Sunday, June 13, 2010
Thursday, June 10, 2010
Friday, May 28, 2010
Sunday, May 23, 2010
The riggings on titanic
An interesting website about the riggings on board the Titanic. Handy for somebody waning to get an idea about the cargo / lifeboat riggings on old cargo ships.
Wednesday, May 19, 2010
The Sire Programme
This pdf file on the OCIMF website gives the details and history of the SIRE / vetting programme.
Also of interest is this small introduction to SIRE on their website.
Thursday, May 6, 2010
Sunday, May 2, 2010
Intertanko - Introduction to COW
Regulations laid down in the 1978 Protocol to the 1973 Marine Pollution Convention (MARPOL 73/78) require the cargo tanks of crude oil tankers to be cleaned using a procedure called crude oil washing (COW). With the COW procedure the crude oil cargo itself is used as the cleaning medium. During the 1960s it was discovered that crude oil, when applied to the cargo still remaining on tank floors and clinging to the tank structures, using tank cleaning machines, effectively dissolves and dilutes these residues and mixes it in with the rest of the cargo which is being discharged ashore by the cargo pumps.
Prior to the advent of COW, cargo tanks were washed with sea water on their ballast voyage to the next loading port. The mixture of oil and cleaning water resulting from this type of cleaning operation could settle out in the tanker’s slop tanks with the decanted water being discharged overboard into the ocean. Consequently, this operation resulted in inevitable operational discharges of oil-water mixture into the sea.
However, the use of crude oil to COW the tanks means that the solvent action of the crude oil makes the process far more environmentally friendly than when water is used. Additionally, after undertaking COW, the volume of cargo residues left in the tanks is greatly reduced removing the subsequent risk of operational discharges at sea.
Modern tankers are designed with segregated ballast tanks (SBT), and there are only a few stipulated occasions on which seawater comes into contact with the oil cargo system during the course of normal tanker operations. The requirement for new crude oil tankers to be built with double hulls, introduced in the 1990s, has further improved the efficiency of COW operations because more of the structural support members are placed outside the cargo tank and on these types of ships, the amount of crude oil residues left in the cargo tank following discharge is much reduced. Overall, the COW procedure and ship design changes have greatly reduced the need for operational discharges from tankers.
Regulation 13B of Annex I of MARPOL 73/78 requires that the COW installation and arrangements onboard a tanker should comply with the provisions of the “Specifications for the Design, Operation and Control of Crude Oil Washing Systems” adopted by the International Maritime Organization (IMO) in 1978. The COW regime requires that before departure on a ballast voyage, after the complete discharge of cargo, sufficient tanks shall have been crude oil washed to preclude the ballasting of a cargo tank without it having been crude oil washed. On SBT ships approximately 25 per cent of the crude oil carrier’s cargo tanks need to be washed, in the prescribed manner, on every voyage for sludge control purposes provided that no tank need be crude oil washed for sludge control purposes more than once in every four months. For tankers with insufficient SBT capacity, the number of tanks to be crude oil washed has to be increased above this minimum level in order to render sufficient cargo tanks “clean” enough (as defined by the regulations) to take onboard enough water ballast to achieve the tanker’s required sailing ballast draught for the voyage.
In addition to the regulatory controls governing the use of COW, commercial or charter party requirements may require the tanker operator to carry out a greater or lesser degree of COW than the specified minimum in order to maximise the discharge of the crude oil cargo. Notwithstanding these commercial pressures for the extent of COW to be undertaken, at no time should a tanker undertake less than the minimum levels specified in paragraph 6 of Section 1 of the mandatory onboard COW Manual.
Although the MARPOL 73/78 COW regime has proved to be eminently successful in minimising tanker operational discharges and improving cargo outturns during the last two decades of the 20th century, the tanker industry has also been learning more about the behaviour of crude oil cargoes over the period. A number of research projects1 have led to a better understanding of the COW process and how it could be further improved. As a result of this work and at the initiative and suggestion of INTERTANKO, in 1999 the IMO adopted amended COW requirements that are laid down in the revised “Specifications for the Design, Operation and Control of Crude Oil Washing Systems”. These revised Specifications can be found in the 2000 Edition of the “Crude Oil Washing Systems” publication, issued by the IMO.
From an operational perspective the changes provide a more realistic and accurate way of determining the suitability of a crude oil for use in crude oil washing.
Saturday, May 1, 2010
Tankers overview
There are today more than 3,500 (Check) oil tankers in operation. They include the world’s largest ships, one of which (the Jahre Viking) can carry more than half a million tons of crude oil at a time. Many other tankers are almost as large.
The Jahre Viking is the world's largest ship at 564,763 DWT. She was built in 1979 at Oppama Shipyard, Sumitomo, Japan.
Yet as a ship type tankers are relatively new. As late as the middle of the 19th century the only oil transported in large quantities by sea was fuel for oil lamps. Most of this was fish, whale and vegetable oil, but in 1859 an unemployed railway conductor named Edwin Drake was hired to drill for mineral oil at Titusville, Pennsylvania. He struck oil at a depth of 21 metres on 28 August 1859 – and in a sense the modern age began.
