Failures - Sleipner a - North Sea Oil Platform Collapse
-
Upload
nils-antoine-freund -
Category
Documents
-
view
179 -
download
2
description
Transcript of Failures - Sleipner a - North Sea Oil Platform Collapse
Actions
Wiki Home
Recent Changes
Pages and Files
Members
Search
Navigation
Home
M Kevin Parfitt - Moderator
Peer Reviewers
Building Failures Forum (Blog)
Wiki Installments
Building Enclosure
Bridge & Infrastructure
Building Failure Cases
Failures in the News
Fire Case Studies
Industrial Disasters
Natural Disasters
Stadiums & Arenas
Systems & Failure Categories
Weird Failures
Wiki Contributions
Contribute a New Case
Comment on a Case
Editing Instructions
Wikispaces Help
Resources
Journal of Performance
TCFE of ASCE
MatDL Failures Case Studies
Links - See Building Failures Forum
In Planning Stages:
Illustrated Glossary
Failures Photo Library
Penn State AE
About AE 537
Sleipner A - North Sea Oil Platform Collapse Edit 8 147 …
Figure 2: Partially completed buoyancy
cells to be floated to a fjord
Table of ContentsBackground
Typical Condeep
Platform
The Sleipner A-1
The Accident
Summary of
Conditions
Timeline
Likely Causes
Verification
Reactions and Outcome
Revised Design
Philosophy
The New Sleipner A
Platform
References
Works Cited
Works Read
Additional Reading
Figure 1: Simplified rendition of a
typical Condeep platform built for
natural gas drilling in the North Sea
Sleipner A-1 Gravity Base Structure23 August 1991 in the Gandsfjord outside of Stavanger, NorwayKsenia Tretiakova -Integrated BAE/MS Candidate (Dec 2012) in the Department of Architectural Engineering, Pennsylvania State
University
Please note: All images in this article are generated by the author; In-text figure references include credit
for any resources referenced during the illustration.
BackgroundThe original Sleipner A (SLA-1)
platform was to be used for oil and
natural gas drilling in the Sleipner
gas field in the North Sea. It was
built in the fashion of typical
Condeep platforms (as seen in
Figure 1[1][7]), but sank during a
controlled ballast test, unlike other
platforms with similar designs. It
was found that a combination of poor
geometry andinadequate design
were the causes of the platform
failure. The failure mechanism was
concluded to be a shear failure that
split open several walls in one of the
platform shafts, which led to rapid
intake of water.
Typical Condeep Platform
Condeep platforms are reinforced
concrete structures meant to float in water up to 300 meters (~1000 feet)
deep. These platforms consist of of a number of buoyancy cells that serve
as the floating mechanism. Water ballast is pumped in and out of the
buoyancy cells to control the depth of the lower portion of the structure (the Gravity Base Structure or
GBS) in the water. Several of the buoyancy cells extend upward ("shafts") to provide structural support to
the deck, which contains all of the buildings, machinery and amenities needed for several hundred people
to live on the platform and drill for natural gas. This deck typically weighs around 55,000 tons[3].
The construction process for a Condeep platform starts on a dry
dock, where all of the buoyancy cells and shafts are cast without
enclosing the cell tops. The dock is then flooded, and the
conglomeration of buoyancy cells is floated out to a sheltered
location with deep water ("fjord"), where the buoyancy cells that
will not be support shafts are capped off, and the GBS is
slipformed upward (see Figure 2[1][8]). As more of the structure is
cast, water ballast is pumped into the buoyancy cells to sink the
structure and keep the construction close to the water. Once the
GBS is completed, the platform structure must undergo a
controlled ballast test to test the mechanical equipment under
service loads and check for minor leaks. A controlled ballast test
requires the platform to be sunk until the GBS is submerged, as
it would be during deck-mating, which would follow at the end of
the test and any necessary repairs. During deck-mating, the GBS
is sunk until submerged once more, and the 55,000 ton deck is
floated above the GBS; ballast is released from the buoyancy
cells, and the GBS lifts the deck into the air. The completed
platform can then be floated to its final location. For deck-mating,
the GBS typically has to be about 20 meters (66 feet) deeper
than it would be at service load.
