Conghuan Le

Motion analysis on integrated transportation technique for offshore wind turbines

With the current state of offshore wind turbine construction technology, a major marine spread is necessary for the installation of the foundation, tower, and turbine. It is advantageous for offshore installation to be integrated into a single operation. The large-scale composite bucket foundation is the basis for a one-step integrated transportation and installation technique using a specialized transport vessel. The proposed transportation and installation technique will minimize the offshore operation and maximize the proportion of work carried out onshore with consequent benefits in terms of cost, quality, and safety. The self-floating composite bucket foundation is towed into the semicircle groove of the vessel and connected to the wire ropes of a stationary crane. Then the tower and turbine are attached to the foundation at the shore and the entire unit consisting of the foundation, tower, and turbine is loaded out from the quayside, transported to the site and set down on the seabed. During transpo...

Hydrodynamic motion of a large prestressed concrete bucket foundation for offshore wind turbines

A large prestressed concrete bucket foundation (LPCBF) was used for the first offshore wind turbine in the Qidong sea area of Jiangsu Province in China. The most critical technique of the foundation is the self-floating towing technique based on a reasonable subdivision inside the bucket. To predict the dynamic behaviors of the LPCBF in waves supported by the air cushion, the hydrodynamic software MOSES is used to simulate the three-dimensional motion of the foundation in the towing construction site. The prototype foundation models are established using MOSES with a water draft of 4 m, 5 m, and 6 m in given environmental conditions. The results show that the hydrodynamic responses of the large floater with air cushions depend not only on the wave conditions but also on the mass of the water column, air cushion height, and air pressure distribution. In addition, the hydrodynamic characteristics can be tuned resulting in small dynamic responses in a particular sea state by changing the draft and water plug height. The floating technique of the LPCBF with supported air cushions in waves is highly competitive for saving cost while using few expensive types of equipment during the towing transportation.

Seismic response of large-scale prestressed concrete bucket foundation for offshore wind turbines

Sandy ocean soil is vulnerable to liquefaction under seismic action. This paper describes the structural design of a new large-scale prestressed concrete bucket foundation (LSPCBF) for offshore wind turbines that take the seismic response of the foundation into consideration. Using an integrated finite element model of the soil, bucket foundation, and upper structure that incorporates infinite elements for the soil boundary, the dynamic responses of the upper structure, the bucket foundation, and the soil surrounding the bucket foundation to three types of seismic wave acceleration time histories were determined using time history analysis. The Shanghai artificial seismic wave was used as an example. This wave causes the most intense seismic response of the seismic waves considered, based on the anti-liquefaction shear stress approach to estimating the area of soil liquefaction. The results showed that 88% of the soil outside the bucket in the range of the bucket depth is liquefied. In contrast, only 9% of the soil inside the bucket is liquefied. As the soil depth increases, the liquefaction range decreases substantially. The simulation results show that the LSPCBF can improve the liquefaction resistance of soil inside and directly below the bucket under seismic loading. Finally, the foundation stabilities under an ultimate load before and after an earthquake were compared. The horizontal displacement of the liquefied foundation increased by 41.1% and the vertical differential settlement increased by 6.2% after the earthquake. A large plastic zone was not formed, which means that an LSPCBF subjected to seismic action is still able to support the ultimate load.

Anti-liquefaction characteristics of composite bucket foundations for offshore wind turbines

Occurrence of liquefaction in saturated sandy deposits under structure foundation can cause a wide range of structural damages from minor settlement to general failure because of bearing capacity loss. By comparing traditional foundations for offshore wind turbines, the soil inside and underneath the composite bucket foundation is subjected to the overburden pressure from the foundation self-weight and constrained by a half-closed bucket skirt. The objective of this paper is to clarify the effects of the soil-foundation interaction on the soil liquefaction resistance around the skirt and under the foundation. The dynamic response of the composite bucket foundation during earthquake, including coupled soil mode of porous media, is calculated using the ADINA finite-element program. A typical configuration of composite bucket foundation is used for the analysis, and two earthquake waves (peak ground accelerations of 0.035 g and 0.22 g) are applied as the base acceleration. The results show that the composite...

Construction and installation technique of large-scale top-bearing bucket foundation for offshore wind turbine

The first offshore wind turbine with the large-scale top-bearing (LSBTB) bucket foundation has been established in Qidong sea area of Jiangsu Province. The critical technique of large-scale bucket-top-bearing bucket foundation includes construction technique, self-floating towing technique, penetration technique with adjusting of horizontal levelness, and removability technique. The prefabrication of LSBTB bucket foundation is in onshore construction base such as wharfs or dock yards; transportation of foundation is towing from the dock yard to the appointed site. The prefabrication of LSBTB bucket foundation needs about two months. And the installation of foundation needs about two hours or so on site. After the proposed technique is industrialized, the cost will be 30%–50% lower than that of existing relevant technique.

