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Oil drilling rig report

Bangladesh University of Engineering and Technology
Report No : 01
Course No: MME-448
Name of The Report: Failure Analysis of Oil Drilling Rig.
Submitted By
Name: Rakib Hasan
Student Id: 1611029
Materials and Metallurgical Engineering
Bangladesh University of Engineering and Technology
Date of Submission: 12.12.2021
Oil Drilling Rig:
A structure above an oil well on land or in the sea that has special equipment attached
to it for drilling and removing oil from the ground. Drilling rigs can be massive structures
housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or
they can be small enough to be moved manually by one person and such are called
augers. Drilling rigs can sample subsurface mineral deposits, test rock, soil, and
groundwater physical properties, and can be used to install sub-surface fabrications,
such as underground utilities, instrumentation, tunnels, or wells. Drilling rigs can be
mobile equipment mounted on trucks, tracks or trailers, or more permanent land or
marine-based structures (such as oil platforms, commonly called 'offshore oil rigs' even
if they don't contain a drilling rig). The term "rig" therefore generally refers to the
complex equipment that is used to penetrate the surface of the Earth's crust.
Components of a Drilling Rig:
Hull – initially rigs were built out
of tanker hulls, so the
terminology remains
Power Module – converts
available fuel into power for the
Process Module – onboarding
and offloading of supplies and
Drilling Module – the traditional
drilling rig apparatus
Quarters Module – where the
crew sleeps and eats
Well bay Module – access to the well and other equipment
Derrick – the oil derrick
The world’s worst offshore oil rig disasters:
Alexander L Kielland, Norwegian North Sea, 1980
Ocean Ranger oil rig disaster, Canadian Atlantic Ocean, 1982
Glomar Java Sea drillship disaster, South China Sea, 1983
Glomar Java Sea drillship disaster, South China Sea, 1983
Bohai 2 oil rig disaster, Gulf of Bohai, China, 1979
Mumbai High North disaster, Arabian Sea, 2005
Usumacinta Jack-up disaster, Mexican Gulf of Mexico, 2007
• Deepwater Horizon disaster, Gulf of Mexico, 2010
The Leading Causes of Drilling Rig Disasters:
Strong winds and rouge waves
Defective equipment and structural failures
Poor Safety Procedures
Blowouts are the uncontrolled release of oil or gas from the well after pressure control
systems have failed. Oil rig blowouts can occur when the rig applies too much pressure
during the drilling, causing the pool of underground oil to erupt. When the drill
encounters a pressurized zone underground and the gravity of the drilling mud fails to
counteract the pressure, it causes a sudden rush of pressure up the system. Once the
pressurized natural gases are exposed to a less pressurized atmosphere, there will be a
kick. Although blowouts are not as common due to modern blowout preventers,
workers should still be trained in how to manage preventer failure to prevent blowouts
from occurring.
Blowout Prevention (BOP):
The BOP is a massive stack of high-pressure valves, weighing nearly 400 tons, that sits
on the sea floor and is designed to stop an uncontrolled release of oil or gas from a well
during the initial drilling. The BOP is located at the top of the well near the seabed and
surrounds the pipe or drill string, which passes through it. The fact that the BOP
surrounds the pipe and stays in place throughout the drilling operation means that it
can be used at any time to shut in or seal off the well. The BOP can shut in the well in
minutes. If activated, the blowout preventer will automatically close hydraulic rams and
activate specialized seals against the drill string to seal the bore. If this does not work
properly, there are other rams which can completely cut through the drill string to seal
the hole. In all, the BOP has six independent shut-in mechanisms. Blowout preventers
are proven to be highly effective in ensuring well safety.
When is a BOP operated?