Welcome. Learn more about what Wikispaces has to offer. (http://www.wikispaces.com/site/signin?goto=http%3A%2F%2Ffailures.wikispaces.com%2FSleipner%2BA%2B-%2BNorth%2BSea%2BOil%2BPlatform%2BCollapse)
Figure 3: Plan view of SLA-1 buoyancy cells, with shafts and tri-cells highlighted
As a result of the need to float a very heavy structure using concrete, extreme care has to be taken in
design. One of the critical design factors for a deep sea platform GBS is the buoyancy cell wall thickness:
if the walls are too thin, they will rupture during deck mating; if the walls are too thick, the structure will
simply not float, or will prove difficult to move to its final destination. To deal with the fine line between "too
thick" and "too thin", low factors of safety are used during the design, requiring both precise load
estimation and precise analysis of buoyancy cell geometry[1].
The Sleipner A-1
The Sleipner A-1 platform was an example of a typical Condeep platform that "did not deviate significantly
from earlier platforms"[2]. It was the 12th platform built by Norwegian Contractors (now Aker Oil & Gas
Technology, Inc.)[5]. SLA-1 was designed using the same software that Norwegian Contractors (NC)
applied to the other 11 deep sea platforms.
Construction for
SLA-1 started in
July of 1989, just
under 2 years
before the failure
of the GBS[6]. As
can be seen in
Figure 3[2][3][4][7]
[8], the SLA-1
platform had 24
buoyancy cells,
four of which
extended into the
shafts that
supported the
deck. Two of the
shafts served as
"drill shafts"
while the
remaining two
served as riser and utility shafts. The GBS was 110 meters (361 feet) tall[1][7], and designed to operate in
82 meters (269 feet) of water[2][7][9]. The deck that would be mated to the SLA-1 GBS weighed
approximately 57,000 tons[9].
The critical design detail in the SLA-1 platform was the connection of the buoyancy cells to one another,
called a "tri-cell" (as shown in Figure 3[2][3][4][7][8]). In Figure 4[2][3][4], the geometry of a "representative"
platform tri-cell (Gullfaks B[2][3][4]) can be observed and compared to the SLA-1 tri-cell geometry. The
reduced thickness of the walls in SLA-1 tri-cells as compared to Gullfaks B is attributed to an effort by the
NC designers to optimize the concrete volume and tricell geometry[3]. Figure 5.1[2][3][4][7][8] magnifies one
of the joints in the tri-cells used for SLA-1 so that the reinforcement details can be observed[2][3][4].
Additional reinforcement detailing is shown in Figure 5.2[1][7]: a section of an elevation of one of the SLA-1
shaft walls[1]. The key items to note in Figure 5[1][2][3][4][7][8] are highlighted in red: the T-headed bar in the
joint (5.1[2][3][4][7][8]) and the location of the last stirrup in the shaft (5.2[1][7]).
The initial cost to build the SLA-1 GBS was approximately $180 million (US).
Figure 4: Tri-cell geometry of a typical platform (4.1)
compared to the geometry of SLA-1 (4.2)
Figure 5: Tri-cell joint reinforcement detailing (5.1) and shaft wall reinforcement detailing (5.2) with key
features highlighted
The AccidentSleipner A-1 was scheduled to undergo deck-mating on 1 September 1991. A second controlled ballast
test was administered on the morning of Friday, 23 August 1991 in the Gjandsfjord near Stavanger
(southwest coastal Norway, in the North Sea) after the platform underwent repairs for minor leaks
discovered during the first test. The platform began taking on water uncontrollably at a depth of 97.5
meters[2][8][9] (320 feet)-- about 5 meters (16 feet) from the anticipated deck-mating depth[1]. The platform
took between 17[3] and 18.5[2] minutes to become fully submerged. The 14 people on board at the time of
the accident were successfully rescued with no injuries[2][4].