Although mineral oil was first used primarily for lighting, the invention of the Diesel and later, the internal combustion engine soon increased its demand enormously. The world’s first true oil tanker is generally accepted to have been the Gluckauf, built in 1886 to carry oil in bulk oil to Europe. The idea of transporting oil in bulk caught on rapidly. In 1885, 99% of the oil exported from the United States was carried in barrels. By 1906, 99% of it was carried in bulk.
The Gluckauf is generally accepted to have been the world’s first oil tanker. But her career was short lived. In 1893 she ran aground on Fire Island, New York and could not be refloated. The remains of her hull can still be seen, just off what is a popular fishing beach.
Demand for oil was encouraged by the invention in 1897 of the Diesel engine, which used oil as a fuel rather than coal. . Within a few years, marine diesel engines were being built-in and by 1911, the first diesel powered ship crossed the Atlantic. By 1927 some 28% of the world merchant fleet used oil for power.
During the next few decades, oil replaced coal as a source of energy and tankers soon formed a major portion of the world fleet. Until 1950, however, most of them were designed to carry petroleum and other refined products. Refineries were generally located close to the fields where crude oil was found. But political and technical developments encouraged the oil industry to move their refineries closer to the markets and this led to an increase in demand for tankers designed to carry crude oil rather than refined products.
In 1950 the standard sized oil tanker was the “T2” tanker, some 620 of which were built in the United States between 1942 and 1946. The tanker equivalent of the famous Liberty ship, many T2 ships were sold after the end of hostilities and formed the backbone of many fleets. They had a deadweight of 16,00 tons and many were still being used in the 1960s. However, by then tanker sizes had begun to grow significantly, a process that was to continue until the end of the 1960s. In 1959 the 114,356 dwt Universe Apollo became the first tanker to pass the 100,000-ton figure: within a decade ships five times that size were being planned.
One reason for this was that tanker owners had discovered how to make use of economies of scale. Unlike petroleum tankers, crude carriers were relatively unsophisticated and fairly simple to build. And, thanks to the square/cube rule, it pays to build them big. If two boxes are built, one with sides 2 meters long and the other with side 4 metres long, the surface area of the first will be 24 square metres and that of the second 96 square metres, or four times as big. But the volume of the first box will be 8 cubic metres and that of the second 64 cubic metres, or eight times as great.
Since it is the amount of steel used that basically determines the cost of constructing the ship it can be seen that using four times as much steel will enable eight times as much cargo to be carried. There are other advantages to be gained from building ships bigger.
The size of oil tankers has grown enormously in the last forty years. This graphic shows, in the bottom left hand corner, a cross-section of a typical 25,000-dwt tanker from the 1950s. By 1963 (second cross-section) tankers of 80,000 dwt were not uncommon, but within ten years many ships of 350,000 dwt had been built or were under construction. The bus in the bottom right hand corner gives an impression of how big these ships are.
One is that crew costs do not rise in proportion to the size of the ship. In fact, from the 1950s onwards crew sizes steadily decreased, as owners took advantage of automation and other technical advances. By the 1980s tankers of 200,000 dwt or more were operating with crews of 24, compared with the 45 required to operate a T2 tanker thirty years before. Other personnel costs, such as shore management, also tended to stay the same, or to fall, since the number of people required to run a fleet depends mainly on the number of ships involved rather than their tonnage.
Fuel costs also tend to fall. A 60,000dwt ship might need about 16,000 horse power to operate at 15 knots. A tanker of 260,000 dwt might require 42,500 hp. In other words, 2.7 times as much energy would enable more than 4.3 times as much cargo to be transported.
In practice, a number of factors helped to prevent tanker sizes from growing indefinitely. In the first place, there was a limit to the number of shipyards capable of building them and the number of ports able to receive them. Secondly, many of the world’s most important shipping routes were unable to cope with very large ships. The Suez Canal, located on what was the most important shipping route in the world in the 1960s, was limited to fully laden ships of 70,000 dwt. The Malacca Strait, separating Malaysia from Indonesia, is too shallow for loaded tankers greater than 260,000 dwt. Larger ships going from the Gulf to Japan, for example, have to go via the Lombok Strait, which adds and extra 1,100 miles to the voyage. Many other straits, such as the Straits of Dover and the Bosporus, present navigational difficulties to large ships.
Developments in the late 1960s however encouraged shipowners to go for big ships. The most important of these was the closure of the Suez Canal in 1967. This meant that ships going from the Gulf to Europe and North America had to go around the Cape of Good Hope instead. At the same time, business and trade were generally booming and, for the first time, the United States had become a major oil importer instead of exporter. Freight rates soared and so did profits. At one time, it was possible for the cost of a new VLCC (Very Large Crude Carrier) of more than 200,000 dwt to be paid off in one year.
