Offshore structures such as wind turbines, rigs, and pilings can be weakened by scour around their base. Scour holes are generated by storms or strong currents and will often appear and disappear on short notice. As a result, storm-driven scour may not leave a detectable signature in a post-storm survey. As such, it can only be properly characterized if monitored also when normal survey operations have to be suspended.
To capture these transient events, Nortek developed the acoustic scour monitor, which allows continuous data collection during scour events, either as real time monitoring system or in an autonomous mode for pre-installation research studies. The scour monitor uses four narrow acoustic beams to detect the along-beam distance from the sensor to the seabed at four points away from the structure (see illustration).
The instrument collects data at a user-specified sampling rate and outputs the acoustic scattering profile along the beam, which provides information both about the changing location of the bottom and the nature of the suspended sediments.
By monitoring scour events, operators can determine the requirement for protective measures such as rock dumping. If combined with vibration sensors it can also relate possible changes in the natural resonance frequency of the structure to changes in the position of the seabed.
Bridge foundations can easily be undermined by scour, critically affecting the stability and integrity of the structure. An online Nortek Scour monitor, can provide continuous and detailed information about the condition of a pier footings, allowing the appropriate actions to be taken in during storm or flood conditions. It also allows bridge owners to make long term structural studies, for planning and maintenance purposes.
The mobility of the seabed can at times cause jack-up rig operators serious problems. In some circumstances, the legs may become buried or suspended. Both scenarios can be critical. The Nortek Scour Monitor is able to provide an early warning system of such events, allowing time for countermeasures to be taken.
Any object placed on or fixed into the seabed, can be affected by scour. In the majority of cases, the effects of scour will be minimal or insignificant, however in some environments scour can be the cause of major structural problems. In potentially troublesome locations, surveying at weekly or monthly intervals can provide useful snap-shots of the situation, but continuous monitoring using a remote sensing device such as the Nortek Scour Monitor, is the only really effective solution. The Nortek Scour Monitor has been designed to continuously measure four vertical ranges at user time selected intervals, thus providing a full time-series picture of the river or seabed as changes occur.
Where there is limited electrical power and communications, the Nortek Autonomous Online System (AOS) makes it possible to power, collect and process data, before transmitting vertical Scour levels via Iridium® to a Nortek server. These real-time Scour levels are ultimately displayed on a dedicated password-protected web site, allowing easy and convenient data access.
The AOS unit is housed in a weatherproof box and can accept inputs from up to three scour monitors.