The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community
Issue link: http://sea-technology.epubxp.com/i/477967
20 st / March 2015 www.sea-technology.com tenance includes cleaning the instruments (especially if a sensor is not self-wiping) and replacing any deteriorating O- rings to prevent water ingress. If using additional parameter sensors (such as conductivity, temperature, pH and DO, or light-based turbidity sensors), further feld servicing may be required. Maintenance intervals are largely dependent on site con- ditions and other variables, such as the potential for biofoul- ing, water temperature and seasonal fuctuations. Common calibration and maintenance intervals are on a weekly, bi- weekly or monthly basis. In case of sensor failure or dam- age, it is useful to have a full complement of spare sensors and sondes on hand. These can be feld swapped during calibration or routine maintenance checks. Having a spare turbidity sensor available will reduce downtime due to un- foreseen sensor failure, which could cause critical and cost- ly interruptions to safe dredging operations. A ratio of 2:1 for deployed units/sensors to spares held is recommended from long experience of extensive dredge projects. Preventing Downtime The requirement for maintenance and spares is not re- stricted to sensors and instrumentation, however. It is often advisable, particularly for long-running projects, to hold a stock of spares for buoy components. Experience has shown that, despite the robust nature of the buoy systems, unfore- seen circumstances, such as thefts, vandalism and collisions with marine vessels (i.e., dredgers), can also cause costly interruptions to dredging operations if external compo- nents, such as solar panels, telemetry equipment or cables, are damaged. Regular checks on buoy components are also essential in order to identify potential issues before they be- come serious: cables and connectors should be checked for wear and tear on a biweekly basis; marine growth should be removed from sensor access areas every two to four weeks; solar panels need to be cleaned on a monthly basis, at a minimum, to remove any built-up dirt that could reduce the supply of integrated systems and monitoring equipment, such as the 1.2-meter Shearwater buoy, and other servic- es (such as analysis of water and sediment samples) to the London Gateway Project for background data and monitor- ing of the dredge project through completion. Other solu- tions from OSIL that have been supplied to dredge projects around the world include standalone monitoring stations for water quality or current measurement; instrumented data buoys, with support frames for moorings in shallow tidal waters; and small relocatable buoys that can follow dredg- ing activity, all of which have been in use on the New Port Project development in Doha, Qatar. Data Collection, Calibration Accurate data ensures that dredging operations remain in compliance with set regulations. For accurate data, all sen- sors should be fully calibrated before the project begins. If using a buoy-based system, the platforms should be fully as- sembled onshore prior to deployment. This includes attach- ing any sensors, towers, solar panels and additional ballast weights if needed. Furthermore, the complete monitoring system (sensors, data logger, telemetry, software) should be tested before the buoy is put in the water. While this process ensures that all equipment is functioning within specifca- tions, it also gives everyone the chance to familiarize them- selves with the system prior to deployment. Issues are always easier to deal with before the buoy platform is deployed in the water. It is also recommended to cross-check sensor accuracy against a separate instrument. This cross-checked data can then be used to show compliance, if needed. Regardless of the instruments chosen to monitor a dredg- ing site, regular maintenance and calibration are required, often by legislation, to maintain accuracy and keep equip- ment functioning within specifcations. Instrument main- (Photo Credit: OSIL) OSIL buoy data display with Google Earth integration.