Summary of Conditions
The initial intake of water was denoted with a very "deep bang-like sound"[2] from one of the drill shafts, D3
(pointed out in Figure 3[2][3][4][7][8]). An eyewitness account indicated that water ingress could be seen in
the lower portion of the shaft, at an elevation of 49 meters (160 feet) or less. The estimated size of the
opening required to sink the SLA-1 platform in the the observed time was approximately 0.4 square meters
(4.31 square feet)[2][9], which translates to a calculated crack length of about 7 to 8 meters (23 to 26 feet).
At the time of failure, the D3 shaft wall was experiencing a differential water pressure of about 67 meters
Figure 6: Probable failure location at tri-cell joint (6.1) and probable failure location on shaft wall (6.2)
(220 feet) waterhead.
Undersea probes were released after the failure to explore the sea floor and observe the wreckage. The
debris of the SLA-1 platform was scattered over an area of roughly 90,000 square meters (approximately
97,000 square feet). The top of one of the shafts could be identified in the debris. 192 pieces of SLA-1 were
mapped across the sea bed, indicating a total collapse of the concrete GBS. As a result of the complete
demolition of SLA-1, no physical evidence could be used to indicate the cause of the failure.
The timeline below outlines key events and actions that took place.
Timeline
Morning of 23 August 1991
05:48:30 - Platform is at a depth of 97.5 meters; a "deep bang-like sound" is heard, followed by
strong vibrations and the sound of rushing water.
05:49:00 (30 seconds later) - another, slightly weaker, bang is heard. Preliminary investigations
follow; Drill shaft D3 is identified as the location of the failure, and emergency deballasting is
initiated.
05:56:30 (8 minutes after the first bang) - The call is given for the 14 personnel on board to abandon
the platform.
05:59:30 (13 minutes after the first bang) - The platform is fully evacuated.
06:07:00 (20 minutes and 30 seconds after the first bang) - The platform completely disappears
underwater.
06:07:45 (21 minutes and 15 seconds after the first bang) - A magnitude 3 (Richter) event is
registered at several local seismological stations.
It was observed that D3 sank first, corroborating the assumption that D3 was the only shaft to experience
rapid intake of water.
Note: Summary of Conditions and Timeline summarized from Jakobsen (1992) [2]
Likely CausesIt was concluded that the tri-cell walls and supports at the cell joints were the weakest points in the
platform; Calculations based on final design geometry showed that the load on these parts was at or very
close to their maximum capacity. The investigators from NC concluded that "final failure was believed to
take place as crushing of the concrete, presumably at the intersection between the tri-cell wall and the cell
joint."[2] The investigators working for Statoil agreed and elaborated on this conclusion, reasoning that
"shear failure occurred due to lack of transverse reinforcement."[3]
Ultimately, the most probable failure mechanism for the SLA-1 GBS was found to be concrete crushing
and shear failure that occurred in the portion of a tri-cell wall located above the transverse reinforcement,
close the the joint. Figure 6[1][2][3][4][7][8] illustrates the probable location of the failure mechanism.
This failure mechanism manifested because of several inconsistencies in the initial conditions defined in
the design software[1][2][8][9][10]. The sinking was partially caused by the inappropriate use of finite element
the design software[1][2][8][9][10]. The sinking was partially caused by the inappropriate use of finite element
(FE) -code NASTRAN[10] with regards to the global analysis of the finalized design[2][9]: the finite element
mesh used to analyze the tri-cells was too coarse to predict the shear stress accurately. The flawed
analysis and post-processing of the tri-cell design led to shear forces being underestimated by some 45%[1][2][9].
Another flaw with the use of the software was the human error involved: the design software involved
considerable complexity, which led to a a very high perception of precision by those who used it. One of
the features of this software was the way it ran calculations: it would only flag certain sections that were
deemed critical by the software presets, and indicate those as sections that the engineers on the design
team needed to check. The probable failure point of the SLA-1 GBS was not flagged.
Additionally, the supports for the tri-cell walls in SLA-1 were designed to only resist lateral forces
indirectly, which meant that the detailing for the tri-cell joints had to be very carefully designed and
analyzed[2]. It was found that the T-headed reinforcing (highlighted in previous figures) in the tri-cell joints
was not long enough to resist the forces on the tri-cell walls. It was also found that the transverse
reinforcing (also highlighted in previous figures) did not extend far enough up to resist the forces on the tri-
cell walls[2][3][9].
