To mitigate the effects of infragravity waves and improve safety and efficiency, ports should consider employing automated mooring systems, writes Trelleborg marine and infrastructure operation president Richard Hepworth
The primary aim for ports and the wider shipping industry is to ensure optimised operational efficiency. But when there are external factors at play – inclement weather, long period waves and the effects of passing ships – this can reduce berth efficiencies by increasing the motions of a vessel at berth. In the worst-case scenario, it can result in reduced throughput and downtime.
However, some ports are incorporating automated and technology-driven equipment to reduce and even eliminate these effects, enhancing safety, efficiency and uptime across berthing and mooring operations.
One met-ocean condition that can have an impact on moored vessels is infragravity waves. These are ocean surface gravity waves, with periods greater than 30 seconds. The wave height is generally smaller than a regular wave when in deep water but can increase in shallow water. Due to having longer periods they are less affected by breakwaters, allowing them to travel uninhibited into harbours and, as a result, can have a significant impact on moored vessels.
There are significant risks associated with these waves, including lines snapping and vessels drifting from berth. This can increase rates of unplanned downtime, as waves have a high level of energy transfer to the vessel and can cause large vessel motions.
In the LNG sector, safety is paramount during transfer operations and greater ship movement caused by such external factors can result in decoupling, triggering damaging unplanned downtime. In some cases, berths experience 15-20% downtime due to met-ocean factors.
One of the ways in which port owners and operators can maximise berth utilisation to allow product or passenger transfers to occur seamlessly and reduce downtime, is minimising the effects of adverse environmental and met-ocean conditions, as well as the effects of passing vessels. This includes static forces; those that are considered more consistent such as short period waves, current and constant wind, and dynamic forces; those that change significantly over time, such as long period waves, gusting winds or the effects of passing ships.
For a mooring system to be effective, it must take all of this into account to restrain vessel motions sufficiently, to allow a big enough window for efficient product transfer. The use of automated mooring technology is becoming more commonplace and is allowing the industry to better manage the challenges faced when maximising berth utilisation.
Traditional versus technology-driven solutions
Trelleborg has conducted two studies which analyse the benefits of using technology-driven solutions, versus conventional mooring arrangements.
The first study compared motions of a 10,000-TEU container vessel when moored in an optimal conventional mooring arrangement at a container terminal located near a breakwater entrance, where it is unprotected from the effects of infragravity waves. Analyses were undertaken on the vessel when secured with mooring lines, versus a rope-free automated mooring system. This was conducted using dynamic mooring analysis software, capable of fast, high-precision hydrodynamic simulations.
Results showed that the automated units successfully moored the vessel with significantly less vessel surge and sway, when compared to mooring with lines. The reduction in vessel motions indicated an increase in berth operability for product transfer from 65% to 95%.
Infragravity waves can have large effects on the motions of the vessel at berth due to their interaction, leading to higher vessel excitation than from a regular sea-state.
Vessels in these conditions will typically have motions that cannot be restrained by mooring lines effectively to maintain cargo operations or a safe mooring, as they can become slack or snap under force.
Dynamic mooring equipment
The second study applied the forces of a passing vessel to a moored vessel and benchmarked a conventional static mooring line arrangement against an automated dynamic mooring system for an upstream facility. The passing vessel was at a distance of 160 m, traveling at 10 knots at complete low tide.
The main motions of concern in a passing vessel analysis is the surge and yaw of the moored vessel and the peak loads on the spring lines and short breast lines.
Results of the study showed that the dynamic system significantly reduced the effects of the passing vessel, as well as parting lines caused by spike loading. When compared with traditional mooring solutions, the dynamic units improved vessel stability and helped to eliminate human error, allowing for a greater product transfer window of operation.
How to choose your technology-driven mooring equipment
When choosing technology-driven mooring equipment, it is important to consider solutions that will make berthing operations smarter, safer, and more sustainable and competitive.
Given the significant impact of infragravity waves on vessels at berth, it is important to specify an automated mooring system which combines vacuum pads and passive damping technology, to rapidly attach and secure the vessel. A solution which can monitor mooring loads acting on the vessel and can provide live data to the operator, will help to optimise day-to-day port and terminal operations. This will reduce risks and unwanted downtime due to damage or injury.
Similarly, the ideal engineered mooring solution should combine a quick release hook with a constant tensioning system, to best handle the effects of passing ships. This will avoid the need for complex pulley systems and route mooring lines to the vessel, which can sag or snap under force. It should incorporate computer-aided design, finite element, and hydrodynamic analysis technologies, to ensure the impact on the vessel at berth is understood completely, to improve safety and optimise transfer windows.
As technology continues to advance and demands faced by the shipping industry grow even tougher, it has never been more important to automate operations where possible. Port owners and operators are continually looking for ways to improve operational efficiency, widen the window for transfers and reduce risks along the way. It is apparent that the more sophisticated, purpose-designed technological solutions can help with this, while improving the sustainability of the industry.
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