Air bubbler system for suppressing ice on inland ferry routes

Ice-free waterways and harbors can be desirable during a greater portion of the year than what might be possible on behalf of nature. A possibility for suppressing ice formation at such locations can be the use of air bubbler systems. SSPA has just finished a project for STA Road Ferries (Färjerederiet) for proposing and comparing different design alternatives and pipe routes for an air bubbler system intended for a ferry route close to Östersund, Sweden.

The ferry route Håkanstaleden, between Håkansta and Norderön, is not intended to be operated all year round unless weather dictates and the purpose of placing an air bubbler system at this particular ferry route is to enable safer and more efficient ferry operations throughout the part of the winter season when ice roads cannot be sustained. The key factor for STA Road Ferries is to find a system design that is well functioning and energy efficient (enabling low operating costs). Three different bubbler system designs were analysed and simulated. Among the three evaluated system designs, the design eventually recommended by SSPA, was found to achieve a significantly better heat flux distribution along the ferry route while also requiring approximately 15%-20% less energy for operation than the worst performing of the investigated designs.

What is an Air bubbler system?

An air bubbler system consists of a set of piping (often plastic tubing) placed on the bottom of the water body. The pipes carry pressurized air that is let out into the ambient water from small holes along the pipe length. The air flowing into the water will find its way to the surface of the water body due to difference in density of the fluids and subsequently will force water to flow along as well. This creates a circulatory motion of the water in the body, resulting in a flow of warmer water from the bottom regions up to the surface. The technique is relatively rare in use at large-scale commercial operations such as ferry routes but smaller and simpler systems are abundant in marinas.

The design of such a system is intricate and depends on several factors such as:

  • Depth profile of the water body
  • Fluid movements in the water body
  • Temperature gradient in the water body
  • Available heat reserve in the water body 
  • Desired ice-free area
  • Required heat flux to surface of water body

STA Road Ferries in brief

STA Road Ferries is a profit centre operating within the Swedish Transport Administration, the governmental authority tasked with planning, construction, operation and maintenance of the national infrastructure.

  • 21 million passenger journeys annually
  • 11 million vehicle transports annually
  • 66 vessels
  • 41 road ferry routes
  • A fully integrated, certified management system
  • An unswerving commitment to the environment

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Illustration shows Isöleden, operated by STA Road Ferries, which has had an air bubbler system installed since the 1950s and operates all year round. Photo: Kasper Dudzik.