then venting to the atmosphere. The nitrogen gas thus generated is fed to a buffer vessel from where it is distributed to the inert gas line. The purity of the nitrogen depends on the flow rate and can be as high as 99.5%. The lower the flow rate, the higher the purity and vice versa.

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Author ARPIT SINGH

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Basic concept on Ship stability and free surface area

Stability Of Ships

Two Principal Forces Which Act On A Ship Floating Freely Are – Weight  &  Buoyancy.

For The Ship To Float, It Must Displace It’s Own Weight Of Water.

Then To Be In Equilibrium Condition, The Centre Of Weight & The Centre Of Buoyancy Must Be Vertically In Line.

External Or Internal Forces Can Move The Ship In Either Transverse Direction Or Longitudinal Direction. But It’s Ability To Return To It’s Original Stable Position I.E., Equilibrium Condition Is Related To The Stability.

Upright Position Of Ship :  A Vessel Is Said To Upright If It Is Rolling Slightly About The Upright Position.
Statical Stability : This Is The Measure Of The Tendency Of A Ship To Return To The Upright Position If Inclined By An External Force.
Upright Position Of Ship : In The Upright Position, Weight Of The Ship Acts Vertically Down Through The Centre Of Gravity ‘G’. While The Upthrust Acts Through The Centre Of Buoyancy ‘B’.
Equilibrium Condition Of Ship : When Weight Of The Ship Is Equal To The Upthrust And The Centre Of Gravity And The Centre Of Buoyancy Are In The Same Vertical Line, The Ship Is Said To Be In Equilibrium Condition.

Consider A Ship Inclined By External Force To An Angle ‘Ǿ’.
               In This Case Centre Of Gravity Remains In The Same Position, But The Centre Of Buoyancy Moves From ‘B’ To ‘B1’.
    Therefore Buoyancy Acts Through ‘B1’  And The Weight Still Acts Through ‘G’, This Creates
    The Righting Moment = Δg X Gz -------------------(1)
    This Moment Tends To Turn The Ship To It’s Upright Position.
    Righting Lever = Gz = Gm Sinǿ
    Now

The Vertical Through New Centre Of Buoyancy -‘B1’ Intersects The Centreline At ‘M’ Which Is Called The Metacentre. The Height  Of ‘M’ From The Centre Of Gravity Is Called The Metacentric Height- ‘GM’

Stable Ship : ‘GM’ Is Said To Be Positive When ‘G’ Lies Below ‘M’. In Such Case Ship Will Roll Back To Original Upright Position When Disturbed By External Or Internal Force.

Tender Ship : A Stable Ship With Small Metacentric Height Will Have Small Righting Lever At Any Angle And Roll Easily. In Such Case Ship Is Said To Be Tender Ship.

Stiff Ship : A Stable Ship With Large Metacentric Height Will Have A Large Righting Lever At Any Angle And Will Have Considerable Resistance To Rolling. In Such Case It Is Called The Stiff Ship.

Unstable Ship : When Metacentre ‘M’ Lies Below The Centre Of Gravity ‘G’, Then ‘GM’ Is Said To Negetive, Which Increases The Angle Of Heel. In Such Case The Vessel Is Said To Be Unstable And Will Not Return To Upright Positon.

Neutral Equilibrium : When Centre Of Gravity And The Transverdse Metacentre ‘M’ Coinside, The Righting Lever Is Zero And There Is No Righting Moment Acting On The Ship. In Such Case Ship Will Remain In Inclined Position With An Angle ‘Ǿ’, And The Ship Is Said To Be In Neutral Equilibrium Condition.

Stability At Small Angle Of Heel
Transverse Metacentre :
The Height Of Transverse Metacentre Above The Keel ‘KM’ May Be Found Out, By Considering The Small Inclination Of The Ship About It’s Centre Line.
For Small Angle Of Heel
Upright And Inclined Water Line Intersects.
Volumes Of The Emerged And The Immersed Wedges Are Equal For Constant Displacement.
    Now
    The Distance Of The Transverse Metacentre Above The Keel Is Given By
    Km = Kb + Bm -------------------------------------------(1)
    Note : ‘KB’ Is The Distance Of The Centre Of Buoyancy Above The Keel. This Can Be Found Out From The Hydrostatic Curve.