The BOP is operated anytime the well needs to be isolated at the seabed. When the BOP
is closed, the well is said to be ‘shut in’ or ‘sealed off’ (similar to a plumber closing a
valve in a pipe). As part of the normal well construction process, it is necessary to shut
in the well to perform routine operations such as pressure testing. A well may also be
shut in for non-routine events, such as a ‘kick’. A kick is a flow of fluid (e.g., oil, gas
and/or water) from the rock layers into the well bore (the hole drilled for exploration). If
no action was taken, these fluids could migrate up the well, past the BOP and escape to
the surface, in what is known as a loss of well control (LOWC).
How does a BOP work?
BOPs isolate the well bore using two types of closure devices — ram preventers and
annular preventers. The BOP will have a minimum of five sets of ram preventers and
two sets of annular preventers. Each set of preventers is designed to withstand the
maximum expected pressure in the wells. As such, only one set of preventers is typically
required to isolate the well from the surface.
Ram preventers consist of two hydraulically activated ‘rams’ that are positioned
opposite each other and designed to seal off a well. A common BOP ram configuration
for deep-water is: › Three pipe ram preventers that can seal the well when drill pipe is
inside the BOP. The pipe rams have opposing half-moon section that allow a rubber seal
to be made around the pipe. › Two shear ram
preventers capable of cutting a wide range of pipe.
Two steel blades are pushed together, pinching the
pipe in the middle (like the cutting action of garden
pruning shears).
Annular preventers have a doughnut-shaped rubber
element that is moved by a large hydraulic piston. The
rubber element is designed to seal around most
shapes and size of pipe and close off an open hole (for
example, if there is no pipe inside the BOP).
During normal drilling operations, all the BOP’s
preventers are open. This allows tools to be lowered
into the well and mud and drill cuttings (generated
during drilling) to be circulated up out of the well,
through the marine riser to the rig. When it’s necessary
to shut in the well, a control panel on the drilling rig
activates one or more of the BOP’s preventers, which
are operated using high pressure water-based hydraulic
fluid. Once the well is shut in, any pressurized fluids in the well hole can be safely routed
through pipes and valves on the BOP to specialized pressure controlling equipment on
the rig.
The Deepwater Horizon Disaster:
The explosion on BP’s Deepwater Horizon rig on 20 April 2010 caused the biggest oil spill
in US history. Four million barrels of oil spilled into the Gulf of Mexico, harming huge
numbers of sea birds and damaging coastlines for hundreds of miles.
The rig worked in 5,000ft-deep waters after completing the drilling of the 13,000ft-long
Macondo exploration well in the Gulf of Mexico.
On the night of 20 April, a sudden rush of natural gas blasted through the concrete core
of the well and caused the rig to explode. This killed 11 of the 126 on board, injuring
several others.
After burning for 36 hours, the drilling rig capsized and sank on the morning of 22 April.
This ruptured the oil riser from the seabed, leaving 1,000 barrels of oil spilling into the
Gulf every day. The damaged well was finally capped with a 40-ton plug on 15 July, with
the oil slick lasting 87 days.
Reasons Behind the Deepwater Horizon Disaster:
The day before the accident, the crew had pumped cement to the bottom of the
borehole, a standard procedure intended to prevent oil leaking out. On the day of the
accident, the team were conducting checks to determine that that the well had been
properly sealed.
The accident was caused by the failure of eight different safety systems that were
meant to prevent this kind of incident:
Dodgy cement
The cement at the bottom of the borehole did not create a seal, and oil and gas began
to leak through it into the pipe leading to the surface.
Valve failure
The bottom of the pipe to the surface was sealed in two ways. It too was filled with
cement, and it also contained two mechanical valves designed to stop the flow of oil and
gas. All these failed, allowing oil and gas to travel up the pipe towards the surface.
Pressure test misinterpreted
The crew carried out various pressure tests to determine whether the well was sealed or
not. The results of these tests were misinterpreted, so they thought the well was under
Leak not spotted soon enough
Whether a well is under control or not, the crew at the surface should be able to detect
a flow of oil and gas towards the surface by looking for unexpected increases in pressure
in the well. Exactly this kind of increase occurred about 50 minutes before the rig
exploded, but it was not interpreted as a leak.
