A Five-Degree-of-Freedom Abrasive Blasting Vehicle for Ship Bottoms

Journal of Engineering Technology, Fall 2006 by Lin, Cheng Y, Crossman, Gary R

Abstract

This paper presents an automatic blasting vehicle for ship bottoms. A blasting head with two degrees of freedom, including roll and pitch angles, is mounted on a moving vehicle through a vertically swinging mechanism. The blasting head is lifted by the swinging mechanism through hydraulic power. With the use of a pressure relief valve in the circuit design, the blasting head is always automatically pushed against a curvilinear surface of a ship bottom during the blasting process. Steel abrasive particles are fed into a hydraulic pump and are projected centrifugally at high speed toward the surface to be blasted. Rubber seals between the blast machine and the hull ensure that all the abrasive, dust, and paint chips during the blasting process are directed back to the machine through a hopper. High-speed air is blown in to carry dust and paint chips into a dust collector while the heavy abrasive particles continue to fall and are recycled into the hydraulic motor. When compared to the open blasting process operated by workers, this automation process drastically reduces the cost of man-hours, abrasive consumption, and total blasting cost. It also improves the working condition of the workers since there is no dusty environment, which is a problem in the current industry practice of open blasting. Engineering Technology faculty members at Old Dominion University were funded by local shipyards to develop the device. The faculty incorporated several senior Mechanical Engineering Technology students to contribute to the design for their capstone design project.

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1. Introduction

Old Dominion University is located in Norfolk, Virginia, home of one of the nation's largest naval bases, as well as several shipbuilding and ship-repair facilities. The university's location in this area has provided faculty numerous opportunities to collaborate on technical projects with the ship-repair industry. The project to design and test the abrasive blasting vehicle described in this paper is such a collaboration. At the same time faculty members are involved in applied engineering projects, senior mechanical engineering technology students are defining their senior capstone project. Senior projects come from a variety of sources, including specific tasks given by the instructor of the capstone course, tasks requested by local industry or members of the industrial advisory committee, projects from a student's workplace or simply ideas that the students have. This project was a perfect opportunity to involve a group of students in the design and initial building stage of a "real-world" project and required a considerable level of effort by the students to satisfy senior capstone project criteria. The faculty members working on the project served as the students' faculty advisors for the course.

2. Background

Ships hulls require periodic cleaning due to corrosion, paint degradation, and incrustation with marine growth such as barnacles. To access the hulls, a ship must first be positioned on several equally spaced position blocks located on the floor of a sunken dry dock. The water is then pumped out from the dry dock, which will be raised to support the ship. Figure 1 shows the photograph of a typical dry dock. Normally, workers perform the cleaning with blasting hoses. One person can blast around 100 to 200 ft^sup 2^/hr with approximately one-half ton of abrasive particles being deposited on the deck of the dry dock.1 The operation is very intense and cleaning of the abrasive particles and incrustation also slows down the cleaning process. In addition, the space between the hulls and the floor of the dry dock is limited. The supporting blocks are impedimental to personnel and cleaning equipment. Vast amounts of potentially harmful dust are generated, so other workers are excluded from the vicinity of the blasting process. Use of the machine blasting head for the cleaning process was proposed by Bergh.2

There have been sonic blasting devices developed, but generally these are unable to provide adequate blasting of both the sides and bottoms of vessels. Sonic applications utilize complex and expensive robotic controls in order to manipulate the blast head.3 The system is quite large and is incapable of passing under a ship on five-foot blocks in a dry dock. The orientation tor the blast head is achieved by sensors, computers, and servomotors. Figure 2 shows a vehicle machine4 using a hull-contacting box designed for blasting ship bottoms. The machine docs not have the capability of blasting non-horizontal sections.

Figure 3 shows an application of attaching the machine blasting on a crane to blast the side of a ship.5

As there are still many unanswered questions with respect to calculation and modeling of the blasting process using appropriate physical laws, most designs are based on experience and empirical observations.4 Figure 4 shows a modified machine blaster that is used in this project. The abrasive particles are fed into a high-RPM wheel (Part 1 ), which is driven by a 30-HP motor (Part 2) with 3450 RPM. The wheel projects the abrasive centrifugally at a speed of 177 ft/s toward the surface to be blasted. Rubber seals between the blast machine and hull ensure that all the abrasive, dust, and paint chips from the blast process are directed back into the machine. A strong jet of air (Part 4) is directed through the stream of the falling abrasive. The heavy abrasive particles continue to fall, but the lighter dust and paint are carried by the air stream through a duct (Part 5) into a dust collector. The dust can be periodically emptied and the abrasive particles can be recycled through an auger (Part 6) to the centrifugal wheel.