Investigation of the load-bearing capacity of suction caissons used for offshore wind turbines

Bagheri, Pouyan,Son, Su Won,Kim, Jin Man Elsevier 2017 Applied ocean research Vol.67 No.-

<P><B>Abstract</B></P> <P>This paper presents the results of three-dimensional finite element analyses of the suction bucket foundation used for offshore wind turbines. The behavior of the bucket and the response of soil supporting the bucket in dense and medium dense sandy soils subjected to static horizontal load are investigated. Field tests results and a centrifuge model test are used to validate the numerical model. Dimensionless horizontal load-displacement and overturning moment-rotation relationships are derived utilizing the Power law and Buckingham’s theorem. The results show good agreement between the numerical analysis results and the straight lines obtained from the Power law until a specific value of horizontal load and overturning moment. Regarding stress behavior of soil supporting the bucket, due to soil densification and bucket movement, maximum stresses are seen near the bucket tip at the right inside of the bucket. The major part of the applied load is transferred by the bucket skirt. Numerical analysis modeling results show that the bucket rotation and displacement are highly dependent on the bucket geometry and soil properties in addition to loading conditions. Normalized equations and figures for the ultimate horizontal load and overturning-moment capacities are presented and can be used for the preliminary design of the bucket foundations in sandy soils.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Preliminary design equations and figures for bucket foundations are proposed. </LI> <LI> Presenting normalized equations and figures for bucket foundations using Power law. </LI> <LI> Bucket rotation and displacement are highly dependent on the bucket geometry. </LI> <LI> Maximum stresses are seen near the bucket tip at the right inside of the bucket. </LI> <LI> The largest soil displacement occurs near the lid inside of the bucket. </LI> </UL> </P>

모래지반에서 버켓기초 설치에 의한 지반교란 평가

김재현,이승태,김동수 한국지반공학회 2018 한국지반공학회논문집 Vol.34 No.11

Bucket foundations are widely used in offshore areas due to their various benefits such as easy and fast installations. A bucket is installed using self-weight and the hydraulic pressure difference across the lid generated by pumping out water from inside the bucket. When buckets are installed in high permeable soil such as sands, upward seepage flow occurs around the bucket tip and interior, leading to a decrease in the effective stress in the soil inside the buckets. This process reduces the penetration resistance of buckets. However, the soil inside and outside the bucket can be disturbed due to the upward seepage flow and this can change the soil properties around the bucket. Moreover, upward seepage flow can create significant soil plug heave, thereby hindering the penetration of the bucket to the target depth. Despite of these problems, soil disturbance and soil plug heave created by suction installation are not well understood. This study aims to investigate the behavior of soil during suction installation. To comprehend the phenomena of soil plug heave during installation, a series of small-scale model tests were conducted with different testing conditions. From a series of tests, the effects of tip thickness of bucket, penetration rate, and self-weight were identified. Finally, soil properties inside the bucket after installation were approximated from the measured soil plug heave.

모형실험을 통한 모래지반에서 석션버켓기초의 관입저항력 평가

김근수(Keunsoo Kim),권오순(Osoon Kwon),오명학(Myounghak Oh),장인성(Insung Jang) 한국지반환경공학회 2014 한국지반환경공학회논문집 Vol.15 No.6

석션버켓기초는 펌프로 버켓 내부의 물을 외부로 배출할 때 발생한 압력차로 지반에 설치되는 기초이다. 버켓기초는 외해의 플랫폼이나 석유?가스 시추시설을 계류시키기 위한 앵커로 주로 사용되었으나, 최근 유럽을 중심으로 해상풍력발전의 기초로 적용되기 시작하면서 국내에서도 큰 관심을 받고 있다. 석션버켓기초의 관입저항력 산정은 석션버켓기초를 성공적으로 시공하기 위해 고려해야 할 주요 사항 중의 하나이다. 본 연구는 석션버켓기초를 관입시킬 때 필요한 관입력을 평가하기 위해 실내모형실험을 수행하였다. 실내모형실험은 압입설치 및 석션설치에서 측정한 관입저항력을 관입성능평가에서 많이 사용되는 기존의 이론식과 비교하여 강도감소계수의 적절한 범위를 검토하였다. Suction bucket foundation is installed with the differential pressure created by pumping water out of bucket. Bucket foundation has usually been utilized in mooring anchor for offshore platform or floating oil and gas production facilities in the open sea. After suction bucket foundation successfully was applied as the foundation for offshore wind turbines in Europe, it recently attracts much attention in Korea, too. To estimate the penetration resistance of the suction bucket foundation is one of the important matters that should be considered during its installation. This study carried out a series of model tests to investigate the penetration resistance of suction bucket foundation. And the mobilized soil strength factor was reviewed through comparing the experimental results by two installation ways (e.g., push-in-load and suction) and the results calculated by the conventional equation.

