November 2000

Two 4g, 2-axis accelerometers at ends of the mine
One 25g, 3-axis accelerometer at center

We conducted impact burial tests at two sites: 17 test drops in 4-m water depth in the Mississippi Sound (20, 25-26 September 2000) and 16 test drops in 11-12 m water depth in East Bay, just south of the East Pass outlet of the Mississippi River (2-4 November, 2000). Both sites were chosen to maximize the potential for impact burial; therefore, sediments were mostly fine-grained silts and clays with low bulk densities, and low shear strengths.

The objectives of these experiments were to better understand the mine free-fall and impact dynamics to be measured and to identify inadequacies in our data acquisition concept.

The East Bay experiments, in 12 m water depth, were aimed at testing the adequacy of paired accelerometers data for quantifying mine fall in the water column and embedment of the mine into the seafloor. The lateral excursion of the mine during fall was also measured. For this test series, weights internal to the mine were moved to shift the CM 42 mm forward of the CV, which more closely model mass distribution of the Mk-52.

The sensor package and data acquisition system of the NRL instrumented mine shape are optimized, by design, for measuring long-term (hours to months) burial in cohesion less sediments including burial by scour, liquefaction, and sand dune migration. To record short-term (microseconds to seconds) dynamics of a cylindrical shape in free-fall within the air and water column and subsequent penetration into soft cohesive sediments, the data acquisition system was reprogrammed to sample at 300 Hz and to sample only the 4-g and 25-g accelerometers. This made possible integration of deceleration data to yield reliable description of velocity and displacement of the mine shape as it falls through the water column and embeds in the seafloor. Unfortunately, increasing the data-sampling rate precluded reliable sampling of the existing magnetic compass and roll sensors. The omission of sensors to describe pitch, yaw and roll of the mine shape limits usefulness of our existing data sets: this deficiency is being corrected for future tests.

Summary of Impact Burial Experiment:

Results:

To measure the lateral traverse, we used a streamlined "dart", with tail fins, which fell vertically through the water column in tank tests. The bearing and distance from the dart to the mine was noted before mine release. The dart was released about 1 s ahead of the mine and the bearing and distance from dart to mine was measured by divers at the seafloor. Divers noted that the mine traversed up to 6 m horizontally during fall through the 12 m water column. Further, they noted that, for every drop, our mine rolled counterclockwise 60 degrees - 105 degrees (viewed from the tail) about its long axis and rotated in heading counterclockwise (viewed from the top) by up to 240 degrees. Laboratory testing showed that the CM of the mine was slightly to the left of axis, accounting for the preference for counterclockwise roll. Once roll about the long axis begins, the change in orientation of the chamfer increases forcing for that roll motion and provides additional forcing for the observed counterclockwise rotation in the horizontal plane.

Our two impact burial data sets, gathered to date, do confirm our initial reservations regarding the Impact Burial Prediction Model description of mine motion in the water column.

Mine Dynamics During the Fall Through the Water

Data illustrating reorientation of cylindrical shape to stable orientation of long axis horizontal: (a) 3-axis, 4-g accelerometer in nose, and (b) 3-axis, 4-g accelerometer in tail. Starting orientation long axis vertical, mine nose (chamfered end) down, and tail 0.3 m below water surface.

Starting orientation long axis vertical, mine nose (chamfered end) down, and tail 0.3 m below water surface. Mine released at 0 sec; rotation to horizontal, 1-2 sec; and embedment in mud, 3.4-3.7 sec.

Predicted vs. Measured Impact Burial Nose Penetration Below Mudline

Predicted vs. Measured Impact Tail Elevation Above Mudline

Predicted vs. Measured Impact Burial Pitch of Mine After Burial