It was hardly surprising, therefore that there was a boom in tanker building. Shipyards in Japan did especially well, but the traditional shipyards in Europe also expanded their tanker building capacity. Inevitably, the oil producers also sought to take advantage of the boom. Between 1970 and 1973 the price of oil rose from $1.70 a barrel to $5.19 a barrel. But then in October 1973 war again broke out in the Middle East and freight rates soared. So did orders for tankers. But then major oil producers (member of OPEC, the Organization of Petroleum Exporting Countries) increased the price of oil to $11.65 a barrel early in 1974. Further increases followed, and the result was a collapse in demand for oil and for the tankers needed to transport it. But many shipowners had already contracted to buy new ships and for the rest of the decade VLCCs and the even-larger Ultra Large Crude Carriers (ULCCs) of more than 300,000 dwt were still being delivered. Most of them went straight into lay-up. It has been estimated that by 1975 the tanker market was so depressed that there was a surplus of 100 million dwt, or around 30% of the fleet.[1]
The imbalance between supply and demand lasted until well into the 1990s. Few new ships were ordered and so the world tanker fleet became progressively older (as did the fleet of bulk carriers and other ships). This not only had economic implications, resulting in many shipping companies and shipbuilders going out of business, it also had safety implications. Statistics show quite clearly that older ships are more at risk than new ones. And by the late 1970s the threat of marine pollution from tankers and other ships was causing considerable international concern.
The Jahre Viking is the world's largest ship at 564,763 DWT. She was built in 1979 at Oppama Shipyard, Sumitomo, Japan.
Yet as a ship type tankers are relatively new. As late as the middle of the 19th century the only oil transported in large quantities by sea was fuel for oil lamps. Most of this was fish, whale and vegetable oil, but in 1859 an unemployed railway conductor named Edwin Drake was hired to drill for mineral oil at Titusville, Pennsylvania. He struck oil at a depth of 21 metres on 28 August 1859 – and in a sense the modern age began.
Although mineral oil was first used primarily for lighting, the invention of the Diesel and later, the internal combustion engine soon increased its demand enormously. The world’s first true oil tanker is generally accepted to have been the Gluckauf, built in 1886 to carry oil in bulk oil to Europe. The idea of transporting oil in bulk caught on rapidly. In 1885, 99% of the oil exported from the United States was carried in barrels. By 1906, 99% of it was carried in bulk.
The Gluckauf is generally accepted to have been the world’s first oil tanker. But her career was short lived. In 1893 she ran aground on Fire Island, New York and could not be refloated. The remains of her hull can still be seen, just off what is a popular fishing beach.
Demand for oil was encouraged by the invention in 1897 of the Diesel engine, which used oil as a fuel rather than coal. . Within a few years, marine diesel engines were being built-in and by 1911, the first diesel powered ship crossed the Atlantic. By 1927 some 28% of the world merchant fleet used oil for power.
During the next few decades, oil replaced coal as a source of energy and tankers soon formed a major portion of the world fleet. Until 1950, however, most of them were designed to carry petroleum and other refined products. Refineries were generally located close to the fields where crude oil was found. But political and technical developments encouraged the oil industry to move their refineries closer to the markets and this led to an increase in demand for tankers designed to carry crude oil rather than refined products.
In 1950 the standard sized oil tanker was the “T2” tanker, some 620 of which were built in the United States between 1942 and 1946. The tanker equivalent of the famous Liberty ship, many T2 ships were sold after the end of hostilities and formed the backbone of many fleets. They had a deadweight of 16,00 tons and many were still being used in the 1960s. However, by then tanker sizes had begun to grow significantly, a process that was to continue until the end of the 1960s. In 1959 the 114,356 dwt Universe Apollo became the first tanker to pass the 100,000-ton figure: within a decade ships five times that size were being planned.
One reason for this was that tanker owners had discovered how to make use of economies of scale. Unlike petroleum tankers, crude carriers were relatively unsophisticated and fairly simple to build. And, thanks to the square/cube rule, it pays to build them big. If two boxes are built, one with sides 2 meters long and the other with side 4 metres long, the surface area of the first will be 24 square metres and that of the second 96 square metres, or four times as big. But the volume of the first box will be 8 cubic metres and that of the second 64 cubic metres, or eight times as great.
Since it is the amount of steel used that basically determines the cost of constructing the ship it can be seen that using four times as much steel will enable eight times as much cargo to be carried. There are other advantages to be gained from building ships bigger.
The size of oil tankers has grown enormously in the last forty years. This graphic shows, in the bottom left hand corner, a cross-section of a typical 25,000-dwt tanker from the 1950s. By 1963 (second cross-section) tankers of 80,000 dwt were not uncommon, but within ten years many ships of 350,000 dwt had been built or were under construction. The bus in the bottom right hand corner gives an impression of how big these ships are.
One is that crew costs do not rise in proportion to the size of the ship. In fact, from the 1950s onwards crew sizes steadily decreased, as owners took advantage of automation and other technical advances. By the 1980s tankers of 200,000 dwt or more were operating with crews of 24, compared with the 45 required to operate a T2 tanker thirty years before. Other personnel costs, such as shore management, also tended to stay the same, or to fall, since the number of people required to run a fleet depends mainly on the number of ships involved rather than their tonnage.
