Standard Equipment

Surface buoy

Surface buoys serve multiple functions in mooring deployments, including acting as a base for surface instruments, retrieval floats, mounting platforms, buoyancy providers, or surface markers for telemetry systems. One of their primary roles is to provide sufficient buoyancy at the top of the mooring line to help maintain a near-vertical orientation of the mooring. A properly designed surface buoy ensures that the mooring system meets depth excursion and instrument inclination requirements, keeping instruments stable and at a consistent depth even through changing tidal conditions and wave action.

The shape and dimensions of the buoy should be carefully selected based on the specific deployment objectives and environmental conditions. In most cases, the buoy should minimize dynamic stress on the mooring line to prevent excessive motion that could degrade data quality or introduce mechanical strain. Since the buoy is subject to wave-induced motion, including heave and roll, efforts should be made to limit the degree of motion transferred to the mooring and instruments. Spherical buoys are commonly used due to their high strength-to-weight ratio, ease of construction, and widespread availability. For durability and reliability, it is recommended to use hard plastic buoys filled with either foam or air, rather than soft plastic air-filled buoys, which are more susceptible to puncturing and failure.

Subsurface buoy

A subsurface buoy plays a critical role in reducing wave-induced motion of the mooring line and helping to keep the mooring as vertical as possible. It also provides support for the weight of both the mooring line and the instrumentation, preventing excessive strain on individual components. Additionally, a subsurface buoy enhances backup recovery capabilities; if the mooring line or cable were to snap, the buoy's positive buoyancy allows the instrument to rise to the surface, facilitating retrieval. By positioning positive buoyancy above the anchor, the risk of entanglement and wear from contact with the seabed is minimized. This is particularly important in L-mooring configurations, where it is recommended to attach a buoy along the mooring line resting at the bottom. If a surface buoy is lost or the mooring line breaks, retrieval from the seabed becomes significantly easier when the mooring line is not fully settled on the ocean floor.

When using a subsurface buoy in the upper water column, it is essential to minimize its density, as its efficiency rapidly decreases with added weight. The floating material density must increase with depth to withstand the higher hydrodynamic pressure at greater depths. While pressure resistance is only a concern in deep-water deployments, selecting the correct buoy material is crucial. Synthetic foam is particularly well-suited for deep-sea moorings due to its high-pressure resistance and suitability for strong current regimes. To reduce drag, one large-diameter buoy is generally preferable over multiple smaller buoys. The buoyancy of the subsurface buoy significantly influences mooring system response, affecting stability, movement, and overall instrument performance.

In some deployments, instruments can be mounted directly within a subsurface buoy. When considering this approach, the shape of the buoy is an important factor:

• Ellipsoid-Shaped Buoys: These have lower drag than spherical buoys, making them a good option for calm environments or when instruments are deployed near the seabed. However, in high-current areas, ellipsoid buoys are not ideal, as strong currents can cause excessive tilting, increasing drag forces. Once the bottom of the buoy faces upstream, the drag contribution becomes severe, compromising stability.
• Streamlined Subsurface Buoy (SUBS): The torpedo-shaped SUBS buoy is designed for high-current regimes, providing greater stability and reducing mooring excursions and inclinations. Unlike ellipsoid buoys, the SUBS is more effective at maintaining a vertical instrument orientation, making it a preferred choice in dynamic ocean environments.

The ellipsoid buoy has a tendency to rotate more than the SUBS, but data from an experiment comparing the two buoys shows that the rotation is slow enough so that the compass is able to keep up with the rotation and provide accurate measurements. Conversely, the SUBS is more stable for the roll (side to side), but the pitch becomes larger during time of increased wave energy.

Mooring line

Choosing the appropriate mooring line is crucial for ensuring the stability and longevity of a deployment. Several factors must be considered, including load type, rope material, ease of handling, deployment duration, and the surrounding environment. Different rope materials offer unique properties such as strength, drag, stretch, endurance, weight, and buoyancy characteristics (floating vs. sinking). Understanding these factors allows for better mooring performance and reduced wear over time.

Drag considerations: deep vs. shallow water moorings
In deep-ocean moorings, drag on the mooring line is a significant factor across the entire length of the system. To minimize excessive drag, it is generally advisable to select a mooring line with the smallest diameter possible. The diameter of the line represents the largest portion of overall drag contributions due to its large cross-sectional area (A in Equation (1) chapter Drag). For shallow water moorings, however, currents (\(v^2\) in Equation (1) chapter Drag) — such as tidal, wind-driven, and wave-induced flows — often exert a greater influence than the diameter of the mooring line. In these cases, the focus should be on managing the effects of these currents rather than solely minimizing the line's diameter. In rough sea conditions, a heavier sinker may be required to prevent the buoy from lifting the anchor.