‘BM’ Can Be Found Out As Follows.
        Let Us Consider A Ship As Shown In Figure, Whose Volume Of Displacement Is ‘▼’, Lying Upright At Water Line ‘WL’. The Centre Of Buoyancy ‘B’ Being On The Centre Line Of The Ship.
        If The Ship Is Now Inclined By An Angle ‘Ǿ’, It Will Lie At The Water Line W1l1, Which Intersects The Original Water Line ‘WL’ At ‘S’. Since ‘Ǿ’ Is Small It May Be Assumed That ‘S’ Is Lying On The Centre Line.
        A Triangular Wedge Of Buoyancy ‘W1SW’ Has Been Moved Across The Ship To ‘L1SL’ Causing The Centre Of Buoyanmcy To Move From ‘B’ To ‘B1’
    Now
    Moment Of Shift Of Ship’s Buoyany From B To B1
    = Moment Due To Shift Of Buoyancy Wedge
Therefore
    ▼ X Bb1  =  V X Gg1
                                         V X Gg1                                        
                Bb1  =   ---------
                              ▼
                                   V X Gg1
             Bm Tanǿ = ---------- -----------------( As Bb1 = Bm Tanǿ )
                                      ▼
                     V X Gg1
        Bm = --------------  ----------------------------------(2)
                        ▼ X Tan∆

    Now To Determine The Value Of (v X Gg1) , Ship’s Length Is Devided In To Thin Strips Of Length
    Δx. The Half Width Of Original Water Line Is ‘Y’
    Therefore
    Immersed C/S Area Of Wedge = ½ Y X Y Tanǿ X Δx
                                                                   = ½ Y2tanǿ X Δx
    Volume Of Immersed Wedge  = Σ ½ Y2tanǿ X Δx

The Volume Of This Wedge Is Effectively Moved From One Side To The Other By A Distance Of (4Y/3)
    Therefore
    Total  Moment Of Shift Of Wedge
                        = Σ ½ Y2tanǿ X Δx  X (4Y/3)
                                                        = Tanǿ (2/3) Σ Y3 Δx
    We Know That,  (2/3) Σ Y3 Δx = Ixx
    (IXX = Second Moment Of Area Of Water Plane         About The Centre Line Of The Ship )
    Therefore
    Total  Moment Of Shift Of Wedge = Ixx X Tanǿ --(3)
    From Equation (2) And (3)
                                                                                     Ixx
    Height Of Metacentre From ‘B’  =  ------ ------------(4)
                                                                                      ▼
Metacentric Diagram
In This Diagram Kb (HEIGHT Of Centre Of Buoyancyfrom Keel ) & Bm ( Height Of Metacentre Above The Centre Of Buoyancy )  Are Plotted Against The Ship’s Draught As Shown In Figure.
As The Position Of Buoyancy-B & The Position Of Metacentre-M Depends Only Upon The Geometry Of The Ship & Draught Of The Ship At Which It Is Floating, Position Of ‘B’ & ‘M’ (HEIGHT Of Metacentre) Can Be Found Out Without The Knowledge Of Loading Of The Ship At Any Intermediate  Draught.

Stability At Large Angle Of Heel
Stability Discussed So Far Is For Small Angle Of Heel With Certain Assumptions ( Two Water Planes Intersect At The Centreline,Wedges Formed Are Right Angled Tringles) The Metacentric Height ‘GM’ Was Taken As The Measure Of Stabilty.
When Ship Heels To An Angle Greater Than 100, Above Assumptions Cannot Be Made & The Principle On Which Initial Stability Were Based Are No Longer True. Insteadthe Righting Lever ‘GZ’, Which Is The Peprendicular Distance Between Vertical Lines Through The Centre Of Gravity & The Inclined Centre Of Buoyancy, Is Used As Measure Of Stability.

