Cathodic Protection limit design to locate Insulation Joint

[subsealinepipe]

Introduction:
For cathodic protection of subsea pipeline, the distance between Lowest Astronomical Tide (LAT) and Highest Astronomical Tide (HAT) is about 80 meter, and the subsea pipeline is buried before shore line into onshore (presuming for the subject length it is buried in both offshore and onshore). The cathodic protection of subsea pipeline (buried) is sacrificial anode and the onshore part (buried) is impressed current.

Question:
Where the location of insulation joint (the limit of sacrificial anode design from impressed current) is to be? (either LAT or HAT?), kindly advise with likely reference


Regards,
Amir

 

[SJones]

The IJ will need to be above ground or in a concrete pit. In addition, the galvanic anode section will still be able to 'throw' current some way up the beach since the seawater and the soil are contiguous electrolytes. It then becomes a question of how far until the pipeline has to come above ground so that you can put an IJ in or deciding whether it is actually necessary to isolate the two systems at all and just let the two systems freely protect the pipeline. Since the offshore section will be polarised by the galvanic anodes, interaction with the onshore section may not be a problem. An IJ would still be required where the pipeline is connected to current drains such as a plant or station.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[SDRME]

Thanks SJones, just for clarity as per DNV-OS-F101(Oct 2007) 6D504, I understand current drain for compatible anode protection systems is not required provided that both have adequate anode protection system, but for various anode protection system insulation for current drain is required.
For example, subsea pipeline with sacrificial anode connected to the offshore jacket which has sacrificial anode system too, current drain is not a matter and Insulation Joint (IJ) is not required, but if the subsea pipeline is a new one connected to an aged jacket current drain is a matter and IJ is required.
Regarding Subsea Pipeline (sacrificial anode) to onshore (impressed current) IJ is required to isolate both systems.

 

[SJones]

Regarding Subsea Pipeline (sacrificial anode) to onshore (impressed current) IJ is required to isolate both systems.

It doesn't say that in my copy of F101; I read it as: "At any landfall of an offshore pipeline with galvanic anodes and impressed current CP of the onshore section, the needs for an insulating joint shall be evaluated" which is what I was saying in my initial reply.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[SDRME]

Thanks SJones, yes DNV calls for evaluation of IJ requirement at landfall.
Regarding offshore system connections I make the question as follows (ref F101 6D500):
Is IJ required between subsea pipeline riser and topside platform piping (in atmospheric air)?, and is there any difference in case both pipeline and topside are newly built, and the case the pipeline is new and the topside is aged” (the cathodic protection design of both subsea pipeline and jacket of topside platform is sacrificial anodes)

 

[BigInch]

Isolate the pipeline riser coming up a platform jacket from the jacket protection system.
Isolate the pipeline from the topsides facilities at all potential grounding points. Don't forget any instrumentation or chemical injection equipment.
Isolate the pipeline at any connection (hopefully only one) to platform piping.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SJones]

Better still: engage a certified CP engineer!

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[SDRME]

With regards, I quote the following statements from DNV-OS-F101 (2007) Sec 6D500:
Quote
Pipeline systems connected to other offshore installations shall have compatible CP systems unless an electrically insulating joint is to be installed
Unquote

Quote
Without insulating joints, some interaction with the CP system of electrically connected offshore structures cannot be avoided. As the design parameters for subsea pipelines are typically more conservative than that of other structures, some current drain from riser and from pipeline anodes adjacent to the pipeline cannot be avoided, sometimes leading to premature consumption. When the structure has a correctly designed CP system such current drain is not critical as the net current drain will decrease with time and ultimately cease; i.e. unless the second structure has insufficient CP.
Unquote

Does not the above imply that IJ is not required for correctly designed CP connecting riser to jacket (in case both have the same CP system, e.g. sacrificial anode)?

 

[BigInch]

Chancy at best. That method is not for me.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SJones]

Yes. That's what it does imply and quite a number of operators proceed on that basis having taken due account of the possible effects on both the pipeline and the structure. If you leave the joint out, it makes submerged pipeline potential measurements from the platform a little more difficult.

The decision to leave out the IJ when hooking up to an existing platform would have to be based on confidence in the original jacket CP design coupled with a reliable CP monitoring history and prediction of system remnant life.

This thread has jumped from onshore IJs to now whether an offshore IJ should be used - does this not indicate that professional help should be engaged?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Deepray]

2 common location to have isolation joint for offshore system.

- Top of riser flange at platform (between riser and topside piping flange tie-in) and

- Shore approach; at tie-in location between onshore and offshore battery limit.

 

[ss770640]

does the code not allow you to use CP protection throughout?

after all, 'evaluation' doesn't mean 'have to'



BEng(Hons) CEng (UK) MIMECHE