Project Requirement
As part of the expansion of Port Canaveral, Cruise Terminal 3 was constructed to accommodate the new generation of LNG-powered cruise vessels operated by Carnival Cruise Lines. These vessels, displacing approximately 180,000 tons, are equipped with three azipod thrusters, each capable of delivering 20 MW of power and full 360° rotation. This enables dynamic manoeuvring without tug assistance but also introduces exceptionally high propeller jet velocities and suction forces at berth.
Given the minimal underkeel clearance and the sensitivity of the quay wall superstructure, the project required a robust scour protection solution that would deliver long-term performance under continuous high-load berthing and unberthing conditions. Traditional rock armour was considered infeasible due to required depth and maintenance challenges under these conditions.
Concrete Mattress Solution
Mott MacDonald specified a tailored in situ concrete mattress solution consisting of multiple mattress types and thicknesses ranging from 200 mm to 600 mm. The variation in thickness addressed the differing levels of propulsion impact across the berth area. These mattresses formed a continuous concrete apron with sealed joints and reinforced edge detailing to prevent underscour.
The fabric formwork system used allowed mattresses to be unrolled underwater by divers, zipped together, and pump-filled in situ with high-strength micro-concrete. This approach created a flexible, high-performance scour protection layer that could be installed to suit the complex geometries and challenging working environment at the berth.
Design Principles for Azipod Propulsion
Proserve’s design approach for azipod-driven vessels is grounded in extensive modelling and testing, which has shown that azipods produce scour conditions comparable to open propellers—with some key differences that drive design.
- Jet and Suction Behaviour: Azipods with bottom fins create concentrated high-velocity jets, while those without fins generate more diffuse, turbulent flow. Both configurations can cause significant suction, especially where clearance beneath the vessel is low.
- Clearance Sensitivity: Low tip clearance increases suction forces dramatically. Mattress thickness is therefore closely linked to clearance ratio, particularly near quay walls and under vessel centres.
- Sealed Protection: Testing confirmed that sealed concrete mattress systems—with interlocking joints and wall seals—offer superior performance by preventing underscour and resisting edge washout, unlike traditional rock armour.
- Surface Profile Control: Proserve’s designs specify low surface undulation to reduce local hydrodynamic loads and ensure even stress distribution. Undulation ratios below 0.16 are preferred.
- Multi-Azipod Interaction: Where vessels use twin or triple azipods, Proserve accounts for the amplified bed velocities caused by overlapping jets and rotating thrust, using conservative spacing and velocity factors based on physical model data.
- Optimised Design Coefficients: Proprietary coefficients for suction and flow design enable accurate mattress sizing, ensuring stability without overdesign.
These principles ensure that Proserve’s insitu mattress systems provide durable, efficient protection tailored to the intense and variable conditions created by modern cruise propulsion systems.
Berth Scour Protection Design for Azipods - White Paper
Installation Challenges
Installation was carried out in collaboration with RUSH Marine and Viking Diving. The environment was particularly challenging due to:
- Operational Cruise Schedule: Divers were only permitted to work during narrow time windows between vessel arrivals and departures. This necessitated careful planning and rapid installation sequences.
- Zero Visibility Conditions: All works were performed in turbid waters, relying heavily on diver skill and tactile feedback.
- Propeller Wash Risk: Partially installed formwork was vulnerable to damage from azipod wash. To mitigate this, the ‘zip flap’ method was employed—each mattress was zipped and filled immediately, avoiding any unfilled fabric being left exposed overnight.
Advantages Over Rock Armour for Azipod Cruise Terminals
Compared to traditional rock armour, in situ concrete mattresses offered several distinct advantages for the azipod-driven cruise terminal at Port Canaveral:
- Reduced Thickness: Concrete mattresses achieve equivalent or greater resistance to scour using less depth than multi-layer rock systems. This was crucial given the tight clearance requirements of the cruise vessels.
- Sealed and Integrated Protection: The interlocked slab system with sealed edges prevents undermining and eliminates the risk of individual element movement, a key failure mode for rock armour under azipod flow.
- Lower Maintenance Requirement: Unlike rock which can become dislodged and require regrading, the mattress system provides a fixed, stable protection with minimal long-term intervention.
- Improved Construction Integration: The mattress system’s reduced thickness enabled savings in quay wall height and pile embedment depth, contributing to overall project economy.
Conclusion
The use of in situ concrete mattress scour protection at Cruise Terminal 3 successfully addressed the unique and demanding conditions created by azipod propulsion. By tailoring the mattress design to suit suction and flow forces and by implementing a highly coordinated installation strategy, the project delivered a long-term, low-maintenance solution that is ideally suited to modern high-power cruise berths where conventional rock protection would be inadequate.