Verification
NC began an investigation into the failure the same day that the SLA-1 GBS sank, as did Statoil (the
company that would have operated the finished platform). The groups that carried out the investigation
attempted to fit potential physical causes for the GBS failure to eyewitness accounts using a "Sequential
Times Events Plot" system that diagrammed in-depth witness descriptions along a time axis to pinpoint
significant events and their locations[6]. The investigative teams then followed up with analytical
calculations to verify their conclusions.
Additionally, the company SINTEF carried out testing of small and full scale models to support the
findings of the investigation team. These models included versions of the existing (collapsed) structure, as
well as modified versions to correspond to investigative theories regarding the initial inadequate reinforcing
at the joint details and in the shaft[2][4][8][9]. The models tested with the "improved" design provided
evidence to suggest that, had the reinforcing been appropriately designed, the load capacity of the tri-cells
would have been about 70% higher[2][3], and the SLA-1 GBS would not have been a total loss.
The findings among the NC investigation team, the Statoil investigation team, and the SINTEF investigation
team coincided very well. As a result of the consistency of the findings in each independent investigation,
Statoil chose to rebuild the Sleipner A platform[9], with some modifications to assure safety and structural
stability.
Reactions and Outcome
The fact that two simples changes to the
reinforcement detailing would have prevented an
extremely expensive structural failure has
resounded widely among structural designers. The
total loss of the Sleipner A-1 platform is an
unfortunate, but excellent, example of the need for
extreme care and detail in design. The SLA-1
accident demonstrates the importance of having
experienced engineers verify computer-generated
design work to ensure the proper use of analysis
and design techniques[9][11].
The initial cost for the SLA-1 GBS was about
$180 million. Statoil estimated their losses
without the SLA-1 platform came in at about $1
million per day. It took over two years to redesign
the Sleipner platform and bring it to fully
operational status[4].
Revised Design Philosophy
The design philosophy after the SLA-1 GBS
collapse was focused on establishing additional,
extensive risk analysis criteria that includes
analysis for impact damage, and design for
platform stability in the case that one of the
Figure 7: Tri-cell geometry of SLA-2 (7.1) compared to
the geometry of SLA-1 (7.2)
platform stability in the case that one of the
buoyancy cells or other compartments is
damaged[3].
Additionally, the revised design philosophy called for greater forgiveness in the initial design in order to
account for any potential future changes that might occur later in the design process. Additional load
factors were applied to all new platform designs. Once the design was completed, the new philosophy
required qualified engineers to check every pre-identified critical section-- not as flagged by any software in
use, but as identified to be a potential weak section in any design by those who investigated the SLA-1
GBS collapse[3].
The New Sleipner A Platform
The new Sleipner platform (SLA-2) was designed entirely using hand calculations. This was the result of a
decision by Statoil to continue through with the design while investigators continued troubleshooting the
software used to design all of the NC platforms in the North Sea. The stipulation behind the SLA-2 design
was that, in addition to hand calculation and design, all of the buoyancy cells in the platform were modeled
and analyzed before they left the dry dock for further construction[3].
The designers of SLA-2 changed the cell wall geometry to minimize stress concentrations (see Figure 7[2]
[3][4] for the differences in geometry). The criteria for the new design included a 10% higher load factor
when accounting for water pressure. The formula for shear capacity was revised as well[3].
Overall, the total time spent on verification of the design was quadrupled in comparison to the design of the
SLA-1 GBS. The SLA-2 platform was up and operation in October of 1993, more than 2 years after the
initial SLA-1 GBS collapse. The overall estimated cost of the SLA-1 GBS collapse totals to $700 million,
after taking into account the downtime losses and cost to redesign the platform[1][3].
