Dissociation Characteristics and Morphology Changes by Using One-Sided Gradient Depressurization in a Rocking Cell System Containing Hydrate Blockages

Gas hydrate blockage poses a severe threat to flow assurance in deep-sea oil and gas production systems. In single-pipeline transport or deep-water environments with restricted access, one-sided depressurization has become an important blockage removal strategy due to its operational simplicity and applicability to single access points. However, the internal dissociation mechanisms and operational safety associated with this method remain poorly understood. In this study, an innovative fully visualized rocking cell system was employed to capture the real-time evolution of hydrate blockages, overcoming the limitations of traditional blind-cell pressure monitoring. The temperature response characteristics of packed-type hydrate blockages under one-sided depressurization were systematically analyzed, and the effects of different depressurization steps and pressure differentials on dissociation behavior were compared. The results reveal a characteristic three-stage thermal response during dissociation (phases I–III). Optimization of the phase II depressurization gradient, by gradually reducing it from 0.5 to 0.1 MPa, significantly increases the minimum system temperature, thereby improving dissociation efficiency and suppressing ice formation. Moreover, the experiments demonstrate that although the large gradient depressurization in phase I can shorten the overall depressurization time, this could increase the risk of secondary blockage in the vent line. Distinct dissociation mechanisms were identified for different blockage morphologies, including radial interface detachment for packed-type blockages, structural collapse or axial–radial synergistic detachment for localized-type blockages, and structural fracture for plug-type blockages. These findings provide mechanistic insights and optimized operational guidelines for safe and controllable hydrate blockage removal in subsea pipeline systems.