A massive amount of cargo is transported by ship with more than 11 billion tonnes moved in 2019, equivalent to roughly 90 percent of the world’s goods, or the weight of 33,522 Empire State Buildings. While shipments dipped slightly due to COVID-19, the international seaborne trade volume is projected to expand at an average annual growth rate of 3.5 percent through 2024, due to growth in world trade. Whether it is dry bulk (e.g., grain, ore), liquid bulk (e.g., petroleum, LNG), big-screen TVs in containers, or general cargo (e.g., break-bulk, cars, heavy machinery), this trade relies on a global merchant fleet of approximately 98,000 ships looking to safely and swiftly load and unload their cargo at a network of ports around the world.
Because a significant amount of capital is required to build and operate a port, port owners and operators rely on quick turnaround to maximize profits, and just-in-time delivery requires efficiency to support a global supply chain network. A key obstacle to reaching that goal is berth downtime—the total time that the ship spends at port when it is unable to load or unload cargo.
Causes of berth downtime
While berth downtime can be triggered by equipment failure on the ship or at the port, disruption in the inland transportation network, or human factors, the following are predominant causes:
1. Wind and waves
Forces due to waves, wind, and current lead to a ship moving from its intended position at the berth. The forces and resulting ship motions typically consist of both static and dynamic components. The magnitudes of these forces depend on:
- Size and draft of the ship
- Wave height and wave period at the berth
- Wind speed
- Current speed
- Orientation of the ship relative to the wave, wind, and current directions
- Water depth at the berth
At some ports, a ship at berth can experience significant forces and motions due to the hydrodynamic disturbance produced by another ship entering or leaving the port. The occurrence and magnitude of passing ship dynamic forces, which are governed by parameters such as ship size, speed, and passing distance, are more deterministic however, in comparison to forces generated by the weather. Passing ship effects can nevertheless lead to downtime and are an important consideration during the design of a new berth.
2. Ship motions and mooring loads
To secure the ship and limit its movement in response to environmental forces, mooring lines (i.e., ropes) are connected from the ship to bollards or mooring hooks installed on fixed structures at the berth. At some ports, the mooring lines are connected to large floating buoys, which in turn are anchored to the seabed using heavy chains. When the ship is berthed alongside a rigid structure such as a quay wall or loading platform, fenders are placed between the ship hull and fixed structure to act as an elastic cushion. The combination of mooring lines and fenders is typically referred to as a mooring system.
When weather conditions worsen, such as an increase in wind speed or wave height, a reduction in loading/unloading efficiency or even a complete stoppage of cargo handling operations can occur due to one or both of the following events:
- Ship movements – displacements, velocities, and accelerations – exceed the safe operating envelope of the cargo handling equipment.
- The tension in the mooring lines or compression force in the fenders exceed allowable values.
In the first type of downtime where cargo handling is no longer safe, operations are temporarily halted while the ship remains moored at the berth. In the latter case however, the ship is forced to disconnect its mooring lines, leave the berth, and anchor in open water until weather conditions improve. This results in even higher operating costs since tugs are usually needed to escort the ship to and from the berth and more time is needed to resume operations once conditions improve.
Estimating berth downtime
Predicting the type, frequency, and duration of berth downtime events is critical in the early stages of a port project, as this parameter governs the port’s economic viability. It also provides insight into the processes driving downtime and helps identify solutions to minimize it.
The ability of a port designer to provide bankable advice to port owners and operators on berth downtime requires that they have access to expertise in a diverse range of subjects including meteorology; statistics; wave hydrodynamics in deep and shallow waters; naval architecture; forces on floating bodies; nonlinear mooring line and fender behavior; anchor-soil interaction, and floating body dynamics.
The analysis of berth downtime involves several phases:
1. Define operational metocean conditions
The operational or day-to-day metocean (meteorological + oceanographic) conditions at the proposed berth location describe wave and wind magnitude and variability with time. Typically, this is a record of wind and wave characteristics at hourly intervals over the past 20-30 years. Other metocean parameters, such as tide elevation, current, air/water temperature, and precipitation can also affect berth downtime for certain types of port and cargo.
The assembly of metocean data can itself involve multiple tasks:
- Field data collection
- Bathymetry survey and coastal Geographical Information System (GIS)
- Wave hindcasting and transformation
- Water level and current circulation modelling
- Climate change effects
2. Determine berth configuration
Selection of a suitable berth arrangement, such as the marine structures, mooring system components, and layout, depends on several criteria including water depth, ship size, type of cargo, target throughput, and metocean conditions.
3. Analysis of ship motions and mooring system forces
Berth downtime prediction relies on estimates of threshold metocean conditions beyond which cargo handling operations are interrupted. Methods include numerical modelling and scale model testing.
