Geo-4D Case Studies
Case Study - Cable Integrity Management (CIM)
Considering a typical 25-year life cycle of the wind farm and associated export cables, and the complex and dynamic nature of the marine environment, a robust and consistent approach to monitoring cable risks after installation is crucial for managing cable assets, and identifying potential issues before they result in cable downtime.
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Bathymetric data may show areas of consistently eroding seabed that will eventually expose a cable resulting in a freespan, vortex-induced vibration, fatigue and cable failure.
Multiple years of bathymetric data along a route can be reviewed to assess sediment mobility and form predictions on future shallow burial.
Figure 2 displays changing seabed levels between 2011 to 2022 along an export cable in the Irish Sea, exhibiting regions with drastic and relatively sudden changes in sediment mobility and seabed levels.
Results - Example 1 Bathymetric Comparison
Cable integrity management (CIM) begins with a detailed forensic review of available pre-installation, installation and post-installation data, to understand the key risks to the cable(s). A ground-model approach (Figure 1) works well, allowing all geophysical, geotechnical, geological and engineering data to be collated and reviewed, typically within a GIS environment.
A baseline report which defines initial site conditions and cable risk profile, is used to define the requirements of the monitoring surveys. The data from the monitoring surveys can then be integrated back into the interpretative report and used to update the assessment of the key risks and guide future surveys. Over time as risks are better quantified and uncertainties reduced, survey frequency may reduce.
Repeat surveys and integrated data analysis enable a deep understanding of the dynamic marine environment and early identification of risks to cables.
CIM Method
Cable depth of cover (i.e. sediment thickness above cable) can be assessed by taking the as-laid cable position and subtracting seabed level from each monitoring survey, to calculate shallow burial and exposures. Acquired SSS data can be used to verify any ‘calculated’ cable exposures from as-laid data.
Where cable exposures are observed in the SSS data, accurate lengths and heights of cable exposures and freespans can be measured. Cable freespans often pose an increased risk due to vortex induced vibration and snagging by fishing gear.
Results - Example 3 Cable Depth Of Cover
Analysis of sidescan sonar (SSS) data may show prevalence of beam trawling close to mobile bedforms, that may expose a cable over time resulting in cable damage (Figure 3).
SSS data can also be used to identify the location of debris, fishing gear and boulders at the seafloor close existing cable routes, that can provide insight into seabed activities that may affect a cable.
Combining SSS data with Automatic Identification System (AIS) data can provide a better understanding of fishing activity; however, it is not mandatory for vessels under 15 m to transmit AIS, therefore it is difficult to accurately quantify fishing activity.
Results - Example 2 Fishing Risk
Sub-bottom profiler data is essential for understanding geological conditions and hazards prior to cable installation. However, during the lifetime of the cable, sub-bottom profiler data is often of more use in evaluation of depth of scourable sediments to understand the extent of future cable exposures in areas of regional scour.
Results - Example 4 Sub Bottom Profiler
By combining interpretations from bathymetric, sidescan sonar, sub-bottom profiler data with cable burial and marine traffic data, the probabilistic risk of a damaging anchor strike from emergency anchoring and drag events from designated anchorages can be calculated
Such modelling can be conducted each time new survey data is acquired to monitor anchor strike risk over time, to highlight areas of increased risk, plan potential remedial works, and provide assurance to cable insurers.
Results - Example 5 Anchor Strike Risk
