ENVIORNMENTAL

Marine Growth

Any structure placed into water is subject to marine growth as shown in the diagram below:

Schematic of critical biofouling stages [L.D. Chambers et al, 2006]

After placing any structure in the water its surface immediately starts accumulating simple organisms creating biofilm, which, in turn provides the food and habitat for more advanced organisms.
Effects of marine growth on loading [Iberahin Jusoh et al, 1996]:

  • increase in structural diameter and displace volume
  • increase in force coefficients
  • increase in structural weight
  • increase in mass andhydrodynamic added mass
  • increased flow instability
  • conceal the member's outersurface
    • For the current project it was vital to investigate the effect of marine growth on a hydrofoil due to its sensitivity to lift and drag.
    Factors affecting marine growth:
  • Environmental
  • Temperature, Illumination, Salinity, Alkalinity
  • Hydrodynamic
  • Flow, Turbulence, Acceleration
    • The most likely species to occur in 0 to 50 m zone from the surface are:
    Mussels (hard growth) - likely to appear in the water range 0-40 m, thickness up to 35 mm. Successful colonizer due to its ability to settle out in high currents (5 knots)

    Seaweedsappear in 0 - 25 m depth

    Anemones, Soft Corals appear in water depth 14 - 100 m, thickness up to 30 mm


    Barnacles - occur in shallow and deep water, can grow up to 80 mm, hard to remove because of the tensile strength in excess of 50,000 p.s.i

    Tubeworms - appear at 35 - 100 m,
    Hydroids 50 -300 mm, appear in shallow and deep water depending on the type of hydroid,
    Tunicates - from the intertidal zone to deep waters

    For this project the following assumptions were made:
  • Hydrofoil surface area 130 m^2
  • Material for hydrofoil - steel
  • or the overall surface marine growth is evenly spread and incompressible
    • Simulation was made in ANSYS program for the hydrofoil with an angle of attack of 8 degrees in order to investigate how much the performance will be affected by marine growth. The expected thickness after one year is 30 mm. The results of the simulation are represented in the table below.

    The sensitivity analysis was carried out with regards to marine growth establishment on the surface of hydrofoil and it was decided to rerun the simulation assuming 10 mm of marine growth. Results are represented in the table below.

    Surprisingly, the results didn't have much difference between each other, the drag was increased by 3 times and lift decreased by 2 times. Based on previous results, the mooring angle from seabed will be decreased by approximately 15 degrees which increases a footprint of the system

    The outcome of the simulation means two things: 1)the marine growth has a significant effect on the performance of the hydrofoil 2) there are great uncertainties about the program simulation such as
  • Marine growth is site & season specific
  • Growth varies with depth
  • Different type of growth has different thickness & distribution
  • The data which was used in the analysis was taken from offshore structures (oil rig platforms) studies. Prediction of marine growth on dynamic structures such as hydrofoils have a very high level of uncertainty.
    • Conclusion.
  • Marine growth has significant effect on performance
  • Issues with stability
  • Alternative material& profile options may improve the performance
  • High manufacturing costs
  • High maintenance costs
    • In order to deploy the hydrofoil in the system with CoRMaT further research is needed (perhaps site testing) to investigate the realistic performance.