Self-Forcing Braided Sleeve Gripper: A Soft Robotics Approach to Offshore Conductor Removal

Abstract

Offshore decommissioning is accelerating asageing oil and gas infrastructure reaches the end of its service life. In theNorth Sea alone, over 2,650 wells are scheduled between 2025 and 2034, and wellplug and abandonment accounted for 49% of decommissioning expenditure on the UKContinental Shelf in 2024. Within this scope, conductor removal, the extractionof large-diameter vertical steel pipes connecting subsea wells to surfacefacilities, remains a persistent operational bottleneck. Current practice relieson the cut-and-pull method, in which the entire lifting load is transferredthrough a single conductor cross-section. Its applicability is thereforegoverned by the structural condition of that cross-section, as corroded casingsand degraded connections may not sustain the concentrated load.

This thesis proposes a compliant grippingmechanism that distributes the lifting load over the outer conductor surface,and examines whether it scales to the multi-tonne loads of conductor removal, aregime far beyond the small-scale applications to which soft robotic grippershave been confined. Following a literature study into soft gripping methods anda conceptual design process, four concepts were evaluated, from which a braidedconstriction sleeve gripper was selected. Its defining property is aself-forcing mechanism, in which the weight of the conductor tightens the braidaround its surface, so that a heavier load produces a stronger grip.

An analytical model relates the braidgeometry to the holding capacity, which grows exponentially with adimensionless self-forcing coefficient. This coefficient is the product of thefriction coefficient and a geometric coefficient scaling with the conductorradius and the square of the number of braid turns, and inversely with thegripping length. The capacity is supplied passively by the weight of a bottomring, requiring no continuous actuation. A parametric campaign across twelveconfigurations, each tested five times, supported the model within the rangeexamined. The measured capacity scaled linearly with the ring weight at fixedgeometry, rose far more steeply with the geometric coefficient than preloadfriction alone permits, following the exponential form characteristic ofself-forcing, and followed the predicted direction for each geometric parametervaried in isolation. A single friction coefficient of approximately 0.135reproduced the measured capacity across differing geometries, supporting thecontact assumptions. The most tightly braided configuration sustained more than75 kg without slipping, giving a payload-to-self-weight ratio of at least 337:1against 16:1 at the baseline.

Applied to the NorthCormorant platform, the model yields an early-phase full-scale design employinga steel wire rope braid of seven turns over a gripping length of 14 metres,preloaded by a 10 tonne bottom ring. Under a conservative design frictioncoefficient of 0.10, the holding capacity reaches approximately 18,800 kNagainst a design lifting weight of 634 kN per conductor section. Slip istherefore not the governing constraint, which is set instead by the ropestrength at the top ring connection and by the deployment of the braid. To theauthor's knowledge, this constitutes the first compliant gripping conceptmodelled, experimentally supported, and dimensioned for offshore removals atthe multi-tonne scale.