^Alexander L. Kielland Semi-Submersible Drilling Rig
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Bad Welds Invite Tragedies
March 27, 1980. Alexander L. Kielland Semi-Submersible Drilling Rig at the Ekofisk Oil Field, Norwegian sector of the North Sea. A howling 40-knot wind was driving rain onto the rig that had just been winched away.
Over 200 of the 212 men aboard were off duty in the accommodation segment of the rig. Many were relaxing in the cinema and the mess hall. Just before 6:30 pm the crew felt a ‘sharp clap’. A little later they sensed ‘some kind of trembling’.
Much like the slight tremors felt before an earthquake lets loose its full fury, the clap and trembling were only the beginning . . . of a painful end. Soon, the rig became the theater of the worst disaster in Norwegian offshore history since the Second World War.
Five of the six anchor cables had broken and caused the sharp clap and trembling as the rig heeled by around 300. What stood between the crew and a total catastrophe was the sixth cable.
But that did not last for long. By 6:53 pm, the last cable standing gave away and the rig capsized. Of the 212 aboard, 123 died. What aggravated the crisis was a safety device that prevented the release of some life boats. And the standby vessel arrived an hour later.
Anatomy of Collapse of the Alexander L. Kielland Semi-Submersible Drilling Rig
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Investigators blamed a fatigue crack in one of the six D-6 bracings. A small 6 mm filler weld with poor profile had precipitated the crack. The weld linked the bracing to a non-load-bearing flange plate. The devil truly is in the detail.
Closer to home, shoddy welding of a bracket that supported a tray inside the reactor of a urea plant in Westlake, Louisiana caused an explosion on July 28, 1992. Such was the impact of the explosion that a fragmented shell of the column was blasted 900 feet away.
The Fundamentals
During welding, metals expand and contract. More than anything else, this introduces undesirables in the process – brittleness, cracks, deformations, porosity, lack of penetration, and what not.
Mild steel or low carbon steel is metal of choice for structural work because it retains its ductility despite journeying to extreme temperatures and back. Sadly, high carbon steel, aluminum, cast iron, stainless steel, and titanium are not so well endowed.
Weld Defects are any of the myriad of imperfections that undermine the utility of a welded joint. ISO 6520 classifies weld flaws while ISO 5817 and ISO 10042 prescribe acceptable limits for such defects.
Welding Failure Caused the Collapse of Seongsu Bridge in 1994
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Issues with welding have been around for decades. The following factors influence and aggravate them:
Ignorance on Weld Processes
Absence of Process Ownership
Needless Belief in Salespersons
In order to prevent failures of welded structures, the American Welding Society (AWS) develops comprehensive welding codes and standards. These define the dimensions, shape, and allowable level of defects.
Other bodies such as the American Society of Mechanical Engineers (ASME), American Society for Nondestructive Testing (ASNT), and American Petroleum Institute (API) make suggestions for the code.
Improper welding procedure causes most weld defects. According to ASME, following factors bring about weld defects:
Poor Process Conditions (45%)
Operator Error (32%)
Incorrect Technique (12%)
Faulty Consumables (10%)
Unsound Weld Gloves (5%)
You can trace most weld defects to two general areas:
Ill-Advised Training and Workmanship: is overcome by providing a detailed Welding Procedure Specification (WPS) that adheres to the fundamentals of welding
Unwarranted Weld Design and/or Material Choice: is nullified by complying with basic design principles and practices
Ignorance is No Bliss in Welding, it’s Deadly
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You can simplify the causes as:
Flimsy Joint Design
Incorrect Shielding Gas or Flow Rate
Faulty Machine or Settings
Erroneous of Defective Electrode / Wire
Incompatible Ambient Temperature or Humidity
Types, Causes, & Prevention
Metallurgically speaking, bad welds are a manifestation of:
Hydrogen Embrittlement
Residual Stresses
Please note, hydrogen embrittlement and residual stresses are complementary – they feed and fatten each other. By eliminating one, you control the other too.
Oxygen and hydrogen are the nemesis of welders. Corrosion, rust, and millscale on the base metals are a result of oxidation. These interfere with welding. Moisture i.e. water in any form invites hydrogen embrittlement – water contains hydrogen and oxygen.
Hydrogen Embrittlement is the entry and spread of hydrogen in a metal structure. It makes metals brittle and causes them to crack. Hydrogen embrittlement manifests as a sizable fall in the ductility of metals during the slow bend or tensile tests.
Solubility of hydrogen in iron (and therefore steel) increases at elevated temperatures. Welding operations provide such high temperatures. Hydrogen remains trapped when metals solidify. Being highly mobile, hydrogen atoms diffuse freely in the metal and gather wherever possible.
Hydrogen Induced Cracking (HIC)
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There is no consensus among researchers on how hydrogen causes brittle fractures. The conventional explanation – an increase in hydrogen concentration builds up pressure and causes cracks – explains only a limited number of fractures.