References
Works Cited[1] Collins, M., Vecchio, F., Selby, R., Gupta P (2000) "Failure of an Offshore Platform," Canadian
Consulting Engineer - Structures, pp 43-48, March/April
Journal Article. Provides a wealth of information on the typical construction and design of a
Condeep platform; Also describes the Sleipner A-1 design, failure mechanism, failure causes, and
the failure aftermath (including information on the construction of Sleipner A-2). Discusses
estimated costs and lessons learned as well.
[2] Jakobsen, B. (1992) “Loss of the Sleipner A platform,” The proceedings of the International Offshore and
Polar Engineering Conference, pp. 1-9
Conference Proceedings. Written by members of the Norwegian Contractors' investigation team.
Describes investigations into the platform collapse; Discusses eyewitness accounts that helped
locate the probable point of failure, as well as the most likely causes for failure and collapse (and
why); Also describes some model tests that have been performed to verify the conclusions in the
paper.
[3] Rettedal, W. (1993) “Design of concrete platforms after Sleipner A-1 sinking,” Proceedings of the
International Conference on Offshore Mechanics and Arctic Engineering – OMAE, pp. 309-316
Conference Proceedings. Discusses new design methods for offshore concrete platforms;
Describes full-scale testing of certain joints to identify failure modes, as well as the revised design
philosophy (including discussion of loads, design criteria, etc.). Also covers the design of the
platform that replaced Sleipner A-1, and the verification of that design. Additionally, discusses risk
analysis and some forethought on future designs.
[4] Jakobsen, B. (1994) “The Sleipner Accident and its Causes,” Engineering Failure Analysis, Vol. 1, No.
3, pp. 193-199
Journal Article. Written by members of the Norwegian Contractors' investigation team. Describes
the initial collapse as well as the factors that determined the probable cause of the failure. Also
discusses the additional model tests to verify failure modes on the joints. Compares results from
model tests to eyewitness accounts to verify the actual failure mode.
[5] Norway.org (1996) "Norwegian Companies in USA - - Aker ," norway.org, n.d., Web
Periodical. News article from the official site of Norway in the United States. Regarding the
acquisition of Norwegian Contractors (NC) by Aker Oil & Gas Technology, Inc. Discusses company
holdings and history, and includes contact information for Aker.
[6] Ynnesdal, H., Berger, F. (1994) "The Sleipner Accident," Proceedings of the Second International
Conference on Health, Safety & Environment in Oil & Gas Exploration & Production, pp. 715-716, 25-27
January
Conference Proceedings. Provides a general summary of the entire story on behalf of the Society
of Petroleum Engineers, emphasizing the usefulness of a "Sequential Times Events Plot" (STEP)
in pinpointing the probable cause and location of the failure based on witness interviews. Briefly
covers the base facts of the accident, then reviews the methodology of the investigations, involving
the "STEP" system to track the course of events of the accident.
[7] Schlaich, J., Raineck, K-H. (1993) "Die Ursache fur den Totalverlust der Betonplattform Sleipner A,"
Beton- und Stahlbetnbau, Vol. 88, pp. 1-4
Journal Article. Article is in German. Title translates to "Causes for the total loss of the Sleipner A
concrete platform." Primary usefulness of this article for this wik i page is the availability of clear
illustrations for portions of offshore platform geometries to reference in figure generation. The article
recounts the accident, describes the conditions during the accident (with great diagrams), includes
diagrams for the finite element meshing of various portions of the GBS, also shows the loading
conditions for the tri-cells.
[8] Arnold, D. (2009) "The sinking of the Sleipner A offshore platform ," Institute for Mathematics and its
Applications (IMA) at University of Minnesota, 7 September 2009, Web.
Website. Produced by the director for mathematics at the University of Minnesota. Provides an
excerpt of one of the elusive SINTEF reports and provides a general summary of the platform, the
failure, the circumstances of failure, and where to look for additional information.
[9] Jakobsen, B., Rosendahl, F. (1994) "The Sleipner Platform Accident," Structural Engineering
International, IABSE, Vol. 3, pp. 190-193
Journal Article. Produced by members of the NC investigation team. Summarizes the accident
and discusses lessons learned. Mentions several important dates for the rebuilding of the structure,
and reasons for choosing to build SLA again. Published in the "Lessons from Structural Failures"
section of the journal.
