1. Introduction
Membrane covers are commonly used as floating covers for clean water reservoirs to prevent evaporation and pollution; for landfills to trap hazardous chemicals and unpleasant odour; and for tailing impoundment [
1,
2,
3]. Floating cover materials such as high-density polyethylene (HDPE) geomembranes are very durable, resistant to many different solvents, and have a high strength-to-density ratio, and if well-designed, long service life (in decades) even in harsh environments [
2,
3]. The Western Treatment Plant (WTP) at Werribee, Victoria, Australia, installed two floating covers to assist with the anaerobic treatment of the raw sewage beneath them, leading to the production of methane-rich biogas, see
Figure 1. The WTP floating covers are made from a 2 mm thick HDPE spanning an area of 450 m by 170 m. The WTP cover is held around its perimeter by clamping strips and mechanical fasteners to provide an airtight seal. In addition to controlling odour, these covers are capable of collecting up to 65,000 m
3 of biogas per day that can generate up to 7 MW of electricity, thereby increasing the treatment plant’s supply of renewable electricity from this sustainable source. Without this cover, the methane-rich biogas would be released into the environment as a damaging greenhouse gas. All sewage inflow is unscreened and passes first through an anaerobic reactor where biogas is produced as the raw sewage undergoes anaerobic digestion, which is trapped below the floating cover and harvested for electricity generation. Solidified sewage substance can amass on the reactor surface to form a
scum-berg which presses against and elevates the covers to a height of approximately 1 metre. The scum-berg can cause lateral displacement, thus causing changes to the stress on the cover, which is partially due to hydrodynamic effects resulting from the sewage inflow. The movement of sewages and the presence of methane-rich biogas require that the proposed inspection methodology is intrinsically safe and, therefore, underpins the requirement of a proactive and effective inspection approach that ensures the integrity and continued operation of this asset. However, for a structure of this size, the definition of its state of stress is currently beyond the capability of current measurement techniques and, hence, requires a more robust approach.
Over the past decades, the acquisition of full-field strain measurement has been an active field for structural health monitoring (SHM). The well-established non-contact strain measurements, such as digital image correlation (DIC), have been extensively investigated and have achieved a high degree of accuracy, primarily for in-plane strain measurement for various applications. However, in using the DIC method at large deformation, the uncertainty in strain measurements increases significantly as the speckle pattern degrades, and decorrelation occurs at such high strain levels [
4,
5,
6,
7,
8]. A more precise full-field strain determination could be achieved by a 3D DIC method for non-planar specimens with out-of-plane displacement and rotation using two synchronised cameras, and recently, one single camera [
9,
10,
11,
12]. However, this method is often limited by the fixed and small depth of field and highly controlled testing environment [
9,
13]. Nevertheless, it is known that the ability to accurately predict the strain field from 3D deformation is significantly challenging, especially for large structures in its operating environment. Recently, photogrammetry has become one of the commonly available non-contact and full-field measurement methods for SHM [
14,
15]. This method, with the aid of photogrammetric software, constructs a digital elevation model (DEM) from sequential 2D digital photographs. Baqersad et al. [
16,
17,
18,
19] integrated photogrammetry to predict the 3D dynamic strain experienced by the blade of a wind-turbine using finite element analysis. Pappa et al. [
20] used optical techniques with reflective markers and projected dots to define the dynamic motion of membranes. Recently, Luo et al. [
21] have investigated the concept of using photogrammetry to measure strain fields of deformed large inflatable structures by combining Delaunay triangulation and finite element methods. However, quantitative evaluations, such as strain and stress measurements, based on photogrammetry are still in their infancy, where many previous studies relied on the monitoring of surface fiducial markers, primarily focused on shape measurement of structural surfaces [
21,
22,
23,
24,
25,
26,
27].
