Introduction
· Requirements for COW system (REG 33 of MARPOL)
· I.G system shall be provided in every tanker having COW
· Only those cargo tanks can be ballasted that have been COW
· Not following the above is a contravention of the International rules
· All ships with COW must have a “Operations and equipment manual” approved by the Administration
· Every new crude oil tanker above 20,000 DWT and every existing tanker above 40,000 DWT shall be fitted with cow system
· To comply with requirements within 1 year after first engaged in the crude oil trade or by the third voyage carrying crude oil suitable for washing, whichever later
· Compliance of cow specifications to be shown in IOPP certificate
· Advantages of COW
§ Increased cargo out turn
§ Reduced sludge accumulation
§ Reduced gas freeing time
§ Reduced manual cleaning
· Disadvantages of COW
§ Does not eliminate water washing
§ Need for additional equipment
§ Specialized man power required
§ Increased stay in the discharge port
§ Increased work load
§ Increases corrosion rate inside the tank
Design of COW system· Parts of a COW system
o Pumps
o Piping
o Fixed washing machines
o Stripping system
· Factors influencing the effectiveness of COW
o Characteristics of the crude oil
§ Wax content
§ Specific gravity
§ Dissolved sediments
o Temperature of oil
o System pressure
o Nozzle rotation
o Shadow sectors in the tank
o Stripping effectiveness
o Location & number of washing machines
·
Number & Location of washing machines is governed by the regulations in MARPOL
o 85% of the VERTICAL surface area inside a tank must be covered by the direct impingement of the jet
o 90% of the HORIZONTAL surface area inside a tank must be covered by the direct impingement of the jet
· Jet length is governed by the washing pressure & nozzle diameter
· Jet length should be such as to be effective in fulfilling the requirements of MARPOL
· Number of machines that can be run simultaneously is given in the COW operations manual
· Running more than above machines causes a drop in system pressure and thereby washing effectiveness
· Minimum system pressure to be maintained is given in the COW operations manual
· Minimum recommended trim during washing given in the COW operations manual
· The given conditions of the COW operations manual must be complied with for a effective wash
· Diameter of piping, Number of machines & fluid velocity in the pipe have a relationship that goes into the designing of the system to satisfy MARPOL constructional & operational requirements
COW Piping· Cow pipes and valves shall be of steel or other equivalent material, of adequate strength, properly jointed, supported & anchored to the vessel
· Washing machine may be used as the end anchor but relevant support to be there for times when the machine is removed for repairs
· Permanent piping independent of fire mains and other systems except cargo system
· Exception to the above on combination carriers’
o Removable machines permitted when carrying dry cargoes
o Above machines capable of being reinstated as original fitted and tested for oil tightness
o Flexible pipes allowed to connect cow system to washing machines
o If so then these hoses to be tested every 2½ years
· Provision to prevent over pressurization in the system
o Relief valve in the pump
· Hydrant valves on tank cleaning line for water washing to be fitted with blanks when COW being done
· All connections for pressure gauges and other instrumentation shall be provided with isolating valves unless fitting is of sealed type
· No part of the Cow system shall enter the machinery spaces.
