Abstract

The Gulfstar® FPS utilizes a standard Classic spar hull form (full depth, cylindrical hull) with a significantly shallower draft than previous Classic spars. Vortex Induced Motion (VIM) is a well-known but complicated phenomenon for cylindrical hulls and model testing is still the norm to determine the platform's VIM response characteristics.

Towing-type tests are widely accepted and employed for VIM model tests; however this test method only simulates a uniform current profile with depth. In the actual ocean environment, only sheared currents are present so the tow test method over-predicts the loading on the cylinder from current, particularly in the deeper portions of the hull. To achieve more realistic test results for VIM testing, it is desirable to use sheared currents, especially for deep draft floaters, such as a Classic spar.

Recent advancements in generating good quality sheared currents involve using a Flume Tank where the model is stationary and the water flows past with its velocity varied with depth. This paper describes a recent test program using this technology in a Flume Tank to explore effects of sheared currents on the VIM response characteristics of the Gulfstar® FPS. In addition to the capability of generating sheared currents, the Flume Tank, completely removes the constraints of towing tank length and the number of realizations for each test case. For benchmarking purposes, a uniform current is also generated in Flume Tank.

Distinct VIM responses have been observed in both types of current profiles. These findings are presented and discussed in this paper.

The value for the offshore industry from better predictions of VIM responses using model tests would be hard to overestimate. Being able to test using sheared currents is a valuable step toward obtaining more accurate test results for VIM type responses.

Introduction

The Gulfstar FPS® is a standard floater solution for the Gulf of Mexico region. The Gulfstar hull has a diameter of 25.9m (85ft) and a draft of 159.7m (524ft) making it much smaller as compared to other classic Spars in the Gulf of Mexico. Cylindrical structures such as Spars are known to exhibit Vortex Induced Vibration (VIV) or Vortex Induced Motions (VIM). VIM has increasingly become critical to the design of riser and mooring system. Low frequency fatigue in mooring system and risers due to VIM often drives the design of riser and mooring systems. Even though VIV has been investigated extensively over past three decades, prediction tools for VIV/VIM response of various systems are still very limited. Model testing is still the industry standard for determining the VIM performance of floaters. VIM is also observed on column stabilized units such as TLPs and Semi-submersibles [18], though not as significant as Spars.

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