Flexible Foundation Model Research Articles

In-place analysis for pile structural response of fixed jacket offshore platform

ABSTRACT In-place analysis for offshore platforms is essentially required to ensure the structural integrity of platform components under extreme environmental conditions. The pile soil interaction is of great concern in structural behaviour of jacket-type fixed offshore platforms. Numerical modelling of a case-study platform with soil-pile-jacket-structure interaction is developed, the pile response demands in terms of lateral displacement and maximum bending moment and shear force, as well as soil reaction are investigated. The topsoil layers have important effects on the responses of the platform and supporting piles. A decrease in the upper soil layers' resistance leads to higher lateral displacement responses at platform base and a decrease in the shear force and bending moment along the pile shaft. The response results of the soil-structure interaction model of the jacket foundation, confirm that the flexible foundation model simulation is necessary to estimate the loads and response demands of the offshore platform well.

Dynamic Soil–Structure–Equipment Interaction (II): Closed-form Analytical Solution for Incident Plane SH-wave Based on Flexible Foundation Model

ABSTRACT We study a two-dimensional structure–equipment model standing on a semi-circular flexible foundation embedded in the homogeneous half-space subjected with plane SH-waves, as a companion to the previous study of dynamic soil–structure–equipment interaction based on rigid-foundation model. We obtain the analytical solution of the system response that demonstrates the influence of foundation stiffness on the response of the soil–structure–equipment interaction system. The results show that, after considering the influence of the foundation stiffness, the effective foundation input motion is amplified, and the maximum amplification percentage can reach to 32.8%. The peak of the floor relative response is amplified by 18.5% and the frequency corresponding to the peak shifts to the lower range with the maximum shift percentage 16.1%. The peak of the equipment relative response can be amplified by 76.7%. There is significant relative deformation in the foundation when the wave is obliquely incident. The equipment-structure frequency ratio and mass ratio are important parameters that reflect the dynamic characteristics of the soil-structure–equipment system. When the equipment-structure frequency ratio is between 0.5 and 1.0, the dynamic characteristics of the structure–equipment system change. We propose a parameter (Ψ) to measure the extent to which the equipment participates in the soil–structure interaction.

Pile-soil-structure interaction effect on structural response of piled jacket-supported offshore platform through in-place analysis

In-place analysis for offshore platforms is essentially required to make proper design for new structures and true assessment for existing structures, in addition to the structural integrity of platforms components under the maximum and minimum operating loads when subjected to the environmental conditions. In-place analysis have been executed to check that the structural member with all appurtenance&#180s robustness have the capability to support the applied loads in either storm or operating conditions. A nonlinear finite element analysis is adopted for the platform structure above the seabed and pile-soil interaction to estimate the in-place behavior of a typical fixed offshore platform. The SACS software is utilized to calculate the dynamic characteristics of the platform model and the response of platform joints then the stresses at selected members, as well as their nodal displacements. The directions of environmental loads and water depth variations have significant effects in the results of the in-place analysis behavior. The most of bending moment responses of the piles are in the first fourth of pile penetration depth from pile head level. The axial deformations of piles in all load combinations cases of all piles are inversely proportional with penetration depth. The largest values of axial soil reaction are shown at the pile tips levels (the maximum penetration level). The most of lateral soil reactions resultant are in the first third of pile penetration depth from pile head level and approximately vanished after that penetration. The influence of the soil-structure interaction on the response of the jacket foundation predicts that the flexible foundation model is necessary to estimate the force responses demands of the offshore platform with a piled jacket-support structure well.

Analysis of block random rocking on nonlinear flexible foundation

In this paper the rocking response of a rigid block randomly excited at its foundation is examined. A nonlinear flexible foundation model is considered accounting for the possibility of uplifting in the case of strong excitation. Specifically, based on an appropriate nonlinear impact force model, the foundation is treated as a bed of continuously distributed springs in parallel with nonlinear dampers. The statistics of the rocking response is examined by an analytical procedure which involves a combination of static condensation and stochastic linearization methods. In this manner, repeated numerical integration of the highly nonlinear differential equations of motion is circumvented, and a computationally efficient semi-analytical solution is obtained. Comparisons with pertinent Monte Carlo simulations demonstrate the efficiency and reliability of the proposed approach. Finally, the survival probability related to toppling of the blocks subject to filtered white noise excitation is investigated, for different kinds of block geometries, foundation materials, and filter parameters.

