| dc.description.abstract | Gas hydrate deposition and subsequent plugging in pipelines pose a significant hazard to the exploration and transportation of deep-water natural gas resources. Deploying passive anti-hydrate surfaces has emerged as a promising alternative, presenting a substantial potential for value creation and positive environmental impact. This approach aims to replace costly and environmentally unfriendly traditional antihydrate methods. A fundamental prerequisite for developing such materials is a comprehensive understanding of the interactions between gas hydrates and various surfaces.
In this thesis, we cope with the scientific challenge of gas hydrate adhesion on surfaces at the nanoscale by proposing a general “sandwich” model. This model encapsulates the essential features of hydrate crystals, interfacial compositions, and surfaces, facilitating a detailed exploration of gas hydrate adhesion behavior. Large-scale Molecular Dynamics (MD) simulations serve as the primary investigative tool. Our study specifically focuses on the atomistic structures of intermediate layers (IMLs) following the adhesion-solidification process and their influence on adhesion strength. Subsequently, we conduct tensile testing for solidified IML structures to reverse-engineer the critical surface parameters governing hydrate adhesion.
This study begins by examining how gas hydrates adhere to atomically smooth solid surfaces. The resulting structure of the IML represents a delicate equilibrium affected by induced growth from both hydrate and surface sides, and modulated by gas concentration. The process of hydrate adhesion-solidification generally occurs in two distinct modes: hydrate-induced growth (HIG) and hydrate/ice competition (HIC). Mechanical testing results show that ice exhibits an adhesion strength approximately five times greater than the lowest hydrate adhesion strength.
Building upon this groundwork, the study further explores the adhesionsolidification process of hydrate on different crystal planes with different wettability. The focus lies on the final IML interfacial structure, recognized as the primary factor determining hydrate adhesion strength on the material surface. Additionally, by establishing an order of adhesion strength for typical solidified interfacial configurations, the study displays the potential of designing anti-hydrate surfaces through the interfacial gas-enrichment strategy (IGES). This strategy entails creating a “gas coating” to significantly reduce hydrate adhesion, offering the potential of facilitating automatic hydrate detachment under dynamic shear flow conditions in pipelines.
The same research philosophy is applied to investigate gas hydrate adhesion on hard surfaces with nanoscale roughness. We systematically explored key determinants of hydrate adhesion, including surface roughness, gas content, and temperature. Unlike macroscale roughness, which typically increases adhesion strength by enlarging contact areas and promoting mechanical interlocking, our findings suggest that nanoscale roughness may instead serve as crack initiators. Consequently, this results in lower ice/hydrate adhesion strength compared to that observed on atomically smooth surfaces. Additionally, we established a more comprehensive relationship between adhesion strength and the nanoscale adhesion interfacial structures of gas hydrate. These results offer new insights into the optimal design of roughness to minimize hydrate adhesion on hard surfaces.
In addition to shedding light on gas hydrate adhesion on hard surfaces, the “sandwich” model is also applied to studying hydrate adhesion on soft materials, represented by organic monolayers. Here, we emphasize the pivotal role of functional groups, particularly when compared to solid surfaces that interact with hydrates solely through van der Waals forces. Notably, our study finds that the flexibility of monolayers exerts limited influence. Hydrophilic functional groups, such as hydroxyl (-OH) groups, emerge as key players, enhancing adhesion through hydrogen bonding interactions. This involvement of hydrogen bonds probably results in a shift from adhesive failure to cohesive failure, significantly increasing adhesion strength.
In conclusion, by leveraging a “sandwich” hydrate adhesion model and large-scale Molecular Dynamics (MD) simulations, valuable insights into the atomistic mechanisms governing hydrate adhesion on surfaces have been gained. The potential of passive antihydrate surfaces as a sustainable alternative to traditional anti-hydrate methods has been confirmed. These findings shed light on the complex interplay among factors affecting hydrate adhesion and the underlying mechanisms. Such atomic insights can guide the design and development of innovative anti-hydrate surface materials, ultimately contributing to enhancing the security and sustainability of oil and gas pipeline systems. | en_US |
