Spar Vortex-Induced Motions Research Articles

Numerical modeling to develop strake design of Spar platform for Vortex-Induced motions suppression

In this paper, a numerical study was performed aiming to reduce the Spar hull surge and sway response due to marine currents for various arrangements of strakes. Spar is a compliant platform consisting of a large-diameter single-cylinder, supporting a deck, moored in place vertically. Spar Vortex-Induced Motions (VIM) suppression is one of the most challenging offshore industries for assessing mooring, risers, and structure fatigue. This phenomenon is mostly occurring due to the interaction of marine currents with Spar platforms. Therefore, Spar VIM response has been the subject and focus of some research in the past few decades. In this research, the interaction of the marine current with a Spar model was numerically simulated. The results were compared with the VIM response of the existing Spars with conventional strake design to validate the model. Different strake patterns and a conventional bare hull Spar armed with strakes (Base model) were used to model the VIM response of the platform. The results demonstrated the possibility of Spar VIM response reduction due to the improving design of Spar strakes. Eventually, the best model was proposed with 58% suppression in sway motion and 9% in surge motion of VIM response and the results were discussed.

An Overview of Relevant Aspects on VIM of Spar and Monocolumn Platforms

Vortex-induced motions (VIM) of floating structures are very relevant for the design of mooring and riser systems. In the design phase, spar and monocolumn VIM behavior, as well as semisubmersible and tension leg platform flow-induced motions, is studied and evaluated. This paper provides a checklist of topics and evidence from a number of sources to justify the selection that should be considered when designing spars or monocolumn platforms regarding the VIM phenomenon. An overview of the influential aspects of the VIM is presented such as heading, external appendages of the hull, concomitant presence of waves and currents, motion suppressor, draft condition (immersed portion of the hull), and external damping due to the presence of risers. Previous works concerning the VIM studies on spar and monocolumn platforms are also addressed. Whenever possible, the results of experiments from diverse authors on this matter are presented and compared.

Mitigation of Vortex-Induced Motion (VIM) on a Monocolumn Platform: Forces and Movements

A great deal of works has been developed on the spar vortex-induced motion (VIM) issue. There are, however, very few published works concerning VIM of monocolumn platforms, partly due to the fact that the concept is fairly recent and the first unit was only installed last year. In this context, a meticulous study on VIM for this type of platform concept is presented here. Model test experiments were performed to check the influence of many factors on VIM, such as different headings, wave/current coexistence, different drafts, suppression elements, and the presence of risers. The results of the experiments presented here are motion amplitudes in both in-line and transverse directions, forces and added-mass coefficients, ratios of actual oscillation and natural periods, and motions in the XY plane. This is, therefore, a very extensive and important data set for comparisons and validations of theoretical and numerical models for VIM prediction.

Active and Passive Control of Spar Vortex-Induced Motions

Spars have become an “industry solution” for deepwater developments. Vortex-induced motion (VIM) of spar platforms in currents remains an important design concern. Although strakes are effective at suppressing riser VIM, all three straked classical spars in the Gulf of Mexico have experienced significant VIM events. These are not examples of poor design but indicate a lack of adequate tools for predicting spar VIM. This paper presents the development and validation of unsteady Reynolds-averaged Navier-Stokes (URANS) methods to predict real-world spar VIM behavior. It includes the ability to address rough surfaces and high supercritical Reynolds numbers. The resulting algorithms are used to assess the effectiveness of active and passive control strategies for suppressing spar VIM. Active control consists of injecting high-pressure water tangentially into the boundary layer and is shown to be extremely effective at reducing drag and VIM amplitudes. Passive control utilizes a sleeve to channel high-pressure stagnation flow into the boundary layer and is found less effective.