Let Us Consider
A Ship Which Is Inclined To Some Angle ‘Ǿ’ From The Vertical.
‘WL’ Is The Initial Water Line And W1li New Water Line When Inclined.
The Volume Of Displacement In Each Case Is Same.
If The Side Of The Ship Were Vertical Along It’s Length, Then The Two Water Line Would Intersect At The Centreline At Point ‘P’
    Further,
Volume Wpw1 Which Has Emerged Will Be Equal To The Volume Which Has Been Immersed. Let This Volume Be ‘v’
The Centroid Of These Two Wedges ‘g’ And “g1’ Be Located At A Distance Of ‘d’

Now
    Moment Of Shift Of Ship’s Buoyancy
    = Moment Due To Shift Of Buoyancy Wedge
    Bc X ▼ = D X V
                       D X V
            Bc = -------  ------------------------------------------------(1)
                         ▼
    &   Gz = Bc – Bg Sinǿ
                       D X V
                 =  -------- - Bg Sinǿ          
                        ▼
                                                   D X V
     Righting Lever ‘GZ’ = -------- - Bg Sinǿ -------------(2)
                                                      ▼
    Note ;  This Is Called Atwood’s Formula. From This Formula If ‘v’ And ‘d’ Are Evaluated For A Range Of Angles Of Inclination ‘Ǿ’ , The Graph Of Righting Lever ‘GZ’ Verses The Angle Of Inclination ‘Ǿ’ Can Be Drawn. The Curve So Drawn Is Called ‘CURVE Of Statical Stability’

Curve Of Statical Stability

In This Curve Righting Lever Can Be Seen, Rising To A Maximum Value And Then Slowly Fall To Zero.
A Ship Inclined Beyond The Point Of Zero ‘GZ’ Will Be Unstable.
Angle Up To Zero ‘GZ’ Point Is The Range Of Ship Stability At That Particular Load Condition.
Ship Operator Should Know This For Safe Operation Of The Ship.

Free Surface Effect

If The Tank On A Ship Containing Liquid Is Not Fully Filled, The Liquid Moved Through The Tank In The Direction Same As The Heel. For This Reason, The Focus Of The Vessels Central Gravity Shifts Away From The Centre Reducing The Righting Lever "GZ" And Height Metacenter Which Results In Increase In Heel Angle. This Effect Is Known As The Effect Of The Free Surface.
We Consider A Full Tank Water In Ship Of Displacement "Δ" With Inclination To Any Angle Ǿ.
The Focus Of Vessel Moves From "G" To "G1", As Wedge Of Liquid Moves Through The Tank

Tank Divisions Use In Tankers:

Largest Ship With Free Surface Effect Must Be Left Space Of Oil Tank For Expansion.
Tanker Was Built Initially With Centeline Bulkhead And Expansion Tanks. Twin Length, Bulkhead Were Introduced Without Expansion Tanks And Successfully Proved Because It Deals With The Loss Of Metacentric Height Of Design Due To The Free Surface Effect.
It Is Not Possible To Design A Dry Cargo Ship In Same Way As C.G Position Varry With The Nature Of Deposition Of Cargo. Effect Of The Free Surface Is Dangerous For A Ship With Small Metacentric Height And Can Make The Ship Unstable.
In These Ship Tanks Required To Be Pressed Up. If The Ship Is Initially Unstable And Helling To Port, Then Any Attempt To Fill The Water Ballast Results In Reduction Of Stability.

# Various books, study material and other online sources has been refereed prior to writing this article but no part is copied or produced  from any of the source but explained same thing in better detailed way.

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Author Arpit Singh and Amit                                                                 Article requested by: Deepak Kumar

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what are the different types of ship | Ship construction

There are many different types of ships, and the differences are mostly based upon the type of cargo the ship transports. According to the study the typical layouts of the main type of Merchant Ships are:
a)  General Cargo Vessels (being phased out).
b)  Bulk Carriers.
c)  Tankers.
d)  Container Vessels. 
e)  Passenger Vessels.
f)  Ro-Ro Vessels.

There are also a number types of vessels designed with a specific purpose of transporting special cargos such as ;
a) Vessels for carrying Project Cargo
b) For Diving Support with D-ynamic P-ositioning Vessels.
c) Heavy Lift Capable Vessels.

Bulk Carriers

Bulk Carrier

The common general bulk carrier will have double bottom, hopper sides or the top and deck wing tanks.  The hopper or the side tanks may be  used for the carriage of light grain cargoes as well as water ballast.
 
On single voyages the vessel may carry high density bulk cargoes only in the short holds to give an acceptable cargo distribution or on general bulk carriers with uniform hold lengths where alternate hold loading or block hold loading may be utilized to stow high density cargoes. With such loading arrangements high shear forces occur at the ends of the holds requiring additional strengthening of the side shell in way of the bulkheads.