Fuel costs also tend to fall. A 60,000dwt ship might need about 16,000 horse power to operate at 15 knots. A tanker of 260,000 dwt might require 42,500 hp. In other words, 2.7 times as much energy would enable more than 4.3 times as much cargo to be transported.
In practice, a number of factors helped to prevent tanker sizes from growing indefinitely. In the first place, there was a limit to the number of shipyards capable of building them and the number of ports able to receive them. Secondly, many of the world’s most important shipping routes were unable to cope with very large ships. The Suez Canal, located on what was the most important shipping route in the world in the 1960s, was limited to fully laden ships of 70,000 dwt. The Malacca Strait, separating Malaysia from Indonesia, is too shallow for loaded tankers greater than 260,000 dwt. Larger ships going from the Gulf to Japan, for example, have to go via the Lombok Strait, which adds and extra 1,100 miles to the voyage. Many other straits, such as the Straits of Dover and the Bosporus, present navigational difficulties to large ships.
Developments in the late 1960s however encouraged shipowners to go for big ships. The most important of these was the closure of the Suez Canal in 1967. This meant that ships going from the Gulf to Europe and North America had to go around the Cape of Good Hope instead. At the same time, business and trade were generally booming and, for the first time, the United States had become a major oil importer instead of exporter. Freight rates soared and so did profits. At one time, it was possible for the cost of a new VLCC (Very Large Crude Carrier) of more than 200,000 dwt to be paid off in one year.
It was hardly surprising, therefore that there was a boom in tanker building. Shipyards in Japan did especially well, but the traditional shipyards in Europe also expanded their tanker building capacity. Inevitably, the oil producers also sought to take advantage of the boom. Between 1970 and 1973 the price of oil rose from $1.70 a barrel to $5.19 a barrel. But then in October 1973 war again broke out in the Middle East and freight rates soared. So did orders for tankers. But then major oil producers (member of OPEC, the Organization of Petroleum Exporting Countries) increased the price of oil to $11.65 a barrel early in 1974. Further increases followed, and the result was a collapse in demand for oil and for the tankers needed to transport it. But many shipowners had already contracted to buy new ships and for the rest of the decade VLCCs and the even-larger Ultra Large Crude Carriers (ULCCs) of more than 300,000 dwt were still being delivered. Most of them went straight into lay-up. It has been estimated that by 1975 the tanker market was so depressed that there was a surplus of 100 million dwt, or around 30% of the fleet.[1]
The imbalance between supply and demand lasted until well into the 1990s. Few new ships were ordered and so the world tanker fleet became progressively older (as did the fleet of bulk carriers and other ships). This not only had economic implications, resulting in many shipping companies and shipbuilders going out of business, it also had safety implications. Statistics show quite clearly that older ships are more at risk than new ones. And by the late 1970s the threat of marine pollution from tankers and other ships was causing considerable international concern.
Thursday, April 29, 2010
Cargo Ventilation and Precautions to Minimise Sweat
"Moisture damage" is the source of a significant number of cargo claims, often involving bagged or bulk agricultural products. Claimants typically allege that failure by the ship to ventilate correctly resulted in the development of condensation ("sweat"), causing the cargo to deteriorate.
However, it is also important to recognise that some commodities may have inherent moisture levels which exceed acceptable limits at the time of loading, making them biologically unstable. Such details may not be known to the ship, and prudent ventilation measures may be insufficient to prevent deterioration of the cargo on passage. Nevertheless, claimants may still maintain that the ship was at fault.
To defend cargo deterioration claims it is necessary for the vessel to produce records showing that customary ventilation routines were followed. Should the necessary evidence be missing or incomplete, it is often difficult for the Club to refute such assertions.
General
Ships are fitted with either natural or mechanical ventilation systems. In addition to minimising the onset and degree of sweat, ventilation may also serve to remove taint and disperse any gases which some cargoes may emit.
The process requires close monitoring throughout the voyage as the moisture content of the cargo coupled with variations in air temperature, cargo temperature and sea temperature can dramatically influence the amounts of water vapour retained by and released into the air inside a hold.
Penetration of ventilating air into a bulk stow on a ship is minimal, and so it is only ever possible at best to provide through-surface ventilation. However, paramount ship stability requirements usually dictate that at least the majority of the holds of any bulk carrier carrying bulk cargoes such as grain are loaded fully into the hatch coamings. For a hold so loaded it is unlikely that any significant through-surface air flow will be obtained.
Whilst bagged cargo stows inevitably have some gaps in them, penetration of ventilating air beneath the uppermost layers of bags in the stow is minimal. Bagged cargoes should always be stowed in such a way that ventilating air can pass freely over the surface of the stow.
Cargoes at risk
Hygroscopic products
Hygroscopic products have a natural moisture content and are mainly of plant origin. They may retain, absorb or release water vapour, and excessive amounts of inherent moisture may lead to significant self-heating and "moisture migration" within the cargo resulting in caking, mildew or rot. Examples of hygroscopic products include grain, rice, flour, sugar, cotton, tobacco, cocoa, coffee and tea.
