E&P Hart: Back To Basics On Seabed Surveys
Seabed images tend to depict the ocean’s floor as a flat, sandy environment. This is not always the case. In fact, companies that want to install a subsea development scheme spend a lot of time and money characterizing the seabed before the first item of kit is installed.
E&P recently spoke to Brian Mackenzie, service line director of Marine GeoConsulting, Europe & Africa, at Fugro Survey Ltd. about the concerns that operators face when planning a subsea development.
E&P: What are some of the main considerations when conducting a seabed survey?
Mackenzie: We would always start with the end-goal of working with our client toward the successful design and installation of seabed infrastructure.
Implicit in ‘successful design and installation’ is its safe and reliable operation and assured asset integrity over its design life. With that in mind, considerations would include what the client is planning to build. The layouts associated with subsea and offshore projects can be complex, and also the shape, size and weight of the installed equipment affect its interaction with the seabed and determine the coverage and type of survey required.
Often the layout, or indeed the equipment to be installed, may not be known at the time of the survey. This certainly makes things interesting because, rather than surveying to a known pattern or layout, we would try to characterize some overall volume of seabed, and from that we would be able to help the client identify the optimum development site within the overall notional project area.
Our first question when receiving an inquiry from a client about a survey is, ‘What is it for?’ Then we start to think about the seabed itself, with the most fundamental question being, ‘What’s the water depth?’, because that would determine the choice of platforms available from which to conduct the survey as well as driving the development concept on the client side.
Once we know the client’s development plans, location and water depth, the next consideration is the seafloor and the sub-seafloor itself as well as its features and ground conditions.
We aim to identify the constraints and hazards that might be posed to the development (for example, any adverse ground conditions). This could simply be excessively soft or sensitive sediment that can’t support infrastructure or, conversely, hard ground or buried rock that poses installation risk. Then there could be a whole host of hazardous features such as faults, steep slopes or gas expulsion features to be taken into consideration.
These are static features that can sometimes be readily identified simply by examining the bathymetry, but it’s also necessary to consider dynamic features such as a mobile seabed. This is something that tends to be talked about more in the offshore renewables sector rather than in oil and gas field development. For example, there is a wind farm development off the U.K. coast that has been built on what turned out to be a mobile sandbank, so the ground is actually shifting.
These are dynamic hazards that you wouldn’t necessarily see in a snapshot survey. And in the so-called ‘frontier developments’ around the world, there may be other dynamic geohazards. The classic threats in such environments include steep unstable slopes characterized by very soft sediment and earthquake-prone environments. These can trigger instability, essentially a soil avalanche developing into a debris flow, with huge amounts of energy and the ability to cause a lot of damage if not destruction. Such geohazards are a crucial consideration in a seabed survey.
Another important point is that in such circumstances the threat to the subsea development can actually originate some distance from the development itself. It could be tens of kilometers away, so in considering and planning the survey, not only are we interested in where our client is going to build, but we may also be interested in the surrounding seabed terrain and the source of threats to the development.
Once the inherent hazards have been examined, we investigate the actual engineering properties of the seabed itself such as the characteristics that tell us, for example, what load it can support, whether its strength might degrade over time and what foundation settlement might be expected. Those engineering properties feed directly into the design process, so they drive the foundation sizing and configuration as well as the selection of installation equipment and therefore any direct financial consequences relating to the characterization.
In the wider context of marine site characterization the environmental habitat conditions are also important, so we can assess the impact of a development against those baseline conditions. Metocean conditions are also important, as waves and seabed currents will determine the load on what the client builds.