Response Of Offshore Structures Research Articles

Experimental Characterization of Stiffness of a Polyester Mooring Rope for a CFPSO

Synthetic ropes are increasingly favored for use in offshore systems. Accurate predictions of the coupled hydrodynamic performance and structural response of offshore structures depends on a thorough understanding of the nonlinear characteristics of fiber materials. The objective of this study is to experimentally characterize the stiffness of a polyester mooring rope for a cylindrical floating production storage and offloading system. The quasi-static stiffness of the ropes and aged ropes after installation and the dynamic stiffness in various loading conditions were computed based on sub-rope tests following the guidelines from the American Bureau of Shipping. The quasi-static stiffness curves exhibited a linear decrease in values as the logarithm of the loading period (in minutes) increased. The dynamic stiffness was, in general, much higher than the quasi-static stiffness. The dynamic stiffness under various loading conditions revealed the complexities of the mechanical properties of polyester rope. Parameters such as the mean load, load amplitude, loading period, and loading cycles all had a notable impact. More tests are required to have a better understanding of their effects.

WAVE SPECTRUM ANALYSIS FOR OPERATIONAL OFFSHORE PLATFORM IN MALAYSIA WATER

As an ageing platform has passed its design life of 20 to 30 years, it is crucial to identify wether it is still fit for operation based on the current environmental behaviour, particularly due to the wave impact. Hence, an analytical approach of wave spectrum analysis between the wave behaviour and offshore structural response will be conducted. The aim is to verify the stability and safety of ageing offshore platforms with the current wave behaviour based on identifying wave characteristics, developing a normalized wave energy spectral density, and analyzing the natural frequency of offshore structural response. The methodology includes data collection from structural dynamic monitoring assessments, wave spectrum development using empirical models, and evaluation of offshore platform stability. The results will provide insights into platform stability, resonance risks, and safety assessment of ageing offshore platforms in Malaysian waters. The findings will contribute to recommendations for modifications or improvements to ensure the safety and integrity of offshore platforms in the specific region.

The SPAR Model: A New Paradigm for Multivariate Extremes: Application to Joint Distributions of Metocean Variables

Abstract This paper presents the application of a new multivariate extreme value model for the estimation of metocean variables. The model requires fewer assumptions about the forms of the marginal distributions and dependence structure compared to existing approaches, and provides a flexible and rigorous framework for modeling multivariate extremes. The method involves a transformation of variables to polar coordinates. The tail of the radial variable is then modeled using the generalized Pareto distribution, with parameters conditional on angle, providing a natural extension of univariate theory to multivariate problems. The resulting model is referred to as the semi-parametric angular-radial (SPAR) model. We consider the estimation of the joint distributions of (1) wave height and wave period, and (2) wave height and wind speed. We show that the SPAR model provides a good fit to the observations in terms of both the marginal distributions and dependence structures. The use of the SPAR model for estimating long-term extreme responses of offshore structures is discussed, using some simple response functions for floating structures and an offshore wind turbine with monopile foundation. We show that the SPAR model is able to accurately reproduce response distributions, and provides a realistic quantification of uncertainty.

A new metamodel for predicting the nonlinear time-domain response of offshore structures subjected to stochastic wave current and wind loads

Offshore structures are subjected to stochastic loads from wave, current, and wind. A long-term response analysis is important for evaluating the structural safety; however, it is computationally demanding to perform the large number of nonlinear time-domain simulations. Metamodeling is an attractive prospect, but there are two existing obstacles to overcome. First, existing metamodels are mostly confined to a single sea state and are unable to predict the response for different wave directions. Second, for floating systems, the inputs are usually vessel loads/motions, which are a priori unknown. This paper develops a new metamodel framework for accurately predicting the time series response under short-term uncertainties from the stochastic process, and seven long-term variables, namely the significant wave height, spectral period, wave direction, and speeds and directions of current and wind. The exogenous input is based only on the environmental variables, which can be predefined. The proposed metamodel is implemented on a floating system and used to predict the time series of riser stress, the extreme stress response and fatigue damage. Four cases with increasing number of long-term variables are tested. Excellent agreement is found between the metamodel prediction and numerical simulation even for the most critical case with all variables included.