Unmanned aerial vehicle (UAV) is a generic uninhabited aerial vehicle that is remotely controlled either manually or autonomously. The ability to install non-contact measurement capabilities on unmanned aerial vehicles (UAV) is creating more rapid and safer opportunities in SHM of large structures such as aircraft wings, dams, vertical tanks, and elevated highways. Recently, there has been significant interest in using UAV systems coupled with photogrammetry to rapidly acquire high temporal and spatial resolution image information for 3D modelling and various disciplines [
28,
29,
30,
31,
32,
33]. This UAV photogrammetry system provides a low-cost commercially available or customisable practical alternative in the close-range aerial domain. This system can be deployed in high risk and inaccessible areas, where it is considered too difficult or impossible for traditional methods and without endangering human life or safety [
29]. Furthermore, this would lead to a more time-efficient inspection process for large areas which include monitoring archaeological sites, environmental and agricultural areas, and traffic [
29,
31]. Despite the advantages in UAV photogrammetry, it is limited by its relatively short flight time and long computational processing due to a significantly large dataset.
In this collaborative project, a single camera mounted on a UAV-based photogrammetry system is deployed as a wide coverage non-contact measurement diagnostic tool to monitor the state of deformation of the WTP membrane covers. The research project proposed to build upon the practical diagnostic tool towards an innovative smarter structure and maintenance, reflecting the digital twin paradigm [
34]. One of the research project objectives is to devise a two-stage global-local monitoring strategy where UAV photogrammetry monitoring is first deployed to rapidly evaluate the global strain response of the cover, thereby identifying critical areas where, subsequentially, a localised-detailed inspection is conducted for further quantitative assessment. This two-stage strategy aims to significantly reduce time and cost for inspection of the entire cover and mitigates workers’ exposure to a high-risk environment. With the current digital deformation and fiducial features on the WTP membrane cover, it is highly advantageous to acquire the strain field of the membrane for a more meaningful measurement of the asset. However, the floating covers DEMs are highly subjected to artefacts from debris, lighting, and weather. Nevertheless, strain fields are extremely difficult to obtain from small measurements difference and as it is derived from displacement information, it requires highly accurate displacement readings [
26]. Thus, the raw DEM requires significant preprocessing prior to further analysis. Furthermore, the WTP regulations, such as flight restriction (minimum and maximum height and duration) and the fact that coating or physical attachment on the cover are not feasible options, limit the ability to enhance the accuracy of acquired measurements through other means, making this problem-specific application significantly challenging.
Our previous works have shown the application of UAV photogrammetry systems into finite element (FE) analysis for strain determination and, furthermore, on-the-field work on the optimisation of flight parameters, which includes overlapping of photos, flight path, and altitude [
28,
35,
36,
37]. In this paper, experimental studies are conducted where UAV photogrammetry is deployed to construct the DEMs of two deformed membrane specimens in a controlled laboratory setting, as a continuation of our investigations on reliable SHM of the WTP floating covers. The DEMs are then preprocessed using a smoothing interpolation with different smoothing parameters and applied as displacement loads for FE analysis to predict its displacement and strain field (see schematic in
Figure 2). As uncertainty and erroneous measurements inevitably arise from all stages of the procedure (data collection and preprocessing of data), a probabilistic approach is considered to provide a statistical prediction of the strain field and its degree of certainty. Hence, the resulting smoothed strain fields are then utilised as training data for a variational inference method approach for heteroscedastic modelling, based on Gaussian process (GP) regression. The findings give insights into the development of a strain monitoring strategy tailored for WTP floating covers and in aiding the decision support for WTP floating cover management.
2. Method
2.1. UAV Photogrammetry Setup and DEM
An unmanned aerial vehicle DJI Spark with an integrated camera was deployed for all the laboratory studies. The 3D DEMs of the membrane are developed by sequential 2D images from the mounted digital camera with pre-determined flight paths. The specification provided by the manufacturer is listed in
Table 1.