A general arrangement of a typical bulk carrier shows a transparent cover with the back of the machine. Large hatches with steel plugs are designed to facilitate the fast loading and unloading of cargo. Since the bulk carrier many rides in the ballast provided a ballast capacity to the right propeller for dipping. A general arrangement of a typical bulk carrier shows a transparent cover with the back of the machine. Large hatches with steel plugs are designed to facilitate the fast loading and unloading of cargo. Since the bulk carrier many rides in the ballast provided a ballast capacity to the right propeller for immersion. The size of this type of ship has also steadily increased and bulk carriers have reached 250 000 tones dead weight.

Ships of the general form, experienced a relatively high rate of losses at the end of the eighties and early nineties, the concern raised about the construction and construction. In the nineties, the safety of the bulk carriers has gained considerable attention in the work of the IMO, classification societies and elsewhere, and this work is still ongoing.

Based on the experience of disturbances with minor consequences, it was concluded that the losses caused by a failure of the local structure resulted in the loss of the sealing of the side shell followed by further flooding excessive bending stress helmet due to damaged partitions.

Much of this work has focused on the details of the structure of the fuselage, the loads derived by loading and unloading, damage to the structure and protective coating of discharge loads, poor maintenance and inspection higher inadequate structure of the ship.

The outcome of this work resulted in provision of guidelines,  adopted by the IMO Assembly in November 1993. This incorporated  a concept of the Enhanced Survey Programme for inspections and surveys of bulk carriers and tankers of 15 yrs or more for incorporation in the SOLAS 1974.

MV Derbyshire, a 1976 built and owned by BIBBY LINES  registered at Liverpool as an ORE-OIL-BULK CARRIER. On 09th Sep1980, during encounter with typhoon  Orchid, vessel sank, 230 miles off  Okinawa with  out any trace or initiating a MAYDAY Distress message. When overwhelmed by the tropical storm, vessel was  carrying 157,446 T of iron ore and was “HOVE TO”.  After nearly 14 years in, June 1994, the wreck of Derbyshire  was discovered. Investigation into the strange orientation of the wreck at a depth of 4000 M and  spread over 1.3 km concluded that the ship sank because of structural failure.

Following this evidence IMO directed a revision of the Load Line Convention 1969, for a close look at the adequacy of bow height and strength of hatch covers in the forward part of these ships.

The safe operation of bulk carriers is dependent on not exceeding allowable stresses in the cycle of loading, discharging, ballasting and de- ballasting.

In November 1997 the International Maritime Organization
(IMO) adopted  new rules covering survivability and structural requirements for bulk carriers of 150 M and above requiring  the bulkhead and double bottom to be strong enough to allow the      ship to survive flooding in hold one.

The size of bulk carriers is often referred by one of the following:

‘Handysize’ the smallest bulk carriers of between 10 000 and 30 000 tones deadweight.
‘Handymax’ bulk carriers of between 35 000 and 50 000 tonnes deadweight.
‘Panamax’ bulk carriers designed to be of the maximum size that may transit the Panama Canal and generally being just under 80 000 tonnes deadweight. Old limitations 12.04/32.2/294 m / New limitations – 15.5/49/366 m.
Capesize’ bulk carriers  80 000 to 150 000 tonnes deadweight which are too large for the Panama Canal and trade from the Atlantic around the Cape of Good Hope.
Suezmax, the typical deadweight of a Suezmax ship is about 160,000 tons capable of transiting the Suez Canal in a laden condition. Suez canal has no locks, limiting factors are draft 20.1 m and height due to the Suez Canal Bridge 68 m and typical beam width of 50 m (164.0 ft).

Tankers

Fig: Tanker

For optimum economic operation of the tanker, the Service speeds of oil tankers have shown an increase from 12 knots o 17 knots. The optimum size of the tanker is also related to the current market economics. The tanker fleet has grown considerably to keep up with the market.

Structurally one of the greatest developments has been in the use of welding, oil tankers being amongst the first vessels to utilize the application of welding which ensures oil tight joints. The same could not be said of riveting.

Welding allows cheaper fabrication methods to be adopted. Longitudinal framing was adopted at an early date for the larger ships and revision of the construction rules in the late 1960s allowed the length of tank spaces to be increased with a  subsequent reduction in steel weight and making it easier to pump discharge cargoes.

As far as the general arrangement is concerned there appears always to have been a trend towards placing the machinery aft. Moving all the accommodation and bridge aft was a later feature and is desirable from the fire protection point of view. Location of the accommodation in one area is also economical.

