rigid conductor well design

Conductor Design for Deepwater and Harsh Environment Jackup Systems

Overview

Traditional wells were generally located in benign environment/shallow water areas and did not require much (if any) tension. Latest generation of wells are in water depths of up to 100m (or more), in areas with storm wave heights greater than 25m. These systems need more exhaustive engineering to demonstrate their adequacy


Sample Wells

A conventional well A well utilising a subsea wellhead single string high pressure riser and surface BOP A conventional well drilled in cantilever mode over a platform


General Analytical Concerns: Main loads either axial or bending loads Loads either internal or external


Axial Loads

External loads cause system to buckle Internal loads do not, but reduce systems capacity to withstand loads External loads: surface equipment and conductor self weight Internal loads: internal casings

Bending Loads

External bending loads are generated by environmental (wave plus current) loads and by structural movement Internal loads are generated by eccentric action of internal casing weight

Checks Performed

Both strength and stability checked Strength reviewed using a combined stress approach Stability reviewed using a modified AISC buckling check developed by Baur and Stahl at Amoco in the 80’s.



Checks Performed



Key Features I

Separate conductor and surface casing May be tensioned, can buckle High bending loads at mudline may require increased conductor size or grade

Key Features II

Contact problems Position of uppermost centraliser on surface casing critical Too high and surface casing may be overloaded Too low and surface casing may contact conductor



Checks Performed



Checks Performed



Key Features III

Fatigue and VIV Both usually acceptable but care has to be taken for long term wells or where well is tied back at a later point



Example Well II High Pressure Riser



Key Features I

Single string Minimum bore usually 18 3/4 inch Usually tensioned

Key Features II

No internal casings hung from surface therefore less buckling load Present pressure limit of 5000 psi to maintain operability of steel system

Key Features III

High bending at upper and lower ends. Stress joints may be required. Fatigue and VIV similar to conventional well



Example Well III Well drilled in Cantilever Mode Over Platform



Key Features I

Usually untensioned Relies on platform guides for stability ‘Rule of thumb’ for guide spacing 1 metre conductor span per inch of conductor pipe diameter

Key Features II

Freestanding surface casing can buckle System stability not just individual strings

Key Features III

Relative deflection Jackup or platform motion Large bending loads generated especially if the diverter ‘locks-up’ Disconnection requirements



Example Well III Well drilled in Cantilever Mode Over Platform



Conclusions

No such thing as a generic well Analysis requirements will increase not recede Most important area is accurate input data

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