Metashape Professional by Agisoft [
38] was used to perform image alignment, construction of dense clouds, mesh, and DEM Agisoft adopted the computer vision algorithms (i.e. Structure from Motion ‘SfM’ and bundle adjustment), described in Barazzetti [
39], Westoby [
40], and Triggs [
41], to develop an autonomous adjustment, realignment of multi-images, and reconstruction of a 3D model with high redundancy. Dominic [
42] also reviewed the basic principles of the modern photogrammetric and procedures of conducting a photogrammetry approach. All the captured images were loaded with their corresponding metadata (e.g., GPS location and camera setting) to Metashape Professional for generating the DEM. All parameters (including alignment, dense cloud, and mesh generation) were set to the highest available setting to construct accurate DEMs.
2.2. Noise Filtering of DEM
All DEMs contain random errors and artefacts and their accuracy depends on the topography, DEM generation method, and resolution [
44,
45,
46,
47]. Maps and models derived from raw noisy DEM are usually readable after some filtering postprocessing of data to filter noise and improve the data quality. It is important to obtain a high-quality DEM with minimal noise in order to accurately obtain the strain field. Common techniques such as attenuating high-frequency noise in DEM can be performed by spatial filtering based (low-pass filter) on a 2D Fourier transform [
44]. Other techniques such as 2D discrete wavelet transform, cutting methods, and smoothing can be found in previous literature [
46,
48]. The DEM smoothing technique, which includes moving average smoothing, median smoothing, etc., is one of the most popular approaches for DEM denoising and widely used in DEM-based geology studies [
45,
49]. One example is the moving average method, which smooths data, where the size of a moving window and the number of smoothing iterations are manually selected. However, this approach has been used with caution as it may produce undesirable high-frequency artefacts. A median smoothing filter, which performs through the signal entry by entry, replacing each entry with the median of the neighbour entry, is also a well-known nonlinear noise filtering process to filter impulsive-like (speckle) noise. However, an additional scheme is required when processing entries at or near the boundary. Cubic smoothing splines interpolation is an alternative approach that embodies a surface fitting technique to create a smooth model of a complex profile while removing noisy data and avoiding Runge’s phenomenon. In this study, the cubic smoothing splines technique is used to preprocess the raw photogrammetry DEM.
In general, the fitted curve spanning each data interval is presented by a cubic polynomial with the endpoints of the adjacent polynomials matching in location and the first and second derivative [
50,
51,
52]. In its basic form, the objective is to minimise the square error and the curvature; refer to Equation (1) for the 2-dimensional cubic smoothing splines form.
where
and
are the data coordinates and
are cubic polynomials, and the weighting parameter
balances the two countervailing constraints and results in a smoothing cubic spline. Succinctly, if
, the smoothing spline passes through all data points resulting in an interpolation spline, and if
, only the curvature of the spline is minimised and hence results in a linear least-squares fit. It should be noted that there are other modified versions of the cubic spline smoothing which include piecewise constant weight function in the curvature measurement and error measure weights. However, for simplicity, only the basic form where
is constant for all directions is considered in this current work.
2.3. Material and Experimental Setups
In this study, HDPE materials from Melbourne Water’s supply were used. A preliminary investigation of a static pull test using static INSTRON 33R 424 was conducted on six HDPE specimens to validate the material properties, Young’s modulus of 125 MPa, Poisson’s ratio of 0.42 and stress and strain curve. Two HDPE materials of 2 mm thickness with different geometry sizes were used for two experimental investigations to represent two common deformations of the membrane cover: out-of-plane (OOP) deformation and in-plane deformation (wrinkle formation). The surface of each test specimen is marked with an internal grid with the discretisation of 20 mm to represent the target features. The test specimens are mounted on a test frame rig, refer to
Figure 3a, and reference points are marked on the frame to align the DEMs to a 3D coordinate system. Rectangular wood blocks sandwich the edge of the specimens and are then secured using multiple screws and clamps to provide the uniform constraints on the edges for the two tests. For each photogrammetry analysis, 18 aerial images were taken approximately 2 m above the membrane specimen with more than 90% overlapping (see
Figure 3b).
