An underinhibited system is defined as a system where an insufficient amount of thermodynamic inhibitor is present to prevent hydrate formation. Underinhibition might occur due to malfunctioning of equipment, temporary limitations in the inhibitor supplies or operational limitations or errors. Understanding the plugging risk of such systems is important in order to take the correct precautions to avoid blocked flowlines. In this paper we summarize the experimental efforts for the last decade within StatoilHydro on the hydrate plugging risk in underinhibited systems. The flow simulator has been used as the main experimental equipment. The overall results for systems underinhibited with ethylene glycol or methanol show that the plugging potential increases up to a maximum at concentrations around 10-15 wt%. At higher concentrations the plugging potential reduces compared to the uninhibited system. The results can be explained as follows: As water is converted to hydrates in a system containing a thermodynamic inhibitor, the inhibitor concentration will increase until the remaining aqueous phase is inhibited. This self-inhibited aqueous phase will wet the hydrate particles, giving raise to the characteristic term of “sticky” hydrate particles. The aqueous layer surrounding the hydrate particles will form liquid bridges, by capillary attractive forces, upon contact with other hydrate particles or the pipe wall. During the hydrate formation period, there is also a possibility that some of the liquid bridges are converted to solid ones, strengthening the agglomerates. Depending on the oil-water interfacial tension, the phase ratio between the aqueous phase and the solid hydrates and the conversion of liquid bridges to solid ones, this leads to increased plugging risk at lower concentrations of inhibitor (< 20 wt%) and reduced risk at higher concentrations as compared to the uninhibited system.

The aim of this thesis is concerned with the influence of sliding velocity on capillary adhesion at the nanometer scale. In ambient conditions, capillary condensation which is a thermally activated process, allows the formation of a capillary meniscus at the interface between an atomic force microscope (AFM) probe and a substrate. This capillary meniscus leads to a capillary force that acts as an additional normal load on the tip, and affects the adhesion and friction forces. The Atomic Force Microscopy (AFM) offers interesting opportunities for the measurement of surface properties at the nanometer scale. Nevertheless, in the classical imaging mode, limitations are encountered that lead to a non stationary state. These limitations are overcome by implementing a new AFM mode (called Circular AFM mode). By employing the Circular AFM mode, the evolution of the adhesion force vs. the sliding velocity was investigated in ambient conditions on model hydrophilic and hydrophobic surfaces with different physical-chemical surface properties such as hydrophilicity. For hydrophobic surfaces, the adhesion forces or mainly van der Waals forces showed no velocity dependence, whereas, in the case of hydrophilic surfaces, adhesion forces, mainly due to capillary forces follow three regimes. From a threshold value of the sliding velocity, the adhesion forces start decreasing linearly with the logarithm increase of the sliding velocity and vanish at high sliding velocities. This decrease is also observed on a monoasperity contact between a atomically flat mica surface and a smooth probe, thus eliminating the possibility of the kinetics of the capillary condensation being related to a thermally activated nucleation process as usually assumed. Therefore, we propose a model based on a thermally activated growth process of a capillary meniscus, which perfectly explains the experimental results. Based on these results, we focused on directly investigating with the Circular mode the role of capillary adhesion in friction mechanisms. We investigated the influence of the sliding velocity on the friction coefficient, and a decrease following three regimes, similar to the sliding velocity dependence of the capillary adhesion, was observed for hydrophilic surfaces that possess a roughness higher than 0.1 nm. Whereas, an increase of the friction coefficient was observed on hydrophilic (Mica) or hydrophobic (HOPG) atomically flat surfaces that posses a roughness lower than 0.1 nm. However, in this latter case, the three regimes are not established. Finally, on a rough hydrophobic surface, the friction coefficient was sliding velocity independent. A direct comparison with capillary adhesion behavior with the sliding velocity is expected to give new insights to explain this interplay. / The aim of this thesis is concerned with the influence of sliding velocity on capillary adhesion at the nanometer scale. In ambient conditions, capillary condensation which is a thermally activated process, allows the formation of a capillary meniscus at the interface between an atomic force microscope (AFM) probe and a substrate. This capillary meniscus leads to a capillary force that acts as an additional normal load on the tip, and affects the adhesion and friction forces. The Atomic Force Microscopy (AFM) offers interesting opportunities for the measurement of surface properties at the nanometer scale. Nevertheless, in the classical imaging mode, limitations are encountered that lead to a non stationary state. These limitations are overcome by implementing a new AFM mode (called Circular AFM mode). By employing the Circular AFM mode, the evolution of the adhesion force vs. the sliding velocity was investigated in ambient conditions on model hydrophilic and hydrophobic surfaces with different physical-chemical surface properties such as hydrophilicity. For hydrophobic surfaces, the adhesion forces or mainly van der Waals forces showed no velocity dependence, whereas, in the case of hydrophilic surfaces, adhesion forces, mainly due to capillary forces follow three regimes. From a threshold value of the sliding velocity, the adhesion forces start decreasing linearly with the logarithm increase of the sliding velocity and vanish at high sliding velocities. This decrease is also observed on a monoasperity contact between a atomically flat mica surface and a smooth probe, thus eliminating the possibility of the kinetics of the capillary condensation being related to a thermally activated nucleation process as usually assumed. Therefore, we propose a model based on a thermally activated growth process of a capillary meniscus, which perfectly explains the experimental results. Based on these results, we focused on directly investigating with the Circular mode the role of capillary adhesion in friction mechanisms. We investigated the influence of the sliding velocity on the friction coefficient, and a decrease following three regimes, similar to the sliding velocity dependence of the capillary adhesion, was observed for hydrophilic surfaces that possess a roughness higher than 0.1 nm. Whereas, an increase of the friction coefficient was observed on hydrophilic (Mica) or hydrophobic (HOPG) atomically flat surfaces that posses a roughness lower than 0.1 nm. However, in this latter case, the three regimes are not established. Finally, on a rough hydrophobic surface, the friction coefficient was sliding velocity independent. A direct comparison with capillary adhesion behavior with the sliding velocity is expected to give new insights to explain this interplay.