Chapter 1 Overview of tank cleaning work

Transcription

1 Chapter 1 Overview of tank cleaning work The overall image of the tank cleaning work should be grasped before implementing it. An overview of individual tasks to be grasped before starting the work is described in this chapter. 1. Tank cleaning work (P.1-2) Tank cleaning refers to the tasks mentioned below. 1) Washing of tanks, lines and pumps 2) Gas-freeing work 2. Objectives of tank cleaning work (P.1-2) The objectives of tank cleaning work are as given below. 1) To wash cargo tanks/ piping after leaving the discharging terminal if workers need to enter the tank/ piping for inspection or repairs, and to create an environment in the tank/piping such that the workers can enter these spaces safely. 2) To create conditions within the tank/ piping such that these conditions match the required standards for permitting work such as hot work to be performed in the said spaces. 3. Tank cleaning work processes (P.1-2 to P.1-15) Implement the processes below in the time sequence of tank cleaning work. 1) Crude oil washing at the previous discharging terminal (P.1-2) 2) Filling of washing water (P.1-3 to P.1-5) 3) Heating of washing water (P.1-5 to P.1-6) 4) Rough washing of cargo line pump (P.1-6 to P.1-7) 5) Line washing (removal of oil traces in the ODM line) and valve flushing (P.1-7) 6) Tank washing (P.1-7 to P.1-8) 7) HC gas purging (P.1-8 to P.1-11) 8) Airing (P.1-11 to P.1-12) 9) Settling and discharging water (P.1-13) 10) Final line washing (P.1-13 to P.1-14) 11) Sludge removal (P.1-14) 12) Slop removal (P.1-14) 13) Primary slop tank washing (P.1-14) 14) Primary slop tank gas freeing, airing and sludge removal (P.1-14) 15) Final line stripping (P.1-14) 16) Acquisition of gas-free certificate (P.1-15) 1. Tank cleaning work (Gas free) 1-1

2 Tank cleaning is generally not performed during the normal operation of SBT tankers and double-hull tankers. It is performed only when it is necessary to enter the tank for inspection and repairs in the event of a breakdown in the cargo handling appliances in the tank, or when the ship is dry-docked. Tank cleaning involves the removal of traces of oil and petroleum gases in the tank and piping, and specifically includes the tasks mentioned below. 1) Washing of the tank and lines with sea water 2) Gas-freeing the environment in the tank so that workers can enter the tank safely. 2. Objectives of tank cleaning work The objectives of tank cleaning work are as given below. a) To create an environment in the tank such that workers can enter the tank and piping safely for inspection and repair work b) To ensure that the environmental conditions in the tank match the conditions required for performing hot work safely. 3. Explanations on tank cleaning work 3.1 Overall flow of work The overall flow of tank cleaning work by time sequence is as given below. (Refer to the Flow Chart.) 1) Crude oil washing at the previous discharging terminal 2) Filling of washing water 3) Heating of washing water 4) Rough washing of cargo line and cargo pump 5) Line washing (removal of oil traces in the ODM line) and valve flushing 6) Tank washing 7) HC gas purging 8) Airing 9) Settling and discharging water 10) Final line washing 11) Sludge removal 12) Slop removal 13) Primary slop tank washing 14) Primary slop tank gas freeing, airing and sludge removal 15) Final line stripping 16) Acquisition of gas-free certificate 3.2 Crude oil washing at the previous discharging terminal Tank cleaning work is considered to begin with the cargo handling work at the previous discharging terminal. In other words, it is preferable to formulate a plan such that crude oil washing of tanks to be cleaned is performed at the previous discharging terminal whenever possible, and the sludge accumulated in the tank is discharged to shore at the same time as the discharging of the cargo. Adequate discussions should be held with the terminal for tank cleaning before dry-docking the ship, in regard to implementing crude oil washing such that no sludge remains in the tank as far as possible. If any sludge does remain in the tank, then the charges for removal of the said sludge will have to be borne by the shipowner Effective crude oil washing It is important to be have thorough knowledge of crude oil washing equipment, tank construction and (Gas free) 1-2

