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Conductor wobble in soft soils - CAN solutions?
27 September 2016

East coast of India has some extreme rapid depositional systems  that deposits very soft clay bound soils into the deep and ultra deepwater environments. In other regions similar depositional systems likely exist. i.e. very soft seabeds.

e.g. In shallow water, jack ups and semis also often experience problems in this first drilling phase, float boxes and other contraptions often needed as a foundation pipe to be able to get cuttings and cement back to surface.

When these areas move into development wells. The stand alone conventional wells in my view will not be enough. Because what we have observed is that conductor movement will create circumferential hole around the conductor and 'conductor wobble' results (although one never sees the conductor move because cyclic movement is so slow). However this gap surely presents a long term problem and potential risk e.g. < fatigue life of conductor pipe.

Thinking suggests that the solution is akin to what we do for suction anchors, support for templates etc. I.e. Self penetrate and pump in a 'Conductor anchor node 'CAN' for short.

These are typically 10-15m in length and up to 6m in diameter. They self penetrate and a simply ROV hydraulic pump sucks out the water from within the CAN and foundation pipe is sucked into the seabed to deliver a secure foundation axial and bending resistant system for the life of the well. 

So why are drillers not using more of these? Where recently offshore Norway and spreading into UK, CAN's are starting to be used/applied.

One for the geotechnical people to share views and thoughts please.


    

 

 

3 answer(s)
Connor
Sr Drilling Engineer
Kebabangan Petroleum Operating Company Sdn Bhd
Total Posts: 1
Join Date: 01/09/14

I have looked into Can technology and think it may be great for some cases, such as avoiding hole washout if drilling and grouting the conductor, or having them drive the conductor for you with a Can foundation to save rig time. 


However, if you are driven by fatigue resistance then the ultra-stiff foundation the Can provides may not be desirable.  I reference a paper by 2H ("Conductor System Fatigue Excitation and Mitigation" 2010-TPC-561) that compares computed fatigue life for against various parameters. The paper presents some initially-counterintuitive results.  A conductor in loose soils has a 300% longer predicted fatigue life than the same conductor in stiff soils.  When you think more about it, it makes sense: a loose system would spread the bending load out over a greater section of pipe, while a stiff system results in a cyclic point load at the interface where the conductor changes from rigid to free.  Fatigue is caused by cyclic loading, not necessarily movement, and so trying to stop the pipe from moving while still applying the same forces may be counterproductive.


I know of a deepwater well that had observable VIV-induced conductor wobble against the seabed when monitored by the ROV.  When compared to the fatigue analysis the behavior matched the model: 2.5" of conductor movement was predicted by the calculated analysis, and that was about the amount that was observed.  But the fatigue calculation also resulted in an adequate fatigue life, so however disconcerting the wobble was to observe, it was not a critical issue.


I've already written too much considering I'm no expert in this field, but suffice to say there are a lot of parameters that interact for a conductor fatigue situation.  If Can has a benefit for wellhead / conductor fatigue life that would have to be factored into the overall design and demonstrated by a conductor analysis.  The last time I spoke with them they were not making that claim.

admin
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Relentless Pursuit Of Perfection Ltd.
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Scott_McNeil
Consultant
SPREADAssociates
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Join Date: 05/03/08
Hi Peter,

From a drilling perspective, has it been proven that this is due to Conductor movement? Or is it perhaps due to erosion of the seabed caused by the tide flowing around the Conductor?

From my experience, a very soft seabed is frequently associated with high tidal currents and can cause significant erosion of the seabed around the Conductor (I've seen up to 40' drop in seabed level around the Conductor).

Our answer was normally to drive rather than jet or drill in the Conductor, plus if necessary, use a thicker joint wall in the conductor at seabed level.

As to why the CAN system isn't used more often, wouldn't it be rather limited in application? I can certainly see the advantages of it's use, but don't you need a rather rare set of circumstances to be able to use it?

Best Regards

Scott
Posted by

Peter Aird

Drilling Specialist/Well Engineer/Training Consultant

SPREADAssociates

Total Posts: 339
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