The requirements of the International Convention for the Prevention of Pollution from Ships 1973 and Protocol of 1978 have greatly influenced the arrangement of the cargo spaces of oil tankers.
A major feature of the MARPOL Convention and its Protocol has been the provision in larger tankers of clean water ballast capacity. Whilst primarily intended to reduce the pollution risk, the fitting of segregated water ballast tanks in the midship region aids the reduction of the still water bending moment when the tanker is fully loaded. It also reduces corrosion problems associated with tank spaces which are subject to alternate oil and sea water ballast cargoes.

In March 1989 the tanker Exxon Valdez, which complied fully with the then current MARPOL requirements, ran aground and discharged 11 million gallons of crude oil into the pristine waters of Prince William Sound  in Alaska.
The subsequent public outcry led to the United States Congress passing the Oil Pollution Act 1990 (OPA 90). This unilateral action by the United States Government made it a requirement that existing single hull oil tankers operating in United States waters were to be phased out by an early date, after which all oil tankers were to have a double hull.
In November 1990 the US suggested to make double hulls compulsory for new tankers. Other IMO member states suggested alternative designs offering equivalent protection against accidental oil spills.
In 1992 IMO adopted amendments to MARPOL which required tankers of 5000 tons deadweight and above contracted for after July 1993, or which commenced construction after January 1994, to be of double-hulled or middeck construction, or of other design offering equivalent protection against oil pollution.

Studies by IMO and the US National Academy of Sciences confirm the effectiveness of the double hull in preventing oil spills caused by grounding and collision where the inner hull is not breached.

However the mid-deck tanker has been shown to have more favorable outflow performance in extreme accidents where the inner hull is breached.

The United States authorities consider grounding the most prevalent type of accident in their waters. Hence thus while MARPOL provides for the acceptance of alternative tanker designs, the United States legislation does not, accept alternative designs.

MARPOL required that the existing single hull crude oil tankers of 20 000 tons or more deadweight and existing single hull products carriers of 30 000 tons or more deadweight not having segregated ballast tanks could operate after June 2007 and those having  segregated ballast tanks will not be able to operate after July 2021.

Passenger Vessel

Several modern passenger ships have had their machinery placed aft; this gives over the best part of the vessel amidships entirely to passenger accommodation. Against this advantage, however, allowance must be made for an increased bending moment if a suitable trim is to be obtained.
Passenger accommodation standards have increased substantially, the volume of space allotted per passenger rising steadily. Tween deck clearances are greater and public rooms extend through two or more decks, whilst enclosed promenade and atrium spaces are now common in cruise vessels. The provision of air conditioning and stabilizing devices have also added to passenger comfort. Particular attention has been paid to fire safety in the modern passenger ship, structural materials of low fire risk being utilized in association with automatic extinguishing and detection systems.
There has been a demise of the larger passenger liner and larger passenger ships are now either cruise ships, short-haul ferries or special trade passenger (STP) ships. The latter are unberthed immigrant or pilgrim passenger ships operating in the Middle East to South East Asian region.

Container Vessels

Container vessel

Containers are re-usable boxes of 2435mm by 2435mm section, with lengths of 6055, 9125 or 12190mm. They are used for most general cargoes and liquid-carrying, gas carrying and refrigerated versions are in use. The latter may have their own independent cooling plant or be supplied with cooled air from the ship's refrigeration system.
The cargo-carrying section of the ship is divided into several holds with the containers racked in special frameworks and stacked one upon the other within the hold space. Cargo handling is by vertical movement of the containers by a special quayside crane. Containers may also be stacked on hatch covers and secured by special lashing arrangements. Cargo holds are separated by a deep web-framed to provide the ship with transverse strength. The structure outboard of the container holds is a box-like arrangement of wing tanks providing longitudinal strength. The wing tanks may be used for water ballast and can be used to counter the heeling of the ship when discharging containers

A double bottom is fitted which adds to the longitudinal strength and provides additional ballast space.
Accommodation and machinery spaces are usually located aft leaving the maximum length of full-bodied ship for container stowage. Cargo handling equipment is rarely fitted, since these ships travel between specially equipped terminals to ensure rapid loading and discharge . Container ships have carrying capacities from 1000 to 2500 TEUs or more. The twenty foot equivalent unit (TEU) represents a 20ft (6055mm) 'standard' container. Container ships are faster than most general cargo ships, with speeds up to 30 knots.

