Humping

Key Takeaways

• Drill string humping in horizontal wellbores occurs when the weight-on-bit (WOB) applied by the surface hookload causes the drill string to go into compression in the horizontal section, and the compressive force exceeds the lateral confinement provided by the mud buoyancy and contact with the borehole wall; sinusoidal buckling (the first mode of lateral instability, producing a sinusoidal shape with the pipe alternately touching the high side and low side of the borehole) occurs at compressive forces above the sinusoidal buckling load (which depends on drill pipe diameter, pipe wall thickness, and the radial clearance between the drill pipe and the borehole wall); helical buckling (the more severe second mode, in which the drill string wraps into a helix that contacts the borehole wall continuously, dramatically increasing contact forces and torque) occurs at compressive forces approximately twice the sinusoidal buckling load; sinusoidal buckling in the horizontal section reduces WOB transmission efficiency (the buckled pipe dissipates applied load in lateral contact forces rather than transmitting it to the bit), increases drag and torque, and can lock up the drill string entirely in severe cases by generating contact forces too high to overcome by rotation; drilling in sliding mode (not rotating the drill pipe while using a downhole motor) in the horizontal section is particularly prone to drill string buckling because rotation significantly increases the buckling load threshold.
• Casing humping during cementation of horizontal sections creates a different problem from drill string humping: as cement is pumped into the annulus behind the casing and sets, the casing is in a state of axial stress determined by the temperature differential between the cement heat of hydration and the initial casing temperature; if the cement develops sufficient compressive strength before the casing is fully supported by bond, and if thermal expansion of the casing during cement hydration is not accommodated by the casing's freedom to move axially, the resulting compressive stress in the casing can cause it to buckle in a sinusoidal hump pattern within the cemented interval; casing humping during cementing is most problematic in horizontal sections where the casing is laying on the low side of the borehole and is not constrained by gravity in the same way it would be in a vertical well; the consequence of cemented casing humping is a deformed casing profile with reduced internal clearance at the hump contact points, which can cause completion tools to hang up during subsequent interventions and can compromise the long-term structural integrity of the casing at the high stress contact locations.
• Upheaval buckling of subsea pipelines is the offshore counterpart of drill string humping and is a significant pipeline integrity challenge for flowlines carrying hot produced fluids from deepwater fields: when a buried pipeline operates at temperatures significantly above the installation temperature (production fluids from deepwater wells can be 80-150 degrees Celsius), the thermal expansion of the steel pipe generates axial compressive force (since the pipe cannot expand freely because it is constrained by friction between the pipe coating and the seabed); if this compressive force exceeds the downward force provided by the overburden (the weight of the soil cover above the pipe), the pipe buckles upward; the critical parameters are the soil cover depth (deeper cover increases the resistance to upward buckling), the pipe temperature and thermal expansion coefficient, the soil friction coefficient at the pipe-seabed interface, and the initial imperfections in the pipeline burial profile (trenches with variable depth, span crossings, spanning sections) that act as initiation sites for upheaval buckling; design mitigation includes sufficient burial depth calculated from the upheaval buckling load, engineered rock dump at potential initiation sites, and deliberate introduction of lateral buckles (controlled buckles designed into the pipeline route) as an alternative to upheaval buckling management in pipelines where burial is impractical.
• Detection and monitoring of pipeline humping uses a combination of regular inline inspection (ILI) with geometry pigs that measure pipeline deformation, AUV (autonomous underwater vehicle) surveys with multibeam sonar that map the seabed around the pipeline and identify exposed spans or humps, and fiber optic distributed temperature sensing (DTS) systems that detect temperature anomalies at buckle locations where the pipe geometry changes the thermal insulation; once a hump is detected in a subsea pipeline, the engineering assessment must determine whether the buckle is stable (growing slowly with operating cycles and manageable within the remaining fatigue life of the pipe) or unstable (growing rapidly toward a condition where the pipe will fail); stable humps are monitored with increased inspection frequency; unstable humps may require operational temperature reduction (reducing throughput or water injection cooling), rectification by additional burial or rock dump, or in severe cases a clamp repair or pipeline replacement in the buckled section.
• Engineering analysis of humping uses structural mechanics models that treat the drill string or pipeline as a beam-column subjected to combined axial compression and lateral loading, with the lateral resistance provided by the borehole wall contact forces (in drilling) or by the soil cover weight and friction (in pipeline upheaval buckling): the classical Lubinski-Woods-Chesney analysis for drill string buckling (developed in the 1950s-1960s) provides the sinusoidal and helical buckling load formulas widely used in torque-and-drag software; the Palmer-Martin analysis for pipeline upheaval buckling (developed in the 1990s) provides the critical cover depth required to suppress buckling for a given operating temperature and pipeline size; both analyses have been extended and refined by subsequent researchers to account for real wellbore geometry, axial friction, dynamic loading effects, and soil nonlinearity, but the fundamental physical insight remains the same: a slender pipe under compression constrained by a lateral boundary will buckle when the compressive load exceeds the lateral confinement, and the compressive load must either be kept below the critical value or the lateral confinement must be made sufficient to suppress buckling at the expected load.