Sea Technology

AUG 2017

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www.sea-technology.com August 2017 / st 29 C ook Inlet in Alaska is one of the most demanding bodies of water for conducting oceano- graphic mooring work. It has some of the largest tides in the world (9.5 m/30 ft.) with currents up to 3 m/sec (6 kt.), along with winter sea ice, boulder fields, fiber-optic cables and pipelines, submerged debris, turbid waters, heavy ship traffic, and mobile sand waves rising several meters high. Alaska LNG is considering a project that would include de- signing an LNG terminal at Ni- kiski and a pipeline crossing in northern Cook Inlet. The design for the Nikiski site requires data to address issues such as vessel berthing. The data needs for the design of the northern Cook Inlet pipeline crossing are focused on near-bottom currents and sedi- ment transport. Mooring Deployments In order to obtain the neces- sary data for operational and project design engineering, ASL Environmental Sciences designed and built custom oceanographic moorings to survive the chal- lenging conditions of Cook Inlet. Moorings have been deployed at various sites, with a six- month deployment rotation. Thirty-three bottom frames and nine taut-line moorings were deployed from December 2013 to October 2016. The bottom frames housed upward- looking sonar and ADCPs, while the taut-line moorings had downward-looking ADCPs that provided high-resolution, near-bottom current measure- ments. The mooring recovery rate has been 100 percent, but not without challenges. Mooring modifications and refinements to ground tackle have been made to improve recovery timing from days to hours over the three years of deployment. For example, the recovery of the first bottom frame off the Kenai River required three days of grappling before the mooring was able to be re- covered. More recent recoveries were within an hour, and if drag- ging was required, within 2 hours. Bathymetry/Sand Waves Early discussions with Jen- nifer Ewald of the U.S. Bureau of Ocean Energy Management alerted us to the threat of burial of bottom frames from mobile sand and sediment deposition, particu- larly in northern Cook Inlet. Fur- ther investigation identified large areas of mobile sand waves that were apparent in multibeam data. This information was taken into account when determining the initial mooring locations. Also, prior to deployment, each site was surveyed using the vessel's single-beam echosounder, and final adjustments to deployment location were made within the limitations of the project and regulatory requirements. The most northerly site off Anchorage was particularly worrisome, and it was decided that the risk of burial of the bottom frames was too high; therefore, two taut-line moor- Current and Ice Measurements In Cook Inlet Monitoring of Extreme Environment for LNG Project in Alaska By ASL Environmental Sciences • Alaska LNG Multibeam bathymetry from northern Cook Inlet show- ing large 10- to 15-m (30- to 50-ft.) sand waves. The data source is NOAA NCEI bathymetry surveys.

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