Non-hygroscopic products
Non-hygroscopic products have no water content. However, certain commodities (eg steel) may be damaged if stowed in a moist environment, and others may be harmed if packaged using a hygroscopic material (eg wood, paper).
By way of illustration a vessel loaded a parcel of glass packed with layers of paper between each sheet. At the discharge port it was found that the paper had absorbed moisture from the air during the voyage, making it impossible for the glass sheets to be separated. The cargo was rejected by the receiver.
Types of Sweat
Cargo sweat
Cargo sweat refers to condensation which may form on exposed surfaces of the stow as a consequence of large amounts of warm, moist air being persistently introduced into a hold containing substantially colder cargo.
However, it is also important to recognise that some commodities may have inherent moisture levels which exceed acceptable limits at the time of loading, making them biologically unstable. Such details may not be known to the ship, and prudent ventilation measures may be insufficient to prevent deterioration of the cargo on passage. Nevertheless, claimants may still maintain that the ship was at fault.
To defend cargo deterioration claims it is necessary for the vessel to produce records showing that customary ventilation routines were followed. Should the necessary evidence be missing or incomplete, it is often difficult for the Club to refute such assertions.
General
Ships are fitted with either natural or mechanical ventilation systems. In addition to minimising the onset and degree of sweat, ventilation may also serve to remove taint and disperse any gases which some cargoes may emit.
The process requires close monitoring throughout the voyage as the moisture content of the cargo coupled with variations in air temperature, cargo temperature and sea temperature can dramatically influence the amounts of water vapour retained by and released into the air inside a hold.
Penetration of ventilating air into a bulk stow on a ship is minimal, and so it is only ever possible at best to provide through-surface ventilation. However, paramount ship stability requirements usually dictate that at least the majority of the holds of any bulk carrier carrying bulk cargoes such as grain are loaded fully into the hatch coamings. For a hold so loaded it is unlikely that any significant through-surface air flow will be obtained.
Whilst bagged cargo stows inevitably have some gaps in them, penetration of ventilating air beneath the uppermost layers of bags in the stow is minimal. Bagged cargoes should always be stowed in such a way that ventilating air can pass freely over the surface of the stow.
Cargoes at risk
Hygroscopic products
Hygroscopic products have a natural moisture content and are mainly of plant origin. They may retain, absorb or release water vapour, and excessive amounts of inherent moisture may lead to significant self-heating and "moisture migration" within the cargo resulting in caking, mildew or rot. Examples of hygroscopic products include grain, rice, flour, sugar, cotton, tobacco, cocoa, coffee and tea.
Non-hygroscopic products
Non-hygroscopic products have no water content. However, certain commodities (eg steel) may be damaged if stowed in a moist environment, and others may be harmed if packaged using a hygroscopic material (eg wood, paper).
By way of illustration a vessel loaded a parcel of glass packed with layers of paper between each sheet. At the discharge port it was found that the paper had absorbed moisture from the air during the voyage, making it impossible for the glass sheets to be separated. The cargo was rejected by the receiver.
Types of Sweat
Cargo sweat
Cargo sweat refers to condensation which may form on exposed surfaces of the stow as a consequence of large amounts of warm, moist air being persistently introduced into a hold containing substantially colder cargo.
Ship’s sweat
Ship’s sweat refers to condensation which forms directly on a vessel’s structure when the air within a hold, made warm and moist by the cargo, comes into contact with cold surfaces as the vessel moves into cooler climates. Cargo may be damaged by overhead drips, by contact with sweat which has formed on the ship’s sides or by condensed water which may accumulate at the bottom of the hold.
Influencing factors
Saturation
The amount of water vapour that air may contain is highly dependent on its temperature. A given volume of air is said to be saturated when no more water can be absorbed. If the air temperature then falls, condensation will occur.
As air rises in temperature so does its saturation moisture content; its capacity to retain water climbs by ever-increasing amounts. Thus, when hot air is cooled, its potential for releasing water in the form of condensation is far greater when it is cooling from higher temperatures than when cooling from lower temperatures.
Apart from periods of fog or rain, ambient air is rarely saturated. Moreover, it will never be totally dry. Within these two extremes the amount of water retained by the air will vary according to the prevailing conditions.
Relative humidity
Relative humidity is the actual amount of water vapour in the air compared with the saturation amount of water vapour in the air at the same temperature and pressure. The figure is usually expressed as a percentage, with saturated air having a relative humidity of 100%.
At main deck level, ambient sea air over the open oceans will normally have a relative humidity in excess of 80%.
Dewpoint temperature
When an isolated volume of air cools, relative humidity increases as the temperature falls. Once the temperature has descended to the level at which saturation occurs, water begins to condense. This temperature is known as the "dewpoint".
Dewpoint temperature may be measured by a variety of methods. Ships generally use a traditional wet and dry bulb arrangement consisting of two identical mercury thermometers, one of which has a damp wick covering the bulb. These are normally housed in a protective marine screen.
