pipeline anode temp 1

[JT100]

how can i calculate the temperature at the external surface of a coated pipeline on the seabed? pretty dure i'm missing something basic here, i've calculated the ohtc but unsure of next step.
its for a CP design, i usually just assume the temp of inner product of the pipeline, however this is to high for the anodes we are using and i need to prove the temp will be within limits.

 

[SJones]

http://en.wikipedia.org/wiki/Thermal_conductivity

Develop a composite U value for the pipe wall, the anti-corrosion coating, and the anode material using a delta T between the product in the pipeline and the seawater temperature (I think!)

Alternatively, just wrap an insulating sheath under the bracelet anodes to prevent excessive conduction.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[jfduroch]

You may have to perform qualification testing of your anode material to guaranty a minimum efficiency or to revisit potentially the metallurgy or deviate from the standard code curves.
The alternative proposed above using a insulation sheath is also possible but there again you will have to perform qualification testing to ensure that the sheath used does not degrade with aging unless supplier can propose you a convenient material for the design life.
To calculate a quick composite U assume a additional virtual coating layer using soil thermal properties but there again the calculation may not tell you much as soil properties may be difficult to get and will be variable.
Investigate in all 3 directions.

 

[JT100]

the pipeline has a five layer coating so would there be any value in applying an insulating sheath?
i have calculated an ohtc for the pipe wall and the coatings however i didnt think the anode should be included as i want to find the temperature at the outer surface of the coating to know what the anode is exposed to.

 

[SJones]

If you are using DNV RP-F103, you will only have to worry about heat transfer if the pipeline is buried. If the pipeline is not buried, then electrochemical capacity automatically defaults to the <= 30 deg C value. In performing your design, are you working on the basis of assuming that all anode dissolution occurs from the outer surface inwards (some specifications demand painting and/or enamel packing between bracelet segments to try to achieve this)? If yes, then your approach at stopping at the coating surface could be considered correct. Designs are based on 'anode surface temperature' - it just has to be made clear which anode surface.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[JT100]

yes i am working to DNV RP F103, and taking the protection potential from is0 15589-2. it is a short pipeline (2km) and so i plan on placing 1/2 anodes at each end of the route, the pipeline is to be trenched and rock dumped however the anodes will be exposed on the seabed. i'm assuming that means i cannot assume the minimum electrochemical capacity?

 

[SJones]

If the anodes are not, or will not be, buried then you are free to use the electrochemical capacity as directed by the standard.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[narenr]

If you have made teh OHTC calculation, then just calculate the heat transfer across each layer till you get the anode surface temperature.

See attached spreadsheet typically used for calculating the anode temperature. This process actually helps in reducing the anode quantity for long pipelines.

In some of my projects I have actually used a layer of neoprene sheet below the anode to reduce the temperature.

Rgds Naren

Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit www.narendranath.itgo.com.

 

[JT100]

thanks for the reponses, and the spreadsheet, thats very helpfull- the Film coefficient is just another name for heat transfer coefficient yeah?