3 piping, and be adequately familiar with the contents of the Crude Oil Washing Operations and Equipment Manual Number of tanks for which crude oil washing (referred to as COW hereafter) is to be implemented MARPOL stipulates the number of tanks for which COW is to be implemented as given below. Note however, that some terminals may have their own rules and regulations related to COW, and the number of tanks that can be washed may be restricted depending on the rules and regulations of the specific terminal. a) COW of about one-fourth of the total number of tanks is to be performed considering effective sludge control. b) However, COW of the same tank two times within four months for the same purpose is not necessary. 3.3 Filling of washing water This includes the work of filling the sea water (washing water), which is to be used for washing the tanks and lines, in the slop tank Selection of filling tank Sometimes only the primary slop tank may be used, and sometimes the tanks on both sides may be used as filling tanks. If all cargo tanks need to be washed before dry-docking the ship, the usual practice is to fill slop tanks on both sides of the ship so as to avoid the washing effect from degrading due to pollution of the washing water. If the number of washing tanks is small or if there are restrictions on filling water in slop tanks or on the discharging time, then only one primary slop tank should be filled. Primary slop tanks/ secondary slop tanks The slop tanks of crude oil tankers are connected by free-flow lines. This is to collect the dirty water after washing on one side, to shift the water only that has been separated from oil at the bottom of the tank to the other tank by gravity and to re-use this water as washing water. The tank in which dirty water after washing is collected is the primary slop tank, while the tank on the washing water driving side is called the secondary slop tank. A heating coil is generally installed in the primary slop tank. For tank washing likewise, the intake ports for washing water are used only on the secondary slop tank side, and the eductor driving water is returned to the primary slop tank side. Only the sea water layer at the bottom of the oil-water separator tank in the primary slop tank is shifted by gravity to the secondary slop tank through the free flow line. This method enables tank washing to be performed efficiently, using washing water in the secondary slop tank, which has smaller oil content Filling method Three methods of filling washing water are available. 1) Method of filling water from the sea by gravity In this method, the sea chest valve is opened, sea water of the required volume is directly filled first from the sea in the No. 5 center cargo tank by gravity, and then shifted to the slop tank using the tank cleaning pump. 2) Method of filling water from the sea by pump In this method, the sea chest valve is opened and the sea water from the sea is directly filled in the slop tank using a pump. 3) Method of filling water by gravity from the ballast tank In this method, the cargo line and ballast line are connected to the spool piece and water is filled by gravity from the ballast tank. (Gas free) 1-3

4 If slop tanks on both sides of the ship are to be filled, water can be filled directly into the primary slop tank and simultaneously shifted to the secondary slop tank by gravity using the free flow line Deciding the filling method Consider the following points to decide the filling method: 1) When adopting the method of filling water from the sea * The method of taking in warm sea water from the sea directly through the chest is effective since the heating time is curtailed. Considerable time is required for shifting the water from the No. 5 center cargo tank to the slop tank. * Since the cargo line is directly connected to the sea chest, errors in valve operation may result in marine pollution. 2) When the method of filling water from the sea using a pump is adopted The volume of the slop tank is generally small. If the planned volume is filled, the difference in levels of tank and waterline becomes small. Oil remaining in the cargo line flows overboard (in the opposite direction) due to ship motion, bringing about the risk of marine pollution. Accordingly, when filling water from the sea by gravity, the slop tank should not be filled directly from the sea. Instead, the No. 5 center cargo tank should be filled with water from the sea. If the cargo pump trips when filling sea water from the sea using the cargo pump, the oil included in the cargo line flows back to the sea as a reaction. Thus, it should be borne in mind that there is adequate likelihood of marine pollution to occur. To prevent pump cavitation, tripping, seizing due to heat generation and erosion of impeller, the line lower than the water level should always be used. The cargo pump should be started after confirming that the petroleum gas on the pump suction side has been removed, and the ballast should be filled. * The method of taking in warm sea water from the sea directly through the chest is effective since the heating time is curtailed. * Since the cargo line is directly connected to the sea chest, pump trippings or errors in valve operation may result in marine pollution. 3) When adopting the method of filling water from the ballast tank * Safety against marine pollution is high since the cargo line and sea chest are not connected. * The ballast tank and cargo line are connected by the spool piece. Therefore, there is a risk of polluting the ballast line and ballast tank with oil if there is an error in valve operation Deciding the quantity of water to be filled Consider the following points to decide the filling quantity: 1) The filling quantity cannot be defined unconditionally because of the differences in the capacity of slop tanks and in the number of washing tanks on each ship. The quantity can be considered however, based on the quantity of oil recovered after tank washing and the quantity that can be recovered in the slop tank on one side of the ship. 2) If the filling quantity is too small, the defects mentioned below may occur. * Reduction in the cleaning effect due to progressive contamination of washing water * Air may be drawn into the TKC pump * Reduction in efficiency of gravity shift through free flow line to secondary slop tank. 3) If the filling quantity is too large, the defects mentioned below may occur. * Increase in the time required for settling and discharge * Water may need to be discharged during the cleaning work. Past records indicate that the appropriate total filling quantity (both sides) is about 3,000 MT Time required for filling The time required for filling water by gravity from the sea chest is about 2 hours. However, the time (Gas free) 1-4

