Deepwater-Riser-VIV Assessment Using Time-Domain Simulation

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18769, "Deepwater-Riser- VIV Assessment Using a Time-Domain- Simulation Approach," by Kevin Huang, Hamn-Ching Chen, and Chia-Rong Chen, Texas A&M University, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May. A computational-fluid-dynamics (CFD) approach was used to assess deepwater-riser vortex-induced vibration (VIV). The studied riser was a top-tensioned riser in 3,000-ft water depth. The proposed CFD approach is feasible for practical riser-VIV assessment. It also is an effective tool to disclose riser-VIV details, provide fundamental understanding and insight to VIV phenomena, predict riser VIV for complex current conditions, evaluate sensitivities including vessel motion coupling and transient effect, and verify riser-VIV design and analysis. Introduction Riser VIV is a design challenge for deepwater applications. Many software tools have been developed to perform riser-VIV analysis. However, most of the tools are based on empirical formulas and rely heavily on model-test data. This approach could provide satisfactory VIV predictions for shallow-water risers with a length/diameter (L/D) ratio that is relatively small, and model tests could be carried out easily to provide input data and/or verification. Deepwater risers are likely to have high-order-mode vibration in strong current. Under such conditions, model tests in a wave tank are difficult, limited either by tank size or model scale, and field experiments are feasible but costly. Further, important characteristics associated with deepwater-riser VIV are not yet studied or understood. Deepwater risers tend to experience multimode vibration; therefore, it would be overly conservative to assume single-mode lock-in. Also, the excited modes in deepwater-riser VIV could be very high; however, higher modes are more sensitive to damping and, hence, demonstrate strong nonlinear behavior. The CFD approach uses an overset-grid technique to handle the data-grid movement caused by riser deflections and vibrations. A typical single-casing top-tensioned riser was sized for 3,000-ft water depth. The riser's dynamic response under different currents was simulated in 3D, and the effects of the riser/fluid interaction were included through instantaneous drag and lift forces. The riser in-line and crossflow responses (including modal shapes and frequencies) and VIV-induced stresses were studied in detail.