DRY DOCK PREPARATIONS



Plan – it is first thing to be prepared. Good plan must be prepared as step by step job schedule and shall include different scenarios (in case of rain, bad weather etc.). First of all go through following subjects:

Preparations for DD should start before last discharging. It is good time to check all equipment, prepare tools, spare parts etc. In discharging port COW of all tanks should be performed. After that drain as much as possible (including brench lines to suction valves, U pipe to slop tanks etc). During DD voyage following operations must be performed:

  1. Line flush.

  2. Washing secondary, filling and heating primary slop.

  3. Educting all cargo tanks and lines (strip all what was drained after lines flushing).

  4. Filling up secondary slop (Clean water. Use ODME line to flush it).

  5. Washing and purging of tanks.

  6. Gas freeing.

  7. Inspection and demucking COT.

  8. Flushing top and bottom lines.

  9. Decanting secondary slop.

  10. Transfer residues to primary slop.

  11. Washing secondary slop (if neccessary – usually skipped).

  12. Gas freeing secondary slop.

  13. Decanting primary slop.

  14. Discharging primary slop to barge / facility.

  15. Washing primary slop.

  16. Stripping all lines to barge / facility.

  17. Purging and gas freeing primary slop.

Ad.1. To flush all lines at the begining use all cargo pumps with minimum pressure (bottom lines in pump room are not designed to be pressurised above 5kg/cm2). Use sea chest but be careful – there can be oily residues between sea chest valves and inside line. Start pump dry (it is only one time when dry start of COP is permissible). Use the nearest COP to sea chest first. Wait for vaccum, in case of any problems with vaccum on suction side, use stripping pump to help. All lines must be flushed. Prepare pipe diagram to mark flushed pipes or mark it directly on the panel in CCR. Flush manifolds first via deck line, x-over valves on manifold. Open all manifolds and drains. After that open drop and flush bottom line to slops. Flush tank's brenches tank by tank via bottom lines and x-over bottom valves. Flush eductors and stripping lines.

Strat another pump to continue. (Use the same technique – dry start with closed last sea chest valve or start second pump before stop first and operate discharge valve to keep suction in line). Flush deck line and drop. Use another pump in same way. If vessel is equipped with tank cleaning pump, flush it as well. Do not miss stripping pump to flush as well.

It is possible to take water for flushing / washing from ballast system (cargo – ballast connection) but be sure that pressure in ballast line before open connecting valve, is higher than in cargo line side.

Ad.2. It is very important to wash secondary slop as good as possible. It is a source of water to wash rest of tanks. In this stage bear in mind amount of water in slop you plan. For calculation purpose, water drained from lines and stripped from tanks after flushing to be counted. Basicaly, amount of water (all included) taken not to extend about 90% of one slop tank capacity. Sometimes it is not clear which slop is primary / secondary. For sure, primary slop must be fitted with heating coils. Anyway, to be sure check set of ballancing pipe. In primary slop ballancing pipe inlet is situated few meters above bottom of tank (water to be taken from), in secondary slop outlet of pipe is situated much higher (water to be given to). Normal operation with ballancing pipe and in close washing method is that secondary slop is a source of liquid (water) for washing machines and eductor(s) and primary slop accumulates that liquid and allows to fill up secondary slop through ballancing pipe.

Start heating water in primary slop as soon as possible, especially when vessel is equipped in heating coils only in that tank.

Ad.5. Commence washing tanks. Set of tanks to be washed depends of vessel design. On VLCC not equipped with TCP, common COP is used (in that case use all pumps during washing all tanks). In that case it is possible to wash and strip pair of tanks simultanouesly when vessel has two separate stripping lines. Basicaly, all depends on capacity of pump in comparision with capacity of washing machines, eductor(s) etc. When TCP is available, one tank at the time is possible to strip and wash. Recommeded sequences of washing: BW-FW-BW-BW (usually 3-4 hrs), anyway it depends on last cargo(es) and condition of tanks. Tanks to be washed with hot water (min. temp. 50DegC but no more than 60degC) heating coils in primary slop to be used. Be sure that all heating coils are in good conditions and test it with steam well before. Heater is mostly used only to wash primary slop at the end of preparations. Water for washing to be taken from secondary slop with ballancing line open. High suction to be used (to avoid filling COP bottom line).