Used Isolation System to Reduce the Response of Offshore Structure Considering Soil Structure Interaction

Abstract Fixed offshore platforms are structures of steel or concrete located in the middle of the sea used to export oil, trade, or military. These are designed to withstand all types of weather and dynamic loads. The response was very high under applied these loads. This paper includes a proposal to apply a new isolation system to be placed in different places on the offshore platform to reduce vibrations from high water waves. Three cases that be used, fixed-based (column connect directly to the pile), isolation system in the pile head (isolation between column and pile), and isolation system in the pile head and the middle of columns. The dynamic movements in waves were simulated using ABAQUS programs. More accurate elements and real material properties were used to bring the results closer to reality. The Computational Fluid Dynamics (CFD) method was used to study the effect of water-structure interaction and soil-structure interaction. The results show that the use of the isolation system at the base of the offshore platform has a very significant effect on decreasing the response of the platform to the loads placed on it. Where the isolation system works as a hinge at the pile head and is not allowed to move and rotate for the pile, also the moments are zero. As for the shear force, it is the least that can be compared with the other cases.

Uncertainty quantification in low-probability response estimation using sliced inverse regression and polynomial chaos expansion

For wave energy converters (WECs), wind turbines, etc., estimation of response extremes over a selected exposure time is important during design. Sources of uncertainty arising from background slowly-varying environmental conditions and from shorter time-scale fluctuations in ocean winds, turbulence, etc. must all be considered. Together, these different sources can comprise a high-dimensional vector of stochastic variables (often on the order of hundreds or thousands). To accurately propagate the influence of these uncertainty sources to model outputs, conventional surrogate model building approaches such as polynomial chaos expansion (PCE), stochastic collocation, low-rank tensor approximations, etc. must consider dimension reduction. We explore the use of sliced inverse regression (SIR) combined with polynomial chaos expansion. SIR first reduces the original high-dimensional problem to a low-dimensional one; then, an optimal polynomial PCE model is proposed and applied on “effective” components in the low-dimensional space. SIR-PCE can mitigate the curse of dimensionality. It is employed here in the prediction of the long-term extreme response of offshore structures; it is demonstrated using classical benchmark analytical functions as well as offshore applications including extreme waves and the response of a wave energy converter. Efficiency and accuracy gains over Monte Carlo simulation and other methods in literature are found.

Prediction of Long-Term Offshore Structural Responses Based on Nonlinear Wave Modeling

Wind-generated random wave loads are the dominant loads to consider for maintaining the reliability of fixed offshore structures. Based on probabilistic techniques, the inherent randomness of the wave loading can be used to predict extreme offshore structural response, which is Gaussian in nature. However, researchers have found that the hydrodynamic component and structural dynamics substantially impact the frequency spectrum, leading to a non-Gaussian stochastic offshore structural response. A finite-memory non-linear system (FMNSNL) has been proven to be an efficient approach to evaluate the non-Gaussian stochastic offshore structural response compared to the conventional method, Monte Carlo time simulation. However, the analysis has been conducted based on short-term distribution only. The most satisfactory analysis is based on long-term distribution. Hence, further investigation in this paper will evaluate the long-term probability distribution of extreme offshore structural responses. As a result, a 100-year extreme offshore structural response prediction achieves 80% to 96% accuracy compared to the Monte Carlo time simulation. The probability distribution has been evaluated using the Gumbel distribution function throughout this investigation. Still, there is a little deviation at the tail end of the distribution between the simulated response values and the fitted line. A different distribution function, such as the Generalised Extreme Value (GEV) distribution, is advised for future work.

Seismic response and liquefaction characteristics of wide and shallow multi-bucket jacket foundation under scour conditions

Scour has a significant effect on the seismic response of offshore structures. Compared to monopiles, MBJFs are shallow foundations and scouring has a more detrimental effect on their earthquake performance. However, the combined impacts of scour and earthquake on the structural response and liquefaction characteristics of the MBJFs have not been completely investigated. Based on the secondary development of the ABAQUS program, this paper proposes a method for calculating the seismic response and liquefaction of MBJF under local scour conditions. The seismic response and liquefaction characteristics of MBJF-supported OWTs in silty soil are evaluated under different scour depths, scour ranges, earthquake motions, soil conditions, and structural damping. In comparison to the unscoured condition, the results show that the maximum horizontal displacement, lateral acceleration, and bending moment of MBJF-supported OWTs increase by 30%, 97%, and 58%, respectively. Liquefaction is generated mostly in the soil outside the bucket rather than inside. An increase in soil density can improve the lateral acceleration of the structure and reduce the proportion of liquefaction in the soil. The maximum reduction in liquefaction can be 70.2% for an increase of 23% in soil densities. Disregarding structural damping will overestimate the structural response and the proportion of soil liquefaction. The findings may shed light on the seismic response and liquefaction characteristics of MBJF-supported OWTs located in complex offshore scour environments.

