This study aims to characterize the hydrodynamic behavior of top-tensioned drilling riser joints with complex cross-sectional profiles when exposed to offshore current environments, such as slick joints and conventional buoyant joints. Computational Fluid Dynamics (CFD) has been employed to analyze these component cross-sectional profiles and define the component drag coefficients (Cd) and lift coefficients (Cl). These coefficients determine the riser external loading, which in turn affects the riser internal stresses. The Strouhal number St is also calculated which characterizes vortex shedding. Validation of the CFD model is performed based on 2-dimensional flow past a cylinder. Following this, 2-dimensional representations of the detailed drilling riser components are created. The flow speed range considered corresponds to prevalent offshore environments, with Reynolds number (Re) = 105 – 106. The average forces are determined by subjecting the riser to various flow angles of attack across the 360 degrees range, giving the Re-dependent Cd and Cl. Variations in vortex-shedding is also analyzed by calculating St to demonstrate the effects of different flow attack angles. Validation of Cd and St for flow past a circular cylinder show good agreement with literature. Variation of the riser component outer profile and flow attack angle show variation in the drag and lift force experienced as well as the vortex shedding behavior. More accurate Re-dependent Cd was produced for the various cross-sections analyzed that could be used in subsequent global riser analysis to simulate the full connected system from vessel, riser system, wellhead, and conductor and casing (interacting with the seabed). Application of these more accurate drag coefficients result in more representative riser motions and loads in the global riser analysis. The benefit is potentially increasing vessel and operational uptime, and accurately calculating the drilling riser system fatigue life. Ultimately, this enables drilling operators and their stakeholders to make better-informed decisions on their offshore operations.

Vortex-induced vibration (VIV) is a significant factor in causing cyclic stress and fatigue damage in drilling risers. A new design of drill riser buoyancy is introduced with tri-helical grooves formed into the module to mitigate against VIV. The focus of this work is to characterize the hydrodynamic performance improvement of this new design helically grooved buoyancy over the conventional buoyancy design. Computational fluid dynamics (CFD) is employed to simulate the buoyancy module hydrodynamic behavior in 3-d space. A current speed representative of offshore environment corresponding to Re = 106 is considered using a Reynolds-averaged (k-epsilon) turbulence model. This methodology is based on previously validated domain, mesh and temporal independence studies for a circular cylinder that also showed agreement with published literature. The hydrodynamic performance is characterized primarily by the in-line and cross-flow drag coefficients Cdx and Cdy. For the helical buoyancy, Cdx is benchmarked between CFD simulation and physical town tank testing which showed similar results. Direct comparison of Cdx for the static buoyancy module (bluff body) sees Cdx being higher for the helical form compared to the circular form due to the circular form's inherent drag crisis characteristic. However simply comparing Cdx would be erroneous. A complete hydrodynamic performance representation requires the consideration of the full flow field and Cdy. This exposes that periodic vortex shedding occurs for the circular form but is mitigated for the helical form. Vortex shedding occurring for the circular form results in significantly larger Cdy values as well as potentially larger dynamic Cdx for a non-fixed bluff body. For the helically grooved form, vortex shedding is successfully mitigated and Cdy is significantly lower. The helical grooves form channels that allow flow along different directions away from the free-field flow directions which function to disrupt the downstream regular flow order. This prevents the regular formation of vortex shedding in the wake of the riser which is a pre-cursor to VIV. The omnidirectional nature of the helical grooves also ensures this mechanism for breaking regular flow is achievable regardless of the environmental current direction acting on the riser. VIV is an increasingly significant problem with deep-water exploration in harsh weather environments. The new buoyancy design provides passive vortex shedding mitigation to top-tensioned risers which would reduce the propensity of VIV occurrence.

