Quantifying seabed dynamics to support resilient offshore infrastructure development

The offshore environment is busier than ever. An increasing number of submarine cables and foundations are being installed offshore in support of the energy transition and global connectivity. As a result, risks to which the installation of offshore infrastructure is exposed, is increasing.

One of the major challenges in designing offshore infrastructure and submarine utilities is the impact of a dynamic seabed. In highly dynamic areas, natural seabed variations up to ten metres can occur due to presence of dynamic sand waves and sand banks, potentially exposing cables and jeopardising the stability of foundations. Insights into future seabed levels is therefore crucial input for developers of offshore infrastructure.

Deltares supports the continuous development and maintenance of offshore infrastructure by providing expertise on seabed dynamics in line with our long-term ambition of Enabling Delta Life. By disseminating the combined insights and knowledge from different contributing sources, we support risk-based decision making and help clients design resilient infrastructure that can cope with natural and structure-induced seabed variability.

Deltares does research and provides expert advice on the following topics:

• Seabed mobility assessments, for example for offshore wind farms, foundations, pipelines, (export, inter-array, data) cables and their landfalls
• Predictions of future seabed levels (both for installation and operation phases)
• Hindcast of seabed levels to assist in detecting Unexploded Ordnances (UXO’s)
• Seabed bedform dynamics analysis
• Burial risk assessment and routing optimisation for cables and pipelines
• Dredging activities and dredge plumes assessment
• Sediment management

Approach

Deltares has developed a novel data-driven framework to track both linear and nonlinear seabed evolution to better understand local systems and their potential future dynamics. For this, we integrate different techniques, such as data analysis, numerical modelling and earth observation. This approach allows us to assess historical trends and forecast future scenarios influenced by both natural processes and human activities. The predicted seabed level estimates - derived through probabilistic techniques relying on historical trends and model outputs - can be used to optimise the design of offshore wind farms. The methodology allows us to consider the full anticipated lifetime of a development, which is typically multiple decades.

The understanding of local seabed dynamics is key in designing resilient offshore infrastructure. Based on the dynamics of relevance observed in data, we bring together different additional sources of information – from earth observation to numerical models – to build the comprehensive system understanding. Our adaptable and scalable methods efficiently process large datasets at high spatiotemporal resolution, allowing us to quantify uncertainties and estimate future system development.

Turning measured data into insights for risk-informed offshore design

Data-driven analyses are often the core of a seabed mobility assessment. Results of these analyses provide a high level overview of the dynamics, but can also be used to estimate future seabed levels for specific periods of interest. With sufficient spatial and temporal resolution in the available data, sand waves can for example be tracked, upper and lower envelopes of bed level evolution determined, and variability statistics quantified.

A single standardised procedure to conduct a data-driven seabed mobility assessment does not exist, as each area of development contains unique elements. However, through our extensive experience in dozens of projects over the world, such as in the North Sea, Baltic Sea, USA and Taiwan Strait, we developed efficient, adaptable and scalable algorithms to handle large datasets in a tailored way. These tools allow us to translate geophysical and geotechnical data of a complex system into a clear understanding of the dynamics and interactions among morphological features. Ultimately, this information is used during probabilistic analysis to quantify historic and future seabed levels including statistical uncertainty bands.

Predicting seabed dynamics with in-house developed numerical models

Quantifying the long-term morphological evolution of the seabed is crucial to offshore infrastructure development, but often remains challenging despite the availability of our novel data-driven techniques described above. For example, how can we also estimate seabed change if there is insufficient data to derive robust trends? And how will seabed dynamics be influenced by environmental (climatic) changes and/or human interventions in the physical system? These questions are often challenging to address through data analysis alone. This is why numerical morphodynamic models can also play an important role in reliable estimates of future seabed levels.

Using our in-house developed Delft3D numerical modelling suite, we can obtain valuable insights into the processes that drive morphological evolution. Delft3D is a process-based model capable of simulating the fundamental physical processes that drive the seabed dynamics, enabling us to achieve reliable estimations of seabed behaviour. Numerical Delft3D models have been widely applied in both commercial and scientific context, with many applications revealing consistency with observed large-scale seabed dynamics. Using numerical modelling, we can anticipate future seabed dynamics and illuminate relevant underlying physical processes.

At Deltares, we promote further research on the processes driving offshore seabed dynamics using numerical modelling through collaboration with academic institutions. We do this by collaborating with leading institutes such as in a joint PhD project with the University of Twente, and several Master graduation projects with Delft University of Technology on modelling sand wave dynamics.

These collaborations not only enhance our understanding of the seabed dynamics, but also help refine methodologies for modelling of seabed dynamics. An excellent example of this is a recent study using Delft3D-FM on sand wave recovery and trench infill mechanisms, demonstrating the power of using numerical modelling in seabed mobility studies in which (future) human interventions are of relevance.

Animation showing sand wave recovery and trench infilling modelled in Delft3D-FM.

Observing sea bathymetry from outer space

Monitoring seabed dynamics is essential input to ultimately design and maintain offshore infrastructure. However, due to the significant costs and efforts related to conventional geophysical (i.e. bathymetric) surveys, temporal and spatial gaps in data often exist, especially in unexplored areas. Moreover, since surveys are only snapshots in time (often multiple months or even years apart), the evolution of the seabed between measurements may still be difficult to assess. As such, potentially relevant aspects such as seasonal variability is typically not captured by conventional surveys.

Emerging technologies offer new ways to fill these gaps, providing an additional source of data with higher temporal and spatial coverage, easily accessible, and at a lower cost. Earth observation, particularly satellite-derived bathymetry, enables time- and cost-effective mapping of shallow seabed features such as migrating sand banks, tidal channels and local depressions. Moreover, by analysing sea wave propagation or sun-glint from optical satellite images, features up to several tens of meters below the water level can be detected - covering most of offshore wind farm areas.

Deltares has developed an algorithm to derive bathymetry from Landsat-8 and Sentinel-2 imagery, with data dating back to 1984. Such abundant data sources can provide additional insights to reinforce system understanding from data analysis. As part of the KPP-CIP initiative, Deltares built a web portal in 2020, which is free to use, to illustrate the satellite-derived bathymetry.

We also have a collaboration with EOMAP and GGS Geo Consultancy to deliver a global coastal bathymetric dataset. On this topic Deltares continues its research, also in collaboration with external organisations to jointly perform research on this topic.

Animation showing our product of satellite-derived bathymetry.