ExxonMobil recently completed a series of Vortex-Induced Vibration (VIV) tests on rigid cylinders. The objectives were to provide insight into the physical phenomena that drive the VIV of riser sections at realistic, riser-scale Reynolds Numbers and to provide data that can be used to develop riser VIV lift and damping models for use in riser VIV analyses. These tests were performed using an innovative, resonant VIV Test Rig designed to provide credible VIV data for fullscale riser sections. We tested a smooth bare cylinder, bare cylinders with three levels of roughness, and a straked cylinder. The tests covered ranges of reduced velocity, Reynolds number, frequency of vibration, and vibration amplitude that are of interest for deepwater risers, at a realistic riser mass ratio. This VIV Test Rig allowed quantification of VIV lift and damping forces for cylinders and provided information on the complex phenomena that affect VIV amplitude. The VIV response of low mass ratio, bare cylinders shows pronounced sensitivity to Reynolds number and roughness when Reynolds numbers are in the critical region. Amplitudes of response can be as large as twice the cylinder diameter for smooth, bare cylinders in certain Reynolds number regions, larger than those reported previously for cylinders with properties similar to those of production risers. Measured motion and force time histories suggest that the strakes we tested completely disrupted the Karman vortex street, as was shown by the random, rather than periodic, nature of the cylinder vibrations. These findings are important for the design of risers subject to VIV. The need to enhance riser VIV prediction methods in light of these discoveries should be evaluated. Introduction As offshore oil and gas exploration and production moves into deep and ultra-deep water, high-current environments are more frequently encountered in the field. Vortex shedding due to high currents may excite high bending modes of risers, resulting in high rates of fatigue damage. VIV suppression is often required to reduce long-term fatigue damage rates to acceptable levels. The behavior of long, flexible risers in ocean currents is complex, with significant hydrodynamic and dynamic nonlinearities. Simplifications are required to make the problem tractable to analysis. In typical riser VIV analyses [1, 2], risers are modeled as tensioned beams and vortex-induced hydrodynamic loads are approximated using strip theory. For VIV, the global response amplitudes are determined by balancing vortex-induced excitations on the riser with dissipative forces. Thus, to predict riser fatigue with confidence, it is important that accurate strip-theory models are used to represent the flow physics of vortex-shedding and dissipative forces. Vortex-induced forces cannot yet be calculated using computational fluid dynamics (CFD) at Reynolds numbers consistent with riser-scale VIV [3], and so are typically based on model tests of rigid cylinders. In April 2003, ExxonMobil performed VIV testing on rigid cylinders to develop VIV lift and damping data at riserscale Reynolds numbers. A wide range of cylinder configurations and flow conditions was tested. The objective was to develop high-quality measurements of hydrodynamic forces and motions for incorporation into VIV response models as one element in the development of a high-confidence, validated design basis for riser VIV fatigue analysis.

Vortex Induced Vibration (VIV) of spar hulls is an essential operational issue in high current environments and an important consideration for mooring and riser integrity. Based on extensive analytical, laboratory, and field studies, ExxonMobil has developed a reliable methodology for predicting the VIV of classic spars, which has been validated by field measurements. This paper extends our previous discussion of key issues related to reliable prediction of classic spar VIV. The main areas addressed in this paper include:Estimation of current drag using field measurements Peak value statistics for the measured time series VIV lock-in dominance Single-versus multiple-degree-of-freedom motion

This paper describes an analytical implementation of the component approach for motion predictions of a deepwater CALM buoy as described in the companion paper “Component Approach for Confident Predictions of Deepwater CALM Buoy Coupled Motions — Part 1: Philosophy”. The implementation of the approach starts with a “model-of-the-model” validation of the analytical tool. Emphasis is given to making an accurate analytical characterization of the model as tested. To capture the strong coupling between the buoy motions and line dynamics the analyses described herein were carried out in the time-domain. This allows a rigorous treatment of the hydrodynamic forces on the buoy as well as the non-linear mooring loads when analyzing the buoy responses in waves. Since the validation analysis is a model-of-the-model practice at model scale, the proper application of the validated tool to the full-scale system is discussed. This involves modeling of the exact full-scale system and the proper selection of the hydrodynamic coefficients for the buoy and lines. In this paper we will present the numerical modeling procedures and the results from validation work to confirm that the analytical tool is validated correctly. Detailed results from validation analysis versus model test data will be shown for system components including buoy hydrodynamics from the forced oscillation test, line tension from line oscillation test, and the motions and tensions of integrated buoy/mooring/riser system. We point out that the hydrodynamic coefficients at model scale can not be directly applied to the full-scale system analysis even though they are from model test measurements. We will present the difference between the results of the model-scale system using model scale hydrodynamic coefficients and those based on a proper range of the coefficients at full-scale. This will highlight the need to design component tests to determine appropriate full scale coefficients in order to improve the accuracy of full-scale design predictions. These results will show the advantages of adopting a component approach over the common industry practices in the areas of correct use of model test data, validation analysis and the analysis of the coupled CALM buoy system responses in waves.

In LNG shipping / offshore offloading, liquid motion within the container sometimes leads to vapor entrapment at the container walls. Dynamic behavior of the entrapped vapor is governed by its thermal or thermodynamic state and profoundly affects LNG sloshing pressure on the container walls. In this paper, the authors will discuss experimental observation of condensable vapor dynamics including steam and natural gas at cryogenic temperatures. Additionally, the authors will also discuss relevant implications in sloshing experiments and the scale up to prototype design.

This paper describes a component approach of coupled motions for design of deepwater CALM offloading system in West Africa environment. Confident offloading buoy motions coupled with mooring and offloading line dynamics is identified as one of the key design challenges. In deepwater systems, components from the wave forces (exciting forces and radiated wave forces), viscous damping forces and mooring forces follow different scaling laws. We can not properly scale up the measured global responses of the coupled system to full-scale to verify the design. Component approaches overcome many of the test engineering and scale up weaknesses associated with truncated physical modeling. An effective application of a component approach develops a model test strategy for the purpose of validating the design analysis tools. In this paper we present a strategy for model testing, design tool validation and full-scale analyses. Differences between the component approach and the current industry practice are highlighted.