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Development of an autonomous mobile inspection vehicle for detecting structural defects in ship's hulls

Final Report Summary - HULL INSPECTOR (Development of an autonomous mobile inspection vehicle for detecting structural defects in ship's hulls)

The HULL INSPECTOR system was created as an inspection crawler that could be attached to the side of a ship, using powerful permanent magnets. The same magnets are then used to form a Magnetic flux leakage (MFL) array for the detection of corrosion in the hull. The crawler can also be used to inspect the ship's welds using Alternating current field measurement (ACFM) and Phased-array (PA) ultrasonics.

The scientific and technical objectives are:
- to overcome the limitations of manual and current automated inspection of ship's hulls by developing an autonomous vehicle, which will carry a range of sensors to automatically inspect ship's hulls;
- to develop a range of electromagnetic and ultrasonic sensors for locating the position of the inspection vehicle on the hull, tracking its movement along features such as welds and detecting defects, such as corrosion and weld fatigue cracks that may be detrimental to the structural integrity of the hull;
- to develop an intelligent inspection system that is able to control inspection coverage according to the geometry of the test piece and the severity of flaws detected;
- to build a prototype vehicle and sensors for inspecting steel hulls, which range from those of small harbour boats, through coastal ships to large ocean going liners, oil tankers and container ships;
- to improve the control of weld quality during hull fabrication;
- to improve the maintenance of hull structural integrity during service.

The project was structured into the following Work packages (WP):
WP-A System specifications and sample procurement: to specify the range of applications for which the equipment will be tested in the project and the main features of the system, and to produce realistic test samples.
WP-B Development of NDT techniques, sensors and systems: to develop and produce a series of NDT inspection techniques, sensors and systems to detect and locate areas of corrosion and weld defects in a ship's hull.
WP-C Sensor integration software and data fusion: to create a software program to integrate the results from the sensors in WP B, D and E.
WP-D Development of mobile vehicle and control software: to produce a fully functioning mobile vehicle, capable of carrying the NDT sensors and systems to the required inspection areas, and to be able to deploy these sensors correctly.
WP-E Robot position and guidance system: to develop a system capable of locating the the ship's hull. This is for the purpose of tele-operating the vehicle to an area of the hull that is to be inspected and of mapping NDT measurements to their spatial location in a weld or on a plate surface. The vehicle will also be controlled by the operator using onboard video feedback.
WP-F Integration: to integrate all the deliverables into the final hull inspector system.

There have been many achievements of the HULL INSPECTOR system. Some notable successes include the following:

- Magnetic flux leakage system for thick sections
MFL consists of an array of electromagnetic sensors located between two powerful magnets. The magnets create a saturated magnetic flux in the hull of the ship. As the sensor array passes over any thinning of the hull - due to corrosion - the flux 'leaks' out of the steel plate and is detected by the array of electromagnetic sensors. This is used in industry to quickly screen and locate areas of corrosion in steel plate.

- Gimballed phased-array and ACFM holder
ACFM is used to inspect the surface of the weld for surface breaking cracks which act as potentially fatal stress raisers. The combination of the volumetric inspection using PA and the surface inspection method using ACFM has been combined to form a powerful technique for the validation of hull welds. Neither of these techniques is new, but the combination of the two, held within a four-way fully gimballed probe-holder with a magnetically attached crawler is. The gimballed holder consists of three holders combined, two for twin phased-array probes arranged on each side of the weld and a variable geometry ACFM array spanning the width of the weld.

- Alternating current field measure (ACFM) variable geometry array
ACFM consists of an array of electromagnetic sensors positioned across a weld. A fluctuating magnetic flux is played across the surface of the weld, produced by coils in the sensor heads. As the flux encounters surface breaking cracks, small magnetic poles are formed which are detected by secondary coils in the sensor array.

- The completed hull inspector system
The completed system combines all of the above mentioned exploitable techniques with a magnetically attached crawler. The system is aimed at any industry that has a requirement to inspect welds - using ACFM, PA and the sensor holder - to industries concerned with corrosion of steel plates - using the auto-calibrating MFL system.

The impact of the HULL INSPECTOR project is the creation of a magnetically attached crawler, capable of inspecting hull welds during fabrication, using PA and ACFM, and for corrosion of hull plates using MFL.

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