For the safety and convenience of sea transportation, X oilfield of 27 m water depth in China Bohai Sea will be developed by subsea production system under Jack-up drilling conditions, which is different from conventional mode of both shallow and deep water area. The stability of shallow-water subsea wellhead system should be the primary concern in this new mode. Three separate well configuration stages, namely as drilling with 30" riser, drilling with 20" high pressure riser and completion, are analyzed. Assessments include axial capacity, strength, wave fatigue and VIV fatigue assessment considering the 30" OD with 1.0" WT conductor size and 13-3/8" OD with 0.43" WT surface casing size. In additional, sensitivity analyses regarding to ice load parameter has been considered for critical load case identified. Results show that the operations are deemed as feasible in X oilfield. The top tension or equivalent CTU tension setting for drilling operations in which accommodates the strength and fatigue requirements is recommended.
In general, jacket platform and surface wellhead are used for shallow water area, while the subsea wellhead is usually for deep water area. However, X oil field in Bohai Bay lays below a wide shipping route, so the jacket platform is not permitted to stand inside. In order to develop the oil field economically, the shallow water subsea model is put forward, as shown in Figure 1.
The wellhead stability issue occurs during the drilling phase when riser or conductor strings are attached to the subsea wellhead. In this new model, the stability of conductor and subsea wellhead should be paid attention. Compared with the conventional jacket platform in shallow water area, the jack-up rig has no lateral supports, so the conductor can be seen as a slender rod model. It means under the same environmental conditions, the conductor and subsea wellhead are more vulnerable. Compared with the subsea mode in deep water area, the current at bottom layer is much faster than that in deep water. For example, the current speed in 27 m water depth X oil field is 123 cm/s for 1 year return period. The same condition of 1,500 m fields in China South Sea is only 30 cm/s. This also means that the conductor of shallow water subsea mode is more susceptible to strength and fatigue failure.
