Advancement of Spar VIV Prediction

Abstract

Abstract 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 measurementsPeak value statistics for the measured time seriesVIV lock-in dominanceSingle- versus multiple-degree-of-freedom motion Introduction Spar technology continues to evolve, and is currently seeing wide application in deep to ultra-deep water 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, or 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 Gulf of Mexico, the presence of the loop current and loop current eddies introduce the potential to cause vortex-induced-vibration (VIV) on the spar hull (Figure 1). Speeds in these currents can remain high well below the surface, exposing the entire hull to large forces. In addition, hurricane currents may also cause VIV motion of the spar. Field measurements have shown that all of the three classic spars (Hoover [1], Genesis [2], and Neptune [2]) have experienced VIV. From these experiences, we have 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 one to two knots have induced VIV. Figure 1 - Gulf of Mexico Currents (Available in full paper) Industrial experience with VIV started many decades ago with the development of VIV suppression devices for smokestacks and chimneys (Figure 2). Over the years, there have been numerous studies on VIV physics as well as VIV suppression devices for onshore and offshore structures [3]- [6]. In recent years, there has been an increased interest in spar VIV design practice, highlighted by the first ever industry-wide workshop dedicated exclusively to spar VIV [2]. The October 2003 workshop addressed a wide range of topics, including regulatory perspectives, current VIV design practices, model test practices, field observations, and technology gaps. The workshop participants concluded that significant progress had been made, yet spar VIV prediction was still a challenging task. A part of the results from the workshop was incorporated into the new Appendix H - Spar Mooring Design for Vortex Induced Motion in RP 2SK Station-Keeping for Floating Structures.