Spar Vortex-Induced-Vibration Prediction

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 16343, "Advancement of Spar VIV Prediction," by T.-W. Yung, R.E. Sandstrom, S.T. Slocum, Z.J. Ding, and R.T. Lokken, SPE, ExxonMobil Upstream Research Co., originally presented at the 2004 Offshore Technology Conference, Houston, 3-6 May. Vortex-induced vibration (VIV) of spar hulls is an operational issue in high-current environments and an important consideration for mooring and riser integrity. On the basis of extensive analytical, laboratory, and field studies, ExxonMobil has developed a reliable methodology for predicting the VIV of classic spars that has been validated by field measurements. Introduction Spar technology continues to evolve and currently is experiencing wide application in deep- to ultradeepwater fields in the Gulf of Mexico (GOM). Eight spars have been installed in the GOM since 1996, and five more are in various stages of design and construction. These include classic spars [deep-draft caisson vessel (DDCV)], truss spars, and cell spars. A spar platform is characterized by a large, usually cylindrical, hull that extends deep into the water. In the GOM, the loop current and loop-current eddies (Fig. 1) have the potential to cause VIV on the spar hull. Speeds in these currents can remain high well below the surface, exposing the entire hull to large forces. In addition, hurricane currents also may cause VIV spar motion. Field measurements have shown that all of the three classic spars (Hoover, Genesis, and Neptune) have experienced VIV. From these experiences, it was observed that both loop/eddy currents and inertial currents (after a severe storm) could cause VIV. For the Hoover DDCV, eddy currents with speeds as low as 1 to 2 knots have caused VIV. Industrial experience with VIV started many decades ago with the development of VIV-suppression devices for smokestacks and chimneys. Over the years, there have been numerous studies on VIV physics as well as VIV-suppression devices for onshore and offshore structures. In recent years, there has been an increased interest in spar-VIV-design practice, highlighted by the first-ever industrywide workshop dedicated exclusively to spar VIV. The October 2003 workshop addressed a wide range of topics. In the workshop, the ExxonMobil spar VIV-design practice, based on work for the Hoover project, was presented. The design practice is based on a combined analytical, model-testing, and field-measurement approach. The discussion was focused on key factors for reliable prediction of classic-spar hull-VIV response and comparisons between model test results and field measurements.