대형 원형강관 수직도 모니터링을 위한 3D 모니터링 시스템

구성민,박해용,오명학,백승재 한국산학기술학회 2020 한국산학기술학회논문지 Vol.21 No.11

A suction bucket foundation, especially useful at depths of more than 20m, is a method of construction. The method first places an empty upturned bucket at the target site. Then, the bucket is installed by sucking water or air into it to create negative pressure. For stability, it is crucial to secure the verticality of the bucket. However, inclination by the bucket may occur due to sea-bottom conditions. In general, a repeated intrusion-pulling method is used for securing verticality. However, it takes a long time to complete the job. In this paper, we propose a real-time suction bucket verticality monitoring system. Specifically, the system consists of a sensor unit that collects raw verticality data, a controller that processes the data and wirelessly transmits the information, and a display unit that shows verticality information of a circular steel pipe. The system is implemented using an inclination sensor and an embedded controller. Experimental results show that the proposed system can efficiently measure roll/pitch information with a 0.028% margin of error. Furthermore, we show that the system properly operates in a suction bucket-based model experiment. 석션버켓기초는 20m 이상의 수심 조건에서 경제적인 시공이 가능한 건설 공법이다. 석션버켓기초는 내부가 비어 있는 양동이가 뒤집어진 형태의 구조물을 목표지점에 위치시킨 후, 버켓 내의 공기나 물을 흡입하여 내/외부의 수압차이에 의한 관입력으로 기초를 설치한다. 안정성 확보를 위해 버켓의 수직도 확보는 필수적이다. 그러나 해저지반의 불균질성이나 편심으로 인하여 버켓의 기울어짐이 발생할 수 있다. 석션버켓기초는 현재 시공과정에서 관입/인발 작업 반복을 통해 수직도를 확보하는 방법이 일반적으로 적용되고 있다. 그러나 이 방법은 현장 작업자의 경험 및 숙련도에 의존적이며 수직도 확보에 비교적 장시간이 소모된다. 본 논문에서는 실시간 석션버켓 수직도 모니터링 시스템을 제안한다. 제안된 시스템은 수직도 데이터를 수집하는 센서부, 수집된 데이터를 처리하여 무선 전송해 주는 컨트롤러부, 원형강관의 수직도 정보를 실시간으로 보여주는 디스플레이부로 구성되어 있다. 제안된 시스템은 기울기 센서와 임베디드 컨트롤러를 활용하여 구현되었다. 제안된 시스템의 실험 결과 roll/pitch가 0.028% 이내의 오차로 측정이 가능함을 확인하였다. 또한, 제안된 시스템을 석션버켓기초 모형실험에 적용하여 수직도 정보의 실시간 확인이 가능함을 보였다.

Evaluation of cyclic and monotonic loading behavior of bucket foundations used for offshore wind turbines

Bagheri, Pouyan,Kim, Jin Man Elsevier 2019 Applied ocean research Vol.91 No.-

<P><B>Abstract</B></P> <P>Suction caissons are considered as an alternative foundation solution for offshore wind turbines. In the present study, three-dimensional finite element (FE) analyses are performed to assess the behavior of a bucket foundation and soil supporting the bucket under cyclic and monotonic loading conditions. A parametric study is also performed for a wide range of bucket geometries and two different soil densities. The results indicate that bucket geometry and soil properties significantly affect the foundation response due to cyclic loading conditions. The bucket with the smallest geometry installed in medium dense soil exhibits the lowest stiffness in initial loading and then with the progress of cyclic loads experiences lower stiffness compared to the caissons with larger geometries. The sensitivity of the foundation response to the soil density is higher than its geometry. The bucket under the lowest vertical load experiences the lowest stiffness in both virgin loading and during the progress of cyclic loads. The highest soil displacement is observed near the lid at the interior of the bucket. Stresses caused by cyclic loading belong to certain ranges. Additionally, increases in the skirt length result in increases in the stress ranges and shift the range to the right side. With respect to the monotonic loading conditions, normalized diagrams are proposed that can be used for the preliminary design of suction bucket foundations.</P>

석션 버켓 지지구조를 포함하는 해상 풍력발전기 고유진동수 추정을 위한 모형화

서윤호,김상렬,김봉기 한국풍력에너지학회 2016 풍력에너지저널 Vol.7 No.2

Impulse noise from hitting the piles and high cost due to construction time prevent offshore wind turbines from advancing over the world. Recently, a supporting structure with suction buckets, which is superior to piles and jackets in terms of low noise and cost, is being constructed and can then make the industries of offshore wind turbines to be more competitive. As for using the supporting structure with suction buckets, it is important to predict the natural frequencies of the whole structure. In this paper, the effective methods for modelling the various components of the whole structure are suggested in order to predict accurate natural frequencies. Spring elements are used for soil and connecting parts of suction buckets. Then, a blade is modelled by a 2-DOF equivalent model which has the same natural frequencies of the single blade. The proposed method is demonstrated by comparing predicted natural frequencies to experiment results.