In the OOP deformation test, a 1200 mm squared HDPE with an internal 1000 mm squared grid marked on the surface is used, as shown in
Figure 4. The membrane cover is mounted on a partially fixed frame, allowing in-plane deformation, and dominantly displaced in the OOP direction by a hydraulic car jack located approximately at the centre of the membrane. The membrane is loaded six times, at first incrementally to a height of 23 mm, 48 mm, and 72 mm, then released to 63 mm and 51 mm, and a final loading to a height of 86 mm, which formed the six OOP deformation tests denoted as
Step 1 to
6, respectively, for this study. Optical fibres were installed and aligned with the vertical lines of 1000 mm at 800 mm and 1000 mm horizontally on the HDPE specimen for benchmarking the strains; refer to
Figure 4.
In the in-plane deformation test, a 1200 mm by 500 mm rectangular HDPE with an internal 1000 mm by 300 mm grid is used, as shown in
Figure 5. The membrane cover is fixed on one end while the other end is translated in the in-plane direction towards the fixed edge by 112 mm, 161 mm, and 267 mm. These imposed displacements caused the membrane to form a wrinkle/fold denoted as
Step 1 to
3, respectively. The maximum fold heights are 190 mm, 223 mm, and 280 mm for each subsequent step test. Optical fibres were installed and aligned with the horizontal line of 1000 mm at 180 mm and 320 mm (80 mm and 220 mm horizontal grid lines) vertically on the HDPE specimen for benchmarking; refer to
Figure 5.
The preprocessing of experimental data is performed using MATLAB. The photogrammetry DEM is transformed into a readable format, and then the fiducial grid is extracted from each test. A contrast threshold technique is used to identify and extract the grid, and a stationary reference outside of the membrane is used to align the DEM to a global coordinate system. The coordinates and elevation of the grid lines from its corresponding DEM and for each loading case are then extracted in-aid of MATLAB’s inbuilt computer vision tool detectSURFFeatures.
The cubic smoothing spline is then employed to smooth the model and filter noise due to artefacts, such as light reflection and dirt on the membrane, as a function of weighting parameter, , from to . Afterwards, the preprocessed displacement fields are used as the displacement grid to load the FE membrane model. In this work, the DEM OOP deformation field is fully applied as the grid load and the in-plane deformation fields are only applied on the top and bottom boundaries of the grid for the OOP deformation test and applied on the left boundary of the grid for the in-plane deformation test while the remaining are excluded and set to freely deform in the in-plane directions and to ensure convergence of the solution.
ANSYS 19.2 is used as the FE computational analysis tool to simulate the deformation of the HDPE membrane using the processed DEM displacement field as displacement load conditions. A shell membrane element was used to model the membrane cover of mesh discretisation of 5 mm. Given the quasi-static nature of the membrane, ANSYS Static Structural analysis is considered. ANSYS Parametric Design Language scripts were developed to assign and displace each node on the grid. For nonlinear FE analysis, Large Deflection is set ON and in the experimental investigations, ANSYS Solver Controls Weak Springs is set to Program Controlled to restrain rigid body motion. The simulated smoothed and raw models’ strain fields are compared to the strain reading from optical fibre for each deformation test and step.
2.4. Statistical Process Approach: Gaussian Process
Significant smoothing of the data or model inevitably results in undesirable amplitude reduction. Photogrammetry data and images are subjected to noise and processed data after smoothing interferes with the precise measurement of the elevation/height causing erroneous strain prediction, especially in areas with high displacement gradients. Nevertheless, unavoidable uncertainties exist throughout the stages from acquisition of data to output of results, therefore, the deterministic prediction of strain values is difficult, and hence, this motivates a probabilistic approach to tackle these inevitable variations. Given the practical application in hand, the only information obtained are the raw data and smoothed models with different weighting parameters constructed from the photogrammetry DEM, and the true strain measurements remain unknown. A statistical process approach using sample data of the smoothed and raw strain field is employed to provide a probabilistic prediction in evaluating the likelihood of strain values of the structure.
