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Subsea Cable Burial Sled

Hanh Pham, Paulina Nol, Joe Mills, Zakary Morgan

Sponsored by

Figure 1. Final Design with Seabattery

Project Background

Spawar Systems Center Pacific develops seafloor hardware that support a wide range of Navy missions. One of the hazards for seabed mounted equipment is commercial bottom trawling (Figure 2), Seafloor hardware must be designed to allow trawl nets to ride up and over without snagging. Power and signal cables must be buried to avoid being snagged, crushed or cut by the trawl’s ground gear. The probability of cable failure in a single crossing by trawl gear is a function of cable burial depth as represented in Figure 2.

Figure 2. Sea Floor Trawling

Figure 3. Cable Burial Depth vs. Probability of Cable Failure (Pfail) 

When Subjected to a Single Trawl Door Crossing

Objective

Design a reliable, low-tech, robust, compact, and inexpensive sled with minimal moving parts, capable of burying several hundred feet of cable sufficiently deep to prevent damage from commercial bottom trawling.

Requirements

• • Sled capable of traversing an obstacle of at least 30.48 cm (1 ft)

  • Cable laying system that works passively as the sled is pulled by a winch

  • Blade should be self digging

  • System should be able to bury 30.48 m (100 ft) of cable with a minimum bend radius of 7.62 cm (3 in)

  • Cable needs to be between 15.24 cm (6 in) below surface

  • Force to pull the sled should not exceed 4450 N (1000 lbf)

  • The length and width of the sled should be about 152.4 cm (5 ft) and 91.4 cm (3 ft), respectively

• Trough cut by the sled should be 1.27 cm (0.5 in) wide

Summary:

. In order to be protected, a 0.64 cm (¼ in) diameter cable needed to be buried 15.24-20.32 cm (6-8 in) in depth. The sled would be deployed from one ocean floor hardware hub and towed to the next subsequent hub by an undersea winch. The hubs can have many functions for the Navy; but overall they are mainly used to help the Navy conduct research and keep their affairs in order in both a domestic and foreign sense. Burying cables can prevent countries in conflict with the United States from damaging communication cables and compromising security information. It would require a maximum of 1780 N (400 lbf) pull force and be capable of navigating obstacles such as anchor chain and rocks. The final design was constructed mostly of steel with a toboggan style sled and a plow style blade. A water jet was also added to help mitigate the pull force needed to get the sled from hub to hub. The reel system was constructed from a simple plastic injection-mold reel and aluminum for the reel supports. A braking system utilizing a belleville washer was also added to help reduce the effects of excessive spinning of the reel causing birds-nesting. The cable feed system was designed simply to make the process of getting the cable into the trough as easy as possible. With this in mind a thin Delrin cylinder, with a radius above the minimum bend radius of the cable, led the cable smoothly into the trough. From the final test, the system as a whole worked as intended and required approximately 1780 N (400 lbf) of pull force to operate the system.

Final Design

Figure 5. Side view of Final Design

Figure 6. Front View of Final Design

The final design was tested at the 25 and 45 degree angles with and without a water jet. From Figures 7 and 8 it is evident having a water jet demonstrated a significant decrease in the overall pull forces. Obstacles such as an angle iron and pipe were also placed in the water to test if the sled could successfully ride over them. The sled was able to overcome the obstacles, and continue burying the cable in the sediment.

Figure 7 and 8. Final Test Average and Maximum Pull Forces for Various Set Ups

                              Figure 9. Final Test 25 Degree Angle with Water Jet Performance                     Figure 10. Final Test Best Fit Pull Forces

Video of Final Test Performance