Sea Technology

AUG 2017

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32 st / August 2017 www.sea-technology.com each turnaround trip, profile data were collected using a CTD that also had fast-response turbidity, chlorophyll and DO sensors. Water samples were collected for analysis of total suspended sediment. These data allow the optical turbidity data to be converted to suspended sediment values in milligrams per liter (mg/L) of water. Data/Results Resultant data products included ice draft and velocity, water column currents with higher resolution near bottom for pipeline design, water level/tide, sediment transport, and water proper- ties (temperature, salinity, turbidity, chlorophyll and dissolved oxygen); all of which will inform the overall design basis for the pipeline crossing and LNG marine terminal. Other interesting features being studied include bottom motion and pressure changes due to ves- sels passing overhead (squat), which could impact pipeline design and maintenance, and tidal-mod- ulated long-period infragravity waves that could affect LNG vessel berthing. Conclusions We found that a high recovery rate of subsur- face metocean moorings in Cook Inlet can be achieved by careful attention to site selection, instrumentation and mooring design. Multibeam and echosounder data can be used to avoid ar- eas with mobile sand waves and large rocks. The moorings and instrumentation must be able to contend with very high current speeds, high sus- pended sediment concentrations, abrasion and submerged debris. High-quality ADCP current direction data can be obtained by minimizing the amount of ferrous material near the ADCP (e.g., use of aluminum bottom frames), degaussing alkaline battery packs, calibrating the ADCP flux gate compass on site, and correcting the ADCP current directions using boat-based ADCP transects. Contributors Rick Birch, senior oceanographer; Todd Mudge, consulting services manager; Ben Garrett, field lead; and Ed Ross, project co-manager, contribut- ed to this article from ASL Environmental Sciences. Brenda Maskos, marine engineer, and Rob Carroll, project controls lead, contributed to this article from Alaska LNG. Chris Yetsko, metocean engineer, contributed to this article from ConocoPhillips. Acknowledgments The authors would like to thank Jeff Raun, John Taylor, Claire Joseph and Bill Maxson at Alaska LNG and Jessy Bar- rette, James Bartlett and Matt Stone at ASL, as well as Mari- time Helicopters and the captains and crew of the Hom- er-based vessel Maritime Maid. We greatly benefited from discussions with Jennifer Ewald, who shared lessons learned during previous deployments by NOAA. ST Standard ASL practice is to use a gauss meter to measure the magnetic signature of each battery pack and ensure it meets established criteria. Substandard battery packs are degaussed again at ASL and retested. Nonferrous material is used for the construction of entire moorings, and any battery packs are located as far as possible from the ADCP. To further improve the accuracy of the current directions measured by the moored ADCPs, corrections were applied based on boat-based transects. Water Properties Water property time-series data (temperature, salinity, turbidity, dissolved oxygen) were obtained with data log- gers on the bottom frames and taut-line moorings. During Deployment of downward-looking ADCP in low-drag SUBS buoys.

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