First of all, some background on the phenomenon itself:
Biofouling occurs when marine organisms build up on submerged surfaces, such as the underside of ships, underwater structures and technical instruments. A thin layer of organic molecules adsorbed on to the surface of an instrument may seem relatively harmless, but that facilitates the attachment of micro-organisms, such as bacteria or algae. These in turn may act as bait for larger organisms such as barnacles, mussels and sponges, as well as more mobile creatures such as crabs, shrimps and snails.
Before you know it, subsea equipment can be covered and potentially rendered useless, requiring a time-consuming and possibly costly extraction and clean-up operation.
A more effective technique for preventing biofouling
One long-standing approach has been to smear the transducer with zinc oxide paste. This is known to many as the stuff used by parents to alleviate baby’s diaper rash, but it is also a biocide widely used in marine industries to deter the build-up of micro-organisms.
However, zinc paste can wash off submerged surfaces over time, reducing its effectiveness, and, potentially, that of the transducer.
Therefore, Nortek wanted to introduce its users to a more recently developed and potentially more effective technique – the application of silicone-based stickers to the transducer windows. The surface of these stickers is hard for micro-organisms to adhere to, enabling marine life to be washed away more easily when water flows over the transducer.
Nortek believed that these stickers worked better than zinc paste but wanted to find out what its customers thought. So, the company sent stickers made by the Japanese company Nitto to ADCP users to try out in some projects where biofouling was a significant problem. The stickers were provided pre-cut to fit Nortek equipment, but they can be applied in many configurations up to 50 cm wide.
Meeting the challenge of biofouling on ADCPs in Texas waters
William Fuller is a graduate researcher working as part of a team at Texas A&M University (TAMU) headed by Dr Jens Figlus. The team has been deploying two Nortek Aquadopp current profilers as part of a project in Galveston Bay that is assessing the hydrodynamic impact of turbulence caused by the wakes of large ships using the nearby Houston Ship Channel on dykes being built in the area to protect wetlands.
Given the warm water and periodic intense currents created by these ship wakes, which churn up marine life, biofouling can be a serious problem in the area.
While zinc paste – the project’s usual method – worked fine in combating biofouling in late winter and early spring, the team noticed it became less effective later in the year as the water heated up and the paste became eroded.
So Fuller was happy to try out the Nitto stickers, applying them to one of the project’s profilers, while using Desitin zinc oxide paste on the other ADCP for comparison.