Automated ultrasonic testing is used to support many inspection code and standard ultrasonic surveys per requirements in American Petroleum Institute API 510 Pressure Vessel Inspection [1], API 570 Piping Inspection Code [2], API-653 Tank Inspection and Repair [3], and American Bureau of Shipping (ABS) Guide for Non-destructive Inspection [4]. Automated ultrasonic testing of ship hulls is used to assess the hull plates for general corrosion, pitting and other wastage that may occur on the outside or inside of the hull. This article discusses some of the technical aspects involved in the ultrasonic c-scan ship hull surveys.
Figure 1: Automated ultrasonic C-scan and A-scan testing of ship hulls.
Automated Ultrasonic Testing C-scans
AUT is used where high-speed, high accuracy, and high resolution corrosion mapping is required. An AUT crawler is configured to evaluate the integrity of storage tanks, pressure vessels, pipelines and ship hulls to support the safe operation of the asset over its lifetime and in a predictive manner to schedule immediate repairs, future repairs, and asset retirement. The standard AUT crawler is a two-axis scanning system that supports ultrasonic thickness survey along the scan axis and index axis. The AUT crawler is drive by high torque stepper motors and powerful magnetic wheel that adhere strongly to steel surfaces of various contours. The probe fixture is configured to allow for some limited displacement in the vertical direction and some rotation in about the scan and index axes to follow contoured surfaces. An example AUT crawler on a ship hull is shown below in the video. The UT crawler advances in the scan axis direction and the transducer is scanned across the index axis. Typical index axes bridges range from 12" to 36" long. Scans distance can range from a few feet to hundreds of feet.
An example user interface is shown below. On the top right is the A-scan that is used to measure the hull thickness. Two signals of interest are observable from left to right. The first signal is the interface echo received from the ultrasonic pulse travelling through the water column to the ship hull outside surface. The second signal is the first backwall reflection, or thickness, of the ship hull. The two ultrasonic signals shown can shift dramatically to the left or right depending on the contour of the ship hull which can increase or decrease the water path. To compensate the variable distance water path, an interface gate is set on the interface echo with an appropriate range. All thickness measurements are made relative to the gate position at which the interface echo crosses the gate. The second red gate to the right is used to measure the thickness of the ship hull and its position is moved relative to the position where the interface echo crosses the interface gate.
The C-scan shown in the middle left is a top view thickness map where each pixel is color coded thickness value. Typical ultrasonic ship hull c-scan resolution is 0.50 x 0.50" or 1.00 x 1.00" based on the customer of applicable code requirement. In some cases, the resolution can as little as 0.25 x 0.25" if pitting discrimination is required. In this example the nominal ship hull plating is 0.625" thick. The color palette to the left of the ship hull C-scan is distributed over 4 colors. The red, yellow, green and blue bars correspond to 100 – 25, 25 – 15, 15- 10, and less than 10% wall loss ,respectively. The red pixels show wall loss exceeding 25% along a vertical frame and horizontal stringer. The ultrasonic testing A-scan selected shows a ship hull minimum wall thickness of 0.232". Thousands of ultrasonic A-scans are generated during a single C-scan with hundreds of thousands and millions of A-scans collected over an entire ship hull C-scan. For example, a 600" long C-scan using a 30" index axis will generate 72,000 A-scans at 0.5" scan and index axis resolution.
Transducer Selection for Ultrasonic Testing of Ship Hulls
Ultrasonic testing immersion transducer selection for ultrasonic ship hull is based largely on the plate thickness. An immersion transducer is a single element longitudinal wave transducer with a 1/4 wavelength matching layer to water. The matching layer permits maximum ultrasonic power transmission at the design frequency. The immersion, or water jet, configuration used in AUT scanning of ship hulls, is designed to transmit ultrasound in applications where the test parts are partially or wholly immersed in water, which allows a uniform and fast coupling technique for rapid scanning of parts. The acoustic lens on the front face of the transducer improves signal-to-noise ratio at the desired depth(s).
For thicknesses in the 0.25" to 0.50", a 0.50" diameter 5 MHz – 2.00" focus transducer is recommended. Hull thicknesses in the 0.50" to 2.00" range required a probe with a 3" focus. Ship hulls with significant outside and/or inside surface pitting may required lower frequency transducers or experimentation with transducer focal depth to determine the optimal transducer. Example ship hull surface pitting is shown below. In this case the surface pitting measured 0.0625" to approximately 0.125" in depth and required a 0.50" diameter 5 MHz 2.00" focus transducer to maintain adequate 1st backwall reflection signal-to-noise (SNR) ratio.
Summary
Ultrasonic C-scan testing of ship hulls can be a challenge compared to more conventional scanning applications like storage tank, pressure vessels and piping due to the total amount of square footage required to be scanned and the various concave and convex surfaces present on a ship hull. Current technology can complete small to large ships in 3 – 10 days depending on access and safety considerations.
[3] API 510 Pressure Vessel Inspection Code: In-service Inspection, Rating, Repair, and Alteration, American Petroleum Institute 2020.
[4] API 570 Piping Inspection Code: In-service Inspection, Repair, and Alteration of Piping Systems, American Petroleum Institute 2020.
API 653 : 2020 TANK INSPECTION, REPAIR, ALTERATION, AND RECONSTRUCTION
American Bureau of Shipping (ABS) GUIDE FOR NONDESTRUCTIVE INSPECTION SEPTEMBER 2020
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