Valve failure no. 2
About 8 minutes before the explosion, a mixture of mud and gas began pouring onto
the floor of the rig. The crew immediately attempted to close a valve in a device called
the blowout preventer, which sits on the ocean floor over the top of the well borehole.
It did not work properly.
Overwhelmed separator
The crew had the option of diverting the mud and gas away from the rig, venting it
safely through pipes over the side. Instead, the flow was diverted to a device on board
the rig designed to separate small amounts of gas from a flow of mud. The so-called
mud-gas separator was quickly overwhelmed, and flammable gas began to engulf the
No gas alarms
The rig had an onboard gas detection system that should have sounded the alarm and
triggered the closure of ventilation fans to prevent the gas reaching potential causes of
ignition, such as the rig’s engines. This system failed.
No battery for BOP
The explosion destroyed the control lines the crew were using to attempt to close safety
valves in the blowout preventer. However, the blowout preventer has its own safety
mechanism in which two separate systems should have shut the valves automatically
when it lost contact with the surface. One system seems to have had a flat battery and
the other a defective switch. Consequently, the blowout preventer did not close.
Defective Equipment and Structural Failures:
The malfunctioning of equipment and the lack of seaworthiness of the drilling platforms
have been attributed as the main reasons for some of the biggest offshore drilling rig
disasters. For example, the capsizing of the Alexander L Kielland platform (March 1980),
Norway’s worst offshore disaster that killed 123 people, occurred due to the failure of
one of the bracings attached to one of the platform legs to withstand strong winds and
high waves. An undetected fatigue crack in the weld of an instrument connection on the
bracing was singled out as the root cause of this disaster.
Another example of how the malfunctioning of a single piece of equipment can lead to a
full-blown disaster is the Deepwater Horizon disaster, which was caused due to the
failure of the blowout preventer. Similarly, the explosions on the Petrobras P-36 semisubmersible oil platform off the coast of Brazil in 2001 were believed to have occurred
due to an emergency drain tank, which was ruptured due to increased pressure.
Alexander L. Kielland: Offshore Disaster:
The Alexander L. Kielland rig was fabricated and completed in March 1976; it collapsed
on 27 March 1980, whilst in service, due to a fatigue crack propagating from a welded
joint, with the loss of 123 lives. The original design of the construction was a standalone
drill rig, with associated service facilities, for service in a shallow marine environment.
The platform was supported vertically by five tubular columns at the apices of a
pentagonal design, each interlinked by robust tubular braces and stabilized upon
pontoon bases. The structure entered service and ended up as a “flotel”
accommodating several hundred rig personnel. The failure sequence was identified as
the loss of one vertical column, resulting in rig destabilization, with tilting of the entire
structure leading to inversion and submersion. All the evidence confirmed the
mechanism of the initial failure as a fatigue process, sited at a defective weld bead in an
inserted hydrophone tube located in a robust brace designated D-6. The fracture
characteristics of the principal failure site were subject to extreme scrutiny, and it was
the conclusions of all the major reports that a progressive cracking in that brace and the
consequent brace separation that led to the ultimate collapse of the entire structure.
Probable causes of capsizing the Alexander L Kielland platform:
A brief examination of the fatigue characteristics defines
the site of crack initiation at and around the inserted
hydrophone tube, in the brace D-6. The site of the
primary progressive fracture together with the later
plastic failures is shown by the circumferential red
markings in Fig. 1.
The primary fracture path is defined in Fig. 2 with
the inserted hydrophone tube remaining in place
beside the drainage port.