The dewpoint temperature may then be determined by using a "Dewpoint Table" (see Annex I). This figure is important when considering cargo ventilation requirements.
Wet and dry bulb thermometers
When using traditional wet and dry bulb thermometers, the accuracy of the dew point temperature will depend on the condition of the equipment. The muslin covering the wet bulb should be clean, the water in the reservoir should be distilled and the bulb itself should be wet.
In order to ensure that the readings are correct, the device should always be positioned clear of any exhaust vents, other draughts and all sources of heat.
Dewpoint measurement
Theoretically, all decisions regarding cargo ventilation should be based on dewpoint temperatures, comparing the dewpoint of the ambient air with dewpoint of the air inside the hold.
Given that most ships are customarily equipped with wet and dry bulb thermometers located close to the bridge, determining the dewpoint temperature of the ambient air is usually straightforward.
However, ascertaining the dewpoint temperature inside a cargo space is more problematic. One of the simplest methods is to use a "whirling psychrometer", swinging the instrument inside the hold until the wet bulb temperature has stopped falling and remains steady.
All readings should be taken well away from any air inlets, ensuring that only hold air is tested. Enclosed space entry procedures should always be observed.
If access to the holds is impossible or undesirable, and provided there is no significant air flow, wet and dry bulb thermometers may be placed in the trunking of an exhaust ventilator or similar pipework leading from the compartment, allowing the device to be drawn out and read from above deck.
Ventilation
Once the above information has been obtained, the rules are simple;
Dewpoint Rule
VENTILATE if the dewpoint of the air inside the hold is higher than the dewpoint of the air outside the hold.
DO NOT VENTILATE if the dewpoint of the air inside the hold is lower than the dewpoint of the air outside the hold.
Three Degree Rule
In many instances it is impracticable to measure hold dewpoint temperatures accurately, or at all.
In such cases ventilation requirements may be estimated by comparing the average cargo temperature at the time of loading with the outside air temperature several times a day. Ventilation may then be carried out on the following basis;
VENTILATE if the dry bulb temperature of the outside air is at least 3°C cooler than the average cargo temperature at the time of loading.
DO NOT VENTILATE if the dry bulb temperature of the outside air less than 3°C cooler than the average cargo temperature at the time of loading, or warmer.
In order to apply the Three Degree Rule, it will be necessary for the ship’s staff to take a number of cargo temperature readings during loading.
Further observations
During periods of heavy weather, steps should be taken to prevent rain and spray from entering the cargo spaces. This may mean suspending ventilation until conditions improve. If so, the circumstances should be logged.
It is important to appreciate that ventilation should also be carried out during the night if the readings indicate that ventilation is appropriate. Ambient temperatures are usually lower at night, therefore the risk of ship’s sweat developing is more likely during the hours of darkness.
In addition to ventilating the holds according to the above regimes, it is important that regular inspections of each compartment are carried out where possible. This need not involve entry into the cargo space itself - for example ship’s sweat may be seen forming on the underside of manhole covers. In such instances, and especially at night, the cargo should be ventilated irrespective of the Dewpoint or Three Degree Rules, weather permitting.
What to expect
In broad terms it is often possible to estimate ventilation requirements in advance by considering the climatic changes likely to be encountered during the voyage. The following examples indicate what may be expected on passage, but do not obviate the need for detailed monitoring and recording;
Hygroscopic cargo - cold to warm climate
If a stable cold cargo is carried to a warm climate, ventilation will always be unnecessary. Indeed, in some circumstances ventilation may lead to cargo damage.
Hygroscopic cargo - warm to cold climate
Vigorous surface ventilation of the cargo spaces will almost certainly be required due to the likelihood of ship’s sweat developing.
Non-hygroscopic cargo - cold to warm climate
Ventilation is never required. Cargo sweat is liable to occur if warm moist air comes into contact with cold cargo. Therefore holds should usually remain sealed to allow the cargo and internal air to warm gradually during the voyage.
Non-hygroscopic cargo - warm to cold climate
Ventilation is largely irrelevant. Development of significant ship’s sweat is very unlikely.
Combined cargoes
Problems may arise if hygroscopic and non-hygroscopic cargoes with different inherent temperatures are loaded into the same compartment. Their ventilation requirements may differ, resulting in damage to one or other of the products in spite of normal routines being followed. As far as possible, hygroscopic and non-hygroscopic cargoes should not be stowed together.
Stowage
Given the sensitive nature of many hygroscopic products and the possibility of sweat, efforts should be made to ensure that such cargoes do not come into contact with hold steelwork. This is particularly important in the case of bagged agricultural produce intended for human consumption such as rice, beans and flour.
For bagged cargo, rows of dunnage or bamboo poles should be laid in the direction of the bilges to aid drainage, not more than 20 centimetres apart. A second layer should be placed on top at right angles to the first before covering the whole area with matting.
If the cargo space is not fully fitted with cargo battens, bamboo poles or dunnage should be positioned crosswise against the frames to keep the bags away from the sides of the ship. Ideally, they should also be lashed together at the intersections to prevent them from becoming disturbed during loading. As an extra but not essential precaution, mats may be placed against this arrangement. In the same context, the top surface of the stow may be covered with thick kraft paper.
