Deepwater Drilling Riser Research Articles

Internal flow effect on the parametric instability of deepwater drilling risers

This study attempts to explore and distinguish the effects of internal flow induced centrifugal force and Coriolis force on the parametric instability of deepwater drilling risers. The governing equation of a fluid-conveying flexible riser subjected to parametric excitations is established at first, which is then reduced into a system of first-order linear equations by use of the Galerkin method with attention to the Coriolis-related skew-symmetric matrix. The stability charts of a deepwater drilling riser at different internal flow velocities are plotted by applying the Floquet theory, and the effects of the centrifugal force and Coriolis force on the primary instability have been identified, distinguished and exemplified. Owing to the centrifugal force effect which can reduce the effective tension and decrease the natural frequencies of the riser, the simple instability zones are enlarged and shifted in the stability chart, and more modes can be triggered in the chosen excitation domain. On the other hand, the simple resonance zones become narrower ascribed to the Coriolis force's damping effect. The most notable finding is that the Coriolis force creates and/or strengthens the coupling effect of excited modes, which can make the combination resonance much more profound.

Analysis on the Operation Fatigue of Deepwater Drilling Riser System

Fatigue is one of main failure modes of deepwater drilling riser system. Analysis models of wave induced fatigue and vortex induced fatigue of deepwater drilling riser are established according to the riser connection, installation and hang-off operations. Characteristics of wave induced fatigue and vortex induced fatigue of riser system in different operation modes are studied. The influence of each operation fatigue on the combined fatigue is also identified. The results show that wave induced fatigue damage and vortex induced fatigue damage of upper riser are large in installation and hard hang-off modes. The fatigue damage of riser system in soft hang-off mode is less than that in hard hang-off mode. The combined fatigue damage of the upper and lower riser is large. The lower riser is under the influence of wave induced fatigue and vortex induced fatigue, while the upper riser is mainly under the influence of wave induced fatigue, especially that in installation and hang-off modes. The fatigue damages of riser in installation and hang-off modes have a great influence on the combined fatigue of riser and cannot be neglected in riser fatigue analysis.

Numerical Simulation of Pressure Loss in Largesize Annulus of Deepwater Drilling Riser

Large difference exists between largesize annular hydraulics and conventional well's. Using numerical simulation, this article analyze annular pressure loss of deepwater drilling riser with large size, describing how various parameters such as annular mud velocity, drill pipe rotation speed, mud property and annular clearance affect the pressure losses in a well, compared with that of a conventional wellbore as well. Theoretical calculations have shown: As for power law fluid in the largesize annular space, when it comes to laminar flow, an increase in drill pipe rotation speed can result in a decrease of annular pressure losses; In the contrary,under turbulent conditions, this may lead to a rise in annular pressure losses. The pressure drop dramatically along with an increasing in the annular space, in other words, the size of annular clearance is a major factor that affect the pressure losses in the annular space.

Movement optimization of freely-hanging deepwater risers in reentry

Due to the complexity of riser dynamic behavior in various sea conditions, the reentry of the deepwater drilling riser to the seabed wellhead is difficult to perform. To facilitate reentry operation, Ant Colony Optimization (ACO) algorithm is adopted to optimize the movement of the riser upper end. Under optimized movement, the distance between the riser lower end and the seabed wellhead is shortened, and the oscillation of the riser lower end is damped quickly. To test the efficiency of reentry motion optimization, the water-tank experiments are carried out. The experimental results show the good efficiency of the movement optimization. For a 1500m riser in deepwater drilling, the numerical simulations of ordinary and optimized reentry motions are shown also.

FEM analysis of deepwater drilling risers under the operability and hang-off working conditions

In recent years, numerous exploration activities of oil and gas industry have been conducted in ultra deep water. The global offshore industry is building systems today for drilling in even deeper water, progressively using new technologies, and significantly extending existing technologies. This is the general trend in the offshore oil and gas industry. So the technology of ultra-deepwater risers, which is the main tool in drilling oil, is more and more standard. This paper manly focuses on the global analysis of the drilling risers. And it is divided into two parts, operability analysis and hang-off analysis that are used to check the design of the riser. In this paper, the rotation angle and stress of the riser in the drilling mode are calculated to determine the operability envelop. The number of the buoyancy modules has been determined and according to the API standard, all the worked out values have been checked out. From all the above, it is concluded that the operability envelop is relatively small under harsh condition and the number of the buoyancy modules is a little large. And above all, the design of this riser is successful.

Vortex-Induced Vibration and Coincident Current Velocity Profiles for a Deepwater Drilling Riser

The objective of this study is to use full-scale field data on current velocities and riser motions to better understand the behavior of deepwater drilling risers. The data are comprised of riser accelerations and coincident current velocity profiles from the monitoring of vortex-induced vibration (VIV) of a drilling riser located at a 1000 m water depth site. Proper orthogonal decomposition (POD), an efficient numerical technique for characterizing the spatial coherence in a random field, is employed here to identify energetic current profiles. The accuracy resulting from the use of only a limited number of the most important POD modes is studied by comparing measured current velocity profiles with those reconstructed based on a reduced-order truncation. In addition to studying current velocity profiles, riser acceleration data from this deepwater drilling riser are also analyzed. In order to analyze the VIV response of this riser, in-line and cross-flow motions in different data segments are studied. Again, empirical POD procedures are employed—this time to derive energetic spatial vibration modes defining the riser motion. Importantly, these modes are identified without the need for either an analytical/computational model of the riser or any physical dimensions and material properties; instead, they are derived exclusively using the field data. Relationships between riser response and coincident current velocity profiles are investigated, especially for those data segments associated with observed lock-in response.