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Passive Sediment Sampler Retrieval System

University of California, San Diego MAE156B Project Sponsored by

Systems Center Pacific

Executive Summary

Significant levels of chemicals have been released into the US Navy’s harbors over time and the projected cost for the cleanup of the chemical contamination is over 1 billion dollars. The presence of these chemicals has created a pressing need to study and quantify the levels of chemical contaminations to monitor the current situation and develop prospective solutions. SPAWAR has developed a low-cost passive sediment sampling system that allows for the monitoring and analysis of chemical concentrations in the sediment and water.

The passive sediment sampler is comprised of a stainless steel frame and a sampler sheet that, over time, absorbs certain types of chemicals. The samplers are deployed into the sediment at a maximum water depth of 15m for a maximum of 2 months, and then they are retrieved for analysis to determine chemical concentrations at different sediment depths, as well as that of the water. The high overhead costs and poor success rate associated with current methods for locating and retrieving the sediment samplers, using human divers, undermine the low-cost nature of the passive sediment sampler. SPAWAR asked UC San Diego’s Mechanical and Aerospace Engineering department to design a retrieval system for the passive sediment samplers to develop a low-cost, efficient retrieval method.

Figure 1: Cross-section view of assembled retrieval system

The retrieval system that was developed attaches to the side of the passive sediment sampler and is deployed concurrently with the sampler. Externally, the retrieval system consists of two major components: a buoy and a pressure housing. The pressure housing contains a battery, microprocessor, real-time clock, motor driver, and motor which enable the retrieval system to release the buoy at a time specified by the user. The buoy, when released, floats to the surface of the water, and it contains a spool of cable that unwinds as the buoy rises to the ocean surface. The cable is attached to the passive sediment sampler on one end and attached to the buoy on the other. When the buoy is located by the user on the surface of the water, the entire passive sediment sampler is hoisted up by the user via the cable.

An emphasis was placed on developing a low-cost design that could allow for the purchase and deployment of hundreds of passive sediment samplers. Therefore, only low-cost, off-the-shelf parts were utilized in the retrieval system design to ensure a unit price of less than $150. As a result, the pressure housing and buoy were constructed of readily available PVC tubing components. In addition the electronic components chosen were simple and low-cost to ensure robust performance within the required price range.

Components of the retrieval system were placed under several tests to ensure required performance requirements are met under operating conditions. A pressure test of the pressure housing was conducted to ensure watertight performance of the pressure housing under water pressures experienced at the seafloor. An un-spooling test of the cable spool attached to the buoy was conducted to assess the risk of tangling associated with a static cable spool made of PVC tubing. An axial force test of the force exerted by the release mechanism in the pressure housing was conducted to analyze the ability of the release mechanism to overcome biofouling conditions that may interfere with the release of the buoy. Lastly, a timing test using the electronic components was performed to test the accuracy and reliability of the electronic components to release the buoy at a time specified by the user.

This project successfully created and tested a low-cost retrieval system for the passive sediment sampler, which allows for the deployment and retrieval of larger numbers of passive sediment samplers. A greater number of samplers deployed would permit the collection of more data on chemical concentrations. This data is vital to understanding how chemical pollution spreads and can be used to make valuable recommendations on how the US Navy can protect its harbors from further environmental damage.

Figure 2: Retrieval system attached to the passive sediment sampler

Figure 3: Passive sediment sampler deployed using a push/pull rod system (Sediment Sampling System Images Courtesy of SPAWAR)

Project Objective

Design a low-cost retrieval system to efficiently find and recover deployed passive sediment samplers.

Statement of Requirements

• • The system must be able to consistently and repeatedly release an easily identifiable buoy from sediment sampler installation depth to water surface for positioning and retrieval of sampler.

  • The retrieval system must be able to be submersible and operate at depths of up to 15.24m (50ft).

  • The retrieval device must be released after a user specified programmable duration for a duration of up to 2 months.

  • The tether cable from buoy to sediment sampler frame must be able to withstand the working load (222N or 50lbf) of pulling sampler out of sediment and retrieval to water surface.

  • The tether cable must be sufficiently long enough, between 22.86m and 30.48m (75ft to 100ft), to enable buoy to reach water surface under currents of up to 1m/s.

  • The system must be able to operate in saltwater and freshwater and resistant to corrosion due to the salinity of saltwater.

  • The system must be made of materials that minimize cross contamination with sediment elements that the passive sampler is sampling.

  • The retrieval system must be relatively low cost (~$100)

  • The system must be resistant to biofouling and operate under such conditions.

Figure 4: Passive sediment sampler retrieval system infographic

Overview video of project