A hatch cover for deployment over a hatchway formed in a floating roof liquid storage tank permits rapid loading and unloading of inspection or maintenance equipment and minimizes vapor emissions from the tank. A first plurality of optoelectronic devices is provided on a surface of a substrate, the first optoelectronic devices being arranged in a 2D array pattern that corresponds to a 2D array pattern corresponding to different light cores of a first multicore optical fiber. By combining the appropriate quantities of drilling mud with base fluid, a riser mud density at or near the density of seawater may be achieved.