UNDRAINED SHEAR STRENGTH ESTIMATION OF THE COVER LAYER OF HYDRATE AT SITE W18/19 OF SHENHU AREA

Undrained shear strength of the cover layer of hydrate is the most important engineering parameter for conductor jetting design, strata settlement and well stability analysis during gas hydrate production test. The piezocone penetration test (CPTU) could provide effective data to understand the vertical change of undrained shear strength from seabed to hydrate bearing layer. In this paper, we conducted CPTU and corresponding laboratory test at site W18/19 of the Shenhu area, northern South China Sea. Based on the data, the estimation method of cone factor was developed to calculate the undrained shear strength. The result shows that the cover layer of hydrate is mainly consist of calcareous clay and the soil strength becomes harder with the increase in depth. The cone factors calculated from total cone tip resistance, effective cone tip resistance and excess pore pressure is 13.8, 4.2 and 14.4 respectively. Based on the CPTU data, the developed method was used to calculate the vertical change in undrained shear strength, which increases with depth within the W18/19 cover layer of hydrate.