Ro-Ro Vessels 

These vessels are designed for wheeled cargo, usually in the form of trailers. The cargo can be rapidly loaded and unloaded through stern or bow doors and sometimes sideports for smaller vehicles. Some have been adapted to carry containers. loading ramp usually at the after end. Internal ramps lead from the loading deck to the other 'tween deck spaces. The cargo may be driven aboard under its own power or loaded by straddle carriers or fork lift trucks. One or more hatches may be provided for containers or general cargo, served by deck cranes. Where cargo, with or without wheels, is loaded and discharged by cranes the term lift-on lift-off (Lo-Lo) is used.

The structure outboard of the cargo decks is a box-like arrangement of wing tanks to provide longitudinal strength. A double bottom is fitted along the complete length. The machinery space and accommodation are located aft.  Only a narrow machinery casing actually penetrates the loading deck.

Sizes range considerably with about 16000dwt to 28000 displacement tonne) being quite common and high speeds in the region of 18 to 22 knots are usual.

When used as ferries, vehicles usually enter at one end and leave at the other. This speeds up loading and unloading but requires two sets of doors. There has been considerable debate on the vulnerability of Ro-Ro ships should water get on to their vehicle decks.

Various means of improving stability in the event of collision and to cater for human error in not securing entry doors, have been proposed. Since the loss of the Herald of Free Enterprise regulations have been tightened up. The later loss of the Estonia gave an additional impetus to a programme of much needed improvements.


# Special credit to the book "Ship Construction" by D.J Eyres and  G.J bruce (7th edition) which has been used as a reference source for few points included and images used.

# If you have any complain regarding the post contact us!

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A,B,C,D In Ship Terminology

A

Access Holes – Holes cut in the ship’s structure to permit entering or leaving various compartments.
After perpendicular – A vertical straight line at or near the after edge of the rudder post
Amidships – At the midship section of the ship
Accommodation ladder – a portable ladder on a ship’s side for people boarding from small boats or from a pier.
Aft – Near or towards the stern of the shipShip’s terms
Aftermost – The furthest aft.
Aft Peak Bulkhead – The first main transverse watertight bulkhead forward of the stern.
Anchor – A heavy object of steel attached to a vessel by a cable and/or chain and cast overboard
to keep the vessel in place.
Athwart ship – Perpendicular to the fore and aft centerline of the ship

B

Ballast – Any substance other then cargo, which is usually placed/loaded in the inner compartment of a ship to produce a desired trim or improve stability.Ship’s terms
Bay – It is an area between adjacent transverse frames or transverse bulkheads
Beam – The width of the ship at its widest point
Bilge – Internally, the lowest part of the ship.
Bilge Keel – Piece of plate set perpendicular to the ship’s shell along her bilge for reducing the rolling.
Bilge strake – It is the strake at the turn of the bilge extending outward to a point where the side rises
vertically.
Bow – The forward end of the ship.Ship’s terms
Boss – The curved swelling portion of the ship’s hull around the propeller shaft.
Breakwater – A guard plate preventing solids or liquid from sweeping into the deck.
Breast hook – A triangular plate bracket joining the port and starboard structural members at the stem.
Bridle – A V-shaped chain, wire or rope attached to a vessel being towed, to which the towline is
attached .Ship’s terms
Body plan – A drawing which shows the frame lines in elevation.
Boat deck – a deck on which the lifeboats are kept.
Bollards – A round tie posts to which the mooring lines are made fast.
Booby hatch – A watertight covering over a deck opening which is used for a stairway or ladder.
Bridge – An elevated structure extending across or over the weather deck containing the stations for the control of the ship and communications.
Bridge deck – A deck of superstructure midship Bulkhead – A vertical partition separating the
compartments in a ship.
Bulkhead deck – It is the uppermost deck to which the transverse watertight bulkheads and
shell are carried.
Bulkhead Structure – Is a transverse or longitudinal bulkhead plating with stiffeners and girders.
Bulwark – It is the vertical plating immediately above the upper edge of the main deck at side.
Butt – The joint formed when two plates are placed edge to edge

C

Cabin – An enclosed compartment of a ship used as living quarter.
Camber – The athwart ship rise of a deck.Ship’s terms

