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Typical anode installation inside a center tank of a VLCC.Anode installation can also be carried out during voyage by employing the abseil or ballast methods. See below for Anode fixing methids

 

 

Use of sacrificial Anodes for external protection
Hulls of ships are very much prone to corrosion of an aggressive nature in the form of pitting corrosion unless they are applied with cathodic protection. Protection with the very best and the most expensive coatings alone is not enough as the applied coating is vulnerable to mechanical damage at sea or in port and to imperfections at the time of application.
ETC has two main types of sacrificial anodes; high purity Zinc and Aluminium which are alloyed with other metals to give performance enhancement. Zinc was the first ever material to be used and is therefore considered the traditional anode material. However, Aluminium has several outstanding values and has fast become the first anode of choice. Both Zinc and Aluminium anodes have a normal design life of one, two or three years to suit the owner's requirements. Hull anodes are usually welded direct to the ship structure, but can be bolted if required.
The efficiency of any anode material depends upon its electromechanical properties. First amongst these is open circuit potential. For Aluminium, the open circuit potential is -1.0 volts with reference to Ag/AgC1 reference electrodes whilst for Zinc it is -1.05 volts. This translates to a better driving potential (voltage) for Aluminium anodes which means that for the same anode configuration, Aluminium anodes can deliver 30% more current than a compatible Zinc anode system. Secondly, the current capacity of Aluminium anodes is 2,500 Ampere-hours/Kg as compared to 780 Ampere-hours/kg for zinc anodes.
As an added benefit, fewer anodes translates into reduced frictional resistance on the hull of the ship which reduces operation costs. The higher the current capacity, the lower the consumption rate becomes and hence the consumption rate for Aluminium is in the order of 3.5 kg/Amp-year as compared to 11.23 Kg/Amp-Year for Zinc Anodes. Thus Aluminium anodes with their higher electro-chemical capacity and lower density, which translates into lesser weight and/or lesser anodes than that of a Zinc system attracts tremendous economies along with improved performance.

Number and Type of Anodes for Hull Protection
To obtain the approximate wetted hull area, the formula below may be used.
(1.8 x LBP x D) + (BC x LBP x B) where LBP =Length between perpendiculars. D=Draft. BC=Block coefficient. B=Breadth

Typical block coefficients for various vessels are:

Tankers

0.8 to 0.9

Naval vessels

0.6

Dredgers

0.8

Tugs  

0.6

Coasters

0.75

Trawlers   

0.55

Cargo vessels  

0.75

Yatchs 

0.4 to 0.5

Passenger Vessels

0.6

Launchers

0.4

The total current requirement is calculated as:

Current (Amps) =

Area (m2) x current density (mA/m2)
    1000

The total weight of anode material is calculated as:

Number of anodes  =

Current (Amps)  x design life x 8760
capacity of material (amps. hrs /kg)
    8760 = Number of hours in one year

The number and type of anodes selected must satisfy both the total current and total weight requirement as follows:

Number of anodes  =

Current required (Total) 
Individual anode current output
Number of anodes  =

Weight required (Total)
Individual anode net weight

Anode locations:
1. Full hull protection
Anodes should be located equidistantly as possible around the hull between 4-6 meters apart. As a guide, 60 % of the calculated anodes should be mounted in the after half of the vessel with further consideration that 25% of the calculated anodes should be placed  around the stern only protection described below. Anodes situated in the forward part of the vessel should be located to prevent damage or removal by anchor chains.

2.Stern protection
Anodes should be positioned on the stern area and rudder adjacent to the propeller; care being taken to minimize disturbance of the water flow propeller. Anodes should not be fitted within 300mm of the line of the propeller tips and should be parallel to the flow lines of the hull. Twenty five percent of the anodes required for the hull protection are required for stern only protection.

Use of Sacrificial Anodes for Internal Cathodic Protection
The corrosion pattern in cargo ballast tanks of crude oil carriers is very different from that on the ships hull but can prove even more damaging and expensive. The cargo is alternated with sea water and in this environment, the corrosion generally takes the form of pitting on the horizontal surfaces such as the upper stringer platforms, the inner bottom shell plating and the face plates of longitudinal and transverse members.

Pitting corrosion almost certainly occurs in lower areas where water may be present beneath oil cargoes and where residual water is present in tanks which for all intensive purposes are empty.  ETC's pit-guard anodes are designed to prevent pitting attacks on the bottom plating of cargo and ballast tanks. Installed very close to the bottom plating, they provide the necessary cathodic protection for the critical areas.

Because the residual water, remaining in the cargo/ ballast tanks will most times be oily, the self cleaning property of ETC's Alinode Aluminium alloy is definitely advantageous for this application. Pit-guard are provided with an integral clamp for ease of attachment to the scallop holes in the bottom longitudinal and can be fitted during voyage.
However, it must be kept in mind that these anodes are relatively small and may be permanently submerged in comparison to other anodes in the same tank which are subject to ballasting factors. Periodic inspection of the Pit-guard is recommended and replaced promptly when ever necessary. The wastage of internal surfaces of " permanent" ballast water tanks is usually uniform in nature and can be inhibited by the installation of sacrificial anodes distributed evenly throughout the internal surfaces.