5 The heat withstanding temperature of the valve seat ring is generally about 65ºC. required subsequently for shifting the water to the slop tank (from the No. 5 center cargo tank to the slop tank) depends on the stripping work in the final stage of shift from the No. 5 COT (C) Filling start period The filling of washing water starts when the preparations before tank cleaning and gas freeing work are completed after departure of the ship from the discharging port. However, decide the start period after considering the points below, if the water is to be filled from the sea chest (refer to Filling method ). 1) Consider the sea area where water is taken into the ship ensuring that sea water at the highest possible temperature can be filled. 2) Adjust the work schedule such that the filling work is performed during day time so that the condition of the sea water surface can be properly observed Filling line diagram Marine pollution may occur in the event of errors in the valve operating procedure and if oil has remained unexpectedly in the line at a position higher than the water level. Study thoroughly beforehand the valve operating procedure in the lines to be used, and clearly indicate this procedure in the line diagram. 3.4 Heating of washing water Washing water is heated so as to enhance its washing effect. Generally, the washing effect (oil removal effect) of heated washing water (hot sea water) is more pronounced than that of cold sea water, although some washing effect can be obtained from cold sea water. Washing with hot sea water has a slightly better washing effect because of the steam generated from hot sea water even at locations that do not come in contact directly with the washing water (locations with high concentration of stiffeners in tank or in the large shadow zones of the washing machine) Judgment on the need for heating water Make an overall judgment on whether heating is required or not, after considering the points mentioned below. 1) Ship type and ship s age 2) Time for gas-freeing work 3) Number of tanks to be cleaned 4) Description of work within tanks 5) Specific fuel consumption Heating method Two heating methods are available. 1) Heating using heating coils Heating coils installed in the slop tank are used to heat the water. Generally, this method of heating is combined with the heating of water using heating coils 2) Heating using tank cleaning heater Washing water is passed through the tank cleaning heater and heated during the washing operation Washing water temperature The temperature of washing water during the washing with hot water is about 50 to 60ºC. Examine the withstanding temperature beforehand and set the upper limit because the valve seat ring and the tank coating may be damaged if the temperature is excessively high. (Gas free) 1-5