· Oil to be drained from the line before water washing
· COW & water washing system may be common, if so
o Piping system may pass thru a steam heater for water washing
o This heater to be fitted with a positive means of isolation (blanks) during COW
· The piping system to be tested to 1½ times the working pressure
Tank washing machines· Only fixed washing machines are allowed except on combination carriers
· Machines may be
o Deck mounted
§ Programmable
q Single nozzle
v Multi stage washing
Ø Subject to reaction forces
§ Non programmable
q Multi nozzle
v Single stage washing
o Bottom mounted (Submerged)
q Multi nozzle
· Deck mounted machines may have
o Fixed drive units
o Portable drive units
· Drive for the machine may be
o Fluid
o Pneumatic
· In case of portable drive units the number of units to be sufficient to ensure that no unit is to be moved more than twice from its original position
·
Verification of movement of machines
o Deck mounted
§ Programmable machines
q Rotation & Angle indicator
§ Non programmable
q Characteristic sound pattern
q Visual (from tank opening)
q Viewing glass
q External indicator
o Submerged
q Characteristic sound pattern
q Visual (from tank opening)
q External indicator
· Each branch in the COW system to have a “Root” valve
· Each machine to have a “Stop” or “Isolating” valve
· In case of removal of a machine for any reason the supply pipe end & tank opening both to be blanked
· To confirm Cleanliness or Effectiveness
o Deposit on the bottom of the sounding rod
· To verify effectiveness of cleaning the ratio of the volume of oil on top of departure ballast to the volume of that tank shall not exceed 0.00085
· Oil content of the arrival ballast not to be more than 15ppm and/or no sheen or discoloration of the surface of water
Pumps· Dedicated
· Cargo pumps
o Bleed off
§ Ensure sufficient & required pressure in the COW line, especially when there is very little back pressure at the manifold
o Delivered from 1 or all pumps
· If cargo pumps are
Stripping system· Effective stripping is very essential for good results
· Positive & recommended trim to be maintained
· Capable of stripping at a rate of 1.25 times the total throughput of all the washing machines that can operate simultaneously in one tank
· Small diameter line (MARPOL Line) to be fitted outboard of manifold valve for stripping lines and pumps
· Local & remote (CCR) gauges to be provided at accessible locations to monitor efficiency of the stripping systems
o Suction gauge of pump
o Suction gauge of suction side of eductor
o Discharge gauge of pump
o COW line pressure gauge
· Checking of tank bottom for being dry
o Level gauges
o Hand dips
· Internal structure of the tanks shall be suitable for efficient drainage to the bell mouth
o Drain holes
Operations· “COW Operations & Equipment Manual” is a must for vessels with COW
· The above manual is in the form of a standard format
· Tanks to be COW
o Departure ballast
o Arrival ballast
o Heavy weather ballast (if anticipated)
o ¼ of the remaining tanks for sludge control
· No tank needs be washed more than once in 4 months for sludge control
· No crude oil washing to be undertaken during a ballast voyage under normal circumstances
· Pressure testing of the COW system during ballast
· Ballasting of crude oil washed tanks to be completed prior departure for examination of water surface for oil content
· For COW operations all tanks must be inerted to or less than 8% O2 and to remain at a positive pressure
· COW operations to be stopped if O2 content goes beyond 8% (local rules may require less O2)
· To avoid static hazard only “Dry Crude” to be used (De bottom)
· Slops decanting process to obtain dry crude
· Saturation of crude in the slops after some time
· Continuous monitoring of slop tank ullages
· Calibration & Measuring of O2 content of a tank
· Recording of COW operations in the ORB
· Importance of maintaining the COW & IG systems
o COW not performed
o Unable to satisfy MARPOL requirements
o Delay to vessel
o Collection of slops (wastage of volume)
o Discharge operations suspended
· Certain crude oils are not suitable for COW
o Viscous
o Wax
o Heated
o High sediments
· Above info can be found from the COW manual
· Such above crude oils may not be carried in tankers obliged to perform COW unless they have SBT as per MARPOL requirements
· PSC can inspect COW operations being performed
· Vapor emission during filling of departure of ballast in washed tanks to be controlled by containment in empty cargo tanks
Qualification of Personnel· Person in charge of COW operations shall have at least 1 year experience on oil tankers having participated in COW operations
OR
· If no COW experience then a training program in COW is required
· Must have participated atleast twice in COW operations one of which should have been on a similar ship
· Shall be fully conversant with the contents of the COW operation & equipment manual
· Others to have atleast 6 months experience on oil tankers and are to be involved with cargo discharge operations and fully conversant with the COW operation and equipment manual
COW checklists
Pre arrival at Discharge Portv Notify Terminal
v Terminal radio check list
v O2 analysing equipment tested & working
v COW system isolated from the heater
v Hydrant valves blanked
v Valves to fixed machines shut
v Lines pressure tested
v Machine drive units
v Pressure gauges
v Stripping system
v Communication system
v COW, Responsibilities & Job description plan
Before COWv Pre arrival checks & conditions
v Meeting with ship & shore staff
v Communication link within vessel & shore
v Abort conditions & Procedures
v Fixed & Portable O2 analysers
v IG system & O2 content being delivered
v O2 content of the tanks
v Positive pressure in the tanks
v Responsible person assigned to check leaks
v Drive units & machines set
v Valves & lines double checked
v Ullage floats housed
During COWv Quality of IG monitored & recorded
v Frequent checking for leaks
v Frequent checking & monitoring of other tanks
v Frequent checks for machine & drive unit operation
v Prescribed wash pressure
v Cycle times
v Responsible person stationed on deck
v Minimum recommended trim
v Recommended tank draining method
v Slops being monitored continuously
After COWv Shut all valves
v Drain COW line
