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Abstract The marine natural gas hydrate is considered to have huge resource potential in the South China Sea, and a sixty‐day production test has been successfully conducted in 2017. As the crucial and most vulnerable connection of floating platform and subsea wellhead, the risers are of paramount importance in safe production test of the hydrate. Due to harsh environment and the possible drive off and drift off events induced by failure of dynamic positioning system of the platform, the unplanned emergency disconnect of risers is an integral preventive measure to avoid accidents and secure the well. Once the disconnect occurs, the huge potential energy stored in the riser column will release and consequently cause violent axial recoil, which will cause serious consequences if the recoil control fails. This paper presented a quantitative risk assessment approach on the basis of dynamic Bayesian networks to dynamically predict the risk of riser recoil control failure during production test of hydrate. By using the proposed method, the dynamic failure risk of the riser recoil control was figured out, and the most contributory factors were identified. The practical application of the proposed methodology was demonstrated through a case study. Results indicated that the probability of riser recoil control failure can be significantly decreased within one year through equipment repair, and the failure and repair rates of shutoff valves and main air isolation valves are the most contributory factors. Some corresponding safety control measures were proposed to further mitigate the risk of riser recoil control failure during the production test of hydrate.

To meet the requirements of riser safety monitoring in offshore oil fields, a new Fiber Bragg Grating (FBG)-based bundle-structure riser stress monitoring sensor has been developed. In cooperation with many departments, a 49-day marine test in water depths of 1365 m and 1252 m was completed on the “HYSY-981” ocean oil drilling platform. No welding and pasting were used when the sensor was installed on risers. Therefore, the installation is convenient, reliable and harmless to risers. The continuous, reasonable, time-consistent data obtained indicates that the sensor worked normally under water. In all detailed working conditions, the test results show that the sensor can do well in reflecting stresses and bending moments both in and in magnitude. The measured maximum stress is 132.7 MPa, which is below the allowable stress. In drilling and testing conditions, the average riser stress was 86.6 MPa, which is within the range of the China National Offshore Oil Corporation (CNOOC) mechanical simulation results.

Offshore drilling activities have been significantly impacted by extreme storm conditions as oil and gas development has moved into deeper water. Under the situation of the imminent arrival of a typhoon, retrieval of the whole drilling riser is time-consuming and tedious. A novel soft hang-off system under hang-off and transit operations is proposed and developed to solve the low-efficiency and time limitation for risers' retrieval. The hang-off joint system suspends the drilling riser and the hydraulic system is designed for dynamic load compensation. Sea trial for enhanced hang-off mode with hang-off joint, non-differential control dynamic load compensation mode at hang-off operation and passive compensation mode at transit operation are conducted to evaluate the load compensation effect equipped with novel soft hang-off system compared with conventional hard hang-off mode. Results on dynamic load compensation effect are discussed. The novel soft hang-off system is of high application value for deepwater drilling engineering. [ABSTRACT FROM AUTHOR]

Extreme storm-safe drilling riser, a solution to overcome high time cost of riser emergency retrieval under the situation of the imminent typhoon arrival, is to modify the existing drilling riser to make it disconnectable and leave the long riser string below in a freestanding mode to survive. The buoyancy can maintains neutral during the normal drilling operation. However, locally the buoyancy can will have some effect on the riser system nearby. Numerical analysis and a hybrid scaled truncated model test in the water tank are conducted for the two critical configuration modes: connected mode and freestanding mode. A series of load cases under various current and wave, buoyancy upthrust are conducted in the experimental test to evaluate the hydrodynamic and strength performance of the riser near buoyancy can. The numerical analysis, experimental model design, data post-process method and result discussion can be a valuable reference for deepwater drilling engineering. [ABSTRACT FROM AUTHOR]

The marine natural gas hydrate is considered to have huge resource potential in the South China Sea, and a sixty‐day production test has been successfully conducted in 2017. As the crucial and most vulnerable connection of floating platform and subsea wellhead, the risers are of paramount importance in safe production test of the hydrate. Due to harsh environment and the possible drive off and drift off events induced by failure of dynamic positioning system of the platform, the unplanned emergency disconnect of risers is an integral preventive measure to avoid accidents and secure the well. Once the disconnect occurs, the huge potential energy stored in the riser column will release and consequently cause violent axial recoil, which will cause serious consequences if the recoil control fails. This paper presented a quantitative risk assessment approach on the basis of dynamic Bayesian networks to dynamically predict the risk of riser recoil control failure during production test of hydrate. By using the proposed method, the dynamic failure risk of the riser recoil control was figured out, and the most contributory factors were identified. The practical application of the proposed methodology was demonstrated through a case study. Results indicated that the probability of riser recoil control failure can be significantly decreased within one year through equipment repair, and the failure and repair rates of shutoff valves and main air isolation valves are the most contributory factors. Some corresponding safety control measures were proposed to further mitigate the risk of riser recoil control failure during the production test of hydrate. [ABSTRACT FROM AUTHOR]