There is no better manner to estimate this than having previous or getting new field data - which, I am sure, you can find, since there are several universities and research institutes nearby - on the fouling development (biomass, species composition - some species are usually defined as "soft fouling", while calcified biofoulers (e.g., barnacles) are called "hard fouling" by people from more applied fields) on such surfaces. Fouling development will also depend on the composition, orientation, roughness and diameter of the pipe in question. 

Bio fouling is a phenomenon in which ecological sequence of biological organisms on the immersed objects. we can estimate the bio fouling using a quadrant (size depending upon the surface 0.25X0.25M ), i don't know method for calculating inside the pipes.  In general, BGA only dominates  inside the pipes.

Ask to PiedraHita at Santa Barbara or in Aquaculture Eingenerin Association 

Hi, did you ever get a straight answer to this? Please share relevant papers that actually list biofouling rates. I would be interested to see them.