Investigation on the sensitivity of flexible foundation models of an offshore wind turbine under earthquake loadings

This paper presents an investigation on the sensitivity of flexible foundation models of offshore wind turbines subjected to earthquake loadings. A novel seismic analysis framework (SAF) is developed and implemented in an open source aero-hydro-elastic analysis tool, “FAST”, for accurately modelling the effects of seismic loadings on offshore wind turbines. SAF has been validated through comparisons against experimentally validated numerical tools, GH Bladed and NREL Seismic. The behaviours of three flexible foundation models, namely, the apparent fixity (AF), coupled springs (CS) and distributed springs (DS) methods, subjected to earthquake loadings have been examined in relation to a fixed foundation. A total of 224 fully coupled nonlinear simulations of the foundation models are performed using a dataset of 28 earthquake records which are scaled using the target spectrum matching technique to represent the actual seismic effects of the selected sites. The results reveal that the AF model appropriately reflects realistic situations in comparison to the CS model. In addition, the amplitudes of vibration induced by the earthquake loadings are larger for flexible foundations compared to the rigid foundation. The main contribution to the out-of-plane bending moment of the support structure at the mudline comes from the wind loading for all the foundation models. This study has also found that the 2nd flap mode of blade is activated by the earthquake loadings for the AF and DS models but not for the rigid and CS models. As a result, the peak blade-root bending moment is found to be more sensitive to pseudo spectral acceleration (PSA) for the AF and DS models. Furthermore, the peak tower-top displacement and mudline bending moment increase linearly with PSA for all the examined models. This study contributes to the evaluation of the wind turbine responses subjected to earthquakes or combined multi-hazard loadings in the operational state.

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Designing support structures for offshore wind turbines is a complex task as these are highly dynamic systems subjected to long-term cyclic loads with variable amplitude. Long-term cyclic loading may cause stiffening or softening of the soil around the pile foundation of an offshore wind turbine, which leads to variations in the foundation stiffness and accumulated permanent rotation of the pile. Although variations in the foundation stiffness can negatively impact the fatigue life, the long-term variability of the soil conditions is normally not considered in the fatigue damage assessment. The main objective of this study is the investigation of the impact of changes in soil parameters on the fatigue lifetime for an offshore wind turbine founded in loose sand. For this purpose, a generic monopile based offshore wind turbine with flexible foundation model was used. The soil-pile interaction was modeled with a distributed spring model using nonlinear API p-y curves. Integrated analyses in the time domain were performed and fatigue damage was assessed in terms of a damage equivalent bending moment at mudline. The fatigue lifetime varies between -9 percent and +4 percent when considering changes in soil conditions, depending on the assumption of soil softening or stiffening, respectively. These results indicate that changes in soil parameters should be taken into account in the fatigue damage calculations of offshore wind turbines for more precise fatigue lifetime estimation. Moreover, it is emphasized that more accurate modeling of soil-pile interaction is required in the design and optimization of offshore wind turbines.

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Designing support structures for offshore wind turbines is a complex task as these are highly dynamic systems subjected to long-term cyclic loads with variable amplitude. Long-term cyclic loading may cause stiffening or softening of the soil around the pile foundation of an offshore wind turbine, which leads to variations in the foundation stiffness and accumulated permanent rotation of the pile. Although variations in the foundation stiffness can negatively impact the fatigue life, the long-term variability of the soil conditions is normally not considered in the fatigue damage assessment. The main objective of this study is the investigation of the impact of changes in soil parameters on the fatigue lifetime for an offshore wind turbine founded in loose sand. For this purpose, a generic monopile based offshore wind turbine with flexible foundation model was used. The soil-pile interaction was modeled with a distributed spring model using nonlinear API p-y curves. Integrated analyses in the time domain were performed and fatigue damage was assessed in terms of a damage equivalent bending moment at mudline. The fatigue lifetime varies between -9 percent and +4 percent when considering changes in soil conditions, depending on the assumption of soil softening or stiffening, respectively. These results indicate that changes in soil parameters should be taken into account in the fatigue damage calculations of offshore wind turbines for more precise fatigue lifetime estimation. Moreover, it is emphasized that more accurate modeling of soil-pile interaction is required in the design and optimization of offshore wind turbines.

