Summary
With current landing-string tensile loads approaching 1,500 klbf, deepwater
drilling programs are quickly eliminating comfortable design margins.
Consideration of static loading alone in these cases can sometimes be
insufficient. This paper provides a mathematical model to predict dynamic
axial loads imposed on a deepwater landing string by a drillship’s response to
ocean waves. The model can be adapted to a given landing string’s geometric and
mechanical constraints. The model will help the user identify the correct
opportunity for pausing a landing operation when seastates are increasing in
intensity, or alternatively, may enable the user to continue deepwater landing
operations with increased confidence during what may be considered marginal
conditions.
Introduction
Landing heavy, large-diameter casing strings from dynamically positioned
drillships in deep water is becoming much more common. As static tensile loads
approach the capacity of today’s landing strings, considering the effects of
additional dynamic loads becomes critical for preventing overload. Dynamic
loads caused by vessel heave are imparted to the drillstring when it is sitting
“stationary” in the slips and when it is suspended from the elevators at loads
greater than those the motion compensator can absorb.
Drillstring vibration, known to be responsible for many drillstring
failures, has been extensively studied in the past. Pioneering works include
the study of dynamic loading of drillpipe (a) while tripping1 and (b) while rotary drilling.2 For
tripping, drillstring vibration (considered impulse excitation) was caused by
adding a stand of drillpipe and then setting the slips. During tripping
operations, the top of the drillstring experiences a sudden change in velocity
within a short period of time (impulse) before reaching constant tripping
speed. For rotary drilling, longitudinal and torsional drillstring
vibration are considered as periodic variation in bit force (acceleration)
caused by bit/formation interaction, commonly referred to as stick/slip and bit
bounce.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
19 May 2004
- Revised manuscript received:
30 September 2005
- Manuscript approved:
2 October 2005
- Version of record:
15 December 2005
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