Cargo hatch – a large opening in the deck which permits loading of the cargo inside Ship.
Cargo port – an opening in the ship’s side used for loading and unloading cargo Cargo area – that part of the ship which carries cargo.
Cargo hold bulkhead – boundary bulkhead separating the cargo holds.
Carlings – supports, usually in a fore and aft direction, of flat plate/bar, welded between transverse deck beams.
Casing – covering or bulkhead around or about any space for protection.
Ceiling wood –sheathing on tank top to protect the cargo and ship structure.
Center line – middle line of the ship extending from stem to stern
Chain locker – compartment in the forward part of the ship for storing the anchor chain.
Chain pipe – A pipe for passage of chain from deck to chain locker
Classification - the certification process administered by a an agency whereby the ship is constructed and maintained to the agency’s requirements.
Cofferdams – any space between two bulkheads or decks primarily designed as a safeguard against oil leakage from one compartment to another.
Collar – a welded plate used to close a beam or frame penetration through plating.
Collision bulkhead - the foremost main transverse watertight bulkhead
Companionway - a covered stairway leading down from the open deck
Compartment - a sub-division of space or room in a ship
Confined space - a space identified by one of the following characteristics, limited opening for entry or exit, unfavorable natural ventilation or not designed for continuous worker occupancy.Ship’s terms

D

Dead rise - a rise or upward slope of the bottom of the ship from the keel to the bilge.
Deadweight – cargo carrying capacity of the ship.
Deck house - A structure on the freeboard or superstructure deck not extending from side to side.
Derrick - a device for hoisting heavy weights or cargo
Displacement - Total weight of the ship, when afloat, including every thing on board.
Discharges - any piping leading through the ship’s sides for conveying bilge water, drains, etc.
Dog - a small bent metal fitting used in closing doors,hatches etc.
Double Bottom - is the structure of the ship bounded by the bottom shell and the inner bottom plating
Doublers plate - a plate fitted over another plate for extra strength
Duct keel - is a keel built of plates in box form, extendingthe length of the cargo hold.Ship’s terms

E

Enclosed superstructure - is the superstructure with bulkheads forward and fitted with weather tight doors and closing appliances.
Erecting - the process of hoisting and joining the various parts of the ship’s hull. These parts could be in various sizes and called micro panels, panels, blocks, mega blocks.

F

Fabricate - to make assemblies from raw steel materials
Face plate - narrow stiffening plate along the edge of any frame, stiffeners or other structural members, either welded or integral part of the member.
Fairing - reforming distorted plating to its original shape

Fender - any device used to absorb the shock and prevent damage to ship’s hull when coming against a
jetty or against any other object.

Forecastle - short superstructure situated at the bow.
Foremast - the forward most mast on a vessel
Forepeak - is the area of the ship forward of the collision bulkhead.
Forepeak Bulkhead - Same as collision bulkhead

Frame spacing - the fore and aft distance between adjacent frames.
Freeboard - vertical distance from the upper watertight deck to the waterline when the ship is fully loaded.
Freeboard marks - making done amidships on both sides of the ship, to mark the position up to which the ship can load
Freeboard deck - normally the uppermost complete deck exposed to weather and sea, which has permanent means of closing all openings.
Freeing port - is an opening in the bulwarks to allow the water shipped on deck to run freely overboard.

Funnel - a smoke stack of a vessel

G

Gangway - is a raised walkway between superstructure such as between the bridge and forecastle or between bridge and poop.
Galley - kitchen of a ship
Garboard strake - the longitudinal plating next to the keel
Girder - is a collective term for primary member supporting structural members.
Girth - any specified length
Grating - light platform or walkway made up of metal bars, used for access to the machinery
Gross tonnage - overall volume of the ship.
Gunwale - is the upper edge of the ship’s side

H

Hatch – An opening in the deck for passage of cargo
Hatch Coaming - is the vertical plating built around the hatchways, to prevent the water from entering the hold.
Hatch Covers - Steel covers fitted over the hatchways to prevent the ingress on water in the holds, and also as supporting structure for the deck cargo.
Hawse pipe - a pipe passage from the deck at bow to the ship side for the passage of the anchor chain
Hawser - a large rope used for towing or mooring
Heel - inclination of the ship to one side with respect to the center line.
Hull - the body of the ship, including the shell plating,decks, bulkheads, and framing Inboard - inside the ship, toward the center line