Influence of Soil Parameters on the Fatigue Lifetime of Offshore Wind Turbines with Monopile Support Structure

Designing support structures for offshore wind turbines is a complex task as these are highly dynamic systems subjected to long-term cyclic loads with variable amplitude. Long-term cyclic loading may cause stiffening or softening of the soil around the pile foundation of an offshore wind turbine, which leads to variations in the foundation stiffness and accumulated permanent rotation of the pile. Although variations in the foundation stiffness can negatively impact the fatigue life, the long-term variability of the soil conditions is normally not considered in the fatigue damage assessment. The main objective of this study is the investigation of the impact of changes in soil parameters on the fatigue lifetime for an offshore wind turbine founded in loose sand. For this purpose, a generic monopile based offshore wind turbine with flexible foundation model was used. The soil-pile interaction was modeled with a distributed spring model using nonlinear API p-y curves. Integrated analyses in the time domain were performed and fatigue damage was assessed in terms of a damage equivalent bending moment at mudline. The fatigue lifetime varies between -9 percent and +4 percent when considering changes in soil conditions, depending on the assumption of soil softening or stiffening, respectively. These results indicate that changes in soil parameters should be taken into account in the fatigue damage calculations of offshore wind turbines for more precise fatigue lifetime estimation. Moreover, it is emphasized that more accurate modeling of soil-pile interaction is required in the design and optimization of offshore wind turbines.

Soil-structure interaction on the response of jacket-type offshore wind turbine

Jacket structures are still at the early stage of their development for use in the offshore wind industry. The aim of this paper is to investigate the effect of the soil-structure interaction on the response of an offshore wind turbine with a jacket-type foundation. For this purpose, two different models of flexible foundation-the p-y model and the p-y model considering pile groups effect-are employed to compare the dynamic responses with the fixed-base model. The modal analysis and the coupled dynamic analysis are carried out under deterministic and stochastic conditions. The influence of the soil-structure interaction on the response of the jacket foundation predicts that the flexible foundation model is necessary to estimate the loads of the offshore wind turbine structure well. It is suggested that during fatigue analysis the pile group effect should be considered for the jacket foundation.

Seismic analysis of offshore wind turbines on bottom-fixed support structures.

This study investigates the seismic response of a horizontal axis wind turbine on two bottom-fixed support structures for transitional water depths (30-60 m), a tripod and a jacket, both resting on pile foundations. Fully coupled, nonlinear time-domain simulations on full system models are carried out under combined wind-wave-earthquake loadings, for different load cases, considering fixed and flexible foundation models. It is shown that earthquake loading may cause a significant increase of stress resultant demands, even for moderate peak ground accelerations, and that fully coupled nonlinear time-domain simulations on full system models are essential to capture relevant information on the moment demand in the rotor blades, which cannot be predicted by analyses on simplified models allowed by existing standards. A comparison with some typical design load cases substantiates the need for an accurate seismic assessment in sites at risk from earthquakes.

The Identification of the Unbalance of a Flexible Rotating Machine From a Single Rundown

The reliable estimation of a flexible foundation model and the state of unbalance (both amplitude and phase) of a turbogenerator from machine rundown measured vibration data is an active research area. Earlier studies on the estimation of both these quantities used the whole frequency range of the rundown as a single band. However, such an identification may be inaccurate for large flexible foundations having many modes in the rundown frequency range. For reliable identification, the whole frequency range has to be divided into a number of frequency bands and the frequency-dependent foundation models have to be estimated together with the unbalance. This paper combines the unbalance estimation with the split frequency range for the foundation model, and the highlights the limitations observed during estimation of foundation models and the state of unbalance. It is shown that the accuracy of the method may be enhanced by judicious choice of a weighting function. Having established the method in simulation, experimental data from a 3-m long test rig, with four journal bearings, is used to test the method. The approach seems to give reliable estimates of the machine unbalance.