Expert opinion is now that biologically stable bagged hygroscopic cargoes do not require ventilation channels, unless specifically demanded by the IMDG Code (eg some types of seed cake, fishmeal). Nevertheless, for certain commodities many charterers still require ventilation channels to be built into the stow. If so, the charterers should be asked for written instructions regarding the number and position of such channels, and these should be followed accordingly.
A check list addressing the carriage of hygroscopic cargo is featured in Annex II.
Bunker tanks
Hygroscopic products may be damaged by localised sources of heat. Incidents have occurred where parts of parcels of grain have been scorched or have become discoloured when lying against hot bunker tanks. As far as possible, the bunkers used during the voyage should be drawn from tanks situated well away from holds containing hygroscopic products. If impracticable, bunker tanks adjoining cargo spaces should be heated only when required, ensuring that the temperature does not rise above normal operational levels.
Records
Ventilation records are crucial. In the event of moisture damage, evidence showing that the vessel ventilated correctly may be instrumental in defending any ensuing claims.
If the Dewpoint Rule has been followed, wet and dry bulb temperatures and dewpoints should be logged once per watch, bearing in mind that these may change considerably over a short period. For the same reason, the sea temperature should also be noted. This information should be recorded for each hold together with the times of commencing, ceasing or resuming ventilation, and the reasons for doing so.
If the Three Degree Rule has been followed, a record should be kept of the ambient air temperature and the sea temperature once per watch together the average temperature of the cargo at the time of loading. Once again, ventilation details should be documented for each hold.
Ship’s sweat refers to condensation which forms directly on a vessel’s structure when the air within a hold, made warm and moist by the cargo, comes into contact with cold surfaces as the vessel moves into cooler climates. Cargo may be damaged by overhead drips, by contact with sweat which has formed on the ship’s sides or by condensed water which may accumulate at the bottom of the hold.
Influencing factors
Saturation
The amount of water vapour that air may contain is highly dependent on its temperature. A given volume of air is said to be saturated when no more water can be absorbed. If the air temperature then falls, condensation will occur.
As air rises in temperature so does its saturation moisture content; its capacity to retain water climbs by ever-increasing amounts. Thus, when hot air is cooled, its potential for releasing water in the form of condensation is far greater when it is cooling from higher temperatures than when cooling from lower temperatures.
Apart from periods of fog or rain, ambient air is rarely saturated. Moreover, it will never be totally dry. Within these two extremes the amount of water retained by the air will vary according to the prevailing conditions.
Relative humidity
Relative humidity is the actual amount of water vapour in the air compared with the saturation amount of water vapour in the air at the same temperature and pressure. The figure is usually expressed as a percentage, with saturated air having a relative humidity of 100%.
At main deck level, ambient sea air over the open oceans will normally have a relative humidity in excess of 80%.
Dewpoint temperature
When an isolated volume of air cools, relative humidity increases as the temperature falls. Once the temperature has descended to the level at which saturation occurs, water begins to condense. This temperature is known as the "dewpoint".
Dewpoint temperature may be measured by a variety of methods. Ships generally use a traditional wet and dry bulb arrangement consisting of two identical mercury thermometers, one of which has a damp wick covering the bulb. These are normally housed in a protective marine screen.
The dewpoint temperature may then be determined by using a "Dewpoint Table" (see Annex I). This figure is important when considering cargo ventilation requirements.
Wet and dry bulb thermometers
When using traditional wet and dry bulb thermometers, the accuracy of the dew point temperature will depend on the condition of the equipment. The muslin covering the wet bulb should be clean, the water in the reservoir should be distilled and the bulb itself should be wet.
In order to ensure that the readings are correct, the device should always be positioned clear of any exhaust vents, other draughts and all sources of heat.
Dewpoint measurement
Theoretically, all decisions regarding cargo ventilation should be based on dewpoint temperatures, comparing the dewpoint of the ambient air with dewpoint of the air inside the hold.
Given that most ships are customarily equipped with wet and dry bulb thermometers located close to the bridge, determining the dewpoint temperature of the ambient air is usually straightforward.
However, ascertaining the dewpoint temperature inside a cargo space is more problematic. One of the simplest methods is to use a "whirling psychrometer", swinging the instrument inside the hold until the wet bulb temperature has stopped falling and remains steady.
All readings should be taken well away from any air inlets, ensuring that only hold air is tested. Enclosed space entry procedures should always be observed.
If access to the holds is impossible or undesirable, and provided there is no significant air flow, wet and dry bulb thermometers may be placed in the trunking of an exhaust ventilator or similar pipework leading from the compartment, allowing the device to be drawn out and read from above deck.
Ventilation
Once the above information has been obtained, the rules are simple;
Dewpoint Rule
VENTILATE if the dewpoint of the air inside the hold is higher than the dewpoint of the air outside the hold.
DO NOT VENTILATE if the dewpoint of the air inside the hold is lower than the dewpoint of the air outside the hold.
