Renewable hydrogen from alkaline water electrolysis (AWE) is central to a low-carbon energy system, yet the efficiency of AWE cells is limited by the gas they produce. Bubbles formed at the electrodes raise the gas fraction of the electrolyte, which lowers its ionic conductivity and adds an ohmic loss that grows as cells are driven to higher current densities. This gas is not spread evenly through the cell but is concentrated in a thin layer near the electrode, where a balance between how quickly gas is produced and how quickly it is carried away sets the build-up, and that region is difficult to reach experimentally: intrusive probes and optical imaging cannot resolve it, and the near-wall profile of a classical cell has not been measured.

This thesis measures the steady-state gas fraction in a classical alkaline water electrolyser using X-ray imaging. The cell was purpose-built for the experiment: a wide inter-electrode gap of 114 mm filled with 6 M KOH, with no forced flow, so that the near-wall layer develops unconfined. Time-averaged radiographs are converted through the Beer–Lambert law into a two-dimensional gas-fraction field ε(x, y), at a pixel size of 45 μm, for current densities j spanning from 36.5 A m−2 to 2434 A m−2, and for both the hydrogen and the oxygen electrode.

The measured fields show a thin near-wall layer that decays into an almost gas-free bulk. The peak gas fraction is set by the current density, growing roughly as √j to about 0.2 at the highest current, while the width of the layer is set dominantly by height, broadening steadily at the highest current from about 0.3 mm at the bottom to 1.5 mm at the top of the 87 mm of electrode height captured in the field of view, and changing only weakly with current. It is therefore the current that fixes how much gas the layer holds at its peak, and the vertical length of the electrode that fixes how far that gas spreads horizontally. At equal current density the hydrogen side carries about twice the near-wall gas fraction of the oxygen side, in line with the reaction stoichiometry; matched for gas production, the hydrogen peak is the higher and the oxygen layer the wider, so the two sides hold much the same near-wall gas but arrange it differently. Only the hydrogen side develops a dispersed-gas background, which builds up towards the top of the cell to a gas fraction of about 0.026 at the highest current, whereas the oxygen side stays close to the detection floor.

Interpreted via a one-dimensional production–removal balance, the profiles imply an apparent lateral dispersion coefficient of order 10−7 m2 s−1. It is not a material constant: it grows with both current and height and varies by over an order of magnitude, and is best read as an apparent, lumped quantity. A likely, but unproven, reading is that a crowding of gas bubbles, a solid pressure, caps the near-wall peak while dispersion sets the width of the layer.

The experimental data from this study thus provide an unconfined, wall-resolved reference for the gas fraction in an alkaline electrolyser, useful for reasoning about the physics of the near-wall layer, for validating transport models and for cell design, though transfer to pressurised, forced-flow or zero-gap cells needs further validation. ... Read more

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. ... Read more

A novel multi-directional piezoelectric vibrational energy harvester (PVEH) is proposed that can harvest from excitation angles in the XY-plane. The design is a parallel three-chain rotationally symmetric folded-flexure (3CR-FF) topology with a PZT-5H patch on each arm, that achieves multi-directionality utilising degenerate in-plane translational modes. Finite-element modelling guides the geometric design, system identification with a single-degree-of-freedom fit determines the near-optimal load, and the prototype is then evaluated experimentally at 15◦ angular increments in the XY- plane. The 3CR-FF prototype delivers an angle-averaged peak acceleration-normalised power of 1.35 ± 0.23 µW/mg^2 at 43.48 ± 0.12 Hz, with a localised performance dip between 330◦ and 15◦. Its performance is benchmarked against an array of three equally spaced cantilever beams (3TC) of equal total proof mass, which achieves roughly three times lower mean peak power and a wider frequency spread across angles (±0.45 Hz). ... Read more

Auxetic metamaterials are engineered structures commonly characterised by their negative Poisson’s ratio, which indicates transverse expansion when the structure is stretched axially. However, Poisson’s ratio becomes increasingly difficult to apply and interpret for non-cubic unit cells with higher-order symmetry or complex geometries. This limitation contributes to the strong focus on cubic unit cells in current research, despite their limited robustness due to a restricted number of symmetry axes. This study proposes volumetric strain as an alternative and geometry-independent measure of auxeticity. The approach is demonstrated through a case study on non-cubic polygon-prism unit cells generated by in-plane copy rotation. The mechanical behaviour of 2-, 4-, 6-, and 8-fold configurations is analysed using an analytical rigid-body replacement model, finite element simulations, and experimental testing. A Hoberman ring is introduced as an intermediary mechanism to enable uniform multi-directional actuation using a one-dimensional tensile tester. The results show that volumetric strain provides a consistent and robust description of auxetic behaviour across all configurations and modelling approaches. In contrast, Poisson’s ratio exhibits strong sensitivity to small deviations near zero strain and leads to inconsistencies between analytical, numerical, and experimental results. The experimental force–displacement response confirms a linear scaling with the number of bases, while the volumetric strain remains independent of the initial polygonal shape of the unit cell. These findings demonstrate that volumetric strain is a reliable measure of auxeticity for non-cubic unit cells and offers clear advantages for the analysis and experimental validation of complex auxetic geometries. The proposed framework provides a foundation for extending auxetic metamaterial design towards more intricate structures, including spatially copy-rotated unit cells and honeycombs based on Archimedean solids. ... Read more

In the field of compliant mechanisms, neutrally stable mechanisms are gaining interest for among others increasing actuation efficiency, vibration isolation and their use in metamaterials. Prestressing these mechanisms is necessary to provide the potential energy they require to reach their state of neutral stability. Current prestressing methods are limited by their need for individual application or relatively long activation times and limited reversibility, limiting mechanisms which utilize a multitude of neutrally stable flexures. This research aims to design a neutrally stable, enclosed volume joint on which prestressing utilizing a negative internal pressure is tested. The joint is first characterized together with a parameter study, used to understand the influence of its geometry, and a finite element model, to assess its predictive accuracy. An experimental setup is constructed, which measures the torque-angle behavior of the joint under different prestress conditions. The results are compared to the expected behavior, which is closely resembled for some of the prototypes, while the simulations prove to be only indicative of the general trend. ... Read more

Acoustic-structural interactions present significant engineering challenges, particularly in the domains of noise reduction and vibration control. At ASML, measurement-based analyses have revealed that acoustic disturbance paths often dominate the dynamic behavior of atmospheric lithography machines. This project focuses on enhancing ASML’s current one-way coupled acoustic-structural modelling approach by developing a two-way coupled, known as vibro-acoustic, modelling framework. However, this advancement introduces substantial computational complexity, necessitating effective model reduction techniques. The primary objective of this work is to reduce vibro-acoustic models in a way that preserves their ability to be modularly coupled with other system components. To this end, three Component Mode Synthesis (CMS)-based reduction methods were evaluated, with only one proving suitable for both academic and industry-scale models. The resulting reduced-order models successfully retained the dynamic fidelity of the full system and enabled efficient coupling with other substructures. When applied to harmonic excitation analyses, the reduced models achieved a dramatic reduction in computational time, from several hours to approximately one minute, while accounting for the cost of model reduction. ... Read more