Efficacy of Stone Filled Marine Mattresses for Cable Protection in a Nearshore Mixed Sediment Environment

Stone filled polymeric marine mattresses were utilized for the protection of solid core submarine conductor cables. The large diameter cables were found partially or fully exposed, and in a few locations were suspended above the seafloor, extending from the shoreline to approximately 305 meters offshore. The mixed sediment shoreline, where the cables make landfall, is highly dynamic and is exposed to storm energy. Multiple alternatives were evaluated for post lay protective burial of the cables. Stone filled polymeric marine mattresses were selected because of their low cost and quick implementation to reduce operational and safety concerns. The stone filled polymeric marine mattresses were designed to prevent uplift and slip during local storm events. A total of 146 stone filled polymeric marine mattresses, measuring 6 meters by 2 meters by 15 centimeters, were used to cover approximately 915 meters of cable. Installation of the mattresses took 11 days. In areas of suspension, stone backfill was utilized to support the cables and the marine mattresses were oriented with lengths perpendicular to the direction of a cable to provide additional scour protection. Post monitoring of the project includes annual surveys. Initial monitoring efforts have observed marine growth on the structures and localized areas of deposition and erosion surrounding the flexible marine mattresses. These changes are expected and ongoing project monitoring continues to demonstrate that the project is successful.