Effects of constitutive soil models on the seismic response of an offshore jacket platform in clay by considering pile-soil-structure interaction

Offshore jacket platforms are widely used for wind energy generation and gas and oil field development. The South Pars gas field, located in the Persian Gulf, stands as one of the largest gas fields in the world. This gas field is situated in a seismically active region predominantly characterized by clay soil. Considering the significant impact of clay soil on the seismic response of offshore structures and their vulnerability to damage, it is essential to conduct comprehensive studies on the seismic behavior of offshore structures in this particular area while considering the nonlinear behavior of soil and pile-soil-structure interaction (PSSI). However, due to the laboratory limitations, it is difficult to experimentally investigate the seismic behavior of these structures on a real scale by considering the actual soil environment. This paper aims to adopt a nonlinear kinematic hardening model for clay in Abaqus software and calibrate this model parameters for the South Pars gas field region to carefully investigate the effects of nonlinear behavior of soil and PSSI on the seismic response of an offshore jacket platform. In addition, the perfectly elasto-plastic Mohr-Coulomb model is employed as a simple and common constitutive soil model to compare the results. To achieve the study goals, firstly, the numerical modeling techniques are validated. Subsequently, the main parameters of the nonlinear kinematic hardening model are calibrated for the South Pars gas field region, which has not been done for this region in previous works. Detailed numerical models, including the fixed-base model and the pile-soil-structure (PSS) model, are then developed and analyzed using different selected ground motion records. The results indicate that considering the effect of PSSI has a significant effect on the seismic responses of the structure. Moreover, PSSI effect has a great dependence on the constitutive soil model. The results of this study are expected to provide useful insights into the seismic design of offshore structures in similar geological conditions.

A Numerical Study of Brace-Viscous Damper System of Fixed Offshore Jacket Platforms Under Extreme Environmental Loads

The Persian Gulf is one of the most common regions where offshore platforms exist due to the presence of oil and natural gas.Wind, current, and wave loading affect the dynamic response of offshore structures, increasing performance uncertainty and catastrophic failure probability. Thus, this study investigates energy dissipation systems, particularly viscous dampers, to solve design and rehabilitation problems of fixed offshore structures. Viscos dampers improve vibrational behaviour and reduce structural response to optimise platform performance. Thus, eliminating costly structural repairs and strengthening components under extreme environmental loads extends structure lifetime. Thus, different viscous damper system configurations are tested to reduce dynamic response under extreme environmental loads in the Persian Gulf. Diagonal and inverted V-shaped brace viscous dampers with ninedifferent arrangements are compared to find the best configuration. The study found that the brace viscous dampers with only three applied dampers at the top three levels are most efficient at mitigating dynamic response. It reduced displacements from level 1 to level 5 by 52% to 64% and connection stresses by 38% to 54%. Finally, viscous dampers reduce structural vibration and provide uniform and constant structural dynamic response, so the oil and gas industry should use them for offshore structure design and rehabilitation.

A developed model updating method based on extended frequency response functions and its application study of offshore structures