6 3.4.4 Time required The time required for heating the water using the heating coil depends on the season (air temperature), temperature and quantity of sea water filled in the tank. Therefore refer to past records, if available, and estimate the time. Past records show that about 20 hours are required for heating sea water at 23ºC to 60ºC using heater and slop tank heating coil Heating start period The heating start period is as given below. 1) Heating using heating coils Start filling water in the primary slop tank. After the coils are fully submerged, start the heating. 2) Heating using tank cleaning heater The tank cleaning heater is installed in the COW line. Pass washing water through the tank cleaning heater simultaneously with the start of tank washing. 3.5 Rough washing of cargo line and cargo pump The rough washing of cargo line and cargo pump refers to the flushing out of oil that has collected in the cargo pump, lines and valves by passing the washing water through these items before washing the cargo tank. Rough washing ensures that a tank that has been washed will not become contaminated by careless valve operation Rough washing line diagram Consider the following points when preparing the rough washing line diagram: a) Formulate the plan to include passing the water through all cargo and stripping pumps. b) Ensure that line washing is performed not only in the same direction as the flow in each line but also in the reverse direction. c) Ensure the lines with dead ends are not included as far as possible. d) From the valves in contact with lines through which washing water passes, clearly mark and identify the valves that should be closed during the said work on the line diagram. e) Write the direction of flow of liquid, description of work and precautions on each line diagram. 3.6 Line washing (removal of oil traces in the ODM line) and valve flushing Line washing refers to the removal of oil traces sticking to the cargo pump, lines and valves using washing water. Valve flushing refers to the washing of lines and valves on the tank side using flushing valves. Valve flushing is to be implemented for each valve during the line washing work. This work constitutes the main washing of the lines and valves, and should therefore, completely remove all traces of oil sticking to the lines and valves Line diagram for line washing and valve flushing Consider the points below when preparing the line diagram for line washing and valve flushing. a) Ensure that line washing is performed not only in the same direction as the flow in each line but also in the reverse direction. b) Ensure the lines with dead ends are not included as far as possible. c) From the valves in contact with lines through which washing water passes, clearly mark and identify the valves that should be closed during the said work on the line diagram. d) Write the direction of flow of liquid, description of work and precautions on each line diagram. e) Plan for a washing time longer than the rough washing time. Take care to ensure that traces of oil sticking to the cargo pump, lines and valves are completely removed. f) Write down the timing for valve flushing on the line diagram. g) Configure the valve flushing lines such that the profile of the washing line does not permit any blockage to the flow of washing water when valve flushing is implemented. (Gas free) 1-6

7 h) Formulate the plan for valve flushing such that all tank valves and stripping valves are flushed. 3.7 Tank washing Tank washing refers to the work of washing the cargo tanks with sea water using a fixed washing machine (if necessary, portable washing machines may be used in combination with fixed washing machines), and simultaneously, recovering the oil in the slop tank by continuously stripping the washing water Items to be studied Study the items related to tank washing mentioned below for efficient washing of each tank. 1) The ship s attitude during the washing work 2) Tank washing sequence 3) Washing pattern Ship s attitude during the washing work Adjust the ship s trim and heel appropriately after considering the position of bell mouth of the tank during the washing work Tank washing sequence Decide the tank washing sequence decided after considering the points mentioned below. 1) Perform as early as possible the internal inspection of tanks, where necessary. 2) Always wash tanks connected to drip lines around the manifold after the drip lines have been washed. 3) Wash one of the forepeak tanks last because oil is pushed into the forward end of the COW line. 4) The tank washing sequence is closely linked to the HC gas purging and airing work sequence implemented after washing. Therefore, decide the washing sequence after considering that the distance of equipment to be moved, such as the portable fans for airing, is minimized during such work. 5) Slop tanks cannot be cleaned simultaneously with the cleaning of other tanks since they are filled with washing water. Therefore, clean the secondary slop tank after shifting the washing water to the primary slop tank after the first discharge. Wash the primary slop tank after discharging the slop to shore at the anchorage before dry-docking Deciding the washing pattern and the number of washes Decide the washing pattern and the number of washes after considering the points mentioned below. 1) Past records 2) Structural features of each tank 3) Type of oil loaded Washing time The washing time is decided by the factors below. 1) Washing pattern 2) Number of washes 3) Rpm of the washing machine. (Gas free) 1-7

