Research On Hydrostatic Deep Sea Sediment Coring Technology

The seafloor sediments play very important roles in revealing the global climate change and the essential interaction between various earth environments and predicting long-term climate change. The commonly used gravity-type sediment corer is featured by simpler structure and convenient deployment. However, it is difficult to use it for hard sediment coring due to its limited energy. The fully plugged condition happened during the penetration of gravity corer is also a big problem for it to get longer sediment sample. Vibrocorer, another type of sediment corer commonly used in the field of geologic detection, is capable of handling the stiff sediment coring but is inevitably equipped with cumbersome framework. When the vibrocorer works in great depth, it become difficult to supply the electrical or hydraulic power to the corer because the length of the electric cable or hydraulic hose would become very long and the energy loss along the cable or hose would be large. Due to the above characteristic, its demand for supporting vessel is relatively high. Based on the analysis, to combine the merits of both types of corer, a hydrostatic sediment coring mechanism is proposed and the sediment corer which is designed based on the coring mechanism can utilize both the gravity potential energy and seawater pressure energy for sediment coring. Because the total amount of energy that the hydrostatic corer can use is nearly doubled relative to the gravity corer, the penetration of the corer and the length of the sediment sampler will be significantly enhanced compared to the gravity corer with the same weight. The penetration model of the hydrostatic corer is similar to the pile driver on land. Hence, the hydrostatic corer can generate impact force in the sample tube which is twenty times the gravity of the corer, and the feature enable the corer to sampler the stiff sediments.The paper focuses on the research about the hydrostatic-driven coring mechanism and the design of the hydrostatic-driven unit, the integration and lab experiments of the hydrostatic sediment corer, the utilization of hydrostatic energy and the conversion of the impact energy, the reliability of the hydrostatic-driven unit, the seatrial of the hydrostatic-driven unit. The goal of the paper is to design a feasibile hydrostatic sediment corer and hence provide a new tool for deep sea sediment coring. The technical difficulties of the research are summarized as follow:(1) Unlike the sediment corers which are either powered by the gravity potential energy or the energy from the surface vessel, the hydrostatic sediment corer is powered by the hydrostatic energy of the seawater. Because of the huge water-pressure, the release and utilization of the hydrostatic energy from the high-pressure seawater is very difficult to control. The high corrosion of the seawater is also a big problem. Hence, how to use the hydrostatic energy of the seawater for seafloor sediement coring is a technical difficulty of the paper.(2) To increase the total amount of the energy used for sediment coring, how to use both the hydrostatic energy and the corer’s gravity potential energy for the sediment coring is another technical difficulty.(3) The available hydrostatic energy of the hydrostatic sediment corer increases linearly with the working depth of the sediment corer. How to use the hydrostatic energy for different type of sediment sampling is one of the technical difficulties.(4) The hydrostatic sediment corer uses the hydrostatic energy to hammer the sample tube into the sediment repeatedly. The modeling and analysis of the stress and strain and the disturbance extent of the sediment under the hammering effect is one of the technical difficulites.The innovative points of the paper are summarized as follow:(1) A hydrostatic-driven coring mechanism is proposed and the design method of the hydrostatic-driven unit based on the mechanism is put forward. The seafloor sediment corer which integrated the hydrostatic unit can use both the gravity potential energy of the corer and the hydrostatic energy of the seawater for sediment coring. Besides, the hydrostatic corer can directly use the seawater as working fluid and the corer have compact size and easy operation.(2) The model of hydrostatic energy utilization of the hydrostatic unit is built and optimization method of the impact energy’s conversion of the corer is proposed. According to the analysis results of the energy utilization and conversion, through adjusting the pay load of the hydrostatic corer and the stiffness of the impact unit, the hammering velocity and number of the corer can be changed, and the shape of the stress wave in the sample tube can be optimized for different type of sediment coring.(3) The disturbance theory of sediments under the hammering effect of the hydrostatic corer is porposed. During the hammering coring process of the sediment corer, the disturbance of the sediment in the horizontal direction should be estimated by the radius of the plastic zone, and the disturbance of the sediment in the vertical direction should be evaluated by the axial strain and yield criterion.The experiment results of the hydrostatic sediment corer demonstrate that the hydrostatic-driven coring mechanism is feasible and therefore provides a new method for seafloor sediment coring.