A numerical solution is proposed for the design analysis of the mooring system of an FSRU in shallow water. Previously. such analysis relied on second-order diffraction theory with viscous damping empirically calibrated from physical model tests. However, both experimental and theoretical methods had to introduce uncertainties in the predicted mooring load because of their physical and theoretical limitations. A complicated procedure had to be introduced to derive design loads considering the uncertainties and limitations. The proposed numerical solutions are developed to minimize those uncertainties by introducing the state-of-the-art numerical tools to accurately model the flow field near the FSRU and the surrounding wave field. A CFD-based numerical wave basin, MrNWB, and a potential-based higher-order Boussinesq wave model, HAWASSI, are coupled together to simulate the near- and outer-field free-surface flows around the FSRU hull. This paper describes the framework of the proposed numerical method, followed by preliminary verifications of the accuracy and effectiveness of the proposed solution. A benchmark model test of an FSRU moored in a shallow sloping beach is used to validate the generation of the low-frequency wave and the slow-drift motion of FSRU from CFD simulation. The numerical results show significant improvement in the low-frequency FSRU responses compared to the conventional theoretical methods.

The objective of this paper is to assess the suitability of a new, open-source, Finite Element Modelling (FEM) program called Object-Oriented Multi-Physics Finite-Element Library (oomph-lib) to study the Fluid-Structure Interaction (FSI) mechanics of a fluid-conveying two-dimensional channel that has a flexible section. Previous studies have shown that this system contains rich dynamics that can include unstable oscillations of the flexible-wall section due to the fluid loading that itself is determined by the wall motion. The fundamental system is relevant to a host of applications in both engineered (e.g. flexible-pipes, membrane filters, and general aero-/hydro-elasticity) and biomechanical (e.g. blood flow, airway flow) systems.The computational model developed using oomph-lib accounts for unsteady laminar flow interacting with large-amplitude (nonlinear) deformations of a thin flexible wall. The fluid loading on the wall comprises both pressure and viscous stresses while the wall mechanics includes inertial, flexural and tension forces. Nonlinear effects in the wall mechanics principally arises through the tension induced by its deformation and the correct modelling of its geometry throughout its motion. The discretised equations for the coupled fluid and structural dynamics are combined to yield a single (monolithic) matrix differential equation for all of the fluid and wall variables that is solved through a time-stepping algorithm so as to generate numerical simulations of the system behaviour.In this paper we present results of a systematic validation of the computational model developed. Meanflow mechanics are validated by comparison against theory for Poiseuille flow through the channel with the flexible-wall held in its undisplaced position. Appropriate comparisons of statically-loaded deformations and in-vacuo vibrations of the flexible wall are made against linear theory and the limits of linear behaviour identified. The steady-state FSI is validated by comparing large-amplitude wall deformations, pressure and skin-friction loadings with published computational results that were obtained using a different computational scheme that is not in the public domain. Finally, some preliminary results of large amplitude dynamic FSI for the system are presented and discussed. Taken together, these results demonstrate the suitability of oomph-lib as a modelling and predictive tool for the study of fluid-conveying flexible pipes.

In this paper we consider a fluid-conveying channel with a compliant insert undergoing large amplitude flow-induced deformations. The objective is to assess the suitability of an open source finite element library oomph-lib for modelling this system. The fundamental system is relevant to a host of applications in both engineered (e.g. flexible-pipes, membrane filters, and general aero-/hydro-elasticity) and biomechanical (e.g. blood flow, airway flow) systems. The structural model uses a geometrically nonlinear formulation of the solid mechanics. Viscous flow is modelled at Reynolds numbers producing unsteady laminar flow. We present a brief summary of previous component validations with oomph-lib. We then focus on the unsteady-state FSI validation by comparing with published results, obtained using different computational schemes. This is done for both small-amplitude and large-amplitude wall deformations. Finally, we look at some preliminary energetics analysis of the flexible wall. The validations demonstrate the suitability and versatility of oomph-lib as a modelling and predictive tool. The flexible wall energetics validation show the possibility of understanding system stability through analysis of the flexible wall and fluid energetics.