| dc.description.abstract | This report presents a stochastic method for predicting the time scale of scour and backfilling, occurring around vertical piles and marine pipelines on the seabed. The erosion occurs due to waves alone or waves com- bined with a current. Existing formulas for the time scale are expanded allowing input of random waves. The waves are assumed to be stationary and narrow-banded such that the statistical distributions Rayleigh and Forristall (2000) can be employed. When applying the Rayleigh distri- bution, the waves are assumed to be linear, while Forristall distributes the wave crest heights representing long-crested (2D) and short-crested (3D) waves where the second-order effects sum-frequency and difference- frequency are included. The waves typically exhibit a nonlinear behaviour in severe seastates and in shallow water.
The time scale is calculated based on typical field parameters and pre- sented graphically for linear, nonlinear long-crested and nonlinear short- crested waves. When second-order effects are included, the wave crests appear higher and sharper than to linear sinusoidal waves. This causes the water particle velocity below second-order waves to be higher, result- ing in shorter time scale, which is reflected in the results. The results for the time scale of long-crested and short-crested waves are also compared, and all the results display that the 3D waves are higher, resulting in lower time scales when the nonlinear effects increase.
The time scale is calculated based on typical field parameters and presented graphically for linear, nonlinear long-crested and nonlinear short-crested waves. When second-order effects are included, the wave crests appear higher and sharper than to linear sinusoidal waves. This causes the water particle velocity below second-order waves to be higher, resulting in shorter time scale, which is reflected in the results. The results for the time scale of long-crested and short-crested waves are also compared, and all the results display that the $3D$ waves are higher, resulting in lower time scales when the nonlinear effects increase. | |