8 The general guidelines for washing times are as given below. 1) Single-hull tankers Top wash 1-2 times, bottom wash 3-5 times: About 4 to 7 hours per tank 2) Double-hull tankers Top wash once, bottom wash 2-3 times: About 2 to 3 hours per tank Washing line diagram The line configuration for tank washing is almost the same as that of COW but the washing line diagram should be planned considering the points given below. a) The washing water should always be taken from the secondary slop tank. The sea water after washing should be stripped by the eductor and returned to the primary slop tank. b) During normal COW, only the tank cleaning pump is used. In case of tank washing work, however, the use of cargo pump simultaneously with washing should also be considered. 3.8 HC gas purging Before airing the washed tank, perform HC (hydrocarbon) gas purging using inert gas to prevent the atmosphere in the tank from reaching the flammable range of petroleum gases. Reduce the HC concentration to below 2% by volume (HC gas does not explode if its volume ratio is less than 2%) while maintaining the atmosphere within the tank in a non-flammable condition. During the HC gas purging operation, the environment above deck becomes extremely dangerous since flammable and toxic HC gas is discharged to the atmosphere from the vent riser. SOLAS Chapter II-2 stipulates that the discharge port for discharging HC gas so that the HC gas concentration becomes less than 2% by volume, should be led more than 2 m above the upper deck and that it should be provided with an appropriate cutout device. HC gas with a concentration of less than 2% by volume can be discharged from the deck level HC gas purging Fig.4-1 method (A) Purging thorough vent riser Four HC gas purging methods are available. 1) Purging through vent riser (see Fig. 4-1(A)) In this method, the IG line and the cargo line on deck are connected, the inert gas is fed from the tank main valve through the direct filling line, and the HC gas in the tank is released to the atmosphere from the vent riser. 2) Purging through the purging pipe on deck (see Fig. 4-1(B)) In this method, the IG line and the cargo line on deck are connected, the inert gas is fed from the tank main valve through the direct filling line, and the HC gas in the tank is released to the atmosphere from the purging pipe installed on the deck. 3) IHI purge mouth method (see Fig. 4-2) In this method, the inert gas is supplied from the IG branch valve, and the HC gas in the tank is released to the atmosphere through the direct filling line from the tank valve using the purge mouth installed in the crossover line 4) Mother tank method (see Fig. 4-3) In this method, the inert gas is fed from the IG branch line taking one tank as the mother tank. The HC gas in the tank is led to other tanks through the cargo line (bottom line) from the tank valves, and released to the atmosphere from the purging pipe installed on the deck. The mother tank method is effective when several tanks are to be replaced simultaneously. (Gas free) 1-8

9 Fig.4-2 IHI purge mouth method Fig.4-1(B) Purging through the purging pipe on deck (Gas free) 1-9

10 When purging HC from the bell mouth via the cargo line in the tank, the water remaining in the tank and the line should always be completely stripped before purging the gas so that the HC gas flow is not blocked in the cargo line Selection of method Implement HC gas purging effectively for a short time by a method in which inert gas is vented all over the tank. Consider the points mentioned below for selection of the method, and select a method that is most suitable for the ship. In principle, implement HC gas purging according to the operation manual supplied by the shipbuilder. 1) Difference in specific gravity (HC gas is heavier than inert gas) 2) Flow of inert gas 3) Piping 4) Structure and equipment in tank 5) Progress status of tank cleaning work Time required The time required for HC gas purging is decided by the items given below. 1) Tank volume 2) Number of tanks 3) Number of IG fans to be used (quantity of inert gas to be supplied) 4) Gas purging method Fig.4-3 Mother tank method Past records reveal that the required time per tank is about 6 to 10 hours when the IG fans are operated in parallel. The time required for purging after washing can be shortened by purging each tank for about 2 to 3 hours using the washing water heating time before the tank washing work. The need for implementing this (Gas work depends free) 1- on 10 the time of start of the cleaning work.