4. Analyze berth downtime statistics
Analysis of monthly and annual berth downtime using the metocean data and operating thresholds can provide guidance on achievable port throughput, and commercial impacts of weather-related delays.
How to mitigate berth downtime
Here are some guidelines to increase the reliability of berth downtime estimates and its mitigation based on lessons I have learned through supporting port projects over the past 26 years:
- Do not underestimate the value of field data collection during the feasibility phase: A program of metocean measurements at the port site enables validation of the hindcast and wave transformation models and greatly increases the reliability of the berth downtime estimate. This is especially important in coastal areas where numerical models may not fully represent the magnitude of phenomena such as infragravity waves, which can amplify ship motions at berth. A relatively small early investment can prevent significant operating cost increases in the future.
- Change the berth location/orientation or explore alternate port sites if possible: The power of numerical wave transformation and ship motion models is that they allow the engineer to rapidly investigate whether a berth location with better natural sheltering exists in the general study area, or whether a simple change in the orientation of the moored ship will reduce its dynamic response to wave forces.
- Consider using the latest mooring systems technology: While the basics of mooring a ship at berth have not changed in centuries, new materials and technology are making it possible to achieve reduced ship motions in more challenging conditions. Examples include:
- Use of mooring lines made with high-modulus polyethylene HMPE fibres (e.g., Dyneema™) which are 15 times stronger and 8 times lighter than steel wire of the same size. The low stretch, high stiffness, and high strength of Dyneema™ lines allow for higher metocean threshold conditions and reduced berth downtime.
- Use of automated mooring systems that use a variety of computer controlled electrical and mechanical systems to increase damping and thereby reduce ship motions. While the capital cost of automated mooring systems is markedly higher than that of their conventional counterparts, the resulting increase in port throughput can quickly offset the initial investment in many cases. In the past, I have written software add-ons for commercial ship motion analysis programs (e.g., ANSYS-AQWA™) to simulate the efficacy of automated mooring systems and have advised clients on whether such technology can benefit their port project.
- Optimize breakwater location and length: For some exposed port sites, the metocean conditions and port throughput requirements may necessitate the inclusion of a breakwater to shelter the berth. Breakwaters can lead to marked changes in patterns of coastal current circulation, water exchange, and sediment transport, and therefore come with a significant environmental footprint. Breakwater configuration can be optimized using both numerical modelling and wave basin tests to achieve target throughput while minimizing cost and impact on the surroundings.
- Increase product storage at export facility: For some ports, it may be possible to achieve annual throughput targets even though there are protracted seasonal downtime events in some months. For example, a combination of berth downtime analysis and discrete event simulation of the processes involved in the production, transport, storage, and loading of iron ore from mine to port can determine the size of the stockpile required at the port. This allows ore production to continue at the mine even though ships are temporarily not being loaded at the port. Once weather conditions improve, the accumulated stockpile can be depleted to normal levels by increasing the number of ship arrivals at port. This method also allows the setup of a ship arrival schedule which significantly reduces the risk of weather delays and associated demurrage costs.
- Monitor everything: The dynamic behavior of a ship at berth is strongly affected not only by the type of mooring system it uses, but by how well the ship’s crew deploys and tends the mooring lines when the ship is at berth. As environmental conditions (waves, wind, current, tide level) and the ship’s draft change, some mooring lines will experience higher tension while others may even go slack. Both these situations lead to increased ship motions and overloaded (or even broken) mooring lines and increased berth downtime. Instrumentation at the port to continuously monitor and relay the loads in the mooring system to the ship’s crew enables them to maintain the correct level of pre-tension in the lines and greatly improve ship operability. Monitoring of the metocean conditions at the port also improves safety and efficiency of ship handling operations.
When developing a new port or expanding an existing port - whether it is building a new port for exporting copper concentrate in Chile or deepening a container vessel quay in Sweden from 14 m to 19 m to accommodate larger ships, it is imperative to consider throughput targets and berth downtime. Understanding the issues and impacts can help port stakeholders determine the best way to minimize downtime without significantly increasing capital outlay or impacting the environmental footprint of the facility. This analysis provides insight into the diversity and complexity of the processes at play and can empower port owners and operators to make cost-effective decisions, which can mitigate berth downtime and ultimately maximize port throughput.
About the Author
Sundar Prasad, Ph.D., is a Senior Coastal and Metocean Specialist at WSP*. He has 26 years of global experience in metocean studies, development of metocean design criteria, ship motions and dynamic mooring analysis, port siting, navigation simulations, and berth availability assessments. Dr. Prasad is presently a contributor to PIANC, The World Association for Waterborne Transport Infrastructure Working Group (WG212) whose mandate is to update existing guidance on the “Criteria for Acceptable Movement of Ships at Berths”.
* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.