When communities on remote islands are connected to an interconnected power system, the reliability of supply gets a boost while customer energy costs are reduced. For the small island of Petersburg, Alaska, U.S., the electrical connection to the Southeast Alaska Power Agency’s (SEAPA) power system reduces customer energy costs by more than 70%. Without SEAPA’s carbon-free connection, Petersburg would be forced to rely on diesel generation that, on average, discharges some 92,250 lb (41,844 kg) of CO2 annually.
Before considering cable replacement, SEAPA investigated the possibility of performing a splice to effectively repair the failed cable. Time-domain reflectometer (TDR) tests were performed to identify the position of the fault. Test results signaled a negative reflection, indicating a break in the cable at nearly 2000 ft (610 m) offshore at a depth of 350 ft (107 m). A smaller positive reflection also was evident, which was either a possible ghost reflection or water ingress from gravity and density differentials between the seawater and oil.
A Replacement Cable
- ROV footage.
- High-resolution multi-beam sonar bathymetric maps.
- Magnetometer sub-bottom profiles.
- Directional peak cable location surveys.
- Using SEAPA-furnished surveys, Sumitomo began the design process for the replacement cable in November 2020.
- Total cable weight
- Challenges associated with transportation logistics
- Time for cable installation
Cable Removal Phase
- Pulling the shoreline ends onto the vessel, the ITB 45, using ropes and mesh pull grips.
- Using dynamic positioning, the existing cable was extracted onto the ITB 45 and loaded into the cable storage system, specifically designed to contain the cable should an oil leak occur.
- Monitoring tensions so as not to exceed the cable manufacturer’s tension limits.
- Calculating fluid dynamics to determine the depth required to maintain equilibrium pressures for CanPac Marine Services Inc. divers to install caps without oil escapement.
- Deploying divers at the faulted cable ends to cap the cable underwater and prevent oil escapement.
- The cable removal phase proved to be successful, with no oil released and 100% of the faulted cable removed. The total start-to-finish time of four and a half days included diving operations, ROV cable location, shoreline extractions and dynamic positioning vessel subsea extractions.