Truss Spar VIM Correlation Between Model Test and Field Measurement
Authors: Beattie Mike , Prislin Igor , Yiu Flora , Maroju Soma , Ocker Colin , Leon Chris 
Published: May 2013

Vortex Induced Motions (VIM) of a floater are the result of the exciting forces by vortex shedding on the hull, causing response near the resonant period of any of the six degrees of freedom motions. VIM is currently a critical design component for risers and moorings on floaters, particularly in the deepwater Eastern and Central Gulf of Mexico (GoM).

There have been a number of publications on spar VIM numerical modeling and on physical model testing procedures and results. Due to complexity of the VIM phenomenon, model tests are still considered the best approach to validate other tools, such CFD models or simplified numerical models dependent on empirical coefficients. However, little work has been reported on assessing the differences between field data and model test predictions.

This paper examines field measurements of motions of a truss spar when the spar experienced multiple high current events in the GoM between the years 2006 and 2011. The motion data are studied to see if VIM lock-in occurred during any of these periods of high current events. Assumptions and procedures for estimating the reduced velocity (UR) and A/D ratio using field observations are summarized. Comparison between the design guide based on the model tests and the field measurements are also presented.

The paper examines possible VIM events for the Constitution truss spar based on several hindcast GoM loop-current events over the duration of five years. The events are selected to match the design practice when VIM is expected to occur.

The field data for these events are processed and the full-scale results compared with the model test results and design basis. The conclusion of this study is that both the model test results and VIM design parameters are conservative for the platform responses observed in the first five years of operation. It is recommended that an industry wide study is needed to establish industry guidelines for VIM prediction that more accurately reflect calibration to measured floater VIM response. These guidelines can then be used in design process not only for spars but also for other types of offshore structures.

Introduction

Vortex Induced Motion (VIM) is a phenomenon known for a long time and it has been discussed in myriad conference papers, journal articles, dissertations, and books. There is no intention in this paper to repeat what is already well described in the literature, nor to cite all these valuable references as we would certainly missed most of them due to lack of space. Thus, there is a logical question as to what is the main objective of this paper? In this paper we would like to present and discuss how VIM of a floater affects the oil and gas exploration and how accurate are design data from model tests when compared with the field measurements.

In this paper we will focus on a truss-spar type of floater in the Gulf of Mexico (GoM). We selected a truss-spar which is a frequent type of production platform in the GoM, but the VIM field data for this type of platform have not been widely discussed in public nearly as much as VIM model test [3] or simulation results based on CFD [4].

VIM imposes additional loads including fatigue damages on mooring lines and risers and also, could bring drilling to a halt. Thus, designers try to account for this phenomenon early enough so the floating systems can be built with enough safety margins [5]. On the other hand, operators are interested in knowing possible occurrences for VIM during platform operations in order to take preventive measures and mitigate the VIM, usually by re-tensioning the mooring, which could de-tune the platform lateral motion natural frequency from the vortex shedding frequency [6].