PROPELLER SCOUR AND HSS BERTHS

Berth scour aprons are often needed to protect: -
• Pile wall span height
• Pile embedment
• Passive sliding resistance
• Underscour to gravity structures
• Scour mounding obstructions in berths with low clearances

The Concrete Mattress system: -
• Creates good interlocking concrete slabs underwater and is often used for berth protection
• Unlike rock armour, it does not suffer from rolling or sliding displacement
• Much lower mattress thickness saves dredging and importantly reduces wall span height

Concrete mattress is principally designed to resist suction uplift in berths. Proserve have appropriate design guides and experience, and undertake scour apron design

Propeller Scour
Concrete mattress systems are increasingly being used for the higher propeller flows from larger container vessels due to their low thickness and effectiveness.

Significant suction uplift occurs under propellers, a design guide by the Dept. of Ship Science, University of Southampton, is used to estimate suction uplift forces for the design of concrete mattresses. The principal parameters affecting suction and therefore mattress thickness are: -

• Propeller tip clearance to bed
• Propeller diameter
• Engine power used on berth

Bow Thrusters
Bow thruster jets against quay walls create a downward flow which causes toe scour to unprotected beds. A concrete mattress apron a few metres wide and of modest thickness usually provides effective protection.

HSS Jet Scour
Large HSS vessels have jet exit velocities of up to 23m/s. Jet rotation during berthing creates high speed jet impact onto the beds. Where berths are unprotected, very significant scour holes, up to 9m deep, have been created in soft deposits. The jets produce large positive pressures and smaller areas of associated suction. Mattress is designed to resist suction uplift based on CFD modelling and comparison with proven usage.

The mattress edge detail and its location are critical for the performance of the mattress. It is critical that the mattress edge embedment is below the expected scour depth at the chosen location, usually in an area of low expected scour.