Precise position and motion control of offshore vessels is often challenging, especially in harsh environment due to highly nonlinear dynamic loads from free-surface ocean waves and currents. In addition, coupled nonlinear effects of risers and mooring cables connected to the vessel can lead to unexpected responses, thus justifying the significance of modeling these nonlinear coupled effects for safer and cost-effective design and operation of offshore structures. In this study, a fully coupled multi-field fluid-structure-interaction (FSI) solver is developed to simulate the wave- and flow-induced vibration of the flexible multibody system with constraints (viz., vessel-riser system) in a turbulent flow. The structural domain with multibody systems is solved using nonlinear co-rotational finite element method, whereas the fluid domain is solved using Petrov-Galerkin finite element method for moving boundary Navier-Stokes solutions. A partitioned iterative scheme based on non-linear interface force corrections is employed for coupling of the turbulent fluid-flexible multibody system with nonmatching interface meshes. Delayed Detached Eddy Simulation (DDES) via the Positivity Preserving Variational (PPV) method is employed for modeling turbulence effects at high Reynolds number. The free-surface ocean waves are modeled by the Allen-Cahn based phase-field method. We address two key challenges in the present variational coupled formulation. Firstly, the coupling of the incompressible turbulent flow with a system of nonlinear elastic bodies described in a co-rotated frame. Secondly, the two-phase coupling based on the phase-field approach to model the air-water interface. We then present the dynamics of coupled vessel-riser system studied in harsh environmental conditions with a view of developing a robust station keeping system. The proposed fully-integrated methodology based on the first principles of variational continuum mechanics removes many assumptions and empirically assigned parameters (e.g. drag and inertia coefficients) for modeling the surrounding fluid flow at high Reynolds number.
In this paper, we focus on vortex-induced vibration (VIV) of a free-hanging riser attached to a vessel under irregular wave conditions. The global in-plane responses of the hanging riser are firstly studied numerically in order to generate the equivalent current profile under vessel motion, and a simplified irregular vessel motion-induced VIV prediction methodology is then proposed based on the understanding from previous experimental observations and literature review. Further comparison on irregular vessel motion-induced VIV and ocean current-induced VIV at the same operation site with the same return period is performed to emphasize the importance of vessel motion-induced VIV. Numerical results highlight that vessel motion-induced VIV can cause similar stresses, fatigue damage and drag amplification similar to the steady ocean current cases, especially to the operation site like Norwegian Sea where strong wave field exists with mild current condition. It should be mentioned that although the simplified methodology proposed in this paper requires further experimental validation, it is believed that the presented numerical pre-study would help the industry and the researchers to have initial understanding on the possible occurrence of vessel motion-induced VIV. We further show the similarities and differences of vessel motion-induced VIV with respect to the ocean current-induced VIV and its implications on riser design and operation.
Drilling riser system provides a short-term connection between subsea oil well and drilling vessel or platform. The analysis of different operability envelopes are required for drilling riser analysis, for example, connected drilling, connected non-drilling and hang-off analysis. The operability envelope analysis could provide operators statistical information for riser operational management. The current practice to calculate the drilling operability envelope is to use deterministic approach. However, deterministic approach could not take the randomness from environmental loadings and structures into consideration. Structural reliability method is an analysis tool to quantify probability of failure of components or systems accounting for uncertainties in environmental conditions and system parameters. It is particularly useful in cases where limited experience exists or a risk-based evaluation of design is required. It is gaining increasing popularity in the offshore and marine industry to predict failure probability. In this paper, structural reliability analysis is adopted to analyze the offshore drilling riser deployment. The uncertainties are mainly from wave and current loadings. The significant wave height HS is modeled by a Weibull probability density function, the zero-crossing wave period TZ conditional on HS is modeled by a lognormal distribution, and the surface current speed is modeled as Weibull distribution. Multiple simulations are performed by software Flexcom [1] and efficient structural reliability methods are adopted to get the failure probabilities. The deployment operability will be calculated based on structural reliability methods and the results will be compared with those calculated based on deterministic approach.
Due to the complexity involved in the vortex-induced vibration (VIV) of long offshore risers, the fundamental understanding of the coupled kinematics and dynamics of the standing and traveling waves is not well established. In the present contribution, a systematic numerical study on slender flexible riser immersed in a turbulent flow is performed on a flexible riser pinned at both the ends to investigate the standing and traveling wave responses. This wake-body resonance problem requires a stable coupling of the Navier-Stokes equation with the low mass flexible riser structure subjected to strong inertial effects from the surrounding fluid flow. A partitioned iterative scheme that relies on the nonlinear interface force corrections is employed for the modeling of coupled fluid-riser problem. The study here includes a flexible cylindrical riser considered as a long tensioned beam via linear modal analysis. Full three-dimensional simulations are performed on the flexible riser exposed to two different inflow conditions: uniform and linearly sheared. At first, the response characteristics of the riser model are validated with experimental measurements under pinned-pinned condition for uniform current. A detailed analysis is performed on the response characteristics and vorticity dynamics at various locations along the span of the flexible riser. Our simulations show that for uniform inflow condition, the flexible riser exhibits a standing wave-like phenomenon. On the other hand, for linearly sheared inflow, a traveling wave response is observed for both cross-flow and inline oscillations. These traveling waves travel from the top point to the bottom point.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.