Paper Title: Monitoring Scour Or Sediment Build-Up at Offshore Wind Farms, Intake Structures of Hydroelectric Power Plants & Dams and Bridge Piers

Abstract

Problems can arise in day to day operations of offshore wind farms and hydroelectric power plants and dams, whereby there are events of high scouring at the base of monopiles or high aggregation of bedload and suspended sediment at intake structures. Although deep water floating wind turbines are being developed, most offshore wind farms still utilize traditional fixed-bottom foundation technologies [1]. The different types of foundation include monopile, gravity based, tripod, suction bucket and conventional steel jacket structures. Of these, monopile foundations have been the preferred foundation type so far, simply because they have an edge over other foundation types in shallow water, with regards to construction, production, assembly and cost efficiency. Over the years, there are numerous papers written on the subject of scour behavior for monopile foundations as well as a number of text books on scour at the seabed [2]–[4]. The effect of scour or erosion of sediment near a structure can potentially be detrimental to the stability of the structure and its fatigue life. On the other side of the coin, equally detrimental, is the effect of sediment build-up at openings and intake structures of hydroelectric power stations. Any impact or shutdown can cause losses of thousands of dollars in revenue per day as well as additional costs for manpower and repairs costs to damaged equipment when suspended sediment that is not removed, blocks and chokes valves and turbines. The core point of monitoring sediment build-up or scour is to understand the problem and carry out intervention work before it becomes unsolvable or the damage costs become multiple times more. In general, the two approaches to the design of rock protection around a monopile structure comprises either a static design or dynamic design, both of which would require monitoring of the natural variations in the level of the seabed due to movement of sand waves, sandbanks and general bed movement. Even when scour protections are applied around monopiles (as in the case of Horns Rev I Offshore Wind Farm in Denmark), it would be prudent to monitor the changes in entire scour protections as wave dominated sea conditions can be unpredictable and irregular. Even in less hostile environments such as bridge piers and docks where accessibility is not a problem, the water around these structures can often be turbulent and murky with very low or zero visibility. The traditional way of using commercial divers for inspection suffers from considerable limitations due to poor visibility and high risks to diver safety in high current and turbulent waters. Underwater inspections are often not possible during the flood events due to high volume and high flow rate of silty flood water as in the case of Bonnybrook Bridge in Calgary, Canada [5] As sonar technologies improve, interest in applying these tools for underwater inspection is increasing. A series of field tests and trial inspections both in the US and UK have been carried out using sonar and laser technology with favorable results. International Civil Engineers UK, (ICE) proceedings [6] and Federal Highway Administration (FHWA), USA [7] described in great detail on the findings of using sonars for purposes of underwater inspections. The K-Observer Cloud system is a fully autonomous siltation build-up or scour monitoring system that uses permanently deployed sonars on site to provide 3D point cloud data and analysis reports of current and historic time series bathymetric data to the client in the office. The client can receive automated change detection analysis with alerts such as emails or sms for identification of changes which may exceed any threshold changes considered safe or allowable. The sonar is deployed at one location and creates a 3D map of the survey area in both horizontal and vertical planes. The rugged design of the dual axis scanning sonars, providing 3D point cloud, is ideal for long-term deployment in harsh environments. With no exposed moving parts, the transducer and rotator are contained within an adiprene, oil filled dome which inherently protects both transducer and rotator from possible damage from impacts of moving objects in the water column. Since the sonar is deployed permanently, it can capture critical changes especially in dynamic conditions which can easily be missed when conducting surveys periodically. Critical parameters of the surrounding riverbed at the intake will be scanned, at predefined and variable time intervals. This allows for a timely action/intervention for necessary remediation work. Together with a cloud based solution called ‘Mapping Cloud’ for storage, all work is done through a web browser. The physical instrumentation in the office is a small pc connected to the Internet. From the pc, datafiles are uploaded to the Mapping Cloud, with secure storage, backup and accessibility to the customer only. The real-time data can be shared between multiple users at different locations, with the owner s security and permission. Combining sonar technology with the management and dissemination of data via a cloud solution, this paper describes an autonomous system for real time monitoring of scour or sedimentation build-up using sonars deployed on site with capability of sending data, analysis reports and automated change detections alerts to users in the office.