Accurate finite element (FE) models are extremely vital to the analysis of the dynamic characteristics of engineering structures. However, it is challenging for the frequency response function (FRF)-based model updating method to obtain reliable numerical results as the limited number of FRFs at the selected peak positions usually leads to an unacceptable or failure of model updating. Moreover, the increased number of updating coefficients in complex engineering problems generates nonlinear optimization models with the local solutions by traditional optimization techniques. To tackle these two issues, a new model updating method is proposed to make full use of FRFs extracted from measured field data of structures and the improved particle swarm optimization (PSO) technique for accurately estimating physical parameters of structures. The novelties of this research include: (1) The signature assurance criterion (SAC)-based FRF is evaluated to eliminate the influence of limited frequency points on the accuracy of the updated model using the normalized acceleration component; (2) The enhanced (PSO) is developed to realize the adaptive selection of inertia factors for the better diversity by introducing an average value of the fitness function, and then accurate predictions of updating coefficients within a smaller number of iterations are achieved using the developed constraint factor. The effectiveness of the proposed method is verified by mathematical model of a jacket platform. Results show that the proposed method can accurately obtain the updating coefficients under spatial incomplete condition, and the maximum error of natural frequencies is 0.779% using the accelerations containing 5% noise. To prove the robustness of the proposed method, experimental studies of monopile offshore wind turbine are also conducted and the maximum error of natural frequencies is 1.887% under the consideration of spatial incompleteness represented by the structural stiffness degradation. Finally, the feasibility of the proposed method is evaluated by a test of a complex jacket platform, whose variation is simulated by weakening the connections of some elements. The maximum error of natural frequencies predicted by the updated model is only 4.831% as compared with experimental results. Throughout these examples, the extended FRF model updating method provides engineers and designers with a useful insight into the development of reliable techniques to accurately predict dynamic responses of offshore structures.

Damage assessment of concrete structures subjected to internal explosion under air-backed and water-backed conditions

There has been an increasing concern about the dynamic response and damage assessment of offshore structures under blast loads due to accidents and terrorist attacks. These concrete structures will face severe threats of internal explosions from powerful bombs. After the internal detonation of a bomb, the blast wave in the concrete structure will gradually propagate to two types of fluid-structure interfaces: the air-concrete interface or the water-concrete interface. The purpose of this paper is to investigate the nonlinear response and damage assessment of the concrete structures to internal explosion considering air-backed and water-backed conditions. Two types of blast tests are conducted to obtain the damage mechanisms and degrees of the concrete targets under internal explosion conditions. A fully coupled numerical approach is used to model the internal explosion and the fluid-structure interaction, and it is validated through comparing the test and the numerical results. The shock-wave and damage propagation of the concrete targets under air-backed and water-backed conditions are compared. The damage ranges of the concrete targets with various explosive weights, buried depths, and fluid-structure interfaces are presented. Its results are used to propose a damage assessment method for the concrete structures to internal explosion under air-backed and water-backed conditions.

A Parametric Study on Effect of Wave Height, Water Depth and Support Conditions on Behaviour of Offshore Jacket Structure

Fixed offshore jacket structures are constructed for facilitating oil/gas exploration and production. These structures and their foundations are designed to resist large vertical and lateral loads. Various factors including water depth, wave height and support conditions would affect the response of jacket structures. However, few studies have focused on understanding the response of offshore jacket structure due to variation in these factors. In this context, the present work evaluates response of typical X-braced, square base, 4-legged battered jacket structure using STAAD Pro. under combined vertical and lateral environmental loading. The variables included are water depth (60 m, 90 m and 120 m), wave height (5 m, 10 m and 15 m) and two foundation modelling approaches (viz., fixed at pile location and with defined pile stiffness). The connection between structure leg and pile location is modelled to simulate realistic connection. Deck loads, wind forces and current velocities are considered constant for this study. The wind and wave loads have been applied in parallel, perpendicular and diagonal directions with respect to jacket structure. The wave forces are calculated by Morrison’s equation. For obtaining pile stiffness, medium dense sand layer is considered as foundation soil. The increase in water depth and wave height results in corresponding linear increase in lateral deflection and support reactions, the effect of water depth being more prominent. Moreover, lateral and vertical deflection, shear force and bending moment in the legs, the axial forces in the lower tie beams and plan bracings, and support moments are observed to increase when pile locations are modelled with appropriate vertical, lateral and rotational stiffness instead of fixed support. The effect of water depth on member forces is higher as compared to wave height. The present work deliberates on the mechanisms/reasons to explain the observed results and contributes in direction of framing decision matrix for design optimization of jacket structures.

