🌊Tidal and Wave Energy Engineering Unit 9 – Mooring, Anchoring & Subsea Cabling

Key Concepts and Terminology

• Mooring systems secure floating structures (wave energy converters, tidal turbines) to the seabed
• Anchors provide the connection points between the mooring lines and the seabed
• Catenary mooring lines form a curved shape due to their own weight and provide compliance
• Taut-leg mooring lines remain under tension and provide a more rigid connection
• Subsea cables transmit power and data between offshore devices and onshore facilities
• Umbilicals are specialized cables that supply power, control, and communication to subsea equipment
• Drag embedment anchors (DEAs) are driven into the seabed by the horizontal force of the mooring line
  • Suitable for soft seabed conditions (mud, sand)
• Suction anchors use a large diameter cylinder that is embedded into the seabed by creating a pressure differential
  • Applicable in a wide range of soil conditions

Types of Mooring Systems

• Catenary mooring systems utilize the weight of the mooring lines to provide restoring forces
  • Commonly used for floating wave energy converters and offshore wind turbines
  • Allows for some movement of the moored structure
• Taut-leg mooring systems maintain a constant tension in the mooring lines
  • Suitable for deeper water depths and locations with limited seabed footprint
  • Requires precise installation and higher-strength materials
• Single point mooring (SPM) systems allow the moored structure to weathervane around a central connection point
  • Used for floating production, storage, and offloading (FPSO) units in the oil and gas industry
  • Potential application in floating tidal energy platforms
• Spread mooring systems use multiple mooring lines arranged symmetrically around the structure
  • Provides a more stable and rigid connection compared to catenary systems
  • Suitable for larger floating structures (wave energy arrays, floating substations)

Anchoring Techniques and Technologies

• Gravity anchors rely on their own weight to resist the uplift forces from the mooring lines
  • Typically made of concrete or steel and can be used in a variety of seabed conditions
  • Suitable for temporary installations or in areas with rocky seabed
• Pile anchors are long, slender steel tubes driven into the seabed using hydraulic or pneumatic hammers
  • Provide high holding capacity and are suitable for permanent installations
  • Applicable in a range of soil conditions, including sand, clay, and soft rock
• Suction caissons are large diameter cylinders that are embedded into the seabed by pumping out the water inside
  • Suitable for soft clay and sand seabed conditions
  • Can be installed quickly and removed easily for reuse
• Vertical load anchors (VLAs) are designed to resist both horizontal and vertical forces
  • Consist of a central shaft with helical plates or flanges to increase the bearing area
  • Suitable for taut-leg mooring systems and can be installed in various soil conditions

Subsea Cable Design and Materials

• Subsea power cables typically consist of a conductor core, insulation, armoring, and protective sheathing
  • Conductor materials include copper or aluminum, chosen based on power transmission requirements
  • Insulation materials (XLPE, EPR) provide electrical isolation and are selected based on operating voltage and temperature
• Fiber optic elements are often integrated into subsea power cables for data transmission and monitoring
• Armoring layers, made of steel wires or synthetic fibers, protect the cable from mechanical damage and provide tensile strength
• Outer sheathing materials (polyethylene, polypropylene) protect against abrasion, corrosion, and marine growth
• Dynamic cables, used in floating applications, incorporate additional layers to withstand the stresses of constant motion
  • Bend stiffeners and buoyancy modules are used to control the cable's shape and reduce fatigue

Installation Methods and Equipment

• Cable laying vessels are used to install subsea cables and are equipped with specialized handling and positioning equipment
  • Vessel types include purpose-built cable ships, barges, and modified offshore supply vessels
  • Dynamic positioning (DP) systems maintain the vessel's position during cable laying operations
• Cable route surveys are conducted prior to installation to identify seabed conditions, obstacles, and optimal cable paths
  • Geophysical and geotechnical surveys use techniques such as multibeam echosounders, side-scan sonar, and cone penetration tests
• Trenching and burial tools are used to protect subsea cables from damage by fishing gear, anchors, and other seabed hazards
  • Ploughs, jet trenchers, and mechanical cutters create trenches in the seabed for cable burial
  • Burial depths typically range from 0.5 to 3 meters, depending on the seabed conditions and risk factors
• Horizontal directional drilling (HDD) is used for shore crossings and in areas where trenching is not feasible
  • Involves drilling a pilot hole from shore, then pulling the cable through the drilled path
  • Minimizes environmental disturbance and avoids obstacles such as rocky outcrops or sensitive habitats