In order to prevent such embrittlement:
clean and dry steels before welding
employ baking treatments to liberate trapped hydrogen
follow preheat schedules and prescribed inter-pass temperature for high-strength-low-alloy steels
Residual Stresses are those that linger in a material even after you remove the cause for the stress. Such stresses are a major reason for premature failure of metal structures.
Three main causes of residual stresses in metals are:
Cyclic Temperature Changes
Plastic / Inelastic Deformations
Structural / Phase Transformations
Weld operations cause the base and filler metal to heat up and expand. After you remove the source of heat, the weld and base material cool down and contract.
Methods to Measure Residual Stress
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Some areas cool and contract more than others. Such uneven cooling introduces residual stresses. This creates large tensile stresses in some parts of the weld that are balanced by compressive stresses in other parts of the same weld.
Typical weld defects include:
Lack of Fusion
Incomplete Penetration
Excessive Penetration
Porosity
Inclusion
Cracks
Undercuts
Lamellar Tearing
Other defects include:
Overlap
Distortion
Arc Strikes, Spatter, and Other Surface Disruptions
These defects can introduce stresses and cause failures below the design load. In case of cyclic loads, they cause failures before the predicted number of cycles.
Weld Bead
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Weld Bead is the filler metal deposited in the weld joint. It can be:
Stringer Bead: a narrow bead created by straight dragging or light oscillatory motion of the consumable
Weave Bead: broader and created by wide oscillation of the consumable during welding
Lack of Fusion, Incomplete Penetration, and Excessive Penetration: insufficient heat input causes lack of fusion and incomplete penetration. Low current, excessively fast traverse, and incorrect welding-torch angle result in low heat input.
Exorbitant heat input however results in excess penetration. Slow traverse of the welding torch and high current often provide such extra heat. This is more of a problem when welding thin sheets.
In order to weld correctly, the fusion zone must include the complete thickness of both materials. Anything less and you are struck with incomplete penetration and lack of fusion. Anything more and you risk excessive penetration.
The aforementioned causes for low heat input do not allow:
the weld bead to penetrate the complete thickness of the plate
the weld bead to thoroughly penetrate the toe of a fillet weld
two opposing weld beads to interpenetrate
A weld bead that looks like a bullet train indicates incomplete penetration. Cut beveled grooves while welding thick materials. Fill them and the joint completely with metal using multiple passes to avoid incomplete penetration.
Maintain the position of the weld-torch or the arc at the leading edge of the weld puddle. If the torch or the arc trails the puddle, the puddle forms a cushion and prevents complete penetration.
Also called Cold Shuts or Cold Lapping, an additional cause for lack of fusion is welding wide joints in a single pass. Reason: insufficient heat input. As a result, the side walls of the base material do not melt.
Design narrow joints where possible. For wider joints and thicker material, employ multiple weld passes – split bead technique gives best results. Also, direct the arc towards the side wall of the base plate.
Aluminum oxide hampers fusion when welding aluminum. With a melting point of 3,5000F (1,9270C), the oxide is insoluble in molten aluminum and interferes with melting. The same is somewhat true for iron oxide when welding iron. Pre-cleaning negates the oxide effect.
You can weld thin sheets in a single pass along their square edges. For thicker materials, you need to cut V edges and weld them in several passes. If both sides are accessible, weld on the reverse side too.
Cracks: follow from the strain i.e. dimensional change in the weld joint. During phase changes and thermal shrinkages, diverse parts of the weld joint cool at different rates and introduce strain. Steels with over 0.2% carbon are particularly prone to cracks because they cool rapidly.
The ability to spread and weaken the entire weld joint makes cracks lethal. The general cause is any combination of improper welding conditions, joint design, and welding technique. These create concave weld beads. Flat or slightly convex beads are desirable.
Generally, step-by-step preheating of both sides of the joint followed by slow after-cooling checks cracks. Filing, grinding, de-burring, and cleaning the edges of both materials aligns base metals and prevents cracks. So does clamping them before welding.
Types of Weld Cracks
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Cracks can be:
Hot / Solidification / Centerline Cracks: appear during or immediately after welding
Cold / Hydrogen Induced Cracks (HIC): show up a day or so after welding
Crater Crack: pop up at the end of the weld bead if you incorrectly break the arc
Microfissures: make their presence felt long after you complete the weld. Metal fatigue, seismic activity, or stresses in the heat affected zone (HAZ) create these fissures. Heat treatment restricts them
Hot Cracks: are formed when the weld bead cannot withstand the stresses unleashed by solidification. Cracking starts during or immediately after welding and the crack surface is blue.
Branch-like in appearance, they infest high-carbon-steels containing sulphur and phosphorous. Transverse cracks are known but these usually show up along the weld centerline. You will find them in the weld joint – never in the base metals.
