Subsea wellhead stability is an important technical issue in deepwater and ultra-deepwater oil and gas drilling operations. Since the soil under mudline is mainly silt and clay with weak cementation, the vertical friction force and horizontal displacement provided by the surface conductor are limited. The mechanical properties of subsea wellhead, including displacement and stress, are mainly distributed near the mud line.
The novel expandable surface conductor can effectively enhance the vertical bearing capacity and horizontal mechanical properties of the subsea wellhead by adding annular absorbent expansion materials to the conductor surface. In this paper, ABAQUS software was used to establish a numerical simulation model of the novel expandable surface conductor. Annular expansion material was installed on the upper, middle, and lower part of the surface conductor, respectively, to analyze its influence on the mechanical properties of the subsea wellhead. At the same time, the mechanical properties of the subsea wellhead of the conventional conductor and the novel expandable conductor are compared and analyzed.
The results show that the novel expandable surface conductor arranged near the mudline can effectively enhance the lateral mechanical properties of the subsea wellhead, but has little effect on the vertical bearing capacity of the wellhead, which is mainly due to the weak soil properties near the mudline. The annular expansion material in the middle and lower part of the conductor has little influence on the lateral mechanical properties of the subsea wellhead, but it is more obvious to improve the vertical bearing capacity of the wellhead, which is because the mechanical properties of the deep formation below the mudline are strong, and the horizontal displacement and bending moment of the conduit will not be transmitted to a greater depth under the mudline. At the same time, the lower end part of the expansion material has obvious influence on the vertical bearing capacity of subsea wellhead.
An exploratory study on the improvement of the mechanical properties of the subsea wellhead with the novel expandable surface conductor and basic rule is carried out through the numerical simulation method, which provides theoretical guidance for the subsequent industrial design and application of the novel expandable surface conductor.
Summary
Jetting of surface conductor installation has become a popular method in the offshore oil and gas industry. However, wellhead sinking, tilting, and other instability problems still exist due to the insufficient bearing capacity of the conductor, resulting in substantial economic losses. Based on the traditional installation process, a novel expandable surface conductor technology is put forward, which can effectively increase the surface area of the conductor, change the bearing capacity mechanism between the conductor and the surrounding soil, strengthen the bearing capacity of the conductor, and reduce the risk of wellhead instability, which is an economical, safe, and efficient method. First, the delayed expansion material in water was evaluated and selected, and the volume change of expansion material with time under different temperatures and pressures was measured by laboratory experiment. The calculation model has been established for the ultimate bearing capacity of the expandable surface conductor considering the volume expansion effect, and experiments of field application have also been performed in the 1500-m water depth area. The application practice shows that the theoretical predictions are consistent with the actual situation. Using the novel expandable surface conductor, the surface area of the surface conductor can be increased by more than 15% and the bearing capacity can be increased by more than 20%. The expandable surface conductor can meet the requirements of field construction, reduce the setting depth and number of surface conductors, and can significantly increase the bearing capacity of subsea wellheads, thus providing technical support for increasing the efficiency of deepwater drilling while reducing costs.
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