Spar Mooring for Ocean Turbulence Measurements

Photo by National Park Service. Source: kpbs.org

Team 10 with the spar base mechanism and 1 of 5 spar segments

Background: This capstone project was sponsored by Geno Pawlak, Ph.D. and the MAE Environmental Fluid Dynamics lab at UCSD in partnership with the Scripps Institution of Oceanography. As part of a National Science Foundation funded research study seeking to quantify hydrodynamic mechanisms by which giant kelp mediates transport and flow conditions in inner shelf habitats, field measurements were necessary using Acoustic Doppler Velocimeters (ADVs) whose recorded data would be used to simulate turbulence and generate circulation models of coastal environments. The implications of this research are of importance for the improvement of coastal zone management and the design of aquaculture systems and natural restoration areas. The design team was tasked with engineering a structure that provided a solution to how to best secure the ADVs in the giant kelp environment.

Objective: The primary objective of this project was to design and fabricate an effective solution for safely housing the research instruments and to conduct buoyancy, stress, and stability analyses for the generated design solution.

Functional design requirements:

8-12m tall structure that could safely hold a series of ADVs at varying heights and be deployed from a mid-sized vessel
Could remain stable and withstand the ocean currents and wave forces it would be subjected to
Maintained the sensor probes securely rigid
Minimized tangling with the surrounding kelp
Allowed for ready access to the instruments for any potential maintenance

CAD drawing of complete assembly of the Spar Mooring system

CAD animation of concept assembly during deployment

Final Design:

The structural spar mooring system consisted of three major components: the spar buoy, the guy-wires that provided rigid stability and a base mechanism that anchored the spar and allowed for its vertical alignment during deployment. The spar’s slender structure and relatively thin stainless steel wire ropes were the optimal choices for minimizing entwinement with the giant kelp while still ensuring structural integrity.

The spar consisted of five 2m-long connectable spar segments that were half-filled with closed-cell pour foam that ensured sufficient buoyancy. Each segment housed a mounted ADV battery and securely held the sensor probe with the developed holding fixture that ensured rigidity of the fastened sensor probes. Access to the ADV mounted inside the spar segment was made possible by the access port cavity that would be capped with a cutout door that mated with the face of the hole where it would be rigidly held in place with a set stainless steel hose clamps as their efficient fastening capabilities were found to be sufficient for many applications on the spar mooring and were used extensively throughout the design.

CAD concept drawing of ADV sensor probe holding fixture

Probe holding fixture on spar mooring prototype

Prototype access port with functional ADV battery mount

The base design had to accommodate the anchoring methods used by the sponsor’s research team, which consisted of using a large railroad wheel that could weigh upwards of half a ton. This posed a challenge in that wherever the wheel was to land, was where the spar mooring would be assembled. There was a high likelihood that the surface on which the anchor wheel landed would not be perfectly flat, requiring the spar base to have 2 degrees of freedom so that the spar mooring could be vertically oriented during deployment. However, the base rotation about the spar's axis was only necessary during deployment and was otherwise not required for the spar mooring's functionality.

With stability and ease of use in mind, the team decided to employ a set of coaxially fitted pipes that were held with set-screws. This ensured the outer hinge base pipe could be rotated then secured to any position permitting the hinge’s range of motion to allow the buoyant spar to vertically align itself. Even so, this design was dependent on the friction of the set-screws to hold it all together. The design decision made to solve this problem was to incorporate an outwardly extruded lip on the end of the standpipe, as seen in the demonstration video, as a safeguard to ensure that in the case of loosened set-screws, the lip would stop the spar from disengaging and floating away.

CAD rendering of the spar base functional design

BaseOper.mp4

Spar base functionality demonstration

Summary of Analyses: Finite element analysis was used to evaluate the performance of the spar mooring. Forces imposed on the spar by the wave-dependent cross-shore motion and steady alongshore flow were quantified and applied to the CAD structure. The base and joints of the spar demonstrated the ability to withstand the ocean current forces

FEA: First Principal Stress

Spar Deflection from imposed flow forces

This flow simulation modeled the drag forces on the spar and illustrated velocity differentials around the spar when subjected to a steady ocean current. This was significant in that it quantified how much of an obstruction the spar itself would be and led to design considerations for optimized placement of the ADV probes prodding out of the spar. A sensor in the wake of the spar where flow separates from the frictional surface can result in adversely affected turbulence measurements and velocity readings. It was essential that no more than one probe sensor would be in the wake of the spar at any particular time warranting the need to have the ADV sensors in different positions around the spar.

Stress and displacement plots for guy wires attached 5m up the spar

Stress and displacement plots for guy wires attached 7m up the spar

The deflection of the spar and how it will behave under the flow conditions was of particular interest. The 4th order beam equation was used to determine the displacement, slope, shear, and moment of the spar, from which stress was calculated. This allowed the team to determine ideal placement of the guy wires to minimize bending and stresses, and use generated values to conduct material testing.

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