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offshore deepwater conductor angle 'after cement displacement'
12 January 2014

This aspect arose during a recent conductor job where supporting views thoughts, comments are appreciated.

Background;

The conductor wellbore was drilled straight (rotary assembly)in less than 4hrs (all MWD surveys less than 0.5deg.)

The 75m tapered conductor 30" (5 joints) 36" x-o and wellhead jt was run to bottom (1450m water depth) without difficulties.

Note: Bulleyes in moonpool on conductor jt 0.5-0.75deg. Atlantic, winter conditions with rig moving around a lot!!

Only light resistance (5-10K) was noted on last 20-30m to bottom.  Very little change on bulls-eyes, i.e. 0.5 to 0.75deg?

Conductor was then cemented with lead and tail slurry with plenty of excess pumped and good returns observed to surface (i.e. seabed) throughout.

Bullseye readings after cementation now however reading 1-1.25deg. A 0.5deg increase!

Q1) How did this result?

Q2) What direction should the rig be moved? (note: very limited current data available)?

Q3) How far should the rig be moved to counteract forces causing the now higher conductor inclination observed? 

Q1) The "penny dropping" is that when cement is placed into the drilled wellbore annulus, the conductors tensional weight is reduce by quite a significant buoyant force (work the no's to conclude for yourself!). 

Thus before cementing the tensional presence and weight of the conductor maintains it straight in the wellbore note: that was known to be drilled straight. The current/pipe force that exists did not suggest a problem to conductor inclination since bullseyes readings had not changed.

However after cementation due to the lightening effect on the conductor. I have deduced that it is more than feasible that the current pipe force present can cause the tilt on the conductor observed. Further aided due to the fact that the conductor was not centralised and could thus offer little resistance to counter act this side force? 

Q2.) If this current force can be estimated then this can be counteracted by engineering the rig, the landing string and moving the system a distance and direction to counteracts this force. 

Q3.) Thus the rig should be moved opposite to the current direction a pre-empted and pre-determined distance to counteract current forces acting on the landing string pipe that exists.

Q2 & 3 verified by the fact that after moving the rig on a required bearing and distance the conductor bullsyes should have returned to moonpool observed reading i.e. 0.5-0.75deg!

If not then a reassessment and further move would be required.

The lessons learned from this recent case was;

- centralising a conductor has it seems many benefits. Is there a simple practicable solution to achieve this?

- Knowledge of current profiles and direction is/can be very useful data to have.

- When a conductor is cemented, the buoyant weight of cement in annulus effect will reduce the tensional load. Here it is feasible that conductor can be moved 'tilted' to some extent by the landing string and side effects acting on the conductor just above the seabed.

If conductor cannot be centralised to mitigate this effect then a Pre-determined rig move and direction (based on current force and direction that exist) can be worked into the drilling program and contingency plans. So in future we should perhaps already have a pre-empted plan in place.

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