Requirements for Monitoring Subsea Engineering Works with Floating Platforms

The monitoring of such sites before, during, and after planned works is essential to the successful completion of activities; either for regulatory reasons, or for practical purposes. For example, water quality parameters such as turbidity are now part of the specification for any planned dredge projects, and sediment distribution and the current flows surrounding (and within) a site play a crucial role in the distribution of spoil and the movement of vessels or equipment.

A considerable advantage of the use of floating platforms for data collection is the ability to collect data continuously, in near real-time, from inshore and offshore locations. To collect such a comprehensive dataset, the continuous use of boats is often impractical both logistically and in terms of cost.

However, there should be no misconceptions as to the level of support necessary in order to maintain a regular flow of high quality data from a single buoy or buoy network. When the specifications are being drawn up, consideration must be given to the installation and on-going maintenance of all aspects of the system. This will include ship time for deployment and subsequent service visits, trained personnel to carry out cleaning and service of the sensors on a regular basis, purchase of redundant probes to minimize down time during maintenance, and holding an adequate stock of spares.

There are some important considerations to be taken into account when operating a successful data buoy monitoring system.

Bio-fouling

This is one of the most common reasons for failure in the supply of data from sensors on the buoy system. Most sensors and sondes carry some form of anti-fouling device from mechanical wipers to chemical leachate to UV irradiation. There are positive features to all of these methods but one thing they all have in common is that they are not 100% effective 100% of the time. With optical sensors such as turbidity, optical backscatter and fluorescence, some form of mechanical wiper is commonly used to clear the window at pre-programmed times and this is generally accepted as the most reliable method. However, removal of biological growth around the wiper is still necessary on a regular basis as a build-up of algae, encrustations etc. can impede its operation. Non-optical sensors such as conductivity, temperature, pH and dissolved oxygen cannot be cleaned in-situ with a wiper system.

Chemical methods using poisons such as Tributyl Tin (TBT) and Copper are sometimes used but regular cleaning is still required. Biological growth on the main body of the sonde can make its subsequent removal difficult or impossible at a later date. This should be cleaned regularly and an antifouling agent applied where applicable. Other forms of bio-fouling can affect the performance of the system as a whole. For instance, a build up of bird excrement can impede the efficiency of solar panels and increase the effect of corrosion on other parts of the buoy superstructure.

Moorings

An essential component of the data buoy that is generally hidden from view is the mooring to which it is secured. All buoys need to be fixed in their required position and this is mainly performed using some kind of sub-sea mooring comprising a large weight and supporting lines or cables. Very often, sea-bed frames are included in the system particularly for the measurement of sub-surface currents and tides. Extreme care should be taken when deploying a buoy for the first time to ensure that cables and connectors are not subjected to undue stress or strain. The design of the mooring should be suitable for the size and shape of the buoy but must also be appropriate for in-situ conditions including water depth, currents, tides and shipping hazards. Regular inspections of buoys in-situ should include the condition of the mooring lines either by lifting the buoy or by the use of divers. The first indication of a mooring failure is often notification of a change of position of the buoy in the data stream. This may indicate that the buoy has broken away and is floating freely in the water, which can usually be tracked if the data transmission continues. However, this may not always be the case. In some instances buoys that appeared to be floating away, on closer inspection, have been seen to be moving against the prevailing current due to the fact that they have been picked up by a passing vessel. In dynamic conditions, such as those found in a tidal estuary, mooring lines may become tangled and twisted which increases the strain and can pose a higher risk of failure so regular inspection is even more important. Busy shipping lanes also carry the risk of collision, which can result in a mooring and its buoy being dragged off station.

Data Transmission

One major advantage of remote buoy networks is the ability to collect data, collate it and transmit it to a receiving station, for viewing and further processing. More often than not the final viewing portal is internet based. This can allow secure access to the data by selected individuals or the data can be broadcast to a website for public access. The method of data transmission is highly site dependent. Near-shore sites often use VHF radio as a low-cost method to collect data but it must be remembered that ‘line of sight’ is essential and even then local topography can interfere. Increasingly popular is the use of the mobile telephone GSM and GPRS networks that allow the transmission of data without line of sight over relatively large distances. An essential component here, of course, is the availability of a data handling network with good signal coverage. In some near shore applications a direct cable can be used which is by far the most reliable method for data continuity but can be expensive and is only suitable in a small number of locations. For remote sites, particularly offshore or open-ocean, the only effective way to collect data is to use a Low Earth Orbit communications satellite such as the Iridium system but this can be expensive and may not always provide coverage in some very remote regions. When data transmission becomes interrupted it is all too easy to jump to the conclusion of sensor failure. However, operators should always check the data link as a first and easy option. For instance, if using VHF, is there a new obstruction to the line-of-sight? In busy port locations the arrival of a large vessel to the port can obstruct the VHF data until it moves again. With GSM the strength and quality of the signal must be checked along with any outage from the network provider. Likewise with systems such as Iridium, satellite availability is crucial.

Power Management

At the heart of every data buoy is its power supply, without which the system dies. Batteries are most commonly used with a constant recharging cycle provided by on-board solar panels. Each buoy network should be assessed for available solar power according to the site location. Calculations are made using latitude, longitude and meteorological records in order to assess the panel requirement. In addition, sensors are selected according to user requirements and power available. It is particularly important for the user to be aware of the power limitations when setting the data volume and interval for the sensors. The temptation to have ‘all of the data all of the time’ must be balanced with the usefulness of the data and the power requirement of the sensors. The components of the power supply should be checked regularly for corrosion, overheating and poor connections. The ingress of seawater into badly fitted connectors can totally disable a system over a short period of time. Batteries should be replaced after 2 years of operation.

OSIL has supplied over 250 buoy systems to clients around the world for various applications. Recent systems have included: a 2.6m buoy installed in a North Sea wind farm off the Belgian coast; multiple buoys (including mobile dredge monitoring buoys) for the new port project in Doha, Qatar; several 1.2m buoys installed in Abu Dhabi for EAD; a 1.2m buoy installed in the Canary Islands for an academic research project; a 1.9m buoy installed off the coast of Venezuela.

Suggested Checklist for buoy maintenance

Component Action Suggested Interval*

Power Supply (batteries) Replace 24 months

Mooring Lift buoy from water to check fixings and lines 12 months

Buoy Hulls Lift buoy from water, clean and apply antifoul if required 12 months

Sensors (calibration) Remove and calibrate according to manufacturer specification 12 months

Sensors (meteorological) Clean and service 6-12 months

Sensors (subsea) Remove bio-fouling & clean 1-3 months

Solar Panels Clean 3 months

*The interval is a suggestion only, and will vary depending on the individual requirements of the buoy systems and installation site and conditions.

Visual checks for damage, corrosion or wear should be carried out on every maintenance visit. A sufficient stock of spare parts and replacement sensors/sondes should be carried in order to ensure minimal down-time for the system.

The cost of maintaining a constant stream of high quality data from a buoy network should not be underestimated. However, when taken in context with the costs for the project as a whole it is a small price to pay for the success overall. If maintained correctly a data buoy network can provide invaluable data for a wide range of monitoring applications, which would be impossible both in terms of logistics and cost by other methods.