Optimization Analysis of In-situ Fidelity Coring Drill Bit Structure for Subsea Gas Hydrates

Deep-sea natural gas hydrates are highly sensitive to changes in temperature and pressure, making them prone to decomposition. Existing international coring equipment primarily achieves pressure-maintained coring, which often fails to meet fidelity requirements, resulting in inaccuracies in the in-situ physical and mechanical parameters of the samples. This significantly restricts the effective development of hydrate resources. As a critical component directly interacting with the natural gas hydrate reservoir, the structure of the drill bit has a decisive impact on the quality of the core samples. This paper presents an innovative design of a four-wing composite stepped coring drill bit configuration, specifically tailored to the unique occurrence characteristics of hydrates. Additionally, a numerical model for hydrate drilling and coring is established, focusing on the mechanisms by which the number of steps influences stress distribution and temperature fields. Simulation and actual experimental results indicate that, compared to a non-stepped drill bit (with maximum core stress of 183 MPa and a peak temperature of $\mathbf{1 0. 1 9}^{\boldsymbol{\circ}} \mathbf{C}$), the composite stepped drill bit significantly reduces core stress to 144 MPa and controls the temperature to $5.14^{\circ} \mathrm{C}$, achieving reductions of 21 percent and 50 percent, respectively. During the experimental drilling process, the temperature in the core area remained stable at $2.1^{\circ} \mathrm{C}$, with significantly lower axial resistance, confirming that this configuration can effectively maintain the in-situ state of hydrates. This study provides a crucial design benchmark for the development of fidelity coring equipment for deep-sea natural gas hydrates.