Predictions of Near-bed Working Conditions
Viewing video images with the human eye remains a key examination tool in many subsea engineering activities, but work can be severely affected by poor visibility. Although industry data are not easy to come by, some projects can suffer significant downtime because of poor visibility, especially near the seabed.
RPS makes sustained, in-situ measurements of factors affecting visibility, which accurately reflect true field conditions. Using leading scientific techniques, we can now combine these measurements with expert analysis and modelling to provide location-specific predictions of near-bed visibility. In many cases, this can support forward projection over periods of years. This allows for improved planning of those subsea operations requiring some level of visibility.
Delivered Products and Advantages to Our Clients
- Long-term predictability in underwater visibility (years).
- Improved efficiency in those ROV operations which require good visibility, with substantial cost savings.
- Increased understanding of the operating environment (in terms of suspended sediments).
- Informed budgeting for contracts and potential downtime.
With pipelines traversing many different seabed environments, visibility conditions can vary greatly along their length. Knowing likely visibility at key locations along the pipeline can lead to:
- more effective use of resources and efficient commissioning of future work.
- better quality and interpretation of ROV video recordings, e.g. of pipeline repairs.
Underwater visibility is especially relevant to the construction and maintenance phases of offshore developments. Key aspects which benefit from good visibility include the installation of seabed structures (trees, risers, anchors, etc.) and the inspection and repair of pipelines. Closer to the coast, visibility may be relevant in port environments influenced by turbid river run-off, and for environmental and archaeological survey work.
Our instruments make direct in-situ measurements near the seabed, where most ROV work takes place, so that parameters do not have to be inferred or assumed. Our work can be combined with laboratory or in-situ measurements of visibility with ROV systems.
Key Parameters are Measured
- currents and waves
- optical backscatter
- optical transmission
- particle size distribution (PSD)
- concentration of total suspended solids (TSS).
Research-level Data Analysis
Work is designed and data are analysed by highly experienced sedimentologists and modellers who ensure the highest quality interpretation and predictions. This is underpinned by our 40 years of oceanographic experience in Australian waters and by our international network of research collaborators.
The Science of Visibility
Seeing objects against their background depends on contrast, which is a function of colour and brightness. However, contrast decreases with distance because light from the object is attenuated by two separate processes: absorption and scattering.
Therefore, measuring near-bed visibility and understanding its controls requires measuring:
- absorption, using optical transmission
- scattering, using optical backscatter.
In practice, close to the seabed both these factors are highly dependent upon sediment transport from the seabed, which means that particle characteristics such as size distribution require measurement through time.
Near-bed visibility is influenced by a mixture of periodic factors, and episodic factors, including tidal currents, storms and internal waves. Our analysis can allow forward simulation of periodic components for many years ahead, together with assessment of the significance and potential timing of episodic components.
Local Controls on Visibility
Changes in seabed sediments and hydrodynamics can make the difference between visibility conditions being workable or unworkable. Thus, samples of the seabed confer the advantage of explaining the visibility regime. Our measurements of suspended PSD allow identification of those different particle sizes that limit visibility at different times. This detailed understanding of sedimentary processes increases confidence in predictions of future conditions, and also produces information on the potential for erosion, transport and accumulation.