Environmental Considerations and Challenges

• Subsea cable routes must be carefully planned to minimize impacts on marine ecosystems and habitats
  • Avoid sensitive areas such as coral reefs, seagrass beds, and marine protected areas
  • Consider the presence of endangered or protected species during route selection and installation timing
• Acoustic disturbance from survey equipment, cable laying vessels, and trenching tools can affect marine mammals and fish
  • Implement mitigation measures such as soft starts, acoustic deterrent devices, and marine mammal observers
• Sediment disturbance and turbidity during cable burial can impact benthic communities and water quality
  • Use appropriate burial techniques and tools to minimize sediment resuspension and dispersal
  • Monitor turbidity levels and implement corrective actions if necessary
• Electromagnetic fields (EMF) generated by subsea power cables can potentially affect marine life
  • Use shielding and burial to reduce EMF emissions
  • Conduct studies to assess the impacts of EMF on sensitive species and habitats
• Decommissioning and removal of subsea cables at the end of their service life must be planned and executed responsibly
  • Develop a decommissioning plan that considers environmental impacts, safety, and waste management
  • Recycle or dispose of cable materials in accordance with regulations and best practices

Maintenance and Monitoring Strategies

• Regular inspection and maintenance of subsea cables are essential to ensure their long-term integrity and performance
  • Use remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) for visual inspections
  • Conduct periodic electrical testing and fault location to detect and diagnose issues
• Fiber optic monitoring systems integrated into subsea cables provide real-time data on cable health and performance
  • Distributed temperature sensing (DTS) monitors the cable's temperature profile to detect hotspots and potential faults
  • Distributed acoustic sensing (DAS) detects and locates mechanical disturbances or damage to the cable
• Condition-based maintenance strategies optimize maintenance intervals based on the actual condition of the cable
  • Analyze monitoring data and inspection results to prioritize maintenance activities
  • Use predictive models to estimate the remaining useful life of cable components and plan repairs or replacements
• Emergency repair procedures must be established and tested to minimize downtime in the event of a cable fault
  • Maintain a stock of spare parts, repair materials, and specialized tools
  • Train personnel in fault location, cable retrieval, and repair techniques
  • Establish contracts with specialized repair vessels and crews for rapid response

Safety and Regulatory Compliance

• Subsea cable installation and maintenance must adhere to a range of safety regulations and industry standards
  • Comply with international conventions such as UNCLOS, SOLAS, and MARPOL
  • Follow national and regional regulations for marine operations, environmental protection, and occupational health and safety
• Develop and implement a comprehensive health, safety, and environment (HSE) management system
  • Identify and assess risks associated with subsea cable activities
  • Establish procedures, work instructions, and emergency response plans
  • Provide training and competency assessment for all personnel involved in cable operations
• Obtain necessary permits and consents from relevant authorities prior to cable installation
  • Conduct environmental impact assessments (EIAs) and stakeholder consultations
  • Comply with conditions and requirements specified in permits and licenses
• Maintain records and documentation of all cable-related activities, incidents, and compliance measures
  • Use a document management system to ensure the availability and currency of critical information
  • Conduct regular audits and reviews to identify improvement opportunities and ensure ongoing compliance
• Participate in industry forums and working groups to share best practices and contribute to the development of standards and guidelines
  • Engage with organizations such as the International Cable Protection Committee (ICPC) and the European Subsea Cables Association (ESCA)
  • Collaborate with research institutions and technology providers to advance the state of the art in subsea cable systems