Extreme Conditions Modeling - Full Sea State Approach
Extreme conditions modeling consists of identifying the expected extreme (e.g. 100-year) response of some quantity of interest, such as WEC motions or mooring loads. Three different methods of estimating extreme conditions were adapted from WDRT: full sea state approach, contour approach, and MLER design wave. This noteboook presents the full sea state approach.
The full sea state approach consists of the following steps:
- Take N samples to represent the sea state. Each sample represents a small area of the sea state and consists of a representative (H, T) pair and a weight W associated with the probability of that sea state area.
- For each sample (H, T) calculate the short-term (e.g. 3-hours) extreme for the quantity of interest (e.g. WEC motions or mooring tension).
- Integrate over the entire sea state to obtain the long-term extreme. This is a sum of the products of the weight of each sea state times the short-term extreme.
See more details and equations in
[1] Coe, Ryan G., Carlos A. Michelén Ströfer, Aubrey Eckert-Gallup, and Cédric Sallaberry. 2018. “Full Long-Term Design Response Analysis of a Wave Energy Converter.” Renewable Energy 116: 356–66.
NOTE: Prior to running this example it is recommended to become familiar with environmental_contours_example.ipynb and short_term_extremes_example.ipynb since some code blocks are adapted from those examples and used here without the additional description.
Obtain and Process NDBC Buouy Data
The first step will be obtaining the environmental data and creating the contours. See environmental_contours_example.ipynb for more details and explanations of how this is being done in the following code block.
% Specify the parameter as spectral wave density and the buoy number to be 46022
parameter = 'swden';
buoy_number = '46022';
available_data= NDBC_available_data(parameter,"buoy_number", buoy_number);
% Slice the available data to only include through year 2012
rows = (available_data.year < 2013) ;
filenames_of_interest = available_data.file(rows);
ndbc_requested_data = NDBC_request_data(parameter, filenames_of_interest);
Hm0 = [];
Te = [];
for field = fieldnames(ndbc_requested_data)'
Hm0 = [Hm0 ; significant_wave_height(ndbc_requested_data.(field{1}))];
Te = [Te ; energy_period(ndbc_requested_data.(field{1}))];
end
% Remove Hm0 Outliers and NaNs
filter = Hm0 < 20;
Hm0 = Hm0(filter);
Te = Te(filter);
[row, ~] = find(~isnan(Te));
Hm0 = Hm0(row);
Te = Te(row);
[row, col] = find(~isnan(Hm0));
Hm0 = Hm0(row);
Te = Te(row);
% Delta time of sea-states in seconds
dt = ndbc_requested_data.year_1996.time(2)- ndbc_requested_data.year_1996.time(1);
dt = seconds(dt);
1. Sampling
The first step is sampling the sea state to get samples (H, T) and associtated weights. For this we will use the waves.contours.samples_full_seastate function. We will sample 20 points between each return level, for 10 levels ranging from 0.001—100 years return periods. For more details on the sampling approach see
[1] Coe, Ryan G., Carlos A. Michelén Ströfer, Aubrey Eckert-Gallup, and Cédric Sallaberry. 2018. “Full Long-Term Design Response Analysis of a Wave Energy Converter.” Renewable Energy 116: 356–66.
[2] Eckert-Gallup, Aubrey C., Cédric J. Sallaberry, Ann R. Dallman, and Vincent S. Neary. 2016. “Application of Principal Component Analysis (PCA) and Improved Joint Probability Distributions to the Inverse First-Order Reliability Method (I-FORM) for Predicting Extreme Sea States.” Ocean Engineering 112 (January): 307–19.
% return levels
levels = [0.001; 0.01; 0.05; 0.1; 0.5; 1; 5; 10; 50; 100];
% points per return level interval
npoints = 20;
% create samples
[sample_hs, sample_te, sample_weights] = samples_full_seastate( ...
Hm0, Te, npoints, levels, dt, "PCA", 250);
We will now plot the samples alongside the contours. First we will create the different contours using contours.environmental_contours. See environmental_contours_example.ipynb for more details on using this function. There are 20 samples, randomly distributed, between each set of return levels.
Hm0_contours = zeros(length(levels),1000);
Te_contours = zeros(length(levels),1000);
for p = 1:length(levels)
contour = environmental_contours(Hm0, Te, dt, levels(p), "PCA", "return_fit",true);
Hm0_contours(p,:) = contour.contour1;
Te_contours(p,:) = contour.contour2;
end
plot_environmental_contours(sample_te,sample_hs,Te_contours,Hm0_contours,"x_label",...
