دانلود رایگان مقاله مطالعه پرسازی ناشی از موج در زیر خطوط لوله زیر دریایی

ABSTRACT

This paper presents results of complementary experimental and numerical studies involving wave-induced backfilling of current-generated scour holes beneath submarine pipelines. The laboratory experiments are conducted in a wave-plus-current flume, utilizing Laser Doppler Anemometry to measure velocities, synchronized flow visualizations using digital image technology, along with live-bed scour and backfilling measurements. Each experiment is based on a two-stage process: (1) initial scour induced by a pure current, followed by: (2) backfilling induced by pure waves (either regular or irregular). The time series of scour depths are closely monitored through video recordings. Systematic analysis of these has resulted in a closed form expression for the backfilling time scale, which is demonstrated to be a full order of magnitude greater than the well-known time scale of scour (with both governed primarily by the Shields parameter). The developed expression is strictly valid for the current-to-wave backfilling scenarios considered, while likely serving as an upper limit for more general wave-induced backfilling circumstances. The experiments are complemented by similar backfilling simulations utilizing a fully-coupled hydrodynamic and morphodynamic CFD model. The numerical simulations demonstrate the ability of the model to predict backfilling towards expected equilibrium scour depths based on the new wave climate, with time scales reasonably in line with experimental expectations.

4. Conclusions

The present paper has experimentally investigated wave-induced backfilling processes beneath submarine pipelines, following initial scouring due to a steady current (so-called current-to-wave backfilling scenarios). Backfilling induced by both regular and irregular waves has been considered, with both yielding similar results. The results demonstrate that the initially scoured profile will back- fill to a new equilibrium scour depth governed by the new wave climate, generally consistent with previous experimental (Fredsøe et al., 1992) and numerical (Fuhrman et al., 2014) results involving wave-to-wave backfilling scenarios. Making use of video camera recordings, the time series of the scour depth for each experiment has been obtained and analyzed, yielding corresponding backfilling time scales via integration. For the first time the (current-to-wave) backfilling time scale for submarine pipelines has been quantified in terms of the governing Shields parameter, leading to the regression equation: T∗ b = 0.3h−5/3, presented as Eq. (15) within the paper. Importantly, this is a full order of magnitude larger than for the corresponding well-known scour time scale. No additional dependence of the backfilling time scale on the Keulegan-Carpenter number has been detected from the present data set. While the new expression is strictly valid only for currentto-wave backfilling scenarios, comparison with previously measured wave-to-wave backfilling time scales (Fredsøe et al., 1992) suggests that it can likely be taken as an upper limit for more general wave-induced backfilling situations, provided that the initial (prebackfilling) scour depth does not exceed that expected from a pure current (approximately S0/D =0.6—0.8). The experimental campaign has additionally been complemented with similar numerical simulations (using regular waves), based on a fully-coupled hydrodynamic and morphodynamic CFD model (Jacobsen et al., 2014), extending previous pipeline scour-related applications of Fuhrman et al. (2014) and Larsen et al. (2016). Comparison of the numerical and experimental results demonstrates the ability of the CFD model to reasonably simulate the current-to-wave backfilling process, both in terms of the achieved new wave-induced equilibrium scour depths as well as the corresponding backfilling time scales.