Crack Induced Weld Failure
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Weld joints are prone to such cracks when you:
subject the weld pool to needless restraint
have weld beads of improper shape or deficient size
weld materials with high impurity content or large thermal shrinkage
design joints with large gaps
weld steels containing sulphur and phosphorous
use high-dilution welding processes such as Tungsten Inert Gas (TIG) welding with no filler
When welding carbon-manganese steel, ensure that it contains less than 0.06% sulphur and phosphorous. And if you are welding these steels in highly restrained joints, ensure that this percentage does not cross 0.03%.
Weld Seam on a Pipeline
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Other prevention techniques include:
set weld parameters that create a depth-to-width ratio between 0.5 and 2
employ techniques and practices that maintain small gaps in the base metals pieces, ensure clean materials, and prevent the welding sequence from introducing thermal stresses
use moderate weld speeds, for high speeds induce transverse strains and segregation
wherever possible, use low-dilution welding processes such as Metal Inert Gas (MIG) and Manual Metal Arc (MMA) / Shielded Metal Arc Welding (SMAW)
Hydrogen Induced Cracking (HIC): occurs in steels during production, fabrication, or service. Also called Hydrogen Assisted Cracking or Hydrogen Cracking, it infests either the weld zone or the Heat Affected Zone (HAZ).
Butt welds exhibit transverse cracks while fillet welds have longitudinal ones. Weld toes are most prone to HIC. Other liable areas include weld beads and weld roots.
Welds of low-alloy steels are sensitive to HIC in the presence of:
hydrogen
vulnerable microstructures
high tensile stress
weld assembly that cools below 1500C
GTAW / TIG Welding Can Promote Cracks
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Crater Crack: is a peculiar crack that appears at the end of the weld where the arc is broken. Small and apparently innocuous, it is dangerous because it can spread into the weld joint.
Improper ending of the arc causes such cracks. Weld a little beyond the end of the joint. Else, reverse the direction of the arc for some length after welding the full joint length. Do not break the arc immediately at the end of the joint.
If your welding control supplies gas for a short while after you break the arc, shield the crater with gas till it solidifies. Melt and re-weld earlier stopping points and tack welds before proceeding longitudinally along the joint.
Tack Welds are the temporary welds used to hold the base metals together after positioning them. Such welds maintain the proper alignment of base metals allowing you to create the crucial first joint.
Porosity: is the correct, technical term for gas bubbles and results from the entrapment of gases in solidified welds. The weld pool is vulnerable to contamination and therefore needs a shielding gas.
GMAW / MIG Welding Resists Cracks
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Aluminum is particularly prone because it oxidizes readily. Nitrogen and oxygen cause porosity in steel. In the absence of nitrogen however steels can contain considerable oxygen and yet be porosity-free.
Atmospheric contamination, insufficient deoxidizing alloys, over-oxidized workpiece surfaces, and foreign matter cause porosity. The source of gases can be damp consumables or oil, grease, and metals in the area around the weld.
Following factors cause atmospheric contamination:
very high and very low shielding gas flow
excessive wind in the welding area that blows away the shielding gas
damaged gas supply system or seriously clogged gas nozzle
Other causes of porosity include:
foreign matter such as extra lubricant on the welding wire can introduce hydrogen in the weld
exorbitant solidification rates caused by low welding current and high traverse speeds trap gases that would normally escape
inconsistent arcs incite turbulence in the weld pool and break the shielding gas envelope
use of incorrect electrode, wire, or gas-mixture
To prevent porosity:
clean and degrease materials before welding and store consumables in dry conditions
ensure leakage-free supply of shielding gas
be extra careful in damp ambience as welding torch coolant can condense moisture and contaminate the shielding gas
remove water-containing anodized coatings over aluminum before welding
Undercutting: occurs when incorrect weld procedures or settings erode the thickness at the toe of either or both the base materials. The toe is already affected by stresses and undercutting further adds to its woes.
Undercuts in Weld Joints
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This is visible as a groove in the base material along the direct edges of the weld. Primary reasons for undercutting are low welding voltage and excessive traverse speed.
Fast traverse solidifies the weld bead more rapidly than necessary. Surface tension in the solidifying bead pulls the molten metal near the edge of the weld creating a groove.
Sides of a joint i.e. the edges of the base material melt faster. This is because, unlike other areas of the weld joint and particularly its center, they cannot conduct heat in all directions. Seasoned welders pause slightly at the sides to prevent undercuts.
Insufficient arc voltage causes similar rapid solidification. However, excessive voltage also results in undercutting because it causes extravagant melting of the base material that solidifies rapidly. Similarly too-low and too-high currents trigger undercutting.