Stretch and elasticity: managing mooring tension
The stretch characteristics of a mooring line play a critical role in determining the position of both the instruments and surface buoy at any given time. In deep-water moorings, maintaining a taut mooring line is essential to minimize movement caused by currents and waves. This is often achieved using elastic synthetic rope, which helps absorb energy from ocean motion and prevents the surface buoy from being pulled underwater during extreme conditions. However, the elasticity of synthetic ropes is non-linear and changes over time due to material fatigue. Additionally, in shallow-water deployments, elastic rope is generally not recommended due to the risk of abrasion from the seafloor. In this case, floats along the line can be used to keep the mooring off the bottom while maintaining stability. Note that synthetic ropes are made of viscoelastic materials, so their stiffness characteristics are not constant and vary with the duration of load application, the load magnitude and number of cycles. In general, synthetic mooring lines become stiffer after long use.

To maintain the strength and durability of the mooring line, it is important to avoid knots on the rope, as this reduces the breaking strength with up to 50%. Additionally it is recommended to use torque-balanced wire rope, which helps reduce rotation in the mooring line, preventing twisting and strain over time.
 

Anchor

The ability of an anchor to hold a position on the seafloor is dependent on several factors; the seabed type, the anchor material, shape and weight. A failure in anchor performance can have severe consequences for the deployment, leading to drift, instability, or even loss of the mooring system. The anchor must be sufficiently heavy to ensure that the mooring assembly sinks properly and remains in place on the seafloor. It must be capable of withstanding both vertical and horizontal forces exerted by ocean currents, waves, and tidal changes. If the anchor weight is too low, the mooring system may shift position during strong currents or high tides, compromising data accuracy and stability. However, the anchor size and weight should also be kept within reasonable limits to ensure practical deployment and retrieval. Site conditions play a crucial role in determining the necessary anchor weight.

The holding power of an anchor is influenced by the type of seabed it rests upon. In general:
 

• Soft, cohesive seabed's (e.g., mud or fine sand) provide higher horizontal friction, improving anchor stability.
• Hard seabed's (e.g., rock, gravel, or shell-covered areas) offer lower holding power, making anchoring more challenging.

Anchor shape also affects performance. Certain designs enhance holding power by increasing seabed friction. If the seabed is too soft, the anchor may sink excessively, altering the mooring's intended configuration. In such cases, mud mats can be used to prevent excessive sinking and ensure that mooring components remain at their configured depth—especially crucial when using acoustic releases, which must remain suspended in the water column. Additionally, the density of the anchor material is critical, as materials experience a loss of weight when submerged. For example, the effective weight of concrete underwater is only 56% of its dry land weight. Due to this reduced buoyant weight, larger concrete anchors may be required to achieve the necessary holding power.

When using acoustic release systems, the anchor is often left behind on the seafloor. To minimize environmental impact, it is recommended to use anchors that can disintegrate over time, preventing long-term obstruction on the seabed. A disintegrating anchor also provides a secondary recovery mechanism—if the acoustic release fails, the system may eventually refloat as the anchor material degrades.

Dead weight anchor: The dead weight anchor (illustrated in Figure 1) is used to resist vertical pull of the system; some weight underneath the instrument will keep it from bouncing around in rough weather. A length of chain is recommended due to the reduced chance of sidelobe interference compared to when using weight with a larger surface area. Ballast underneath the surface buoy has the same effect, and the risk of entanglement of the mooring line is reduced. A length of chain placed above the anchor will reduce the tension the anchor experiences.

Bottom frame

When designing or selecting a bottom frame, you should consider a few things before you drop your gear in the water.

Stability: The frame should sit on the bottom and not move or rock back and forth. One common solution is to use a tripod leg configuration. Stability naturally improves with a larger frame "footprint" and a lower profile. The frame will normally have the battery canister on one of the legs. If possible, put the battery inside the leg, and not outside. Have an equal amount of weight on each side / leg; the frame might tip over if the weight is too little and / or uneven. Make sure that the weight is correct underwater.

Mobility: Clearly, the frame should not move while on the bottom. Movement can come from strong mean currents or wave generated currents. Wave generated currents can be particularly troublesome in shallow waters or environments where waves are large and long. The frame must also be firmly attached to the bottom if the bottom type is relatively soft. Note that shifting sediment or sand can undermine bottom fasteners.

Ease of deployment: This should be considered if the frame is to be deployed by divers or handled by a small boat.  If a large amount of weight is used to eliminate mobility, it may not be manageable by a small boat or crew.

Orientation: The instrument should be placed vertically and in the case of the AWAC, with AST, a tilt less than 5 degrees should be the aim. For other instruments, the performance deteriorates quickly for tilts greater than 10 degrees. A gimbal is the best solution if you do not use divers to ensure the proper orientation of your frame / instrument.

