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Summary The Y basin in the South China Sea has 15 trillion m3 of natural gas and represents typical ultra high-pressure high-temperature (ultra-HPHT) subsurface conditions, with the maximum bottomhole pressure (BHP) of 142 MPa (equivalent density = 2.38 sg), the highest bottomhole temperature (BHT) of 249°C at a depth of 5638 m, and an extremely narrow drilling mud density window. Therefore, there are several kinds of technical challenges during drilling operations. In recent years, managed pressure drilling (MPD) has been successfully applied in the basin with reduction of risks and well cost. Presented research focuses on a successful field case study of MPD in the Y basin. The case study consisted of two parts: MPD operational designs and MPD operational procedures. MPD operational designs included two steps: the precise calculation of drilling fluid equivalent circulating density (ECD) and the optimization of operational parameters. Four models were used in calculating drilling fluid ECD: wellbore heat transfer model, drilling fluid equivalent static density (ESD) model, drilling fluid rheological property model, and incorporating the effects of cuttings in the pressure loss model. The second step included two key operational parameters: mud weight (MW) and surface backpressure (SBP). MPD operational procedures were performed in three steps: roles and responsibilities of key personnel during MPD operations, wellsite training program and emergency procedures. Emergency procedures included unexpected drilling events, downhole equipment failures, and surface equipment failures. Taking X-1 Well as an example from the field case study, the following important observations were made. First, the MPD equipment was installed after remodeling of the drilling platform, installation of the wellhead assembly, and pressure testing of the MPD equipment. Next, the MW was set to 2.15 sg and the SBP was maintained at 525 psi while drilling the 8⅜ in. hole section on the basis of the operational design. In addition, the possible risks and corresponding preventive measures were taken into account and the MPD casedhole fingerprinting exercises were carried out. Finally, the MPD was successfully applied to X gas field featuring offshore ultra-HPHT. In addition, the dynamic formation integrity test (DFIT) and stripping out of hole with SBP were conducted. The well was successfully drilled in the narrow drilling mud density window of 0.05 sg without incidents and a sharp drop in the nonproductive time (NPT) was observed. Also, the casing structure was optimized from 7 to 5 strings with a significant reduction in the well cost and drilling time.

The jack-up rig is the most widely applied mobile platform in the shallow-water offshore drilling. Since several predetermined target wells are in the soil stratum of sand overlying clay, the punch-through accidents occur frequently during the jack-up rig spudcan penetration leading to disastrous consequences and sometimes even casualties. Although a few centrifugal model experiments and indoor large-scale experiments have been conducted to simulate the spudcan penetration, these experiments expose significant knowledge gaps that deviate from the actual field operations due to the limitations of a constant displacement speed plan instead of multi-stage preloading plan. In this study, seventeen (17) field experiments simulating spudcan installation in sand overlying clay in in-situ soil strata in the intertidal sea of the Bohai Bay are reported to address the knowledge gaps. A systematic study of the spudcan penetration is presented through key influencing factors such as soil strength ratio of upper sand to lower clay, geometric ratio of spudcan diameter to upper sand thickness, ballasting speed and holding period time. Results indicate that the ultimate (peak) soil bearing capacity increases almost quadratically with the increase in strength ratio of upper sand to lower clay; the final spudcan penetration resistance decreases as a power law relationship with the increase in geometric ratio of spudcan diameter to upper sand thickness; the instantaneous spudcan penetration velocity increases as a logarithmic function with the increase in ballasting speed and the pore water pressure decreases as an exponential function with the increase in holding period time. From these experiments, three novel schemes are proposed to prevent the punch-through. Lastly, this study provides a design basis and theoretical guidance for precise calculations of the ultimate bearing capacity, accurate analysis of punch-through risk and effective prevention of the punch-through accidents in the real jack-up rig.

Shaking table tests (STTs) provide an effective method to realistically assess seismic performance of nonstructural components (e.g., ceilings, water pipe supports and hangers, and glass screen walls), whose damage losses have been found to exceed those of structural components in recent earthquakes. During a shaking table test, nonstructural components respond upon the floor motion of a supporting frame where nonstructural components are attached to. Accurately reproducing floor response spectra of the supporting frame is therefore critical to achieve consistent and high-quality test results. This paper proposes a multi-directional decoupling iteration (MDI) control method to achieve this demand. The principle of the proposed MDI method is explained first. The effectiveness of the proposed controller to overcome the dynamic coupling between the supporting frame and the shaking table and to accurately reproduce floor response spectra along multi-directions is validated through a series of STTs of water pipe supports and hangers.