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Tuesday, January 21, 2014

High Temp Offshore Field Joint Coating

Had a meeting recently where an engineer was writing a specification for an offshore pipeline that will operate at a high temperature (121C).  The line will be coated with a three layer polyethylene coating and the infill would be an SPU (and obviously the line is concrete coated).   His question:  what should I use as a field joint coating on this high temp, offshore pipeline -- and why should I use Covalence Shrink Sleeves?

In addition, his initial leanings were toward a three layer system utilizing a PP (polypropylene heat shrink backing) - but at the same time, his own initial evaluation was that maybe he'd just use our WPCT product (which is designed for ambient temperature pipelines!)  

Here are some of my thoughts:

Anytime a pipeline is going to be operating at 121C, it narrows the field down considerably (as a quick aside - WPCT is only rated for use at 108F so would absolutely not be approved for use on a line this hot).

If this were my line - the first product I would consider is WPC120 (click here for product specific information about WPC120).  WPC120 has a long successful use history all over the world.  It has been used often both onshore and offshore.  It has excellent technical properties and is very field friendly. 
Now I want to bring up a few things at a time here.  Your preliminary drawing indicates a plan to use an epoxy / PP sleeve system.  First, I'm wondering why you are leaning that way?
I understand that utilizing an epoxy as the primary corrosion coat can be a commonly made decision.  But at the same time, I wonder if it is really necessary.  You must consider that using an epoxy offshore can present a significant challenge.  The epoxy has a short pot life (depending on the scope of the project - you may or may not be at a quantity that would make bulk epoxy an option cost wise).  In addition, painting on the epoxy requires time.  In addition, it is possible you will need to force cure the epoxy (which means both extra time - and having an induction coil available) prior to installing the shrink sleeve. 
So, is the epoxy truly necessary?  The facts (as I see them) are that epoxies certainly improve shrink sleeve performance.  That cannot be argued.  Epoxies improve cathodic disbondment results (sometimes), they improve shear strength and peel strengths (sometimes) but in this case, the shrink sleeve is going to be subsea, underneath 30mm of SPU, while the concrete bears the weight of the pipeline.

I say all of that to point out that there is a definite cost to utilizing the epoxy as a layer (material cost, time, equipment, labor, etc) - and I'm not sure the technical advantage is truly worth that cost.
In addition, you are specifying a PP backed sleeve.  That is a bit unusual.  I understand that PP exhibits better abrasion resistance than a PE typically does (and sometimes lesser heat aging properties), but in this case, abrasion resistance really becomes a non-factor.  This sleeve is going to be underneath 30mm (or more) of SPU.  I don't expect that the shrink sleeve will ever be exposed to abrasion.
It looks like in the drawing, you have the sleeve spanning from the taper of one side to the taper of the other side (it could also be an option to run the shrink sleeve from PE backing to PE backing which (in my opinion) would be the better choice...assuming that doesn't cause any problem for your infill contractor.
Now - looking at sleeve widths.  If you are simply running from 1 taper to the other taper - you will need a shrink sleeve that is ~27" wide (686mm) (would need to know final tolerance levels before actually quoting something). 
If you were going to try to run the shrink sleeve from (near because of tolerances) one concrete to the other concrete; you would require a shrink sleeve that is ~33" wide (838mm).
So what would the advantages of the WPC120 product be?

The WPC120 requires no special equipment other than high powered torches.

The WPC120 requires no epoxy - this lowers costs; install times; storage space; potential field problems; potential material shortages; etc.

WPC120 can be easily and readily cut to a custom width with minimal (or no) extra charge
WPC120's backing comes complete with Raychem's patented PCI (Permanent Change Indicator for Shrink Sleeves).  This is an accurate measure for installers and inspectors that the shrink sleeve being properly and evenly heated.  There is no question.

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