Corrosion of these examples can be controlled simply and economically with the installation of either Zinc or Aluminium sacrificial anodes or a combination of both, depending on the particular environmental conditions of each installation.  There is no restriction on the positioning of zinc anodes but it is a recommended practice to ensure that the potential energy does not exceed 540 kg-rn. Aluminium anodes are only permitted in cargo tanks of tankers where the potential energy does not exceed 28 kg-rn. All anodes should have mild steel inserts and these should be sufficiently rigid to avoid resonance in the anode support and be designed that they retain the anode when it is cast.  The steel inserts are generally attached to the structure by means of a continuous weld. Anodes may alternatively be attached to the structure by pre-welded supports or brackets by way of bolting with a minimum of two nuts and bolts per anode.

Design and Installation of sacrificial anodes for ships' internal tanks:
The system life should not be for less than 4 years utilizing an estimated ballast factor. Provision should be made for additional anode consumption if it is anticipated that residual ballast water will remain in the bottom of the tanks. The resistivity of the ballast water will vary due to world wide variations in climate. Generally, 25 ohm-cm is considered in European and Scandinavian waters whilst 15- 20 oh for varm-cms would be applicable in Middle and for Eastern waters. The system should be designed accordingly.

Current densities applicable for various tanks:

Cargo/clean ballast tanks

86 mA/m2

Ballast only and Ballast white oil cargo tanks

108 mA/m2

 

Upper wing tanks   

120 mA/ m2

 

Fore and Aft peak tanks

108 mA/m2

 

Coated surfaces

5 mA/m2

 

Lower wing tanks

86 mA/m2

 

Double bottom tanks, ballast only    

86 mA/m2

 

Cargo/ Dirty ballast tanks

 Dependent on trade

Individual anode current output calculations:
Any anode output (ampere) is the difference in potential (voltage) between the anode material and the steel structure polarised to protection levels by the resistance (ohms) of the anode in the electrolyte. This is expressed as:
I = E/R  where I =Amperes, E = Volts, R =Ohms

Higher outputs from anodes are simply associated with the cross section of the anode being a lot less than the relationship to their length.
The type and number of anodes required:
The total current requirement is calculated as:
Current (Amps) = Protected Area (m2) x current density (mA/m2)
                                                  1000

The total weight of Anode material is calculated as:
Weight (kgs) =   Current (Amps) x design life x 8760
                           Capacity of material (Amp Hrs/Kg)


8760 = No of hours in one year
The number and type of anodes selected must satisfy both the current and total weight requirement as follows:

Number of anodes =      Current required (Total)       
                                    individual anode current output

Number of anodes =     Weight required (Total)         
                                     Individual anode net weight

Resistance Calculation for Hull Anodes
For flat hull type anode the following formula can be used:
R =  
   p      
      
 2 x s         Where p = Resistivity of water (ohms.cms). S = Arithmetic mean of anode length and width

Resistance calculation for tank anodes
The resistance of slender tank anodes in an electrolyte can be obtained by applying the following formulae:

R =      P         
        2
L      
 
( Ln 4 L - 1)
          r
where R = Resisitivity of water (ohms cms.) L =Length of anode (cm) p =Resistivity of water (ohms.cms) r -Mean effective radius of anode (cms.) where R =   ∫ Cross section Area    x 60
                                                        ∏                                           
100 

INSPECTION

The cross sectional area of the anode to be used in determining the mean effective radius is that corresponding to the anodes consumed by 40%. This procedure is traditionally used in calculating anode requirements in the marine industry as being indicative of anode output during its life.  ETC has a team of experienced engineers available to inspect external and internal parts of any vessel, and to advise on the most effective way of employing cathodic protection to combat corrosion.  Periodic inspection of hull external surfaces and tank internals installed with cathodic protection enables our engineers to confirm that existing systems that  are operating as intended, and/ or recommend improvements that can be made to ensure that all surfaces are protected efficiently and effectively until the next programmed dry docking inspection.

TANK ANODE AND HULL ANODE INSTALLATION
ETC has an arrangement with a local Dubai based company that can install tank anodes during ballast voyages using teams working from rafts, divers or by abseiling.  The diving team can also carry out underwater inspection of the hulls of vessels, particularly when the vessel is bunkering in local waters and if necessary, sacrificial anodes can be welded in place under water.

PIPELINE PROTECTION
The pipeline used in larger tankers (i.e ballast lines, stripping lines, cargo lines, etc) have given rise to substantial corrosion problems. The control of external corrosion on pipelines can be achieved by bonding each pipeline length to each other and to the structure at reasonable intervals and as extra insurance, by adding pipeline an ode bracelet assemblies at regular intervals. Effective bonding of the lines is carried out without gas freeing the internals of the lines. The system is installed by stud welding brass studs to each section of the line, adjacent to each flexible coupling and connected by removable if any coupling repair becomes necessary.  For an extra protection, anode bracelet assemblies can be applied directly to the pipelines. Bonding of the bracelet assembly to the pipeline is either by hardened steel earthing screws or by studs being welded to both the pipeline and anode assembly. 

TYPICAL ANODE FIXING METHODS


The above are standard fittings which meet most requirements but special fittings will be designed where necessary.

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