Contact:
Dr. Mario Hoppmann
Website:
Lilian Schubert
What are “moorings”?
Moorings are measuring systems that remain in a specific location in the ocean for several years. A mooring initially consists of a base weight, a long rope and some buoyancy bodies that hold the rope vertically in the water column. Various measuring instruments are attached to the rope, which can independently measure water temperature, salinity, current speed and direction and many other important parameters. There are also devices that can measure ice thickness, collect suspended particles in the water, take water samples or even record the sounds of marine mammals.
Moorings can be several kilometers long. They are lowered into the water during ship expeditions using cranes, sliding beams and winches. The base weight is placed on the seabed and the buoyancy bodies attached to the rope hold the rope (almost) vertically in the water column. The rope is precisely measured so that the uppermost buoyancy bodies are at a safe distance from the water surface. The measuring devices are programmed beforehand with a computer and attached to the rope during lowering. They then measure the water properties independently, e.g. every hour. The batteries in the devices usually last for several years.
How do you get the devices back?
The geographical position of such a mooring must be precisely recorded when it is deployed. Within the next 1-5 years, this location will be visited again by a ship to retrieve the mooring, and in particular the measuring devices with their valuable data. But how do you get the whole thing out of the water again?
At the very bottom of the rope, directly above the bottom weight, a special device is attached: a so-called (Posidonia) releaser. This is an instrument that can be activated by means of acoustic signals in order to open a hook. This hook is connected to the base weight. This “release” allows the entire rope to detach from the bottom weight and float up with all its buoyancy bodies and equipment.
The signal is transmitted from on board the ship using a so-called Posidonia system, which is even able to track the rise of the mooring in the water. Acoustic communication with such a trigger works even at depths of up to several kilometers. Over the course of the next few minutes after the release, the buoyancy bodies gradually rise to the surface and the mooring can be collected. The bottom weight, which weighs several tons and usually consists of several railroad wheels, remains on the seabed. There it is gradually colonized by organisms. The individual devices are detached from the cable and the data read out by computer.
What is the reason for all this effort?
Ship expeditions, especially to the polar regions, are very expensive and logistically complex. At the same time, however, they are urgently needed in order to better understand the climate system and its dramatic changes. In marine research in particular, there are three special challenges with regard to collecting observation data:
1. most scientific ship expeditions take place around the summer (May - October in the northern hemisphere, November - April in the southern hemisphere). This is when the sea ice cover is small and the ice is comparatively thin. Even capable icebreakers can only move efficiently in the polar oceans then. In winter, this is much more difficult, or not possible at all. As a result, there is virtually no data in these regions during the winter months.
2. the number of suitable research vessels is very small, which means that the spatial coverage of ocean measurements is also very limited. Sometimes several ships are part of a scientific project and can measure the same thing at different locations at the same time. However, this does not happen very often.
3. In addition to the physical system, i.e. temperature, salinity and currents, there are many other aspects and processes. These include, for example, the chemical composition of the polar oceans, pollution with microplastics and changes in ecosystems. Only the simultaneous observation of all these aspects (or as many of them as possible) allows a comprehensive understanding of the process. However, many different experts are needed to measure all these aspects.
Only by deploying autonomously measuring sensors on suitable platforms (such as moorings, but also on measuring buoys) can all these different aspects be addressed. An example of this would be the operation of several moorings with physical and biogeochemical sensors distributed over a certain area (e.g. in the Fram Strait or in the Weddell Sea) over a period of several years or even decades. This has been done at the AWI for a long time and forms the backbone of our long-term observations.
Can anything go wrong?
It all sounds pretty simple in theory, but in fact a lot of problems can arise. Here is a small selection:
The design of an anchorage must be meticulously calculated during the planning phase. In particular, the ratio between the air-filled (but pressure-resistant) buoyancy bodies and the weight of the attached equipment must be correct. The buoyancy bodies must also be installed at suitable points on the rope. The same applies, of course, to the type of sensors and the depths at which they are to be suspended. This is determined in advance by the scientist responsible. It can sometimes happen that the buoyancy bodies burst, water penetrates the sensors, ropes break, shackles or rings (or even sensors) corrode. Fortunately, this rarely happens.
