ARA Incorporated,a specialist in high hazard explosives and environmental remediation, was recently tasked with supporting a subsurface survey of Terminal 91 in the Port of Seattle,Washington. Used by the Navy during World War II and as a berthing place of the USS Missouri,Terminal 91 was a port of embarkation for troops and supplies, and continued to be used by the military up until 1970. Following its acquisition by the Port of Seattle in 1976,Terminal 91 is now home to both short-and long-term moorings for many different vessels.
Aerial of Piers 90 and 91, U.S. Navy owned
2009 saw further development of Terminal 91 with a new cruise ship facility, Smith Cove, opening on Pier 91 and homeport to a multitude of luxurious liners including Carnival, Princess Cruises and Royal Caribbean International. Prior to the beginning of the cruise season, a routine diver sweep of the seafloor surrounding the piers unearthed World War II-era military ammunitions, including a 5” training round and, subsequently, 20-90 mm rounds, a 3” armour-piercing round and a mechanical timer fuse.With the Terminal’s history as a naval pier, it was not considered uncommon for ammunition to be lost in the bay.These initial sweeps of the ocean floor using multibeam, side scan sonar and stationary scanning resulted in a Time Critical operation status, cutting planning time to less than six months instead of the usual two to three years.This operation took place throughout the first quarter of 2011 during which further munitions were also detected.
Kitting out the survey
To undertake further subsea surveys of the terminal used by cruise ships to berth, ARA was supported by Sonardyne’s Houston office from which it purchased a Scout USBL acoustic tracking system for shallow water ROV positioning and ViewPoint navigation software.A complete vessel based acoustic positioning system, Scout USBL is designed for tracking divers, ROVs and towfish in waters up to 1,000 metres. Calculating the position of a subsea target by measuring its range and bearing from a vessel mounted transceiver to a small acoustic transponder fitted to the target, the system offers high accuracy performance with efficient subsea tracking operations. Ralph Gall,Technical Sales Manager at Sonardyne remarked,“Although commonly used by the offshore survey and ocean scientific industries, Scout is equally at home tracking a diver or target in just a few metres of water so we were confident that it was the right solution for ARA to use on this unusual project. John Coughlin, Director of Engineering and Sciences at ARA commented:“With the development of the cruise ship terminal and the high-powered manoeuvring bow thrusters of modern day cruise ships disturbing the ocean sediment with their powerful wake, it was decided that the site under Pier 91 required further investigation. As a result, our role in this operation was to identify and clear magnetic anomalies in support of the subsea survey using magnetometers.”
Navigating the waters
The Munitions Response Area of which ARA supported the Remedial Investigation was an area equivalent to 86 acres of open water. ARA’s Remotely Operated Vehicle (ROV) ROUMRS (Remotely Operated Underwater Munitions Recovery System) was fitted with a Sonardyne WSM6 transponder and a Scout transceiver was deployed from the side of the dock on a simple pole arrangement, feeding ViewPoint with position output data to graphically display the ROV’s position on an electronic chart of the port.ViewPoint allowed ARA to explore, visualise and share positioning data from the USBL system, transforming the coordinates of surface vessels, subsea vehicles and structures into geographical information that is overlaid on easy-to-use guidance displays. John continued:“The results of our subsea survey were fantastic and we were particularly impressed with the sub-metre accuracy we achieved in this challenging environment. It wouldn’t have been possible without the help of the team at Sonardyne; not only did they provide the best equipment for the job but their invaluable expertise, integrated software and on-site training meant that our objectives were fulfilled in a timely and cost-effective manner.”