Novel reliability method validation for offshore structural dynamic response

The paper validates novel structural reliability Gaidai-Fu-Xing (GFX) method, particularly suitable for multi-dimensional structural responses, versus well established bivariate statistical method, that is known to accurately predict two-dimensional system extreme response levels. Classic reliability methods, dealing with time series do not have an advantage of dealing easily with system high dimensionality and cross-correlation between different dimensions. An operating Jacket located in the Bohai bay was taken as an example to demonstrate the proposed methodology. Novel state of art bimodal extrapolation technique was applied to predict system reliability with five years return period, which is of practical importance for design of fixed offshore structures.Unlike other reliability methods the new method does not require to re-start simulation each time system fails, in case of numerical simulation. In case of measured structural response, an accurate prediction of system failure probability is also possible as illustrated in this study.Jacket offshore platform subjected to large environmental wave loads, thus structural stresses in different structural critical locations were chosen as an example for this reliability study. The method proposed in this paper opens up the possibility to predict simply and efficiently failure probability for nonlinear multi-dimensional dynamic system as a whole.

Effects of Parameters on Non-Linear Seismic Analysis of Fixed Offshore Platform

Fixed offshore structures in seismically active zones are likely to have deformations large enough to exceed the elastic limits of materials under strong ground motions. Therefore, investigation of nonlinear response of offshore structures under dynamic loadings is very interesting to structural engineers. Nonlinear behavior of structures is highly related with the soil properties and soil behavior. The main purpose of this study is to investigate and show the methodology for inelastic dynamic analyses of fixed-offshore structures under the 10,000-year earthquake loadings. The investigation in this study includes the effect of soil radiation damping, soil-pile gapping, and soil degradation on fixed offshore structures. Additionally, the effect of the direction of earthquake is included in this study. An offshore structure model ‘Platform A’ is modeled in CAPFOS. 10,000-year earthquake events are considered and for this purpose, two different earthquakes are used: Hector Mine Earthquake (1999) and Landers Earthquake (1992). These ground motion records are scaled to have a resulting earthquake that has a return period of 10,000-year. CAPFOS software is used to complete time-history analyses with these earthquakes. Results indicate that: (i) Considering the soil radiation damping negligible is not correct, because without the radiation damping, the total damping ratio is estimated as % 3, while including radiation damping raises the ratio to % 4.5, (ii) soil-pile gapping and soil radiation damping generate adverse effects on analyses outcome, and (iii) using high values of soil degradation factors is not always conservative.

An accurate and efficient time-domain numerical model for simulating water-axisymmetric structure subjected to horizontal earthquakes

The seismic response of offshore structures is generally influenced by water-structure interaction. This study focuses on the earthquake-induced hydrodynamic forces on axisymmetric offshore structures. Based on the finite element method, an accurate and efficient time-domain numerical model for simulating water-axisymmetric structures subjected to horizontal earthquakes is developed. First, according to interface boundary condition and using the method of separation of variables, the three-dimensional (3D) wave equation governing the compressible water is transformed into a two-dimensional (2D) governing equation in the vertical and radial directions, where the solution in the circumferential direction is analytical. Secondly, an exact artificial boundary condition (ABC) in frequency domain is developed by using the separation of variables method. Third, a high-accuracy ABC that is local in time but global in space is obtained by applying the temporal localization method. Furthermore, the high-accuracy artificial boundary condition is discretized and coupled with the finite element equation of the near field, leading to a symmetric second-order ordinary differential equations. Finally, the coupled finite element equation for the water-axisymmetric cylinder is presented. The proposed method is used to study the effect of water compressibility on the seismic response of and evaluate the seismic responses of a composite bucket foundation.

Rapid decoupling Laplace-domain algorithm for dynamic response estimation of offshore structures with asymmetric system matrices

The vibrating differential equation of offshore structures with asymmetric system matrices cannot be decoupled using normal or complex modes. The dynamic response analysis of offshore structures is employed only using the frequency-domain method or step-by-step integration method, owing to which we cannot avoid the limitations of those methods, such as relying the on calculation time step, low calculation efficiency, and steady-state response analysis. To address these problems, a Laplace-domain algorithm based on the poles and corresponding residues of a decoupled vibrating system and exciting wave force is proposed to deal with the dynamic response analysis of offshore structures with asymmetric system matrices. A theoretical improvement is that the vibrating equation with asymmetric system matrices is decoupled by the lower parts of the left and right eigenvectors, providing the possibility of solving the vibrating equation based on the decoupled equations. Meanwhile, this method analyzes the dynamic response of offshore structures in the Laplace domain, indicating that the proposed method is insensitive to the calculation time step and has a higher calculation accuracy. Considering that this algorithm can calculate the transient response caused by the initial conditions, the wave force can be divided into a series of small segments, and the dynamic response is connected by the initial conditions of the small segments, thereby considerably improving the computing efficiency of the dynamic response analysis. Three test cases were applied to evaluate the performance of the proposed algorithm. The results show that (1) the proposed algorithm can estimate the dynamic response of offshore structures with asymmetric system matrices both in the process of transient and steady-state response analysis; (2) the computational accuracy of the proposed algorithm is insensitive to the calculation time step, which has a more stable calculation result compared with the Newmark-β method; and (3) the proposed algorithm has a higher calculational efficiency owning to the transient analysis capability.