Estimating unbalance and misalignment of a flexible rotating machine from a single run-down

Earlier studies have suggested that the reliable estimation of the state of unbalance (both amplitude and phase) at multiple planes of a flexibly supported rotating machine from measured vibration data is possible using a single machine run-down. This paper proposes a method that can reliably estimate both the rotor unbalance and misalignment from a single machine run-down. This identification assumes that the source of misalignment is at the couplings of the multi-rotor system, and that this will generate constant synchronous forces and moments at the couplings depending upon the extent of the off-set between the two rotors, irrespective of the machine rotating speed. A flexible foundation model is also estimated. The method is demonstrated using experimental data from a machine with two bearings and a flexible coupling to the motor. A sensitivity analysis has also been carried out for the proposed approach with perturbation errors in the rotor and bearing models, to confirm the robustness of the method.

교각의 내진설계를 위한 말뚝기초의 모델링 기법 비교

실무에서 교량 교각의 내진해석을 수행할 때 계산의 편의와 보수적인 설계를 위하여 기초의 연성을 고려하지 않고 지반면을 고정단으로 가정하여 해석하는 경우가 많다. 이러한 고정단 모델은 대부분의 경우 설계력 측면에서 매우 보수적인 결과를 주므로, 최근 기초의 연성을 고려하여 현실적으로 교각 기초를 모델링하려는 연구가 수행되고 있다. 본 연구에서는 기초연성의 모델링 방법, 입력응답스펙트럼의 종류 그리고 지반조건을 달리하여 응답스펙트럼 해석을 수행하였다. 수치해석을 통하여 교각에 발생하는 전단력, 모멘트 그리고 변위를 비교하여 기초의 연성을 고려하는 기법과 지반 및 지진파 특성들이 해석결과에 미치는 영향을 분석하였다. 대부분의 경우 고정단 모델에서 큰 전단력과 모멘트가 발생하였지만, 연약 점성토층에 장주기 지진파가 작용하는 특수한 경우에는 기초의 연성을 고려한 모델에서 고정단 모델보다 큰 전단력과 모멘트가 발생하였다. 또한 기초의 연성을 고려하는 여러 모델들의 해석결과를 지반-구조물 동적 상호작용을 고려한 정밀 동해석 결과와 비교하여 그들의 적용성을 평가하였다. In the seismic designing of bridges, the pile foundation of bridge piers generally have been modeled to have a fixed end for its convenience and conservative designing. The fixed-end assumption, however, produces very conservative results in terms of the pier forces. Therefore, many other design methods are evolved to consider the flexibility of the pile foundation. In this study, the response spectrum analysis was performed for a bridge pier having a pile foundation. The shear force, moment, and displacement, which occurred at the pier column under an earthquake loading, were compared to analyze the effects of the modeling method, soil condition and the input earthquake response spectrum. In most cases, the fixed-end model gives larger design forces than flexible foundation models. However, when a long period earthquake is applied to the bridge pier on a soft clay foundation, it is found that the flexible foundation models give larger design forces than the fixed-end model. In the end, the reliability of several flexible foundation models was verified by comparing their results with those of a numerical analysis that considers the soil-structure interaction phenomenon in a rigorous manner.

THE IDENTIFICATION OF THE UNBALANCE AND THE FOUNDATION MODEL OF A FLEXIBLE ROTATING MACHINE FROM A SINGLE RUN-DOWN

The reliable estimation of a flexible foundation model and the state of unbalance (both amplitude and phase) of a turbogenerator from machine run-down measured vibration data is an active research area. Earlier studies on the estimation of both these quantities used the whole frequency range of the run-down as a single band. However, such an identification may be inaccurate for large flexible foundations having many modes in the run-down frequency range. For reliable identification, the whole frequency range has to be divided into a number of frequency bands and the frequency-dependent foundation models have to be estimated together with the unbalance. This paper combines the unbalance estimation with the split frequency range for the foundation model, and highlights the limitations observed during the estimation of foundation models and the state of unbalance. A simulated example is used to validate and assess the advantages of the proposed method. This method is compared to the previous method using the whole frequency range as a single band. Having established the method in simulation, experimental data from a 3m long test rig, with four journal bearings, is used to test the method. The approach gives reliable estimates of the machine unbalance, even in the presence of modelling errors and measurement noise.