Dredging work to deepen the harbour basin and channel into Dublin Port as part of the Alexandra Basin Redevelopment Project has begun in recent months. The €230 million project involves the construction of approximately 3km of quay walls, deepening of the harbour basin and channel to accommodate larger seagoing vessels, as well as works associated with the conservation of the port’s Victorian industrial heritage.
A comprehensive environmental monitoring programme has been established, including live monitoring of marine mammals during the dredging activities. As part of this monitoring programme, Techworks Marine, an award-winning Dún Laoghaire-based company that has been contracted by Dublin Port Company to provide environmental monitoring services, asked TfI Marine to come up with a silent mooring solution for the deployment of its Coast Eye monitoring buoys.
Techworks Marine chose Mobilis DB 8000 buoys for this deployment. The 2.7 tonne DB 8000 buoys have a 3m diameter hull constructed from multiple-section polyethylene floats. These are bolted around a central steel structure and have through-hull access for underwater instrumentation and cabling.
They provide a platform for the acoustic monitoring system as well as giving real-time detailed sea-state data on turbidity, wave and current profiles. The buoys are strategically positioned within Dublin Bay in approximately 20-25m water depth.
TfI Marine designed a taut mooring solution incorporating its innovative elastomeric tethers to produce a more silent mooring system (i.e. no clanking chains or shackles), thus enabling improved acoustic monitoring.
Choosing the right mooring solution
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CLICK TO ENLARGE Fig 1: Elastomeric tether with bypass[/caption]
Typically, mooring systems for monitoring buoys at these depths would use a catenary chain system, which can result in a lot of underwater noise. Alternative fibre rope solutions can be complex, with multiple floats and clump weights often required. For this deployment, a mooring system that was silent yet robust was required. This combination of silent but strong and reliable is difficult to achieve.
TfI Marine’s elastomeric tethers – indicated in the mooring line in black in Figure 1 – enable the monitoring buoys to respond to changes in the sea surface topography, ensuring the survivability of the buoys by keeping them on station and protecting against storm damage and, at the same time, providing a quieter mooring solution.
The elastomeric tethers can elongate from 4.5m to 12m under high loads. This dynamic response significantly reduces peak loads and provides high protection against shock loads. Consequently, the fatigue loading within the mooring system is minimised, resulting in a dual benefit to the monitoring buoys and to the other components within the mooring system.
Optimised mooring solution
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CLICK TO ENLARGE Fig 2: Mooring configuration incorporating elastomeric tethers[/caption]
TfI Marine worked closely with Techworks Marine and undertook simulation work to determine the optimum mooring configuration for the Dublin Bay site. The Mobilis DB 8000 buoys were modelled in Orcaflex, a marine-based software program that simulates the wave and tidal conditions as well as buoy and mooring line motion, to calculate and optimise the loads in the system throughout a range of environmental conditions. There is a significant tidal range of up to 4m in Dublin Bay, as well as wave heights of up to 6m.
Consideration was also given to the span between the anchors of the mooring system. This was modelled across a range of sea conditions to determine buoy behaviour. Parameters such as tension in the mooring line, surge, heave, sway, pitch, roll and yaw were all evaluated. The elastomeric mooring tethers were also modelled and compared against the alternative traditional mooring options and hundreds of simulations and design iterations were analysed to optimise the mooring solution.
Figure 2 shows a sample configuration of the Mobilis DB 8000 data buoy, with the elastomeric tethers in red, along with the bypass, floats and anchors. A taut mooring configuration (i.e. when a minimum tension is maintained in the mooring line and both vertical and horizontal loads are applied to the anchor) was chosen.
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CLICK TO ENLARGE Fig 3: Chain mooring; Fig 4: Polypropylene-only mooring; Fig 5: Elastomeric tether mooring[/caption]
This design provides the benefits of the traditional catenary system without the lifting of the chain on and off the seabed. A chain catenary system would produce significant amounts of noise whilst a fibre catenary system would be unlikely to survive the abrasion of the seabed.
The elastomeric tethers can withstand high loads without tearing and can reach a maximum elongation of 167%. At maximum extension, a bypass becomes engaged and transfers the load through the mooring line. The bypass provides some redundancy within the mooring system and acts as a failsafe. The elongation of the elastomeric tethers can be tailored and is determined by its pre-designed response curve.
The images shown in Figures 3-5 compare the simulated tensions in the mooring line using the elastomeric tether mooring system compared to typical traditional catenary mooring systems.
The response of the elastomeric tether results in a significant reduction in peak loads and shock loads. The two highest spikes in the elastomeric tether response graph in Figure 5 represent instances when the tether reaches full operational extension and the bypass becomes engaged. The frequency of this shock loading is significantly less in the elastomeric tether mooring when compared with the other two mooring systems.
Deployment
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CLICK TO ENLARGE Fig 6: Monitoring buoys, complete with TfI Marine’s elastomeric tethers, being prepared for deployment[/caption]
Once the optimum design solution was finalised, the mooring lines were assembled in Dún Laoghaire. The elastomeric tethers were combined with polypropylene lines using shackle connections. The shackles were whipped and polymorph was applied at the interfaces to eliminate any additional acoustic sounds that may arise from metal-on-metal connections.
The monitoring buoys were successfully deployed using the
Commissioners of Irish Lights vessel
ILV Granuaile, in Dublin Bay on Saturday, 16 September 2017. The buoys were deployed in approximately 20m water depth, with the elastomeric mooring system proving very quick and easy to install. The initial acoustic profiles from the monitoring buoys have returned very positive results.
The elastomeric tethers have already proven their performance in extreme and rare storm events. A month after deployment, two of the most extreme weather events in the past 50 years hit Ireland.
In October 2017, ex-Hurricane Ophelia, closely followed by Storm Brian, passed over the country bringing with them extreme storm conditions. Very high winds together with low central pressure helped generate extremely high waves. A 4.6m high wave and a maximum gust of 135km/h was recorded in Dublin Bay, putting the elastomeric tethers to the test. The elastomeric mooring system performed very well and kept all the monitoring buoys on station.
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CLICK TO ENLARGE Fig 7: Environmental monitoring buoys in Dublin Bay[/caption]
This project has been a very successful deployment overall for TfI Marine, with the silent mooring system proving to be very robust as well as quick and easy to install. Charlotte O’Kelly, managing director and co-founder of Techworks Marine, noted: “We have deliberately made an effort to team up with other innovative Irish companies to provide cutting edge best-in-class solutions, to ensure our clients will have robust, reliable and secure marine data.”
Noel Halloran, CEO of TfI Marine, was equally delighted that this project has showcased TfI Marine’s capability. “Our dynamic tethering system reduces significantly peak loading, and simultaneously, being elastomer, provides a more silent mooring thus enabling improved acoustic monitoring,” he said.
TfI Marine’s products are also suitable for many other applications in areas such a marine renewable energy, aquaculture, navigational aids and other large structures in offshore environments. If you would like to find out more information about TfI Marine’s products, please visit our website www.tfimarine.com or email us at info@tfimarine.com.
Authors:
Barry Doherty, chief operations officer, TfI Marine
Eve Johnston, ocean engineer, TfI Marine