Optimal Design of Riserless Test String for Deepwater Gas Hydrate

The South China Sea contains abundant gas hydrate resources buried in shallow layers due to geological reasons, and these resources have been difficult to utilize for a long time. In order to effectively develop subsea gas hydrate, riserless test strings are used to test the production of exploratory wells, which provides a basis for large-scale development of gas hydrate in the future. A new test stub is developed and installed at the contact position between the test string and the subsea wellhead. The interaction model of the test stub and the subsea wellhead is established, and the force characteristics of the subsea wellhead are calculated. Based on the test string structure, a riser-free test finite element model is established considering the sea wind, waves, currents and seabed soil characteristics, and the stress and deformation of test strings of different sizes under different offsets are calculated. The stress of 3.5 in, 4 in, 4.5 in and 5 in drill pipes is calculated, and it is shown that with the increase of drill pipe diameter, the extreme value of stress decreases and the extreme value of bending moment increases, but the overall variation range is not large. For the drill pipe of the same diameter, with the change of the offset of the upper platform, both the stress and the bending moment extreme value of the drill pipe increase in different magnitudes, the stress extreme value increases greatly, and the bending moment extreme value increases less. With reference to the strength of drill pipes of different sizes, a safety factor of 1.5 is used to check the test string, and it shows that drill pipes of all sizes meet the conditions of use. The force of the subsea wellhead is calculated when 6-5/8 in and 5-7/8 in weighted drill pipes are used at the wellhead, and it is shown that when the platform offset is 3°, the subsea wellhead stress exceeds the allowable value and cannot be. To meet the conditions of use, it is necessary to appropriately reduce the platform offset. The mechanical calculation of the test string without riser and the mechanical analysis of the underwater wellhead provide a basis for the design of the pipe string, which can effectively guarantee the gas hydrate test operation and lay a foundation for the large-scale development of gas hydrate in the South China Sea.