Distortion in Weld Joints
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Overlap: is the diametric opposite of undercutting. Herein, the weld puddle flows across the base metal at the toe but fails to melt them. This prevents proper connection between the base metals. To avoid overlap, use sufficient welding voltage and direct heat towards the toe.
Inclusions: usually occur when welding thick materials in several passes with flux coated or flux cored rods. Each pass creates slag that covers the weld pool. If you do not remove this slag before the subsequent pass, it enters the weld.
Lamellar Tearing: mainly bothers low quality steels. Infested with oxides and sulphides that are elongated during rolling, such steels cannot stand transverse contraction stresses.
T joints are most susceptible to such tearing because they offer a fusion boundary parallel to the rolling plane. Butt and fillet joints however are not completely immune.
You can prevent lamellar tearing by:
using superior grade steel
redesigning joint
peppering the weld area with ductile material
Distortion: is the change in position of the two sides of the joint. This happens due to the expansion and subsequent contraction of base plates during the welding process.
Stainless steels are particularly exposed to distortion. The chances of distortion increase when plates get smaller and welds get larger. A natural prevention technique is to start welding with the plates tipped in the direction opposite to the expected distortion.
Distortion in Weld Joints
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Correct heat input and traverse speed prevent excessive heating of base metals and the resultant distortion. Employing fewer passes, and modifying the joint location and weld sequence are similarly useful.
Spatter, Arc Strikes, and Other Surface Disruptions: spatter includes the tiny bits of molten metal that fly around the weld zone during arc welding. It falls on the base metals and solidifies. It is visible as minute surface projections.
Excessive voltage and wire-feed rates with longer-than-necessary arcs create spatter. They can cause injuries, impede the mechanical assembly of the (welded) workpiece with other components, obstruct the flow of weld puddle, and spoil the appearance.
If you strike an arc outside the joint, it can create a dent that grows into a crack. Discoloration around the weld zone is more an issue with heat-sensitive metals such as stainless steels and aluminum. It can however unfavorably alter the chemical composition and mechanical properties.
Inspection Techniques
Despite all the aforementioned preventive measures, weld defects will raise their ugly head. Before you can undertake curative measures, you need to detect them.
How X-Rays Detect Flaws
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Professional weld inspection techniques boost weld productivity while minimizing rework and the possibility of weld failures. Such failures not only cause loss of life, limb, business, and property, but also create avoidable, vexatious litigation.
Visual Inspection: with a magnifying glass in adequate lighting is a reasonable starting point. Before welding, inspect the materials and fillers for cleanliness and alignment. Check machine settings. After welding, undercutting and excess penetration are immediately visible.
Liquid Penetrant Inspection: reveals flaws undetected by visual inspection. Clean and dry the weld and its vicinity. Spray a colored liquid die with good penetrating power.
When you wipe the die thoroughly off the surface and sprinkle white powder, any surface with fine flaws underneath will draw out the die that has penetrated inside it. You see these as powder wetted and colored by the die.
X-Ray Examination: diagnoses sub-surface defects and inclusions that do not extend to the surface. Visual or liquid penetrant inspection cannot help such detection. Although expensive, these are absolutely essential for weld joints in nuclear plants and submarines.
Ultrasonic Inspection: uses ultrasonic waves to detect surface and sub-surface defects. Engineers direct these waves through the base metal and the weld joint along a known path.
Ultrasound Inspection of Welded Joint @ Pipe
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Discontinuities reflect these waves. The transmission-reflection time gap indicates the location of the defect. Interpretation of reflected waves is a skilled task for you need to tell between genuine flaws and gas bubbles – both reflect waves.
Magnetic Particle Inspection: uncovers above and below surface flaws in ferromagnetic materials only. Magnetic particles (powder) accumulate more densely at the location of defects when you pass a high current between probes stationed across the to-be-inspected area.
Fluorescent magnetic powders in combination with ultraviolet radiation provide more sensitive and visible results. You can conduct this process in wet or dry settings – the wet one is more sensitive as you can use finer powder to expose minute flaws.
On account of the involved electromagnetism, you cannot use this process to unmask fractures in non-ferromagnetic materials such as austenitic stainless steels.
Focus on inspection is not the only link in the chain for the safety of welded structures. Managements need to do more in the form of executing best process controls and weld practices.
Repair usually involves ground out the flawed portion before you re-weld the area to the required specification. And yes, you also need to inspect the weld again. No shortcuts here.
Finally
With myriad considerations involved, fine welding is more of an art. It is not exactly rocket science. Mastering it takes ages of practice coupled with tons of patience.
But before you get to that stage, remember to err on the side of caution – stop welding if you notice something unusual. Resume only after expert consultation. Else, there is a disaster waiting to happen.
If you want more of the power of knowledge on welding, don’t forget to visit our blog. And for sublime marine fabrication services, marine pipe fitting, and large scale custom metal fabrication, contact Kemplon Engineering.
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