Traditionally, rock cutting models for land-based and shallow water applications assume that the rock is cut under atmospheric and dry conditions. For deep sea applications this assumption is not valid, especially because the hydrostatic pressure can be of the same order of magnitude as the unconfined compressive strength of the rock. Therefore it is necessary that both the effect of hydrostatic pressure and the presence of a pore fluid are included in the rock cutting models as well. This paper focuses on extension of the existing phenomeno-logical rock cutting theory for deep sea applications. The suggested improvements of the model are based on observations from both experiments and simulations. The ideas presented can be helpful for the design of equipment for the drilling, deep sea mining and subsea trenching industries.@en

This paper introduces a robust observer-based output feedback control strategy that enables the generation of complex three-dimensional borehole trajectories created by directional drilling systems, while avoiding undesired transient behavior. The model-based controller relies on a set of nonlinear delay differential equations describing the borehole evolution. Herein, only local orientation measurements of the bottom hole assembly of the drilling system are employed. Controller and observer gains are synthesized by optimizing the location of the rightmost pole of the closed-loop dynamics, using a spectral approach for delay differential equations. Moreover, the strategy is extended to cope with the uncertainty of key system parameters in the directional drilling process. The effectiveness of the designed controller is tested in an illustrative benchmark study.

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Directional drilling systems generate complex curved boreholes in the earth's crust for the exploration and harvesting of oil, gas and geothermal energy. In practice, boreholes drilled with such systems often show instability-induced borehole spiraling, which negatively affects the borehole quality and increases drag losses while drilling. This paper presents a dynamic state-feedback controller design approach for the stable generation of complex, three-dimensional borehole geometries, while avoiding undesired borehole spiraling. The design is based on a model for three-dimensional borehole propagation in terms of nonlinear delay differential equations. After casting the problem of borehole propagation into a tracking problem, it is shown that complex, three-dimensional borehole geometries can be asymptotically stabilized with the proposed controller. The effectiveness of the proposed approach is evidenced in an illustrative benchmark study.

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To enable access to unconventional reservoirs of oil and (shale-) gas, geothermal energy and minerals, complex curved boreholes need to be drilled in the earth's crust. Directional drilling techniques, incorporating down-hole robotic actuation systems called rotary steerable systems, are used to generate these curved boreholes. In practice, however, boreholes drilled with such systems often show instability-induced borehole spiraling, which negatively affects the borehole quality and increases drag losses while drilling. As a basis for controller synthesis, we present a directional drilling model in terms of delay differential equations. Next, the problem of curved well-bore generation is formulated as a tracking problem and a model-based robust control strategy is developed, solving this tracking problem while guaranteeing the prevention of borehole spiraling. The effectiveness of the proposed approach is illustrated by representative case studies for the generation of curved boreholes.

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