Storm effects on megaripples in the North Sea and interaction with infrastructure

Introduction
Megaripples commonly occur superimposed on tidal sand waves (Passchier & Kleinhans, 2005) and can constitute a large part of the seabed change where sand waves are relatively stationary (Larsen et al., 2016). Several studies suggest quick and large interactions between a changing hydrodynamic environment during a storm and the dynamics of megaripples (Bellec et al., 2019; Jones & Traykovski, 2019; Passchier & Kleinhans, 2005). At pipeline scales, such dynamic megaripple behaviour complicates burial depth prediction and increases exposure risk (Larsen et al., 2016).

Once exposure of a pipeline occurs, free spans may develop which are sections of a pipe that are no longer supported. These sections are then subject to dynamic loading from waves and currents, which may lead to fatigue and ultimately failure (Fredsøe, 2016; Zang et al., 2010; Zhang et al., 2025).

While the interaction of sand waves and ripples with subsea infrastructure has been studied before (e.g., Morelissen et al., 2003; Zhang et al., 2025), the potential influence of megaripples on the development of pipeline free spans remains to be investigated. Here, we present new data and aim to quantify the pipeline-megaripple dynamics during storms in the North Sea using data analysis.

Methodology
Multibeam echosounder (MBES) data for the seabed near a pipeline in the North Sea has been acquired with a 1 or 2-year interval. In some sections, the pipeline is buried while in other sections it is exposed with a rock berm. The data of 2024 can be used to estimate storm effects as this survey yielded two partial datasets with overlapping areas from before and after a storm event as shown in Figure 1.

First, the data is processed to obtain megaripple height, length, orientation, asymmetry and correlation length along the pipeline for each survey.

Then, using the Dutch Continental Shelf Model (Zijl et al., 2023), hydrodynamic conditions such as wave height and current strengths are obtained along the pipeline. This can be combined with information on sediment properties and a technique like regression analysis quantify the effect of other environment variables on megaripple geometry.

Results
The study is expected to show variations, trends and clusters in megaripple geometry depending on the relative positions to the pipeline and it’s exposure,
with evidence of enhanced asymmetry after storm conditions. Further analysis should reveal underlying correlation patterns between megaripple geometry and hydrodynamic or sedimentary conditions. This result can also be compared with equilibrium megaripple characteristics based on the dominant forcing mechanism, as collected by Hendriks et al. (2022).

Conclusions
With the findings of this study, we will be able to understand how the geometry and dynamics of megaripples near pipelines depend and respond to each other and to storms. Additionally, the findings will also help to understand the consequences of storm megaripple dynamics on free span development.