Oil Tankers
From MarineWiki
Contents |
Description
The International Maritime Organization (IMO) MARPOL convention defines an oil tanker as a ship constructed or adapted primarily to carry oil in its cargo spaces and includes combination carriers and chemical tankers when they are carrying oil in bulk. Oil is defined as pertroleum in any form, including crude oil, fuel oil, sludge, and oil refuse and refined products.
History
In the early 1870s, three passenger/oil ships were built to Lloyd's Class- VADERLAND (2,748 GRT), NEDERLAND (2,839 GRT), and SWITZERLAND (2,816 GRT). These vessels had similar tank arrangement to the modern tankers, i.e., oil were carried in bulk in tanks. However, since the vessel was carrying passengers also, the permit for carrying oil was revoked considering it as too dangerous. in 1886, GLUCKAUF was launched, and is considered as the first oil tanker compared to the modern oil tankers. The next radical change in the ships came in 1906 when Joseph Isherwood introduced the concept of longitudinal framing and PAUL PAIX was the first ship build with this system.
Mission & Design constraints
Tankers are designed to carry the maximum amount of oil for a particular dimension, and where the speed is not a prime concern (unlike container ships). Hence the tankers have a block coefficient close to 0.95 and have a speed of around 15 knots. The sizes vary from small of around 5000 TDW to the worlds largest ships of 550,000 TDW and within these certain standardization have occured (refer the tanker sizes). Apart from the standard tankers there are also versatile vessels that the owners tend to built so that these do not become obselete, and vessels for specific requirement. Shuttle tankers and lightering vessels fall in to this category.
Types of Oil Tankers
Oil tankers can be divided into the following types:
(i) Crude Oil carriers
They carry crude oils from the production fields to the refineries. The voyages are generally longer in length, and these ships are generally larger in size. Normally they are designed to carry three grades of oil.
(ii) Lightering vessels and Shuttle Tankers
They have special requirements. Lightering is the mooring of two vessels for the purpose for transferring cargo oil. The vessel to be lightered may be a floating storage unit that is permanently moored and connected to production facilities, or a vessel that is too large or has too deep a draft to enter the port where the cargo is to be delivered. The receiving vessel is called the service vessel. Lightering involves a series of operations, including the approach maneuver and mooring, hose connection, cargo transfer, hose disconnection and un-mooring. These specially designed shuttle tankers usually have bow or stern loading and dynamic positioning to allow easy transfer of cargo oil even during heavy weather.
(iii) Coastal Tankers
They are small tankers used for the coastwise movement of products and crude oils. These vessels often transit confined waterways, and therefore have effective navigation system and good maneuverability.
(iv) Tank Barges
They are not usually self-propelled, and require a tug for the movement. In the U.S, 60% of water-borne movement is through this.
(v) Product Tankers
They move refined products from the refineries to the consumer.Clean product carriers are designed for the carriage of clean products such as gasoline and lube oils.Common features coating in cargo tanks,heating systems for the cargo oil,and segregated piping systems allowing for the carriage of a number of grades of cargo.Dirty product tankers move black oils such as residual products.
Oil Tanker systems
Hull form
- Propulsion
Tankers require power for propulsion,cargo and ballast handling,and other ship systems.Most tankers are single screw,powered by a single slow speed diesel engine connected to a fixed pitch propeller.Over 30% of tanker casualties are attributable to loss of propulsion or steering.Therefore,tankers operating in particular sensitive regions or otherwise requiring a high degree of redundancy and maneuverability are often designed with twin screw propulsion systems.The major classification societies offer notations for steering and propulsion machinery redundancy.
- Maneuverability
Good maneuvering characteristics are critical for all oil tankers. Most tankers are directionally unstable. In other words the center of lateral resistance in pure sway is forward of the center of rotation in yaw and they need active rudder movements to maintain their heading.This is a consequence of their relatively low length/beam ratios and high block coefficients. The twin skeg arrangements will improve course keeping as will a large rudder or any other fin at the aft end. Turning ability is the converse of course keeping and a poor course keeper will be a good turner. The tactical diameter of most tanker hulls is between 2.5 and 3.5 times their length comfortably within the IMO criterion of 5 times length. Dynamic positioning systems are standard for shuttle tankers which are often required to maintain station for extended periods of time.
- Subdivision and stability
The mission requirements, applicable regulations, preferences of the owner and construction cost are factors that should be considered when developing a tanker's cargo block arrangement.Optimizing the arrangement frequently involves tradeoffs.
- Intact stability
Single hull tankers of conventional design are inherently stable.These tankers require longitudinal bulkheads throughout the length of the cargo tanks for longitudinal strength.The MARPOL regulations for hypothetical outflow,tank length and damage stability tend to encourage an arrangement of the longitudinal bulkheads such that wing tanks and centre tanks have comparable width. In contrast to the inherent stability of single hull tanker,some double hull tankers will become unstable under certain load conditions.A number of factors contribute to the reduction in stability on double hull tankers.The double hull raises the centre of gravity of oil and the free surface effects of double bottom tanks can be quite substantial,particularly when U tanks are fitted.Also, the inner hull of double hull tankers provides sufficient longitudinal strength for ships upto about Suezmax size.This allows the use of single tank across cargo tank arrangements,which further increases free surface effects.
