Analysis of ship parametric roll has generally been restricted to simple analytical models and sophisticated time domain simulations. Simple analytical models do not capture all the critical dynamics while time-domain simulations are often time consuming to implement. The model presented in this paper captures the essential dynamics of the system without over simplification. This work incorporates various important aspects of the system and assesses the significance of including or ignoring these aspects. Special consideration is given to the fact that a hull form asymmetric about the design waterline would not lead to a perfectly harmonic variation in metacentric height. Many of the previous works on parametric roll make the assumption of linearized and harmonic behavior of the time-varying restoring arm or metacentric height. This assumption enables modeling the roll motion as a Mathieu equation. This paper provides a critical assessment of this assumption and suggests modeling the roll motion as a Hills equation. Also the effects of non-linear damping are included to evaluate its effect on the bounded parametric roll amplitude in a simplified manner.
The Vortex Induced Motion (VIM) phenomenon is one of the well-known and important behavior experienced by all Floating Production Systems (FPSs) in operation under the effect of uniform current. In this phenomenon, under the effect of the formed vortices around the FPS hull, the platform oscillates primarily in a direction perpendicular to the flow resulting in a significant fatigue damage to the risers and the station keeping mooring system. Scale towing tank test has been the standard industry tool for predicting the VIM response, but it has its own limitations and has showed to significantly overpredict the VIM response when compared to the field measurements. In this paper, an interactive numerical-experimental approach has been developed to more accurately predict the VIM response and avoid the shortcomings of the model tests to bridge the gap between the model test predictions and the field measurements. Numerical and experimental data are presented to demonstrate the application and advantages of the proposed approach
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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