'Energy Period (s)', "y_label",'Significant Wave Height (m)',"data_label",'NDBC 46022',...
"contour_label",string(levels'));
2. Short-Term Extreme Distributions
Many different methods for short-term extremes were adapted from WDRT, and a summary and examples can be found in short_term_extremes_example.ipynb. The response quantity of interest is typically related to the WEC itself, e.g. maximum heave displacement, PTO extension, or load on the mooring lines. This requires running a simulation (e.g. WEC-Sim) for each of the 200 (H, T) sampled sea states.
For the sake of example we will consider the wave elevation as the quantity of interest (can be thought as a proxy for heave motion in this example). Wave elevation time-series for a specific sea state can be created quickly without running any external software.
NOTE: The majority of the for loop below is simply creating the synthetic data (wave elevation time series). In a realistic case the variables time and data describing each time series would be obtained externally, e.g. through simulation software such as WEC-Sim or CFD. For this reason, the details of creating the synthetic data are not presented here, instead assume for each sea state there is time-series data available.
The last lines of the for-loop create the short-term extreme distribution from the time-series using the loads.extreme.short_term_extreme function. The short-term period will be 3-hours and we will use 1-hour "simulations" and the Weibull-tail-fitting method to estimate the 3-hour short-term extreme distributions for each of the 200 samples.
For more details on short-term extreme distributions see short_term_extremes_example.ipynb and
[3] Michelén Ströfer, Carlos A., and Ryan Coe. 2015. “Comparison of Methods for Estimating Short-Term Extreme Response of Wave Energy Converters.” In OCEANS 2015 - MTS/IEEE Washington, 1–6. IEEE.
NOTE: For calculating the short_term_extreme of each state, we set the optional argument output_py to true in order to return the native python output that we can then feed into our long term extreme function. Consequently, we can put arbitrary inputs to the x and method inputs of the short_term_extreme function.
% create the short-term extreme distribution for each sample sea state
t_st = 3.0 * 60.0 * 60.0;
gamma = 3.3;
t_sim = 1.0 * 60.0 * 60.0;
for i=1:length(sample_hs)
tp = sample_te(i) / (0.8255 + 0.03852 * gamma - 0.005537 * gamma^2 + 0.0003154 * gamma^3);
% time & frequency arrays
df = 1.0 / t_sim;
T_min = tp / 10.0; % s
f_max = 1.0 / T_min;
Nf = int32(f_max / df) + 1;
time = linspace(0, t_sim, 2 * Nf + 1);
f = linspace(0.0, f_max, Nf);
fprintf("Sea state %g/%g. (Hs, Te) = (%.2f m, %.2f s). Tp = %.2f s\n",i,200,sample_hs(i),sample_te(i),tp);
% spectrum
S = jonswap_spectrum(f, tp, sample_hs(i), gamma);
% 1-hour elevation time-series
data = surface_elevation(S, time);
% 3-hour extreme distribution
ste_all{i} = short_term_extreme(time, data.elevation, t_st, "peaks_weibull_tail_fit", 100, "cdf", "output_py",true);
end
3. Long-Term Extreme Distribution
Finally we integrate the weighted short-term extreme distributions over the entire sea state space to obtain the extreme distribution, assuming a 3-hour sea state coherence. For this we use the full_seastate_long_term_extreme function. The integral reduces to a sum over the 200 bins of the weighted short-term extreme distributions.
Similar to the short-term extreme functions, the output of long-term extreme function is a probability distribution. Here, we will look at the survival function and the 100-year return level.The value of the survival function at a given return level (e.g. 100-years) may be calculated using the return_year_value function.
t_st_hr = t_st / (60.0 * 60.0);
t_return_yr = 100.0;
probability_of_exceedance = 1 / (t_return_yr * 365.25 * 24 / t_st_hr);
x_t = full_seastate_long_term_extreme(ste_all, sample_weights, 1-probability_of_exceedance, "ppf");
fprintf("100-year elevation: %.3f meters", x_t)
% plot survival function
x = linspace(0, 20, 1000);
lte_sf = full_seastate_long_term_extreme(ste_all, sample_weights, x, "sf");
figure;
semilogy(x, lte_sf);
xlabel('elevation [m]'); ylabel('survival function (1-cdf)'); ylim([1e-10, 1]); xlim([0, x(end)]);
% add 100-year return level to plot
hold on
st_sf = full_seastate_long_term_extreme(ste_all, sample_weights, x_t, "sf");
xline(x_t, '--')
yline(st_sf, '--')