Three Degree Rule
In many instances it is impracticable to measure hold dewpoint temperatures accurately, or at all.
In such cases ventilation requirements may be estimated by comparing the average cargo temperature at the time of loading with the outside air temperature several times a day. Ventilation may then be carried out on the following basis;
VENTILATE if the dry bulb temperature of the outside air is at least 3°C cooler than the average cargo temperature at the time of loading.
DO NOT VENTILATE if the dry bulb temperature of the outside air less than 3°C cooler than the average cargo temperature at the time of loading, or warmer.
In order to apply the Three Degree Rule, it will be necessary for the ship’s staff to take a number of cargo temperature readings during loading.
Further observations
During periods of heavy weather, steps should be taken to prevent rain and spray from entering the cargo spaces. This may mean suspending ventilation until conditions improve. If so, the circumstances should be logged.
It is important to appreciate that ventilation should also be carried out during the night if the readings indicate that ventilation is appropriate. Ambient temperatures are usually lower at night, therefore the risk of ship’s sweat developing is more likely during the hours of darkness.
In addition to ventilating the holds according to the above regimes, it is important that regular inspections of each compartment are carried out where possible. This need not involve entry into the cargo space itself - for example ship’s sweat may be seen forming on the underside of manhole covers. In such instances, and especially at night, the cargo should be ventilated irrespective of the Dewpoint or Three Degree Rules, weather permitting.
What to expect
In broad terms it is often possible to estimate ventilation requirements in advance by considering the climatic changes likely to be encountered during the voyage. The following examples indicate what may be expected on passage, but do not obviate the need for detailed monitoring and recording;
Hygroscopic cargo - cold to warm climate
If a stable cold cargo is carried to a warm climate, ventilation will always be unnecessary. Indeed, in some circumstances ventilation may lead to cargo damage.
Hygroscopic cargo - warm to cold climate
Vigorous surface ventilation of the cargo spaces will almost certainly be required due to the likelihood of ship’s sweat developing.
Non-hygroscopic cargo - cold to warm climate
Ventilation is never required. Cargo sweat is liable to occur if warm moist air comes into contact with cold cargo. Therefore holds should usually remain sealed to allow the cargo and internal air to warm gradually during the voyage.
Non-hygroscopic cargo - warm to cold climate
Ventilation is largely irrelevant. Development of significant ship’s sweat is very unlikely.
Combined cargoes
Problems may arise if hygroscopic and non-hygroscopic cargoes with different inherent temperatures are loaded into the same compartment. Their ventilation requirements may differ, resulting in damage to one or other of the products in spite of normal routines being followed. As far as possible, hygroscopic and non-hygroscopic cargoes should not be stowed together.
Stowage
Given the sensitive nature of many hygroscopic products and the possibility of sweat, efforts should be made to ensure that such cargoes do not come into contact with hold steelwork. This is particularly important in the case of bagged agricultural produce intended for human consumption such as rice, beans and flour.
For bagged cargo, rows of dunnage or bamboo poles should be laid in the direction of the bilges to aid drainage, not more than 20 centimetres apart. A second layer should be placed on top at right angles to the first before covering the whole area with matting.
If the cargo space is not fully fitted with cargo battens, bamboo poles or dunnage should be positioned crosswise against the frames to keep the bags away from the sides of the ship. Ideally, they should also be lashed together at the intersections to prevent them from becoming disturbed during loading. As an extra but not essential precaution, mats may be placed against this arrangement. In the same context, the top surface of the stow may be covered with thick kraft paper.
Expert opinion is now that biologically stable bagged hygroscopic cargoes do not require ventilation channels, unless specifically demanded by the IMDG Code (eg some types of seed cake, fishmeal). Nevertheless, for certain commodities many charterers still require ventilation channels to be built into the stow. If so, the charterers should be asked for written instructions regarding the number and position of such channels, and these should be followed accordingly.
A check list addressing the carriage of hygroscopic cargo is featured in Annex II.
Bunker tanks
Hygroscopic products may be damaged by localised sources of heat. Incidents have occurred where parts of parcels of grain have been scorched or have become discoloured when lying against hot bunker tanks. As far as possible, the bunkers used during the voyage should be drawn from tanks situated well away from holds containing hygroscopic products. If impracticable, bunker tanks adjoining cargo spaces should be heated only when required, ensuring that the temperature does not rise above normal operational levels.
Records
Ventilation records are crucial. In the event of moisture damage, evidence showing that the vessel ventilated correctly may be instrumental in defending any ensuing claims.
If the Dewpoint Rule has been followed, wet and dry bulb temperatures and dewpoints should be logged once per watch, bearing in mind that these may change considerably over a short period. For the same reason, the sea temperature should also be noted. This information should be recorded for each hold together with the times of commencing, ceasing or resuming ventilation, and the reasons for doing so.
If the Three Degree Rule has been followed, a record should be kept of the ambient air temperature and the sea temperature once per watch together the average temperature of the cargo at the time of loading. Once again, ventilation details should be documented for each hold.
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