Ocean mining activities have been ongoing for nearly 70 years, making great contributions to industrialization. Given the increasing demand for energy, along with the restructuring of the energy supply catalyzed by efforts to achieve a low-carbon economy, deep seabed mining will play an important role in addressing energy- and resource-related problems in the future. However, deep seabed mining remains in the exploratory stage, with many challenges presented by the high-pressure, low-temperature, and complex geologic and hydrodynamic environments in deep-sea mining areas, which are inaccessible to human activities. Thus, considerable efforts are required to ensure sustainable, economic, reliable, and safe deep seabed mining. This study reviews the latest advances in marine engineering geology and the environment related to deep-sea mining activities, presents a bibliometric analysis of the development of ocean mineral resources since the 1950s, summarizes the development, theory, and issues related to techniques for the three stages of ocean mining (i.e., exploration, extraction, and closure), and discusses the engineering geology environment, geological disasters, in-situ monitoring techniques, environmental protection requirements, and environmental effects in detail. Finally, this paper gives some key conclusions and future perspectives to provide insights for subsequent studies and commercial mining operations.

Due to seawater intrusion, submarine sediments and slide masses routinely evolve into fluidized mudflows during long-distance migration and seriously endanger marine engineering facilities. However, few studies of the rheological properties of mudflows have considered the low-temperature seafloor environment and natural sediment properties. Hence, a series of rheology experiments of sediment samples from the northern South China Sea were performed. The apparent viscosity and shear stress of mudflows at 0.5°C were 36.3% higher than those at 22°C. In addition, at a given temperature, the apparent viscosity and shear stress of mudflows with a 90% water content are on average 24 times greater than those of mudflows with a 151.2% water content. The internal variation mechanism of mudflow rheological behaviors was well analyzed on the basis of Brownian motion and interparticle interactions. The Herschel-Bulkley model was employed to fit the test results, and equations of the yield stress and consistency factor were established by considering the temperature. Furthermore, the relationships between rheological parameters and water content conditions are thoroughly discussed. A rheological model that considers the temperature and water content is proposed, and the results provide scientific support for the assessment of mudflow migration ranges, sediment transport, and marine engineering design.

Submarine landslides have destroyed many ocean engineering facilities and induced catastrophic tsunamis, causing considerable loss of lives and properties. However, the evolution process of submarine landslides is highly complicated and difficult to describe with a unified mechanical framework. For fluidized landslides with wider influence ranges, faster migration velocities, and stronger impact forces, it is necessary not only to describe the constitutive model but also to determine the flow state. First, based on the essential rheological test for determining constitutive models, the relevant method and principle are thoroughly analyzed, and the formula for calculating the Reynolds number of fluidized submarine landslides in the entire shear process is derived. Taking the critical Reynolds number as the evaluation standard, a methodology to distinguish the flow state (laminar or turbulent) of fluidized landslides is quantitatively proposed. Second, through the low-temperature rheological test, the proposed formula and methodology are briefly applied to the natural submarine sediment samples in the South China Sea. In addition, the point of contraflexure of the rheological curve is consistent with the critical Reynolds number calculated by the formula, and the validity of this methodology is further verified. Finally, the mechanism of the flow-state transition is systematically analyzed by the internal evolution of the structure of the fluidized sediment samples in the shear process. This study provides important support for understanding the migration of fluidized submarine landslides.

The instability of seabed slope sediments is the main factor influencing the safety of marine resource development. Therefore, to ensure the safe operation of submarine pipelines under complex and uncertain seabed rock and soil conditions, a reliability model was developed to elucidate the trend of impact-related pipeline damage due to submarine slides. Then, a risk assessment of the damage process of submarine slides impacting pipelines was conducted, which is of great significance for the in-depth safety assessment of pipelines impacted by submarine slides. Based on the copula function, a joint probability distribution model considering the correlation among risk variables was established for rational correlation characterization. A probability analysis method of impact-related pipeline damage attributed to submarine slides based on the copula function was proposed. The Monte Carlo simulation (MCS) method was employed to simulate the random uncertainty in limited observation values and accurately determine the reliability of safe pipeline operation under the action of submarine slides. The conclusions were as follows: (1) Based on the copula function, a joint probability distribution model of risk variables with any marginal distribution function and related structure could be developed. (2) The copula function could reasonably characterize relevant nonnormal distribution characteristics of risk variables and could simulate samples conforming to the distribution pattern of the risk variables. (3) The failure probability calculated with the traditional independent normal distribution model was very low, which could result in a notable overestimation of the reliability of submarine pipelines.

With the consumption of land resources and a shortage of land space, the exploration and use of the ocean have increased. However, a new type of geotechnical engineering problem has been encountered in the development of marine engineering: the problem of ultra-soft soils, such as the dredger fills extracted from sediments in coastal reclamation projects and submarine landslides formed by the movement of sediments. Ultra-soft soils usually have very low strength with high water content, and flow easily. These features enable ultra-soft soils with high water content to exhibit physical properties of both solids and fluids. Reliably testing their mechanical properties presents new challenges for the traditional geotechnical test methods. To address these problems, a test method is proposed that combines a full-flow penetrometer test for soils and a rheological test for fluids. Kaolin samples with water content between 110 % and 157 % (approximately two to three times the liquid limit of kaolin) are tested as an example, and a detailed description and verification of the combination test method are presented. The results show that the combination test method can reliably measure the undrained shear strength of an ultra-soft soil at a wide range of shear strain rates in a short time, which is difficult for traditional methods. The combination test method proposed in this study provides a new reference for mechanical property testing of ultra-soft soils and hazard assessments related to ultra-soft soils.