[10] Huckle, T. (2011) "Collection of Software Bugs ," Institut fur Informatik, TU Munchen, 7 November
2011, Web
Website. Produced by a professor at the university. A large and relatively comprehensive list of
accidents and disasters that occurred as a result of an error in the design software. Provides
names, dates, summaries of the failures, and reasons for failure. Also includes additional resources
to find more information on each accident. SLA-1 is #15 in the list.
[11] Holand, I. (1996) "Structural analysis of offshore concrete structures ," IABSE Congress Report,
IABSE, Vol. 15, pp. 879
Technical Report. A report made available to the public on the web by the International
Association for Bridge and Structural Engineering. Discusses how the Sleipner accident is a good
example of the necessity for verification, risk analysis, and quality assurance.
Works Read
Stead, B. (1992) “Importance of fabrication engineering in the early phases of the Sleipner a development,”
Proceedings of the International Offshore Mechanics and Arctic Engineering Symposium, Vol. 1, pp. 543-
548
Conference Proceedings. Describes the conclusions drawn that led to the initial design choice for
the offshore platform (discussions include topographical factors that led to the choice of a gravity-
only system, costs for construction as well as the various hookups required for the typical offshore
platform, and considerations leading to the choice of concrete)
Vecchio, F. (2002) “Contribution of Nonlinear Finite-Element Analysis to Evaluation of Two Structural
Concrete Failures,” Journal of Performance of Constructed Facilities, pp. 11-115, August
Journal Article. Describes 2 failures, one of which is Sleipner A; Recounts the event itself,
summarizes what happened, includes some discussion of loads and dimensions in the summary.
Discusses the nonlinear finite-element modeling analysis the author performed, and how it
accurately portrays load capacity and failure modes: justifies use of computer modeling as a way to
analyze a structure.
Wackers, G.(2004) “Resonating Cultures: Engineering Optimization in the Design and (1991) Loss of the
Help · About · Blog · Pricing · Privacy · Terms · Support
Contributions to http://failures.wikispaces.com/ are licensed under a Creative Commons Attribution Share-Alike 3.0 License.
Portions not contributed by visitors are Copyright 2013 Tangient LLC
Wackers, G.(2004) “Resonating Cultures: Engineering Optimization in the Design and (1991) Loss of the
Sleipner A GBS ,” Center of Technology, Innovation and Culture, University of Oslo, May, Web
Web-based Discussion Paper. This is a work in progress paper, with detailed discussions of… the
accident itself, as well as some discussion of the theory behind design parameters. Also includes
discussions of the appropriate way to approach investigations for failure modes and mechanisms.
Extrapolates to some cultural stuff, and then continues with an evaluation of changes to the design
of the new platform. Also talks about different forms of analysis, as well as the engineering of the
platform design.
Thompson, M., Jirsa, J., Breen, J. (2006) “Behavior and Capacity of Headed Reinforcement,” ACI Structural
Journal, Vol. 103, No. 4, pp. 522-530, July/August 2006
Technical Report. Mainly discusses design recommendations for headed reinforcement, and why
they are recommendations in the first place. Uses the Sleipner A failure as a model for the
importance of appropriate design of headed reinforcement.
Additional Reading
Holand, I., Lenschow, R. (1996) “Research Behind the Success of the Concrete Platforms in the North
Sea,” Proceedings of Mete A. Sozen Symposium – A Tribute From His Students, ACI SP-162, pp. 235-
272
Selby, R., Vecchio, F., Collins, M. (1996) “Analysis of Reinforced Concrete Members Subject to Shear and
Axial Compression,” ACI Structural Journal, Vol. 93, No. 3, May/June
Bea, R. (1994) “The Role of Human Error in Design, Construction, and Reliability of Marine Structures,”
Ship Structures Committee
Gudmestad, O., Holand, I., Jerson, E. (2000) “Design of offshore concrete structures,” 1st ed., Spon Press
The SINTEF reports on Sleipner A (STF22): A97725, A97854, A97857, A97859, A97861, A97833 (there
are 13 more available somewhere)