Welding Defects and Prevention

Multilayer Coatings

• Most common form: FBE primer + polyurethane foam + HDPE
• Prevents hydrate sheath formation and wax deposition in cold seawater
• Syntactic material: hollow glass microspheres in a polymer matrix

Cathodic Protection (CP)

• Stabilizes metal by replacing electrons in the corrosion process

Non-Destructive Testing (NDT)

Introduction

• Ensures integrity and safety of underwater structures and equipment
• Techniques: Visual Inspection, NDT, and Destructive Testing

Classification Society Requirements

• Annual Survey: Focuses on visual inspection
• Special Survey: Includes mandatory NDT at selected locations, conducted every 3 to 5 years

Inspection Methods

Visual Inspection

• Ranges from manual examination to sophisticated TV monitoring systems
• Pros: Wide applicability, no significant equipment required
• Cons: Highly dependent on human factor, reliability can be compromised by non-defect marks

Ultrasonic Thickness Gauging

• Uses sound propagation to detect, locate, and evaluate defects
• Pros: Effective for a wide range of materials, can detect internal defects
• Cons: Ineffective for thin materials, geometric complexity can lead to spurious signals

Dye Penetrant Test

• Uses capillary action to reveal surface discontinuities with a dye
• Pros: High sensitivity to small discontinuities, few material limitations, rapid inspection of large areas
• Cons: Only detects surface-breaking defects, pre-cleaning is critical

Magnetic Particle Testing

• Identifies defects by applying magnetic particles on a magnetized test object
• Pros: Sensitive to fine surface flaws, can detect subsurface defects to some extent
• Cons: Removal of magnetic particles required after testing, magnetic field orientation and strength critical for detection accuracy

Ferrous Wire Rope Testing

• Types: Traditional and computerized methods for detecting external and internal wire breakages
• Pros: Computerized testing offers detailed analysis of wire integrity
• Cons: Traditional methods are slow and laborious

Radiography Inspection

• Uses X-rays or gamma rays for internal defect detection based on radiation absorption variations
• Pros: Comprehensive internal examination, suitable for a wide range of materials and defect types
• Cons: High operational costs, radiation hazard

Eddy Current Inspection

• Detects defects by inducing and monitoring changes in eddy current flow
• Pros: Portable and lightweight equipment, immediate assessment of crack depth and length
• Cons: Limited to near-surface flaws, requires calibration prior to inspection

Alternating Current Potential Difference (ACPD) Inspection

• Assesses the depth of surface-breaking defects through potential difference measurements
• Pros: Accurate measurement of crack depth on flat surfaces, can be used to monitor crack depth during service life
• Cons: Requires surface cleaning before testing, complex geometries can lead to false indications

ROV System Components

• Control unit, manipulator arm, cable
• Launch & recovery unit, garage
• Ongoing issues with failures and wear

Internal Welding Defects

Slag Inclusion

• Description: Slag in the weld metal, compromising toughness
• Causes: Incorrect angle and speed, poor pre-cleaning, low current density
• Prevention: Higher current density, optimal angle and speed, consistent cleaning

Incomplete Fusion

• Description: Poor welding leading to pre-solidified gaps
• Causes: Low heat input, wrong angles, large weld pool
• Prevention: Higher current, slower travel, correct positions

Incomplete Penetration

• Description: Gaps in butt welding joints
• Causes: Incorrect technique, wrong electrode size, low deposition rate
• Prevention: Correct technique, higher deposition rate, proper electrode size

Other Welding Defects

Whiskers

• Description: Electrode remnants on the weld joint's root side
• Causes: Fast feed speed, high travel speed
• Prevention: Snip electrode before welding, reduce feed and travel speed

Necklace Cracking

• Description: Incomplete penetration in electron beam welding
• Causes: Specific metals, improper technique, high speed
• Prevention: Better material selection, constant speed, correct technique

Detecting Welding Defects

Non-Destructive Testing (NDT)

• Observes discontinuities without damage
• Methods: Visual inspection, liquid penetrants, magnetic particles, eddy currents, ultrasonics, acoustics, radiography

Destructive Testing

• Tests finished projects to their limits
• Methods: Acid etch, guided bend, free bend, back bend, nick break, tensile strength