3D Laser Scanning for Thickness Measurements of Hull Structures

Abstract

3D scanning technology uses lasers to scan and capture object surfaces without physical surface contact. Laser scanning is gaining acceptance by many, including owners of marine or offshore assets as a viable inspection and validation method. Laser technology reduces operational times compared to traditional pit gauging techniques, particularly for large areas of widespread wastage or pitting. This paper studies the use of 3D scanning technology for inspection, thickness gauging, and steel wastage measurements of hull structures. Pilot tests were conducted on coated and uncorroded plates in Houston, USA, and uncoated and corroded plates and uncoated and deformed plates in Perth, Australia. Manual Ultrasonic Testing (UT) was conducted, which is the method currently accepted by International Association of Classification Societies (IACS) for thickness measurements of hull structures. For the coated plate, the coating thickness was measured on both sides of the plate. The coating thickness was deducted from the total thickness from 3D scanning before the plate thickness was compared with the UT results. Acceptance criteria are proposed to compare the Manual UT measurements with the 3D scanning measurements to determine if 3D laser scanning is a possible alternative thickness measurement method. The difference of thickness measurements from 3D scanning on coated and uncorroded plates is within 13% when compared with those from UT. The discrepancy is attributed to equipment accuracy tolerances, errors from data post-processing, and measurement errors due to coating surface roughness. For uncoated and corroded plates, the difference reduces to 3%, making the results of 3D scanning acceptable based on acceptance criteria. In addition, the higher accuracy of using 3D scanning to measure plate deformation is demonstrated over traditional methods which use stringlines or laser levels to create a reference surface. Comparisons of the coefficient of variation (CV) on all plates demonstrate the higher precision of 3D scanning technology than that of manual UT. The main limitation of 3D laser scanners is their inability to directly obtain steel thickness for structures that have been coated or painted, especially in watertight/oil-tight structures. The study identifies capabilities, accuracy, and limitations of using 3D scanning technology for thickness measurements of hull structures in the marine or offshore industries. Scanning technology can support inspections providing fast and precise means of thickness measurements of corroded plates without coating. It provides the potential for producing 3D models and analysis for follow-up inspections. Plausible use cases in the maritime industry include defect analysis, fitness for service assessment, damage assessment, and corrosion monitoring.