Hydroelastic Response Analysis of Pneumatically Supported Floating Structures

Abstract

The expansion of infrastructure and energy facilities caused by continuous economic growth has been required, but it has also been caused difficulties in securing location due to an increase in land compensation costs and a lack of coastal spaces. The need for a very large floating structure (VLFS) capable of utilizing marine spaces has been suggested as a solution to these problems, and recently, studies have been conducted on the VLFS to make practical use of the water-front space and develop ocean energy. For the conventional types of the VLFS, pontoon and semi-submersible types are considered. However, the VLFS has a limitation owing to excessive hydroelastic responses which are caused by incident waves. Alternative design concepts of VLFS are required to overcome this problem. This study aims at characterizing a pneumatically supported floating structure (PSFS), which utilize an aircushion below the structural bottom, for potential mitigation of excessive hydroelastic responses in regular waves. To achieve this, a finite element-boundary element (FE-BE) coupling method is proposed by incorporating the pneumatic factor at the fluid-structure interface. The response reduction effects of the PSFS are analyzed by using this method for various design variables. The main variables are the pneumatic factor depending on the pneumatic draft and height, pneumatic area ratio, and number of pneumatic modules. To examine the efficiency of the PSFS modeling, two modeling methods are considered, which are the 1D problem consisting 1D beam-2D fluid coupling and the 2D problem consisting 2D plate-3D fluid coupling. The analysis method presented in this study for PSFS is validated by comparing the numerical results of the pontoon-type platform for which the pneumatic factor becomes 1 as a limiting case, as well as by examining the tendency of responses when the factor varies from 0.2 to 1. In the case studies, the numerical results show that the response reduction performance of the PSFS is, basically, most affected by the incident wave conditions and pneumatic factor. The response reduction effect increases as the pneumatic factor decreases at a relatively short incident wavelength for which the hydroelastic effect is dominant. It is shown that the PSFS can significantly reduce the hydroelastic response for the actual incident wave on an inland sea. A resonant responses caused by the incident wave are also avoided due to the larger natural frequencies of the PSFS compared to those of the pontoon-type VLFS. The proposed analytical method can help to intuitively apply various variables necessary for the PSFS design and can be also used in a performance analysis. In addition, when the floating structure has a large aspect ratio, the efficiency of the numerical analysis can be increased through the 1D problem.|지속적인 경제성장으로 인해 사회기반시설 및 에너지 시설의 확충이 요구되지만 연안 공간의 부족, 보상 비용의 증가 등으로 인해 입지확보에 어려움이 발생하고 있다. 이에 대한 해결책으로 해양의 공간을 이용할 수 있는 초대형 부유구조물(Very Large Floating Structure, VLFS)의 필요성이 지속적으로 제시되고 있으며, 최근 해양에너지 개발과 수변공간의 활용을 위해 VLFS에 대한 연구가 지속적으로 이루어 지고 있다. VLFS의 대표적인 구조 형식으로는 폰툰식과 반잠수식이 있다. 하지만 이러한 VLFS는 형식에 따라 확장성이 제한되거나 입사파에 의한 응답이 과도하게 발생하기 때문에 이를 해결하기 위한 방안이 필수적이다. 본 연구에서는 부유구조물의 하부에 aircushion이 설치된 공기안정식 부유구조물(Pneumatically Supported Floating Structure, PSFS)을 통해 공간의 확장성과 과도한 유탄성 응답을 해결하고자 하였다. 이를 위해 PSFS의 하부에 가두어진 압축공기의 특성을 pneumatic factor로 적용한 유한요소- 경계요소 결합 해석법(FE-BE Coupling Method)을 제안하고, PSFS의 다양한 변수에 따른 응답 감소 효과를 분석하였다. PSFS의 성능 분석을 위해 내부 공기층의 조건에 따른 pneumatic factor, 공기에 의해 지지되는 면적비, 모듈의 개수를 대표적인 변수로서 결정하였다. 