Clearly, part of the insert weld remained integral since the components are still rigidly
attached after some 3 years of service. The entire surface fracture characteristics were
scrutinized to establish, unequivocally, the changing mechanisms of fracture along this
path, Fig. 3. The results of this study are conclusive, with two fatigue fracture initiation
sites (I and II see Fig. 3), at diametrically opposite points within the overall fracture
plane. The cyclic fatigue crack paths were characterized by “clam shell” markings which
arise from minor changes in the direction of the crack tip during progressive crack
extension. These pathways were approximately 312 cm (II) and 113 cm (I) in lengths,
prior to the more rapid crack extension by both ductile fibrous tearing and latterly by
brittle fracture shown by the chevron markings. In total, the fatigue crack components
represented about 53% of the crack surfaces. Investigators reported the presence of
paint layers within the
intended weld bead, which
confirmed the defective weld
condition before the structure
entered service. The lifetime of
the brace D-6 is significant; it
also confirms and identifies
that the quality of the steel
used in its fabrication was
It was and remains a fact that the
weld was produced in a zone of
designated and low nominal stress
level, using materials which met the
required chemical and mechanical
property specifications. The
hydrophone support was inserted
and welded into the brace as a “set
through, pipe to pipe, double sided
fillet, branch weld”. The dimensions
of the intended joint configuration
are shown in Fig. 4.
The welding engineer would regard
the joint as a point of difficulty and
would not have recommended a simple fillet weld between thick wall components,
which involved inserting a large circular hole into the brace, i.e., a cavity of 325 mm
diameter. The weld protocol requested a 6 mm throat thickness fillet, alternatively
expressed as an 8 mm leg length fillet weld, when estimated by a weld gauge. It was
known that the thick-walled brace had been manufactured with both longitudinal
and circumferential “full thickness penetration butt welds” and had been inserted
into the fabrication as such. Why was the hydrophone insert tube not required to
also have full penetration butt welds, perhaps with the reinforcement as indicated in
Fig. 5?
Design Flaws:
A design flaw is something that is built into the DNA of the product. It is the Main reason
for some of the major oil rig disasters. Ocean Ranger oil rig disaster is one of them.
Ocean Ranger Oil Rig Disaster:
On February 14, 1982, the Ocean Ranger was battered by the strongest storm it would
experience. Close to 6:45 p.m., the platform motion exceeded 15 feet of ‘heave’
(vertical travel). At approximately 7:45 p.m., a large wave impacted the Ocean Ranger
and shattered windows in the Ballast Control Room. Salt water spewed into the room,
soaking the ballast control console. Soon after, the ballast control panel was noticed to
be malfunctioning. Mayday calls began minutes later.
Probable Causes of Ocean Ranger Oil Rig disaster:
Semisubmersible oil rigs were still evolving in the early 1980’s. At the time, the Ocean
Ranger was considered the most advanced offshore drilling unit of its day.
Unfortunately, flaws in the design, particularly of the chain lockers and of the ballast
control console played key roles in the disaster because these components were
designed not for failure, but for ideal conditions. Designers could have prevented this
failure mode by including gates or covers for the chain locker openings. Ideal conditions
for the ballast control room meant the area would never be exposed to water. But the
presence of the breakable portlights meant that a risk of water ingress existed.
Therefore, designing against failure meant designing against the possibility that the
ballast control console could be damaged by seawater. Had the console been insulated
or otherwise waterproofed, the damage it undertook might not have occurred, and the
confusion concerning the valve status might not have ensued.
• https://en.wikipedia.org/wiki/Drilling_rig
• https://www.offshore-technology.com/features/feature-the-worldsdeadliest-offshore-oil-rig-disasters-4149812/
• https://www.offshoreinjuryfirm.com/oil-rig-explosions/common-causesof-explosions/equipment-failure/
• https://www.offshore-technology.com/features/the-leading-causes-ofdrilling-rig-disasters/
• https://www.arnolditkin.com/oil-rig-explosions/common-causes-ofexplosions/
• https://www.newscientist.com/article/dn18881-oil-industry-failed-toheed-blowout-warnings/
• https://maintenanceandcure.com/maritime-blog/what-caused-thedeepwater-horizon-oil-spill/
• https://safety4sea.com/cm-alexander-l-kielland-norways-worst-offshoredisaster/
• https://link.springer.com/article/10.1007/s11668-019-00680-4