The Cell Spar and Vortex Induced Vibrations

Abstract

Vortex Induced Vibrations (VIV) of Spars under current flow is an important consideration of Spar mooring design. Model tests and Field data show that Spar VIV is a complex issue that can involve a large number of parameters which include Spar hull characteristics, dynamic characteristics of the moored system and current characteristics itself (profile, turbulence, direction). The Cell Spar is a new design that has several physical characteristics that are different than those of the present classic and truss Spars. Some of these characteristics allow the structure to be constructed in a manner that does not degrade the VIV performance in currents. A series of model tests were performed to study the effects of Spar hull and mooring system characteristics on the VIV response of the Cell Spar. This paper presents an overview of the tow tank model tests and the VIV response that was measured. The paper describes in detail the experimental setup and the Spar details that were altered to achieve a VIV response that is superior to all existing Spars. Introduction A new configuration of a Spar platform called the Cell Spar is presently being fabricated in Texas. This new hull form is composed of a bundle of cylindrical members called cells that are held together by a number of horizontal and vertical structural elements located in the interstitial space between the cells (see Figure 1). Since the outer surface of the upper portion of the Cell Spar does not form a regular cylinder, there was a concern that the strakes used on the existing Spars might not work. In addition, it is more difficult to fabricate the conventional Spar strake members to fit on the multisurfaced exterior of the Cell Spar. A number of plausible new strake configurations were proposed as possible solutions, but since VIV-suppression effectiveness has proven to be difficult to predict, a series of model tests were conducted to adequately quantify performance. This paper describes the various strake configurations that were tested and presents the VIV test results that were obtained. Figure 1 Sketch of the Cell Spar Platform.(AVAILABLE IN FULL PAPER) Description of Cell Spar The upper portion of the Cell Spar under construction is composed of six outer cells surrounding a central cell. These upper cells provide the buoyancy required to float the vessel. The lower portion of the Spar is formed by extending three of the outer cells down to the keel. Fixed ballast is contained within the bottom portion of these legs to ensure that the structure has adequate stability. Like the existing Spars the structure is unconditionally stable since the center of buoyancy is above the center of gravity.

Flow Induced Motions on Multi Column Floaters

“…Evaluation of vortex induced motion (VIM) in offshore floating structures has been largely been carried out for spars. Classic spar, truss spar and cell spar hulls are all known to exhibit VIV (VIM) [1], [2], [6], [7], [8], [9], [10], [12], [13], [14], [15], [16], [19], [27], [28], [29]. [31], [32],…”

Experimental Comparison of Two Degrees-of-Freedom Vortex-Induced Vibration on High and Low Aspect Ratio Cylinders with Small Mass Ratio

“…Moreover, despite focused on mitigating the effects of VIM on such a type of platform by adding strakes, those authors also showed that, without suppressors, the maximum response amplitude occurs in a range of reduced velocities between 5 and 9, making explicit mention to eight-shaped trajectories, in a quite evident coupling between the in-line and transverse motions. Additionally, experiments by Finn et al [2] established a direct relationship between hull geometry and VIM response, highlighting the correlation between towing tests and those performed at water channels. According to those results, a formal procedure for testing VIM on spar-type platforms was presented in Irani and Finn [3], including concerns about the right consideration of the restoring system in small-scale tests.…”

Large Floating Structures

“…A third generation spar, the ''cell spar'', was installed for in 2004 (Finn et al 2003). This design embodies a new construction technique using ring stiffened tubulars assembled in a hexagonal formation to form a spar.…”