Time Scale for Scour Beneath Pipelines Due to Long-Crested and Short-Crested Nonlinear Random Waves Plus Current

This article derives the time scale of pipeline scour caused by 2D (long-crested) and 3D (short-crested) nonlinear irregular waves and current for wave-dominant flow. The motivation is to provide a simple engineering tool suitable to use when assessing the time scale of equilibrium pipeline scour for these flow conditions. The method assumes the random wave process to be stationary and narrow banded adopting a distribution of the wave crest height representing 2D and 3D nonlinear irregular waves and a time scale formula for regular waves plus current. The presented results cover a range of random waves plus current flow conditions for which the method is valid. Results for typical field conditions are also presented. A possible application of the outcome of this study is that, e.g., consulting engineers can use it as part of assessing the on-bottom stability of seabed pipelines.

Joint Distribution of the Wave Crest and Its Associated Period for Nonlinear Random Waves of Finite Bandwidth

The theoretical treatment of statistical properties relevant to nonlinear random waves of finite bandwidth, such as the joint distribution of wave crest and its associated wave period, is an overdue task hampered by the complicated form of the analytical model for sea surface elevation. In this study, we first derive the wave crest distribution based on the simplified version of the Longuet-Higgins’ wave model and proceed to derive the joint distribution of the wave crest and its associated period, and the conditional wave period distribution with a given wave crest, which are of great engineering value. It is shown that the bandwidth of the wave spectrum has a significant influence on the crest distribution, and the significant wave crest is getting larger in an increasing manner as nonlinearity is increased as expected. It also turns out that the positive correlation of wave crest height with its associated period is extended to more massive waves as nonlinearity is enhanced contrary to the general perception in the coastal engineering community that the wave crest is a statistically independent random process with wave period over large waves. The peak period decreases due to the destructive interference of second-order free harmonics.

Fatigue Analysis of a Jacket Structure to Linear and Weakly Nonlinear Random Waves

Jacket structures have been widely used in oil and gas industry and are increasingly becoming competitive as a support structure of wind turbines at different water depths. These types of structures usually fix in transition or shallow waters where numerous field observations and experiments have shown that water particles tend to exhibit non-Gaussian characteristics. However, current engineering practice ignores the wave nonlinearity for the analysis and design of these structures. The application of linear irregular models might result in considerable uncertainties in the obtained wave loads and consequently the dynamic response and thus it is highly questionable. Therefore, it is crucial to calculate the dynamic response of jacket structures under both linear and nonlinear wave models to investigate the validity of linear wave models in different sea states. In this paper, the finite element (FE) model of a jacket structure located in Persian Gulf (SP17 jacket) is setup and applied to perform a comparative study of the dynamic response to both linear and weakly nonlinear random waves. The fatigue life of the jacket structure is then calculated under both wave models. This paper will substantially improve the understanding of the dynamic response of jacket structures under fatigue damage.

Burial and Scour of Short Cylinders and Truncated Cones Due to Long-Crested and Short-Crested Nonlinear Random Waves Plus Currents

This paper provides a practical stochastic method by which the burial and scour depths of short cylinders and truncated cones exposed to long-crested (two-dimensional (2D)) and short-crested (three-dimensional (3D)) nonlinear random waves plus currents can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall second-order wave crest height distribution representing both 2D and 3D nonlinear random waves. Moreover, the formulas for the burial and the scour depths for regular waves plus currents presented by previous published work for short cylinders and truncated cones are used.

Burial and Scour of Short Cylinders and Truncated Cones due to Long-Crested and Short-Crested Nonlinear Random Waves Plus Currents

This paper provides a practical stochastic method by which the burial and scour depths of short cylinders and truncated cones exposed to long-crested (2D) and short-crested (3D) nonlinear random waves plus currents can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall [1] wave crest height distribution representing both 2D and 3D nonlinear random waves. Moreover, the formulas for the burial and the scour depths for regular waves plus currents presented by Catano-Lopera and Garcia [2, 3] for short cylinders and Catano-Lopera et al. [4] for truncated cones are used.