Abstract.In this work the dynamics of a parametric pendulums system has been studied with a view to its application for sea wave's energy extraction. The idea is based on the conversion of the oscillatory motion of the waves into rotation of the pendulums. The system approximating a floating structure with two pendulums mounted on it has been modelled and analyzed. In the first stage of the study the general dynamics of the parametric pendulums has been investigated numerically and experimentally. The focus lies on synchronized rotational solutions, representing a most energetically favourable state of motion. The target state is to achieve a synchronized counter rotation of both pendulums. The controlling strategy, with the aim of initiating and maintaining the desired response, has been developed and verified numerically and experimentally. Different methods based on the delayed-feedback control have been suggested. The numerical and experimental results showing the difference in the system dynamics with and without control have been presented. Finally the energy extraction from the system has been simulated numerically and energy extraction control has been discussed.

Vibrations of cables with a small bending stiffness concerns many engineering applications such as the fatigue assessment of stay cables. With the finite element (FE) method, the analysis can be performed with nonlinear truss elements, but bending effects are not taken into account. Otherwise, beam elements can be used, but the smallness of the bending stiffness may lead to numerical instability and mediocre results in boundary regions. In this context, the paper presents an alternative method to calculate the time evolution of the profile of bending moments in boundary layers of cables, avoiding heavy FE analysis. The developments combine the theory of vibrations of extensible rods with asymptotic methods. The equations are decoupled between the slow dynamics of the boundary regions and the fast dynamics of the span. Then, a composite solution is constructed by means of a matched asymptotic procedure.

Drilling into harsh environment with heterogeneous formations including chert or conglomerate is usually a boundary that can't be crossed with standard PDC bit technology. This paper will show how an innovative PDC cutter shape combined with a novel 3D approach of cutting structure design have withstood this challenge and successfully replaced 16-in. traditional roller cone application in United Arab Emirates by the latest PDC technology delivering an average 35% improvement on Rate Of Penetration (ROP) while continuously drilling to Total Depth (TD) on each section. When drilling chert or conglomerate type of formation with a PDC drill bit, uneven load per cutters is detrimental to their integrity and results in short runs or brutal stop in the drilling operation triggering a trip for drill bit change. The new technology shown in this paper includes a unique hybrid combination of cutter shapes with a design arrangement of the cutting structure to allow for the pre-fracturing of any hard formation heterogeneity by 3D shaped cutters while standard cutters ensure a high level of cutting efficiency through their shearing action. This innovative concept has been intensively tested in the lab through single cutter and full bit scale drilling testing. In addition, in-house 3D bit simulation software has been used to optimize the cutting structure and assure performance within a wide range of drilling scenarios. Based on these simulations, an optimized design was manufactured for 16-in. directional applications usually tackled by roller cone drill bits and known for having heterogeneous cherty formations to drill throughout the end of the 5,000 ft section. Simulation results helped to validate the unique shaped cutters placement on the cutting structure to maximize the pre-fracturing effect. This design was run on Rotary Steerable System (RSS) and Positive Displacement Motor (PDM) assemblies and successfully drilled 5 wells in a challenging field of the United Arab Emirates offshore operations. 100% successful rate to reach TD in one run was achieved while increasing drastically the average ROP of the section by at least 35%. Moreover, the unique design configuration allowed to better control the directional behavior of the drill string, which resulted in a significant reduction in the overall cost per foot. A new boundary has been breached in several wells of a complex 16-in. chert and conglomerate application in the United Arab Emirates thanks to a years-long effort combining an innovative cutter technology, an optimized bit design process including a state-of-the-art 3D simulation software with lab and field experimental testing campaigns. By looking at the micro level structure of the rock destruction mechanism, a huge improvement has been obtained at the macro level of drilling operation economics.