Retrieval: This is perhaps more of a deployment consideration; however, some method should be kept in mind for a retrieval system that does not interfere with the instrument's performance. Surface buoys floating above the instrument can interfere with the acoustic beams; moreover, they may be dragged away or lost due to shipping traffic or curious mariners. Alternative methods of retrieval include acoustic releases (from bottom weight), pop up buoys with releases, drag lines and/or offset mooring systems.

Trawl resistance: Bottom fishing is perhaps one of the most challenging issues for bottom-mounted systems. Trawl resistant bottom frames are designed for protecting the instruments from trawler gear. The frames can reduce the risk of being impacted by trawlers, but the risk is not completely eliminated. A good alternative is the  Miniaturized Trawl Resistant Bottom Mount (MTRBM) systems.

Multi-sensor mounting: Since Nortek instruments provide the possibility to integrate external sensors, this should be considered when deciding on a frame.

Ease of shipping: Some frames can be separated into sections for easy shipping. This of course reduces operational costs when testing is performed at different locations.

Corrosion resistance: All materials of the frame must be corrosion resistant. Materials such as fiberglass, stainless steel (316), aluminum, and plastic are good alternatives. You must always isolate any metal you use, and make sure not to mix metals to reduce the risk of corrosion.

Burial: This is also a challenging issue for bottom-mounted instruments. Low profile frames may not be the best alternative in locations with large sediment deposits or moving sand. Sometimes it is just a matter of time before the frame slowly sinks or sand builds up; in these situations, some users (of online systems only) have found sensors measuring distance from the instrument to the bottom useful. If mean currents and wave-induced currents are manageable at the bottom, a solution against burial is to mount the instrument on a subsurface buoy located just above the bottom.

Magnetic influence: Some materials can influence the magnetic field around the instrument, causing problems with the heading reading. A compass calibration should always be done before each deployment to account for any magnetic influences. Stay away from galvanized steel in particular, as this can have a very unstable effect on the magnetic field, so even with a compass calibration it may still for example change the magnetic effect of the battery discharging, introducing errors. Magnetic deviation in the instrument compass due to a magnetic source on the bottom frame should be taken into account by calibrating the compass attached to the frame before deployment.

Selecting and managing metals

Choosing the right metals for a mooring system is essential for both structural integrity and the reliability of deployed instruments. Key considerations include corrosion prevention, material compatibility, mechanical strength, and avoiding magnetic interference. Addressing these effectively ensures the durability of your deployment and the accuracy of your measurements.

Shackles, swivels, and links serve a critical role in the structural integrity of a mooring system. They are used to connect various components of the mooring line and ensure that instruments remain securely in place throughout deployment. When selecting these connecting elements, the primary consideration must be their strength—each component must be able to withstand the maximum expected tension from the parts it joins. Always bear in mind: a mooring is only as strong as its weakest link.  Swivels are commonly used to allow relative rotation between two connected components. This rotational freedom helps reduce torsional stress on the mooring line, which can otherwise build up over time due to ocean currents or instrument motion. Reducing torsion improves the longevity of the line and the reliability of the entire deployment.

Corrosion prevention and material compatibility
Corrosion poses a serious risk to the mooring system. Mixing different metals in the presence of an electrolyte like seawater can result in galvanic cells that accelerate the degradation of more reactive metals.

Best practices for corrosion mitigation:

• Avoid Mixing Metals: Use the same type of metal for connecting components whenever possible.
• Isolate Dissimilar Metals: Use nylon washers, plastic sheeting, rubber gaskets, or synthetic rope to physically separate different metals. Even inserting a small length of rope between metal components can provide effective electrical insulation.
• Apply Sacrificial Anodes: Attach zinc anodes to vulnerable components such as battery canisters. These anodes corrode preferentially, protecting the surrounding metal parts.
• Inspect Regularly: Periodically check for early signs of corrosion and replace compromised parts before failure occurs.
• Electrically Isolate Battery Canisters: Use non-conductive materials to prevent galvanic contact with other metals

Avoiding magnetic interference
Your instrument relies on magnetic sensors for orientation and positioning. The presence of magnetic metals in the rigging can interfere with these sensors, leading to inaccurate directional readings and compromised data quality. Be mindful of material selection when choosing clamps, shackles, and mounting structures near your instrument. Non-magnetic materials such as stainless steel or titanium are preferred over ferromagnetic metals like iron or standard carbon steel. Perform a thorough compass calibration after installing the instrument in its final configuration. Even if all precautions are taken, minor local disturbances can still affect the compass, making calibration essential. The correct steps for a successful compass calibration can be found here.