이와 더불어 PSFS 모델링의 효율성을 분석하기 위해 제안한 해석법을 1차원 보-2차원 유체 조합의 1D 문제와 2차원 판-3차원 유체 조합의 2차원 문제로 적용하여 수치해석을 수행하였다. 본 연구에서 제안한 해석법의 검증은 pneumatic factor가 1인 폰툰식 VLFS의 실험결과와의 비교와 0부터 1사이의 범위를 가지는 pneumatic factor를 적용한 PSFS의 응답 경향을 통해 수행하였으며, 이를 통해 해석 결과의 타당성을 확인할 수 있었다. 이와 더불어 다양한 변수의 조합을 고려한 수치해석 결과에 따르면 PSFS의 응답 감소 성능은 기본적으로 입사파의 조건과 pneumatic factor에 가장 큰 영향을 받으며, 유탄성 효과가 두드러지는 짧은 입사파 조건에서 pneumatic factor가 작아질수록 응답감소 효과는 더욱 커진다. 그리고 부유구조물의 변장비가 클 경우에 1차원 문제를 통해 수치해석의 효율성을 높일 수 있다. Aircushion이 적용된 PSFS를 활용할 경우, 부력에 의한 지지강성을 낮춰 줄 수 있을 뿐만 아니라 부유구조물의 하부가 수면에 직접 접하지 않아 부가질량을 감소시킬 수 있기 때문에 폰툰식 VLFS보다 고유진동수 범위를 높여 입사파에 의한 공진을 피할 수 있다. 그리고 실제적인 해역의 입사파 조건에서 유탄성 응답을 효과적으로 감소시켜줄 수 있다. 이와 더불어 본 연구에서 제안한 PSFS의 유탄성 해석법은 PSFS 설계에 필요한 다양한 변수를 좀 더 직관적으로 적용 가능하며, 응답 감소 성능 분석에도 활용할 수 있을 것이다.; The expansion of infrastructure and energy facilities caused by continuous economic growth has been required, but it has also been caused difficulties in securing location due to an increase in land compensation costs and a lack of coastal spaces. The need for a very large floating structure (VLFS) capable of utilizing marine spaces has been suggested as a solution to these problems, and recently, studies have been conducted on the VLFS to make practical use of the water-front space and develop ocean energy. For the conventional types of the VLFS, pontoon and semi-submersible types are considered. However, the VLFS has a limitation owing to excessive hydroelastic responses which are caused by incident waves. Alternative design concepts of VLFS are required to overcome this problem. This study aims at characterizing a pneumatically supported floating structure (PSFS), which utilize an aircushion below the structural bottom, for potential mitigation of excessive hydroelastic responses in regular waves. To achieve this, a finite element-boundary element (FE-BE) coupling method is proposed by incorporating the pneumatic factor at the fluid-structure interface. The response reduction effects of the PSFS are analyzed by using this method for various design variables. The main variables are the pneumatic factor depending on the pneumatic draft and height, pneumatic area ratio, and number of pneumatic modules. To examine the efficiency of the PSFS modeling, two modeling methods are considered, which are the 1D problem consisting 1D beam-2D fluid coupling and the 2D problem consisting 2D plate-3D fluid coupling. The analysis method presented in this study for PSFS is validated by comparing the numerical results of the pontoon-type platform for which the pneumatic factor becomes 1 as a limiting case, as well as by examining the tendency of responses when the factor varies from 0.2 to 1. In the case studies, the numerical results show that the response reduction performance of the PSFS is, basically, most affected by the incident wave conditions and pneumatic factor. The response reduction effect increases as the pneumatic factor decreases at a relatively short incident wavelength for which the hydroelastic effect is dominant. It is shown that the PSFS can significantly reduce the hydroelastic response for the actual incident wave on an inland sea. A resonant responses caused by the incident wave are also avoided due to the larger natural frequencies of the PSFS compared to those of the pontoon-type VLFS. The proposed analytical method can help to intuitively apply various variables necessary for the PSFS design and can be also used in a performance analysis. In addition, when the floating structure has a large aspect ratio, the efficiency of the numerical analysis can be increased through the 1D problem.