In-line mooring

Two general in-line mooring methods are presented, referred to as an L-mooring and an I-mooring.

The L-mooring, illustrated in Figure 1&2, is a deployment setup that is suitable for shallow water moorings. The advantage of the L-shape is that the instrument is protected from the winds, waves and currents that the surface buoy is exposed to. This will prevent waves and bad weather from influencing the data quality (depending on the distance from the surface of course). Having the hardware submerged and away from the most dynamical part of the water column leads to a considerable reduction in component fatigue due to surface-wave action. The surface buoy works as a signature of where the instrument is located, and as an aid in the retrieval of the system.

It is recommended to attach a subsurface buoy along the mooring line resting at the bottom. If something should happen with the surface buoy or the mooring line attached to the surface signature, the bottom line can be used as a backup for recovery. It is easier to catch the line if it is lifted off the bottom by the buoy compared to when it is resting along the bottom. Another improvement recommended is to add a subsurface buoy above the anchor to the left, as illustrated. This will prevent abrasion of the line during e.g. low tide. The magnitude of the buoyancy must be such that no part of the cable touches the ocean floor under slack conditions.

The bottom mounted surface penetrating I-mooring can be used at all depths. Figure 2 shows a typical I-mooring with the instrument pointing downward. An alternative is to mount the instrument looking up. The disadvantage of using a mooring type with the instrument directly connected to a surface buoy like this, is that the buoy is exposed to storms, waves, winds and strong surface currents. The motion transferred to the buoy by the environment makes this method tortuous, as this motion contaminates the instrument measurements. One must thus consider the effect of surface waves, ocean currents, bio-fouling and other factors that can vary with the time of year, location and regional climate and weather patterns. Storms, ships, ice, and vandalism can damage surface buoys. A good alternative is to use a subsurface float together with an acoustic release.

An I-mooring extending from the surface to the bottom must include a mooring line that has the ability to stretch, to compensate for the large vertical excursions that the buoy may experience during the change of tides and with passing waves and swells. It has the advantage of reducing the buoy watch circle (the freedom of movement of a buoy, defined by the mooring length) and sensor motion, and it is relatively easy to deploy. The disadvantage is that the wave action and high static tension under severe current conditions cause high dynamic loading. The latter can be reduced by replacing a part of the mooring line with a slack part, as the figure above illustrates, referred to as a semi-taut mooring. The disadvantage then is that the motion of the buoy and the sensor may become considerable, thus introducing errors in the measured velocity. In the above illustration, a chain (working as a dead weight anchor) is added right above the slack part to reduce excessive motion. In addition, it is a good idea to include a subsurface buoy along the slack line, to prevent the slack part to be exposed to rubbing against the anchor and bottom during e.g. low tide.

A I-mooring is recommended where there is minimal variance of water level, low currents and small waves. The dynamic loading requires use of a larger size anchor than semi-taut mooring. The L-mooring enables more buoy movement than the I-mooring, but it absorbs better the energy that might disrupt the performance of the I-mooring.

It is possible to attach several instruments along the same mooring line. The vertical distance between each instrument attached (and the along beam range of each instrument) must determine if the instruments should transmit signals out of step with each other, to avoid interference. (For details see the  Acoustic interference chapter) To ensure vertical orientation for each instrument along the line, you should include a subsurface float above each instrument. These floats as well as other elements on the mooring line should be placed at a sufficient distance from the instrument, so that it does not interfere with the beams. (See the Physical beam interference chapter)

The design of the mooring must focus on minimizing the motion of the instrument and disturbance in the flow, so that the sensor does not sample in its own turbulent wake, as this may generate significant errors in the data. When working with an in-line instrument like an Aquadopp, it is useful to deploy the sensor using a Aquafin, which will maintain a constant upstream orientation.