2.4.1. Gaussian Process Regression
In this study, GP regression is demonstrated as a probabilistic prediction approach for strain determination. GP, which assumes that the joint probability distribution of model outputs is Gaussian, is a Bayesian approach used in applications for model approximation, multivariate regression, forecasting, and experiment design and as an algorithm for regression and classification for machine learning [
53,
54,
55]. The fundamentals of GP regression are briefly introduced, and a more comprehensive report can be found in previous literature [
53,
54,
55,
56].
GP is a stochastic process in which any finite subset through its domain follows a multivariate normal distribution [
56]. Consider the following nonlinear regression model with homoscedastic noise (constant variance) of observed data
where
represents the measurement error, which is independent and identically distributed normal random noises with zero mean and variance
, and
is an unknown function.
The GP regression model can then be denoted as
by the GP method,
is related as a random function and assumed to have a GP prior with a mean,
, and covariance
, where
denotes the set of hyperparameters.
Hence, the joint distribution of the
is multivariate normal.
where
has entries
and
is a
by
matrix whose
element is given by
where
is Kronecker delta.
For a new input
and
, the corresponding response value is calculated as
where
.
Therefore, the lower and upper limits are, respectively,
A simple assumption is that the correlation between two points decays with the distance between the points according to an exponential function. One prevalent choice of a kernel fulfilling this assumption is the squared-exponential kernel function.
where
is the Euclidean distance between two input points and
denotes the collection of hyperparameters associated with the kernel function, in this case
and
.
is the signal deviation and
is the characteristic length scale, which defines how far apart the input values of
can be for the response values to become uncorrelated. The hyperparameters are usually not assumed to be known but are trained by maximising the marginal log-likelihood from the dataset [
54,
55],
2.4.2. Heteroscedastic Gaussian Process Regression
It is highly desirable to fit a regression model with non-constant variance than the standard GP, where a constant variance is assumed, which is often unrealistic in many applications. There have been developments in novel GP approaches to regression with input-dependent noise rates and demonstrated higher accurate modelling of real-world datasets compared to other regression methods [
57,
58,
59,
60,
61]. In this study, variational heteroscedastic GP (VHGP) regression is considered and a brief explanation of the theory will be shown in this section. More detail can be found in the work by Gredilla and Titsias [
58].
Similar to the Equation (2), instead of the GP prior on
and the error term, we have
where
is an unknown function and the function is defined as
to ensure positivity and without losing generality, and
Once the kernel functions are defined, heteroscedastic GP is fully specified and depends only on its hyperparameters (
. Unfortunately, the exact inference in the heteroscedastic GPs is no longer analytically tractable and, thus, a marginalised variational approximation bound [
58] for the likelihood function is given by
where
is a diagonal matrix with elements
, and KL is Kullback-Leibler divergence.
Furthermore, the stationary equations
must be satisfied at any local or global maximum and the two equations are obtained as follows,
for some positive semidefinite diagonal matrix
.
and are expressed as a function of
. So, the marginalised variational bound can be a function of
,
which needs to be maximised with respect to the
variational parameters in
. By implementing the marginal log likelihood for model selection, we can maximise
with respect to the model hyperparameters for a new test point.
This study employs the VHGP regression method for data modelling to deal with the different sources of uncertainty that originated from photogrammetry, preprocessing, and postprocessing analyses. The objective of using an inference statistical approach is to provide descriptive insight and prediction to the strain field. In this work, automatic relevance determination squared-exponential kernel function is defined as
and employed for
and
. All strain results for all
-values were used as the training data set and the predicted VHGP models are compared to the experimental optical fibre strain data.