I

Inner bottom - plating forming the upper surface of the double bottom

K

Keel - main structural member or the backbone of a ship running longitudinal along the center line of the ship.
Knot - speed unit of one nautical mile (6080 feet)
Kort nozzle - a steel ring surrounding the propeller

L

Ladder - inclined steps used on ships
Launching – process of putting the vessel in water by allowing it to slide down on greased skids.
Length between perpendiculars - length measured between the forward and aft perpendiculars.
Length overall - length of the ship measured from the extreme forward to the aftermost point of the ship.
Lightening hole - a hole cut tin a plate of a structural member to reduce the weight

List - to lean over to one side
Load waterline - line of surface of a ship when loaded to its waterline
Longitudinal - a shell, deck or a stiffener running in a fore and aft direction

M

Main Deck – usually the uppermost continuous deck
Mast – a pole or similar structure on which the lights, and signals can be placed or displayed.
Manhole – a opening in a deck or any other member, which provides access for a man
Midship – at or near the middle point of a ship
Midship section – a cross section through the middle of a ship.
Mooring – securing a after deducting the total volume for certain ship by several lines or cables

N

Net tonnage – the figure spaces which can not be used for cargo

O

Outboard – away from the center line of the ship
Overboard – outside, over the side of the ship into water

P

Peak – a narrow compartment at either end of the ship
Pillar – a vertical member which provides support to a deck girder
Pintle – a pin on which the rudder hinges
Poop – a space below an enclosed superstructure at extreme aft end of the ship
Poop deck – first deck above main deck at aft end.
Port – The left hand side of the ship when looking towards forward side
Porthole – a circular opening in ship’s side for ventilation and natural lighting.
Profile – a side elevation of the ship
Propeller – a rotating device which drives the ship through the water

R

Rigging – ropes, masts, wire ropes, lashings etc.
Roll – motion of the ship from side to side
Rudder – a flat piece of steel attached upright to the stern post at the stern by pintles, gudgeons or hinges, so that it can be turned, causing the vessel to turn.
Rudder Post – after post of the stern frame to which the rudder is attached.Ship’s terms
Rudder Stock – shank of the rudder which extends from the rudder to the steering gear

S

Sagging – straining of the ship which makes the middle portion of the ship lower than the aft and the forward end

Scupper – a deck drain
Scuttle – a small opening in deck or shell usually fitted with a cover, for access to a compartment
Sea chest – a compartment through which the sea water is admitted or discharged
Shedder Pates – slanted plates fitted in dry cargo holds to prevent undesired pockets of cargo.
Sheer strake – top strake of a ship’s side
Shell Expansion – a plan showing all the shell plates.
Skeg – a framed steel plate structure which acts as a fixed rudder under the stern
Sheer – an upward curvature of a ship’s deck in the fore and aft direction
Shell plates – plates forming the outer skin of the ship
Skylight – an opening in the deck to give to give air and light to the compartment below
Starboard – the right hand side of a ship when looking forward.
Stem – a piece of bar or plating on which the ship’s outside plating terminates
Stern – aftermost part of the ship.
Stern frame – a heavy strength member in single screw ship combining the rudder post
Stern tube – a long bushing or bearing through the stern to support the end of the propeller shaft
Stiffener – a collective term for the secondary supporting structural members
Stool – a structure supporting the cargo hold and tank bulkheads
Strake – a row of shell, deck, bulkhead or other plating
Strength deck – normally the uppermost continuous deck
Stringer – a fore and aft member used to give longitudinal strength
Stringer plate – deck plating which contacts the shellShip’s terms
Superstructure – decked structure on the freeboard deck extending for at least 96 % of the breadth of the ship

T

Tank top – plating over the double bottom
Tail shaft – a short portion of the propeller shaft passing through the stern tube and carrying the propeller
Tween decks – abbreviation for between decks, placed between the upper deck and the tank top
Trunk – a small casing passing through the deck, used for ladders or ventilation

V

Void Space – an empty space in a ship

W

Weather deck – a deck exposed to the weather
Web frame – a frame with a deep web
Windlass - a machine used to hoist the anchors.

# Various books, study material and other online sources has been refereed prior to writing this article but no major part is copied or produced  from any of the source but explained same thing in better detailed way.

Main reference source: Ship Terms by " Society of naval architecture students, Cochin" By Arjun k bharath.

(If you have any problem / Complain regarding the post please do contact us !)

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Author Amit                                                                            

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