Air lubrication systems are gaining popularity within the maritime industry. However, there are many complexities and challenges involved in the design of such a system. Additionally, the physical phenomena of resistance reduction provided by air lubrication systems is not fully understood.
Shipwright recognizes that such a system is more than the simple sum of its parts. An efficient air lubrication system must be designed with all of its components and their interactions taken into account.
Computational Fluid Dynamics
Computational Fluid Dynamics (CFD) is a powerful tool that can be used in a wide variety of marine applications, including full-scale vessel resistance and propulsion power predictions. Often, the results of CFD analyses are more accurate than contemporary model tests.
Shipwright utilizes in-house CFD software to analyze a vast number of marine scenarios ranging from simple resistance estimations, to complex seakeeping and propeller performance predictions.
Specialized Testing Facilities
Shipwright has developed a one-of-a-kind testing facility designed to simulate real-world conditions at the bottom of a ship hull. This facility allows us to analyze bubble interactions with hull plating and to precisely determine the effects of these interactions. Using data acquired from such tests, Shipwright can validate predictions and tune CFD model physics as necessary to develop more accurate and effective solutions.
The air supply system is an integral part of the ALS and must be designed to meet the unique requirements of each installation. Maximizing the efficiency of the air system while delivering the air to the optimal injection points is the key to optimizing the efficiency of the entire air lubrication system.
Shipwright has extensive experience in project management for exhaust scrubber installations and diving operations for vessels in service. This expertise can be applied to ALS installations to ensure a smooth and efficient transition from concept to operation.
Optimizing the Entire System
Decreasing a vessel’s resistance decreases the amount of power required to drive the vessel. This reduces the vessel’s fuel consumption. Reducing fuel consumption reduces the cost to operate the vessel. Therefore, all other factors held equal, a decrease in vessel resistance results in a direct reduction of cost.
THE COST OF AIR INJECTION
Compressors or blowers are required to force air out of a vessel’s hull. The higher the air pressure and the more air being injected, the more power required to drive the system. This power must be accounted for when designing the system. If the compressor requires more power than the air injection is saving, the system isn’t useful.
OPTIMIZING THE NET POWER REDUCTION
The net power is simply the power saved by the ALS system, minus the power required to achieve that saving. In any given condition, there is an optimal point at which the net power requirement is minimized. In other words, more air would cost more than it would save and less air would provide lower savings than the system is capable of.
Air Lubrication Study
Shipwright has conducted test cases using CFD to demonstrate the potential for analyzing the resistance reduction effects of ALS.
A test hull similar to that of a cruise ship was simulated to run at 17 knots in calm water. The vessel was free to heave and pitch, identical to a standard calm water resistance test conducted in a towing tank.
The resistance of the bare hull was determined and the hull was modified to have a slot through which air was injected.
The slot and air injection parameters were then modified to maximize the resistance reduction effect while minimizing the required air injection rate.
The ALS Version 2 hull was used in an optimization study, again utilizing CFD. The same hull model and injection slot parameters were used, with the vessel running at 17 knots as in the previous simulations.
The air injection rate was varied and a required propulsion power curve was developed from the resulting data. The required compressor power was calculated for each injection rate and a corresponding net power curve was created. The propulsion power curve is depicted in orange, while the net power curve is depicted in blue.
The gross propulsion power is minimized at an air injection rate of approximately 5 kg/s. However, the net power consumption is minimized at a lower rate of approximately 4 kg/s.This is the optimal operating point for the system at a vessel speed of 17 knots.
This study further highlights the importance of optimizing the air lubrication system to provide the maximum environmental and economic benefit, rather than minimizing the vessel’s resistance. The point of minimum resistance may require more power than it’s worth.
This simple CFD study shows that more air does not necessarily result in lower resistance. It is more important that the air be distributed to maximize its effectiveness.
In the ALS Version 2 study, the smaller injection slot with a lower air injection rate provided more than twice the resistance reduction effect of the ALS Version 1 study.
BARE HULL // Total Resistance: 945 kN
ALS VERSION 1 // Injection Slot Width: 22 m Air Injection Rate: 6 kg/s Total Resistance: 896 kN Resistance Reduction: 5.2%
ALS VERSION 2 // Injection Slot Width: 15 m Air Injection Rate: 5 kg/s Total Resistance: 819 kN Resistance Reduction: 13.4%
Shipwright’s engineers and project managers have the ability to take an air lubrication system from early concept design to completed installation using in-house resources.
This results in an efficient design process with reduced cost and product delivery time for our clients.
Air Lubrication: From Concept to Operation
Here is an example of a design process for an ALS system.
1 // FIRST IN FLEET — INITIAL DATA COLLECTION
The first step in a design process is understanding the physical constraints and the client’s goals. The first step in an ALS installation is working together with the client to identify which of their vessels would benefit most from an air lubrication system. Once a vessel is selected, a CAD model of the vessel is developed and operational performance data collected. This data provides Shipwright engineers with the hull geometry and operational factors for concept design.
2 // INITIAL CFD MODELING
The hull form is analyzed with CFD to determine the original resistance characteristics. The results are compared with the operational data to ensure the model is accurate. Shipwright engineers also use the results of these analyses to target air injection points for initial ALS tests.
3 // ALS MODELING AND FLOW CHANNEL TESTING
Using CFD Shipwright conducts analyses to determine optimal air injection locations and flow rates. Testing is conducted in our purpose-built flow channel to validate these CFD models and improve solution accuracy
4 // AIR SYSTEM DESIGN
The air supply system is designed to minimize parasitic losses and leverage existing ship equipment wherever possible, to minimize cost to the client. The compressor or blower will be selected for each ALS installation to ensure that the efficiency of the entire system is optimized within the operational envelope set forth by the client.
5 // PROJECT MANAGEMENT
Shipwright employs its team of experienced project managers to oversee the installation of the ALS system. These project managers are backed by the same engineering team responsible for the system’s design, ensuring it is installed to specifications. Keeping the entire process from concept to installation in-house ensures smooth and efficient workflow.
6 // CONTINUED MONITORING AND SUPPORT
After a first-in-fleet ALS installation is complete, vessel performance data is continuously collected to determine the effectiveness of the system in various operating conditions. Shipwright engineers will analyze this data and work with the client to identify potential improvements for future air lubrication systems on target vessels.