4. Discussion
The UAV photogrammetry inspection method is shown to be effective in monitoring the state of deformation of the floating covers at Melbourne Water WTP [
37]. In comparison to non-contact methods on measuring strain fields, although DIC methods may yield higher accuracy, DIC methods require the membrane to be coated with random patterns and multiple fixed cameras continuously monitoring to capture the whole asset—these requirements are not practicable for WTP. Furthermore, DIC methods are not effective for monitoring wrinkle and fold formations, which makes photogrammetry methods more suitable for monitoring membrane cover [
27]. To further facilitate the development of smart SHM assessment on this highly valuable asset, the digital displacement information of the covers can be further processed into strain measurement to allow a more elaborative quantification of its integrity. Overall, this paper introduces a strain determination method proposed for WTP floating cover and the UAV photogrammetry deployment and its general application for large structures are briefly discussed.
The proposed strain determination method is suitable for global inspection in the two-stage monitoring strategy, as it demonstrates the capability to produce a higher quality strain distribution profile of the deformed membrane for detecting critical areas. Furthermore, the VHGP offers a probabilistic prediction approach that supplements the decision support to justify further localised inspection in concerning areas. The results have shown that the use of cubic smoothing interpolation technique can reliably denoise the DEM, improving the quality of the strain field of deformed membranes to locate high strain areas in both OOP and in-plane deformation tests. However, the smoothing procedure adversely degrades fine detail information and flattens peaks, which is seen for increasing deformation test steps. As a result, there is a difficulty in capturing the numeric strain values at high strain concentrated areas. It should be worth noting that the optimisation of the grid resolution should be carefully considered in both the capability of the photogrammetry, camera, and image resolution and in accurately capturing critical deformations or details of the specimen or structure when implementing this approach for strain determination via UAV photogrammetry [
21]. In our application, WTP regulations restrict additional markings of the floating cover and only existing features can be used as targeted fiducial markers, which limits the ability to enhance the resolution for an accurate reading. Furthermore, due to the disturbance of wildlife, fire hazard, and WTP regulations, UAV photogrammetry cannot be performed too close to the floating cover for more detailed measurements. To facilitate the strain evaluation, a probabilistic approach is implemented to predict the strain values with statistical measurement by using the smoothed and raw model strain fields as data samples. A VHGP method is demonstrated to provide uncertainty measurements to assist in evaluating the strain field within the 95% confidence level by using only the available smoothed and raw models. As VHGP searches a distribution over the possible functions that are consistent with the data, large variances occur in the vicinity of strain concentrated areas and noise due to its relatively high changes in strain value as smoothing weighting parameter changes, thereby corresponding to higher uncertainty variance in those areas.
A factor contributing to inaccurate strain, as discussed earlier, is the reconstruction of DEM using UAV photogrammetry in the region with a steep slope. As reported in Wong et al. [
36], UAV photogrammetry has difficulties in reconstructing the DEM model with sharp corners and high slope gradients. The photogrammetry process may consider the steep region as a discontinuity in the reconstructed model and automatically apply a smoothing function to construct a smooth DEM. For this laboratory experiment, the aerial images were only taken directly above the region of interest. More viewing angles can improve the accuracy of close-range photogrammetry analysis.
The lack of in-plane direction information on the model may have contributed to the inaccuracy of the strain field. However, it was found that by including the experimental full-field displacement as the applied load led to either, mostly, un-converged solution or highly inaccurate strain results with no resemblance to the measured strain profile and values. It was revealed that there were inconsistent feature detections and extractions of points along the grid line. Some data points were extracted within the 1.5 mm width of the line rather than consistently from the centre. These are artificially induced erroneous in-plane deformations, resulting in extremely noisy and unreliable data in the in-plane direction. Therefore, only the in-plane displacements on the boundaries of the grid were manually identified, selected, and loaded to provide sufficient constraint for FE analysis. Further improvement in the object-identification algorithm can accurately identify and extract the markers. Nevertheless, ongoing future work will investigate improving the quality of DEM, to explore feature identifying and optimize selection on the fiducial markers on the WTP floating covers, as well as other strain prediction algorithms.