I have developed different mooring systems, from very short and simple once to very long once, which are sitting in over 3500m of water. The different mooring systems also vary in there complexity, some just carry one scientific instrument like an Acoustic Doupler Current Profiler (ADCP), other have multiple systems and are quite complex.
As all our moorings systems are deployed in Antarctic waters means, that they are all so called sub-surface moorings. Because we always face the risk of Icebergs pasing over the moorings the top mooring buoy has to sit below the surface. Therefore the moorings are designed that the top mooring buoy sits at about 200m water depth. But even this is sometimes not deep enough as we had at least twice that a mooring was pushed furhter down by a pasing Iceberg. The design of the mooring buoy is so that the instruments are nicely protected inside it and so the icebergs did not damage any instruments or the main buoy.
Based on my experince and knowledge of mooring systems, I am a member of the NERC Expert Scientific Moorings Assessment Group.