Model tests on sinking technique of composite bucket foundations for offshore wind turbines in silty clay

The composite bucket foundation (CBF) is a new type of for offshore wind turbines, which can be adapted to the loading characteristics and development needs of offshore wind farms due its special structural form. The composite bucket foundation in the large-scale tests has an outer diameter of 3.5 m and a clear wall height of 0.9 m. There are seven rooms divided inside the CBF by steel bulkheads, which are arranged in a honeycomb structure. The six peripheral rooms with the skirt have the same proportions while the middle orthohexagonal one is a little larger. With the seven-room structure, the CBF has reasonable motion characteristics and towing reliability during the wet-tow construction process. Moreover, the pressure inside the compartments can control the levelness of the CBF during suction installation. Several large-scale model tests on suction installation and adjusting levelness of CBF were carried out in saturated silty clay off the coast of Jiangsu in China. During the sinking process under negative pressure, the tubes valves of all the compartments were controlled to level the foundation in a timely operation. When the foundation started to tilt, the feasibility of the bucket tilt adjusting technique was explored by applying suction/positive pressure and intermittent pumping among the seven rooms. When the required levelness of the foundation was achieved by suction-assisted lowering the high compartments and/or positive pressures raising the low compartments, all of the valves were opened so that the entire foundation could sink. Test results show that the reciprocating adjustment process can be repeated until the CBF is completely penetrated into a designed depth.

Penetration and Removal of the Mooring Dolphin Platform With Three Caisson Foundations

Mooring dolphin platforms (MDPs) with three caisson foundations were installed in the ice-drifting Bohai Sea of China. Before installation, prototype tests of penetration and removal processing were conducted near the design site. To determine the lateral soil pressure and skin friction of the caisson, soil pressure and strain gauge transducers were fixed along the external skirt of caisson B of MDPI. The shaft skin friction was calculated from the strain difference between any two points of the strain gauges. The transducer results indicated that when the soil property determined by unconsolidated and undrained (UU) triaxial tests was used to calculate the unit skin friction resistance, a value of the adhesion factor α of 1.5―2.0 is recommended. The factor α. is 1―0.4 during the suction-assisted penetration phase. The lateral earth pressure coefficient K decreased with penetration depth, most likely due to seepage caused by underpressure. In addition, the difference between the measured values obtained from the soil pressure transducers represented the small tilt of MDP1 during the installation phase. The skin friction and lateral earth pressure significantly decreased in the removal phase, 12 h after the penetration phase, mainly due to the soil disturbance caused by suction penetration around the caisson.

Dynamic response analysis of a floating mooring system

An innovative floating mooring system with two or more independent floating mooring platforms in the middle and one rigid platform on each side is proposed for improving efficiency and safety in shallow water. For this new system, most of collision energy is absorbed through the displacement of floating platforms. In order to illustrate the validity of the system, a series of model tests were conducted at a scale of 1:40. The coupled motion characteristics of the floating mooring platforms were discussed under regular and irregular waves, and the influences of wave direction and other characteristics on dynamic response of the system were analyzed. The results show that the mooring system is safest at 0° of wave incident angle, whereas the most dangerous mooring state occurs at 90° of wave incident angle. Motion responses increase with the increase of wave height, but are not linearly related to changes in wave height.

Preliminary Analysis on Integrated Transportation Technique for Offshore Wind Turbines

With current construction technology of offshore wind turbines, there is a need for a major marine spread to install the foundation, tower and turbine. There is a clear benefit that offshore installation can be integrated into one operation. The large-scale composite bucket foundation is a basis for the one-step integrated transportation and installation technique with a special vessel. The proposed transportation and installation technique will minimize offshore spread and maximize the proportion of work carried out onshore with consequent benefits in terms of cost, quality and safety. The composite bucket foundation with self-floating property would be towing into a semicircle groove of the vessel and connected with wireropes of the fixed crane. Afterwards, tower and turbine are attached onto the foundation at shore and the whole unit of foundation, tower and turbine is loaded out from the quayside, transported to site and set down on the seabed. During transportation, half of the unit weight is taken by the hoisting system of the vessel and other weight is supported by air cushions inside the composite bucket foundation. A detailed case is studied to determine the motions of the vessel with two units to confirm the viability and feasibility of such a method of the integrated transportation. The transportation and installation methodology are developed to reduce the time spent on offshore works in order to fit the installation work within the time windows with great economic benefits.Copyright

Numerical analysis of seepage field of bucket foundations for offshore wind turbines

ABSTRACT As an alternative supporting structure for wind turbines, bucket foundations (BF) have been widely applied to offshore wind farm projects. Considering the importance of installation phase which is supposed to be stable in design, detailed analysis should be conducted for each new site. The penetration (installation) process is closely related to the seepage flow forming around the bucket foundation and the seepage problems of the composite bucket foundation with multiple subdivisions (CMBF) is seldomly studied including the seepage differences between BF and CMBF. In this paper, based on the three-dimensional heat conduction law, numerical simulations are conducted for BF and CMBF from the dimensional effect and various working conditions perspectives. The results indicate that the seepage distribution patterns are different for the multiple subdivisions; higher tilt angle and smaller length-diameter-ratio lead to bigger hydraulic gradient; and the diameter of mid-compartment has a slight impact on the BFs compared with the length-diameter-ratios.