11 3.8.4 Number of IG fans to be used If the IG fans are run in parallel, the HG gas purging time can be significantly shortened and the inert gas supply can be increased. On the other hand, the oxygen concentration of the supplied inert gas increases and the fuel consumption increases. It is preferable to run the IG fans in parallel and shorten the working time because the main purpose of this work is to purge HG gas Start period Start HC gas purging in parallel with the tank washing work. By parallel operation, the HC gas concentration can be brought down below 2% by volume more quickly, and airing can be started earlier. 3.9 Airing Airing refers to supplying fresh air into the tank in which HC gas purging has been completed, and replacing the gas in the tank so that the environment in the tank is made conducive to work to be performed by workers Airing methods Two airing methods are available. 1) Method of using portable fans and air ducts In this method, portable fans are installed in the tank cleaning holes on the upper deck, air is supplied to the tank and the inert gas is discharged. Air ducts are fitted to the portable exhaust fans and gas is exhausted from the bottom of the tank 2) Method of using IG fans In this method, fresh air is supplied using IG fans. When the method is implemented, a blind flange is inserted in the IG branch line of the tank that has already been gas-freed. For this reason, a spool piece connecting the IG line and cargo line is used and air is supplied to the tank from the tank main valve through the direct filling line Selection of method In the actual operation, the IG fan is used preferentially for HC gas purging of the tank. Airing cannot be performed until the washing and HC gas purging of all tanks except the slop tank are completed. Accordingly, airing should be performed using the portable fan and air ducts Arrangement of portable fan and air ducts The positions where these items are to be arranged depends on the arrangement of structural members in the tank. An example of the position is given below. Forward side: Supply fan Aft side: Exhaust fan (with duct) Fresh air is supplied from the tank cleaning hold on the forward side. The gas at the bottom of the tank on the aft side is released to the atmosphere by portable exhaust fan through the air duct. This arrangement is used considering that the mixture of gases in the tank (mainly inert gases) has a specific gravity greater than the supply air Time required Airing requires the maximum time among the tank cleaning tasks. The time required for airing depends on the volume of the tank, but in general, this time is about 12 to 24 hours. (Gas free) 1-11

12 The use of a simple ventilator such as a half-cut drum contributes to shortening the time considerably and is worthy of consideration. Air may not reach all parts of the tank, depending on the arrangement of structural members in the tank. In some sections (particularly the bottom corners), gas may accumulate and the HC gas concentration may not reduce at all. In such cases, the air supply position or the height of the air duct should be suitably changed so that the air flow changes. Dispersing the gas is also effective in shortening the gas-freeing time Airing start period 1) When portable fans and air ducts are used Airing can be started when the HC gas concentration in the tank drops below 2% by volume due to HC gas purging. 2) When IG fans are used Airing using IG fans can be started when the tank washing (except the primary slop tank) and HC gas purging have been completed Continuing the airing work Even if the target values (O 2 21%, HC below 1% LEL, H 2 S below 10 ppm) of the atmosphere in the tank have been reached, HC gas is always generated from the sludge accumulated in the tank. Therefore, airing work should be continued on the ship until it reaches the dry-dock. (However, it is necessary to switch over to inerting when water is being discharged from slop or when the primary slop tank is being washed at anchorage before dry-docking the ship.) Particular care is necessary during daytime when the temperature rises because a large quantity of the gas is likely to be generated at this time of the day. Airing should be systematically carried out in areas other than cargo tanks where HC gas is expected to accumulate (cargo line, cargo pump, vent riser system, coffer dams), gas detection performed and safety should be confirmed Settling and discharging water Settling is the process by which the washing water containing a mix of crude oil and sea water, which has been collected after cleaning each tank, is allowed to remain still in the slop tank so that oil and water can be separated. According to MARPOL, discharging water from slop refers to the work of discharging only the water that has been separated from oil-water mix by settling using ODM equipment. Settling and discharging of water are carried out several times intermittently, but the washing water that remains finally will be discharged to the barge. Therefore, the water from the slop should be discharged as far as possible so that the total quantity of the slop water decreases Settling time The time required for settling varies depending on the type of oil loaded on the ship and the ship motions, but generally, 24 hours minimum are required Implementation period and sea area When all tanks except slop tank have been cleaned, settling can be started. After the separation of oil and water is confirmed, discharge the water from slop. However, discharging of water from slop can be performed only in sea areas that are more than 50 miles away from the nearest land and in Generally, more than 85% of the oil-water mix separates into oil and water about one hour after settling. However, at least 24 hours are required for the emulsion layer to become an oily mixture of less than 100 ppm. It is important that adequate time be given for settling so that the slop water (dirty ballast) can be separated into oil and water. For efficient separation into oil and water, the washing water should be heated using the heating coils provided in the slop tank. However, precautions are necessary because if the slop water is heated excessively, convection may occur causing the oil and water to mix again. Generally, the temperature at which ideal separation of oil and water occurs is considered to be between 45 to 55 degrees Celsius. (Gas free) 1-12