- Damage stability"
The MARPOL damage stability regulation applies constant damage extents, which are derived as a function of the vessel's length and beam. The regulation is roughly equivalent to a two - compartment standard as the longitudinal extents of damage are generally less than the length of the cargo tanks.The damage extents are applied anywhere along the ships length for tankers above 225 meters in length.Fro tankers between 150 meters and 225 meters in length the machinery space is treated as a single floodable space, and for tankers under 150 meters in length flooding of the machinery space need not be considered.
- Double hull requirements
MARPOL regulation 13Fspecifies the hull configuration requirements for tankers contacted after July 6,1993.Oil tankers between 600 DWT and 5,000 DWT must have a double bottom.If wings are not fitted,the size of cargo tanks is limited to 700 m3.Tankers above 5000 DWT must either have a double hull over the full extent of the cargo block,a mid-deck with double sides,or an alternative arrangement specifically approved by IMO as being equivalent to double hulls.The cargo tank length,also referred to as the length of the cargo block,is assumed to extent from the aft point of the aft most cargo tank to the collision bulkhead.The double hull reduces the likelihood that a collision,or grounding will penetrate into the cargo tanks and cause an oil spill.
Most of the crude oil tankers constructed since 1990 have wing tank and double bottom clearances in excess of the minimum requirements.
Some of the reasons for increasing these dimensions are : a) To comply with IMO ballast draft requirements. b) To obtain deeper ballast drafts an reduce the frequency of slamming c) To provide adequate space for structure while maintaining suitable clearances for access,inspection and maintenance purposes d) To reduce the likelihood of a collision or grounding penetrating the inner hull e) To simplify construction,by allowing the prismatic portion of the inner hull to be extended fore and aft beyond the parallel midbody.
Hull Piping system
- Cargo piping system
A standard crude oil carrier is able to load,segregate and discharge three grades of cargo with separation between each grade ensured by two closed valves.Typical design requirements would be loading a full homogeneous cargo including topping off,within 10 hours and discharging a full homogeneous cargo including any standard tank washing and stripping within 24 hours. Crude oil carriers are normally arranged with a pump room just forward of the engine room.The pump room houses the pumps which are usually steam turbine or electric motor driven.Because of the hazardous atmosphere of the pump room,the electric motors are located inside the engine room and are connected to their respective pumps in the pump room through intermediate shafts and stuffing boxes.Lighting must be explosion proof. Product carriers are normally able to load segregate and discharge at least four grades of cargo with two valve separation.One submerged pump per tank is certainly favored for product carriers for reasons of cargo purity and the same system should be considered for black products carriers in order to eliminate the pump room.Drive for submerged pump is usually hydraulic and engined and pumps are fitted in the engine room.The normal precautions for hydraulic systems need to be taken to ensure that it is clean and that any particles are trapped or filtered out. Cargo oil pipe lines may not run through ballast tanks,so that cargo and ballast systems are completely segregated and there is no chance of ballast water becoming contaminated with oil.It is not possible to do this when bulkhead stools are fitted between the cargo tanks and the stool space is part of the double bottom ballast tank.This arrangement is commonly used on parcel tankers.Heavier steel,fully welded with no flanges and a good coating system for that part of the pipe within the stool should be adopted.
Steel can be used for cargo pipes.Piping in tanks can also be glass filament reinforced epoxy resin (GRP).Cargo loading can be carried out either by letting it flow back down the discharge piping,with a by - pass round the pump,or through drop lines directly from,or near ,the manifolds through the deck into the mains inside the tanks.This approach reduces the generation of volatile organic compounds (VOCs) during loading. In principle sufficient piping and valves should be provided to enable any single cargo pump to serve any cargo tank.A by-pass should be arranged so that during cargo loading incoming oil can flow directly from the manifold into the cargo tanks.High level and overfill alarms are required and the overfill alarm (at about the 98% level) should be set so that the person controlling the operation has enough time (usually 1 minute) to prevent the tank overflowing.
Stripping suctions should be placed within a sump at the lowest point of the tank.The cargo system needs to be carefully designed and suctions carefully positioned to ensure that out turn quantity is maximized.Cargo heating is usually not necessary for crude oil carriers,except for the slop tanks.Cargo heating is often required on product carriers.The latter system can only be used in conjunction with a submerged pump in each tanks for circulating contents slowly through the heater.Slop tank heating should be by heating coils so that the contents are not agitated.Steam is the most commonly used heating medium and usually the most convenient when there is a steam plant provided for other purposes.Thermal oil is an alternative system,which is less corrosive and easier to maintain.A main engine exhaust gas boiler provides another source of heat for the system,which is available at sea.The exhaust from the boilers can be used as the source of inert gas,but a heat dump system must then be provided to cater for situations when cargo heating is not required while the inert gas is being generated.