Efficient derivation of extreme non-Gaussian stochastic structural response using the finite-memory nonlinear system (FMNS NL ). Part 1: model development

ABSTRACT For offshore structural design, the load due to wind-generated random waves is usually the most important source of loading. The most accurate method that is commonly used to evaluate the offshore structural response on the subjected load is the Monte Carlo time simulation which can account for all sorts of nonlinearities without introducing any approximations. However, it is computationally very demanding due to its complex procedure. A simple method using finite-memory nonlinear system (FMNS) has been introduced. The method is, however, only applicable based on the linear wave analysis. Hence, by taking advantage on the efficiency of FMNS method, a new model needs to be developed by apply the FMNS method with a nonlinear wave analysis for a more reliable result. The most appropriate model can then be used to determine the probability distribution of response extreme values with great efficiency and accuracy.

Methodology for developing a response-based correction factor (alpha-factor) for allowable sea state assessment of marine operations considering weather forecast uncertainty

The concept of the alpha-factor, a correction factor on the significant wave height limit, was developed by DNV to consider the effect of weather forecast uncertainty in planning and executing marine operations. In this paper, a new defined response-based correction factor, called the response-based alpha-factor αR, is proposed to account for the forecast uncertainty of both significant wave height Hs and peak wave period Tp, and quantify their effect on dynamic response of offshore structures. A general methodology for developing αR for the use in assessing allowable sea states for marine operations is presented, with emphasis on considering the effect of the weather forecast uncertainty. It consists of uncertainty quantification of the sea state forecast, statistical analysis of dynamic responses of the coupled system for marine operations and allowable sea state assessment using response-based criteria. Based on the methodology, αR for an operation can be derived, in terms of the ratio between the characteristic values of the operational limiting response parameter in the condition with or without the weather forecast uncertainty. Then, the allowable sea states can be assessed in terms of the forecast lead time, to include the effect of the weather forecast uncertainty on the operation decision-making. The workable weather windows can finally be identified and selected through a comparison between the allowable sea states and weather forecasts in the execution phase. Followed by the detailed description of the proposed methodology, a case study dealing with single blade installation for offshore wind turbine using a semi-submersible crane vessel is conducted. The crane tip motion is regarded as the operational limiting response parameter to illustrate the methodology based on frequency-domain response analysis approach. The uncertainty in sea state forecasts generated by two machine learning-based forecasting methods is considered in this paper. Results show that in addition to the forecast uncertainty of Hs, that of Tp also requires to be addressed for marine operations with floating structures (such as the semi-submersible crane vessel). The proposed method provides an efficient way to incorporate the weather forecast uncertainty into the allowable sea states assessment for marine operations. The obtained allowable sea states can provide a good reference for the decision-making of the operation in the execution phase.

Damage detection for offshore structures using long and short-term memory networks and random decrement technique

A damage detection method is presented which combines the random decrement technique (RDT) with long and short-term memory (LSTM) networks. The method uses the measured vibration response of offshore structures subjected to random excitation and is able to locate and assess the damage with accuracy, even in noisy conditions. The applicability of the proposed RDT-LSTM method is verified through a numerical example and laboratory tests. The numerical example consists of a jacket platform subjected to random wave excitation. The simulated damage cases encompass single and multiple damage locations not only on whole segments but also on local elements (one-fifth of the whole segment) of the numerical structure, with minor (1%–5%) severity, and different noise levels. RDT is applied first to process the noisy random data, and then the damage detection is carried out using LSTM. After the numerical example, the proposed method is applied to laboratory tests of a jacket platform model under random loading produced by a shaking table. Minor and major damages and their combination at different locations are discussed. Both the numerical simulation and laboratory test show that the proposed RDT-LSTM method has an outstanding performance in structural damage detection.