13 specially-designated areas, otherwise the implementation period has to be adjusted. Moreover, this discharging work has to be performed always during the day time so that the sea water surface can be monitored when the water from slop is discharged into the sea Final line washing Final line washing refers to the flushing out (rinsing) of all traces of oil remaining in each cargo pump and cargo line after the washing of all tanks except the slop tank has been completed. The final line washing process consists of drawing the sea water directly into each cargo pump from the outboard sea chest valve, washing all lines, and sending it to the primary slop tank Implementation period Final line washing is generally implemented after the first discharge of water from slop Line diagram for final line washing Consider the points below when preparing the line diagram for final line washing. 1) The objective of final line washing is the rinsing of the lines, so all the lines are to be washed. However, the procedure may be completed simply and within a short time by performing rough line washing and hot water line washing. (Washing may be performed by passing water in one direction only) 2) Start all cargo pumps and tank cleaning pumps sequentially to pass water through them. Wash all the pumps in this way. 3) Do not flush the tank valves. However, thoroughly wash the valves or lines that are to be removed during dry-docking Sludge removal Sludge removal refers to the work of discharging to shore the sludge remaining in the tank after gas-freeing the tank at the designated anchorage and before dry-docking the ship Time required The time required for sludge removal is decided according to the quantity of accumulated sludge and the number of sludge gangs available. Generally, about 100 to 200 MT of sludge are generated. Past records show that such quantities can be removed in about 2 days Slop removal Slop removal is the process of discharging to shore the slop water remaining in the primary slop tank Slop water measurements Measure the slop quantity and retrieve slop samples (oil samples) before slop removal. These actions should always be witnessed by the ship supervisor. Decide the slop quantity finally after discussion with the sub-contractor. (Refer to Document 6) If the API of the slop oil and the oil-water interface are accurately measured, and the SLOP CERTIFICATE is prepared beforehand by the ship, some of the sub-contractors may use the measured values Primary slop tank washing Primary slop tank washing refers to the washing of primary slop tank that has not been washed after the slop water has been discharged to shore Washing method (Gas free) 1-13

14 Perform primary slop tank washing by drawing the sea water from the sea chest into the tank cleaning pump. Strip the washing water used for washing the tank using the stripping pump and discharge it to barges using the direct MARPOL line. Hot water washing may also be implemented using the tank cleaning heater but in this case, a hose connected to the COW line and leading overboard should be used for adjusting the flow rate of the COW machine. This however, has the risk of discharging oil in the COW line into the sea. Moreover, the boiler load adjustment is difficult. Thus, cold water washing is generally implemented Gas-freeing, airing and sludge removal of primary slop tank Perform gas-freeing, airing and sludge removal of the primary slop tank after washing is completed, similar to the procedure implemented for other tanks Final line stripping Final line stripping refers to the washing and stripping of the line (MARPOL line) used for discharging slop water in the primary slop tank and the stripping pump Acquisition of gas-free certificate After gas-freeing all the tanks, receive gas inspection by the Gas Chemist on site before the dry-docking of the ship, obtain a gas-free certificate and complete the tank cleaning work. (Gas free) 1-14