Purging can be commenced during washing first tank(s). It is not recommended but it can be a good idea to save a time. There are three methods of purging: dillution, displacement, both together. NAIL is the simple rule to know which gas is heavier than another (Nitrogen, Air, Inert gas, Liquid – HC). Method of purging depends on vessel construction and stage of DD preparation. At the beginning of operations it is not possible to use cargo lines to introduce IG inside tanks. Fixed IG installation must be used. Time of purging depends on many factors like IG fans capacity, number of tanks to be purged, method of purging, specification of last cargo and how effective was last COW nad washing. Rule of thumb is to change atmosphere inside tank three times. After that safe atmosphere should allows to start gas freeing. Safe atmosphere means: 1% LEL of hydrocarbon, less than 1ppm of benzen, less than 5ppm of H2S and about 21% of oxygen. It is recommended to start purging from forward tanks and separate branches after complition by means of blind flange to avoid accidental IG leakage due to brench valve. During washing it is only possible to use IG line and purge pipes to obtain safe atmosphere. When cargo line is not used for washing and dry it is possible also to connect IG line to cargo line and purge through suction valves at the bottom of tanks. Another method is to introduce IG via common brench line, close all openings in tank and allow gases to escape via suction and bottom lines to another tank(s).

Ad.6. Although simultaneous washing and purging is not recommended but acceptable, simultaneous washing and gas freeing is not acceptable. Gas freeing to be conducted by means of portable fans (usually water driven). Acceptable hydrocarbons level for commencing of operation is 2% by volume. Main goal of the operation is not to pass through dangerous area during dillution (see drawing). Dangerous area (1-10% of HC and more than 11% of oxygen) indicates explosive atmosphere. Static electricity may be a good source of ignition in that conditions. It is quite important to commence gas freeing as soon as first sequence of purging is completed and atmosphere in tank checked against HC. As soon as the first tank is ready to enter, it is possible to get demucking gang inside. Bear in mind that for gas freeing other tanks less fans to be used (some of fans must be used in tanks where demucking gang is inside). Checking of atmosphere on few levels of tank is necessary, anyway some “gas pockets” may exist withing the tank. When all tanks are ready for inspection, apply for Gas Free Certificate. Importance of mentioned certificate means that after Gas Free Certificate is signed, dry dock takes over responsibility of tank atmosphere.

Ad.7. Acceptable amount of residues after water washing for tankers (total):

Ad.9. Minimum time to decant secondary slop is about 24 hours. Usually there is only few cm of oil on top of water. Anyway, to be on “safe side” keep 1 – 1,5 m of liquid below water interface level. Cargo pump (minimum revolutions) may be used to discharge clean water from secondary slop over board via ODME. At the end of operation stripping pump can be used. It is quite important to flush ODME line well before discharging decanted secondary slop to avoid make ODME dirty. Keep eyes on over board valve to be sure that clean water is discharged.

It is permissible to discharge over board in case of ODME failure but additional precautions must be undertaken:

Ad.14 & 15. Discharging primary slop to barge is performed at the anchorage or to shore facility via marpol line. Stripping pump is used to pump out sludge. Becouse of quality of residues from primary slop after DD preparations, stripping pump strainer can be easly clogged. So, it is a good idea to prepare tools and have crew stand by to open and clean strainer. Water used to wash primary slop is given via sea chest and heated when passing through heater. Quantity of ordered barge is factor to bear in mind. To calculate barge capacity add amount of water for washing (and flushing after it) to total quntity of primary slop. After washing and stripping drain marpol line to primary slop again.