HT-S mooring design assumptions and general recommendations

• The HT-S hydrophone shall be positioned at mid-water column depth to support optimal acoustic performance while minimizing exposure to surface- and seabed-related noise sources.
• The mooring design shall allow for safe, quick, and efficient maintenance and inspection activities, including sensor retrieval without requiring the vessel to berth directly onto the mooring line, as berthing-induced loads may dislodge the anchor.
• The use of chain elements shall be minimized, as chain motion can introduce unwanted mechanical noise. Where chain components are required, appropriate noise-mitigation measures (e.g. protective cover) should be implemented.
• Anchor selection shall reflect a balance between deployment practicality and long-term stability. Heavier anchors generally improve resistance to displacement but may require specialized vessels, lifting equipment, or buoyancy aids during deployment.

Alternative Mooring Solutions: S-Shape vs Catenary design 



HT-S s-shape mooring design

HT-S s-shape mooring design



HT-S catenary mooring design

HT-S catenary mooring design

Executive takeaways

• S-shape mooring:Preferred for long-term acoustic monitoring in environmentally sensitive areas, minimizing seabed interaction and mechanical noise while improving survivability, at the cost of higher complexity and a larger watch circle.
• Catenary mooring:Preferred for deployments near critical infrastructure or in spatially constrained areas, offering simpler installation and a smaller watch circle, but with increased seabed interaction and mechanical noise.

Mooring Elements

Smart mooring elements: provided by blueOASIS, redardless of the mooring design.
Traditional mooring elements: shall be arranged by you depending on the preferred mooring design.

Set 1 - Smart Mooring

• Smart mooring cable: The smart cable allows data transmission and power supply between the Spotter and the HT. Generally, two sections of smart cable are required:  
• Section 1: One of either 5m or 10m connecting the Spotter to the node where the surface buoy is attached. 
• Section 2: The bottom section of the smart cable determines the operating depth of the HT, with a target length of half the depth, but constrained by the maximum operational depth of the HT of 65 m.
• Surface buoy: Required to provide the correct buoyancy. The larger the buoy, the higher the loads on the anchor. However, it must withstand both static load from the weight of the line and sensor, as well as dynamic loads from currents and waves. A buoy with approximately 30 to 50kg of positive buoyancy is generally provided to allow proper sensor operation. 
• In-line buoy: Required to keep the smart cable in the correct position. The number of in-line floats depends on the length of the smart mooring cable and mooring configuration.

Set 2 - Traditional Mooring

• Anchor:
• For s-shape mooring configuration gravity anchors with an effective holding capacity of approximately 75–200 kg are recommended. These may be concrete blocks or modular weights, with sizing based on submerged (wet) weight rather than air weight (e.g. a 100 kg concrete block provides ~55–65 kg effective weight underwater).
• For a catenary mooring configuration, where chain elements are included, lighter anchors, such as small drag anchors commonly used for dinghy moorings, may be considered, as the chain contributes to load reduction at the anchor. The chain section should typically be 8–12 m in length with a 10 mm bar diameter, and the use of a noise suppression cover is strongly recommended.
• Mooring line: for the main mooring line it is recommended a three-strand braided polypropylene rope or equivalent with a nominal diameter of approximately 18 mm.
• For an S-shape mooring configuration, a practical guideline is to select a line length (measured from the anchor to the Hydrotwin) of approximately 1.1 to 1.4 times the water depth allowing the line to reach the Hydrotwin without lifting the anchor and thereby simplifying recovery and maintenance operations. The submerged flotation element should be positioned near the mid-water column.
• For a catenary mooring configuration, the combined length of the mooring line and bottom smart mooring section should be approximately 1.5 to 1.8 times the water depth.
• Auxiliary equipment: auxiliary components include jaw-to-jaw swivels, shackles, and thimbles, selected to match the line diameter and expected loads. 

General Considerations and Mooring Maintenance

• Rope friction can lead to failure, use thimbles in eye splices to prevent abrasion. 
• Include at least one jaw-to-jaw swivel along the mooring line to prevent torsion. 
• Use corrosion-resistant materials suited for the marine environment. We recommend 316 stainless steel for all hardware components and three-strand braided polypropylene for mooring lines. 
• Attach floats to the mooring with a spliced-in secondary line to prevent chafing of the main mooring line. Or use a hosepipe to prevent direct contact with the rope.
• Marine fouling can increase loads on the system and interfere with sensor performance. Plan regular cleaning at least every three months. 
• When planning a deployment, ensure that all ropes, and especially any smart mooring sections, are fully laid out and inspected prior to going offshore to avoid twisting or entanglement. Handling long smart mooring sections (e.g. 50–65 m) can be challenging in confined working areas at sea, and adequate space and preparation are therefore essential.



Thimble in eye-spliced rope.

Thimble in eye-spliced rope.



Jaw-jaw swivel.

Jaw-jaw swivel.



Submerged float on the mooring line, with a hosepipe to prevent abrasion of the rope.

Submerged float on the mooring line, with a hosepipe to prevent abrasion of the rope.

General Mooring Guidelines 

Final Considerations