- Ballast piping system
Ballast pumps are usually fitted in the pump room but where there is no pump room submerged pumps are usually fitted in one of the ballast tanks.These pumps can then serve all the ballast tanks except the aft peak,for which a separate pump in the engine room is used. A stripping system should also be provided.The stripping eductor is usually fitted in the pump room.Mud build up can be a problem especially in cellular type double bottom tanks,in ships which regularly take on ballast in estuarial waters.One solution is to fit spray lines inside the ballast tanks to wash out the mud before it solidifies. As with cargo pumps each ballast pump should be provided with the flexibility to serve each ballast tank. Ballast pipelines are exposed to corrosion from both sides.Coated steel can be successful in this service,but the coating is often eroded at bends.GRP is a better choice but it is essential that the piping be well supported in order to withstand the shock loads that can be imposed. Ballast pipelines also provide a means for gas detection,inerting and ventilation of the ballast tanks.When the tanks are empty the lines can be connected to the gas detection system and if hydrocarbon is detected the line is changed over to inert gas (I.G) system and the tanks flooded with I.G.The disadvantage with this system is that the detection system cannot immediately tell which tank is affected.In order to ventilate the ballast tanks air is blown through the main by the I.G fan and the tank hatches on deck are opened.Care must be taken in checking the tank atmosphere before entry.
- Tank cleaning system
Crude oil carriers are required to be able to wash their cargo tanks by circulating the oil cargo back into the cargo tanks through rotating spray guns suspended from the deckhead and mounted on the tank bottom.The operation is termed crude oil washing[COW].Twenty five percent of the cargo tanks are usually washed during each discharge.This helps to prevent the accumulation of sludge in the bottom of the tank and improves the cargo outturn by reducing the amount of clingage.A separate tank cleaning pump is generally used for this but back up should be available from any of the cargo pumps.Capacity of the tank cleaning pump should normally be about 25% to 35%of the capacity of the cargo pump.The stripping system should be sized at not less than 1.25 times the crude oil washing capacity. A system for hot or cold water washing of the cargo tanks should also be provided.This system is not required when the tanks need to be cleaned prior to inspection or docking or in the case of product carriers changing from one grade to another.The tank cleaning pump can also be used for this and a suction provided to sea chest.Discharge should be through a heater which should be able to raise the temperature of the sea water from 20 degree Celsius to 80 degree Celsius.
- Tank venting and Inert gas system
Hull outfitting
- Mooring system
The mooring operation is one of the most hazardous and labor intensive activities on a tanker.The number needed for a safe mooring operation will often determine the size of a tankers crew.It is therefore essential that the equipment and its layout are selected and designed for use in a manner that is both safe and efficient in the use of crew. The mooring system enable the tanker to moor safely at a variety of terminals,such as a SBM (single buoy mooring),a CBM (conventional buoy mooring),piers and sea islands.The normal arrangement comprises a combination of breast lines to control transverse motion and springs to control longitudinal motion.At a SBM all the load is taken on one stoppers designed to accept standard 76 mm chain.
Mooring equipment should be located as close to the extremities of the tanker as possible.This keeps it clear of cargo operations and increases its effectiveness.However,on large tankers it is often necessary to mount some of the winches within the cargo tank area of the deck.Automatic tensioning winches are not generally favoured.
Hull Structure
- Structure arrangement
Tankers above 20000 tons deadweight are in general longitudinally framed over the full length of the cargo block.
- Structure analysis
The classification society rules provide a good basis for initial scantling development.A full ship 3-D finite element analysis should be performed to assess the overall response of the vessel,and then localized fine mesh analyses should be carried out for webs,horizontal girders on the bulkheads and other high stress regions.Yielding,fatigue and buckling should be assessed.Buckling is a common failure mode and should be carefully evaluated particularly for the transverse bulkheads,bottom and inner bottom plating. For larger tankers sloshing should be evaluated.Although the direct analysis of sloshing using CFD code is a complex process,the classification societies have developed simplified approaches the have been benchmarked against numerical analysis and model tests.
- Fatigue analysis
Fatigue life assessment is particularly important on tankers,as small fractures leading to leakage of cargo oil can have serious consequences.Spectral fatigue analysis should be performed taking into consideration the expected environmental conditions and dominant loading patterns. The use of symmetrical shapes,the fitting of soft toed brackets the use of double integral or luggged connection for stiffeners and the profiling of welds are a few of the ways in which fatigue life can be augmented. Tankers trading in particularly harsh environments must be given special attention.For instance,the fatigue life of side longitudianl connections for tankers operating in the North sea/North Atlantic service or Taps trade is less than half of the expected life if operating in world wide service.
Machinery arrangement
The majority of oil tankers built since 1990 are propelled by a single slow speed diesel engine connected to a fixed pitch propeller.These ships generally have three and sometimes four ship service diesel generators to provide electric power.Cargo and ballast pumps may be electric or steam driven.Outside the pump room,the machinery arrangement is similar to other large commercial vessels. Alternative powering arrangements for twin screw tankers include:
- diesel engines direct connected to the propellers,with large ship service diesel generators,
- diesel engines direct connected to the propellers,with power principally supplied by large shaft generators and
- diesel-electric propulsion system
Sharing