석션 버켓 지지구조 해상 풍력발전기의 고유진동수 예측과 구조물 건전성 감시 시스템을 이용한 고유진동수 관찰

서윤호,마평식,김상렬,김봉기 한국풍력에너지학회 2019 풍력에너지저널 Vol.10 No.2

As the reliability of a tripod support structure with suction buckets should be evaluated by avoiding resonance, an efficient method to predict natural frequency, which is also known as an effective tool to show the existence of damage in wind turbines, is suggested. The whole structure is modeled by beam elements, while a nacelle with blades is considered as a mass. A boundary condition of the suction buckets is described by a serial combination of stiffness elements for soil and a connecting part of a suction bucket. Then, an effective structural health monitoring (SHM) system is developed to validate the proposed modelling method. Accelerometers, strain gauge and various sensors for measurement of sea environment are installed in proper locations, and measured natural frequency is extracted by measured bending strain. It is validated that the measured natural frequency is the almost the same as the predicted one, with an error rate below 1%, and the natural frequency has sufficient margin to major excitation frequencies such as 1P and 3P. Also, it is observed that the measured natural frequency is constant for 3 months.

파라미터 변화에 따른 석션버켓기초에 발생하는 세굴현상에 대한 수치해석 연구

박선호(Sunho Park),송성진(Seongjin Song),왕해청(Haiqing Wang),정태환(Taehwan Joung),신윤섭(Yunsup Shin) 한국해양공학회 2017 韓國海洋工學會誌 Vol.31 No.4

In the case of fixed offshore wind turbines, scouring phenomena have been reported around sub-structures as a result of currents, which seriously damage the structural stability. A parametric study of the various sub-structures of a fixed offshore wind turbine was performed to investigate their effects on the scouring phenomena. For a suction bucket foundation and monopile, the effects of the stick-up heights and water depth were studied, respectively. The open source libraries, called OpenFOAM, were used to simulate a violent flow around a foundation. The numerical methods were selected based on a two-dimensional analysis of a suction bucket. Based on the results for various stick-up heights, a larger scouring region was observed with an increase in the stick-up height because of the down-wash flow around a foundation. Based on the results for various monopile water depths, the water depth had an insignificant effect on the scouring.

실내실험을 통한 트라이포드 석션버켓의 수직도 자동제어 기술의 검증

신진화,이주형 한국풍력에너지학회 2022 풍력에너지저널 Vol.13 No.4

In general, a tripod-shaped foundation secures its verticality by repeating penetration and drawing when suction penetration is performed. These repetitive tasks not only require a skilled operator, but are also less efficient. In this study, an automatic slope control system was developed to verify the slope control performance based on tripod suction in a sandy soil environment. The slope of the tripod suction base was measured, and the relative height of each suction bucket was calculated from the center of the tripod with a formula. The control program reduces the pump output of the suction bucket that penetrates too quickly by controlling the suction pressure of each suction bucket by sending an on/off signal to the suction pump according to the relative height. With such repetitive work, the relative height converges to 0 and the verticality of the structure can be secured while suction penetration is performed. As a result of the experiment, the effect of controlling the slope depending on the optimal limit setting height and penetration depth was confirmed, and a vertical degree within 0.5° was ensured. When installing a tripod suction bucket, the automatic tilt control method using the relative height is efficient without relying on the experience of the operator.

수치해석을 이용한 점성토 지반에 설치된 버켓기초의 지지력 분석

이치홍,김성렬,정재욱 한국지반공학회 2011 한국지반공학회논문집 Vol.27 No.10

Suction buckets have been widely used for offshore structures such as anchors for floating facilities, and the foundations of offshore wind energy turbines. However, the design guidelines for suction buckets have not been clearly suggested. Therefore, this study performed the numerical analysis by using ABAQUS(2010) to evaluate bearing capacities and load-movement behaviors of the suction bucket in NC clay. For the numerical analysis, the depth ratio L/D (L=embedded length of skirt; D=diameter of a bucket) was varied from 0.25 to 1.0. The analysis results showed that the L/D ratio has a significant effect on the bearing capacity, and the vertical and horizontal capacities respectively increased by about 40% and 90%, when L/D ratio increased from 0.25 to 1.0. At the vertical loading, the bucket showed the similar failure mode with a deep foundation, so the shaft and toe resistances can be separately evaluated. At the horizontal loading, the bucket with L/D=0.25 showed the sliding failure mode and the bucket with L/D≥0.5 showed the rotational failure mode.