When estimating the marine growth for an offshore structure in service for 25-30 or more years, it is important to look at the oceanography data for that particular region. Each region will produce different amount or type of marine growth.
In my past decommissioning, abandonment and removal of the offshore structures, we used an estimate of nearly equal or exceeding the weight of the steel structure itself. Some structures may have more marine growth than other depending on the water depth and its particular area.
Hope that provides some feedback.