Marine growth is the unwanted colonization of marine structures by marine organisms. The current standards prescribe a thickness of 100~mm whereas in the literature, layers up to 200~mm thick have been reported for structures in the North Sea.
Marine growth is the unwanted colonization of marine organisms (zooplankton or phytoplankton) that is attached to marine structure. It should be considered in the design/analysis of the platform since marine growth leads to escalate wave forces and mass of the structure.
Based on the terminology Oldfeld, marine growth is classified into Mussels (frequently dominant. Hard exterior, firmly attached); Kelp (long frond-like growths); Algae (soft, branching); Barnacles, tubeworms, limpets (Hard, firmly attached growth, mostly one organism thick, difficult to remove); Hydroids, Bryozoa (Soft, branching and compressible growths); Sponges, anemones, sea-squirts, alcyonium (Soft bodied, often bulky organisms). A lot of factors influence the type and amount of marine growth including physical and biological factors. On the physical itself, temperature, salinity, water depth, and wave action are the main cause. On the other side, predator, larval supply, nutrient and food availability, individual species are the main factors in affecting marine growth type.
As Marine growth has a higher specific gravity than seawater, the dead load of the structure will be potentially increased. For references, the density of marine growth in Arabian Gulf and West Java Sea (Indonesia) are 1.23 MT/m3 and 1.40 MT/m3, respectively. Although marine growth will raise the total mass, the structural stiffness will not be contributed.
According to API RP 2A, all elements of structure (members, conductors, risers, appurtenances, etc.) are increased in cross-sectional area by marine growth. The effective element diameter (D) is combination between clean outer diameter (Dc) and the average growth thickness (t) that would be obtained by circumferential measurements with a 1 inch to 4 inch wide tape.
An additional parameter that effects the drag coefficient of elements with circular cross-sections is the relative roughness, e=k/D, where k is the average peak to valley height of “hard” growth organisms.