OTC 18325 Assessment of Fixed Offshore Platform Performance in Hurricane Ivan Puskar, F.J., Spong, R.E. and Ku, A., Energo Engineering; Gilbert, R.B. and Choi, Y.J., The University of Texas at Austin Copyright 2006, Offshore Technology Conference This paper was prepared for presentation at the 2006 Offshore Technology Conference held in Houston, Texas, U.S.A., 1–4 May 2006. This paper was selected for presentation by an OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject t o correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435 .Abstract Hurricane Ivan is one of several hurricanes that have damaged or destroyed fixed offshore platforms in the Gulf ofMexico i n rec ent years. These events provide a unique opportunity to determine the effectiveness of structural design standards and regulations and develop recommendations for changes, if neede d. Specifically, Ivan provided an opportunit y to evaluate the API RP 2A (RP 2A) design process for fixed platforms to ensure that it provides for well designed structures. The first part of this paper describes the general impact ofIvan on fixed platforms in terms of survival, damage or destruction. Specific findi ngs and trends are reported related to global platform performance as well as component performance. The second part describes a quantitative assessment to determine the adequacy of the RP 2A design process. The approach uses a probabi listic based proces s that compares analytically predicted platform damage and survival to that actually observed during Ivan. The result is a Bias Factor that reflects how well RP 2A predicts platform behavior under hurricane loads. The work was funde d by the Minerals Management Service (MMS). Introduction Ivan was one of several hurricanes in the last dozen years that have significantly damaged or destroyed fixed offshore platforms. Prior hurrica nes are Andrew i n 1992 and Lili in 2002. Katrina and Rita in 2005 also caused si gnificant platform damage and destruction. These ty pes of events provide an opportunity to determine how fixed platforms in the Gulf of Mexico perform in hurricanes on both a qualitative and quantitative basis. The qualitative basi s includes a revi ew of the typical types of damage to topsides and jacket, as well as the general trends observed, such as the number and type ofplatforms with wave-in-deck damage. The quantitati ve basis involves the comparison of the observed damage with what would have been predicted by RP 2A which is the basis for design of fixed plat forms i n the Gulf of Mexico. This provides a quantified assessment of the accuracy of RP 2A and if it is adequate f or design. This paper des cribes these assessments for Ivan based upon an in-depth study performed for the MMS focusing on fixed platforms (no caissons) [Energo Engineering, 2006]. These types of assessments have been performed previously for Andrew and Lili [Puskar, et. al., 1994 and 2004]. Recent hurricanes Katrina and Ri ta in 2005 provide similar opportunities, but have yet to be studied. Ivan Characteristics Ivan developed off the west coast of Africa in late August 2004. By September 5th it was a hurricane about 1,100 miles east of the southern Windward Islands. The hurricane strengthened running south of the Dominican Republic and passed within about 20 miles of Grand Cayman on the 12th. By the late afternoon on September 15th, Ivan was in the east- central Gulf of Mexico approaching the deepwater offshore oil and gas f acilities. During this time, the hurricane was a Category 4 storm on the Saffir-Simpson scale, with maximum sustained wind speeds of more than135 mph. Figure 1 shows the storm track through the key offshore oil and gas blocks. Also shown i n the figure are the fi xed platforms that were destroyed during the hurri cane. Ivan tracked North over the deepwater facilities in the Mississippi Canyon blocks and up into the Viosca Knoll (VK) and Main Pass (MP) block areas. The majorit y of the destroye d or damaged fixed platforms resided in the VK and MP blockareas. Ivan continued its Northerly track through the eastern edge of the Mobile block area, making landfall as a major hurricane with maximum winds of 130 mph on the early morning of September 16 th just west of Gulf Shores, Alabama. General Platform Damage A total of seven fixed platforms were destroyed in Ivan as shown in Table 1. Figure 1 shows the locat ion of the platforms destr oyed. One of the seven (MC 20A) was toppl ed by a mudslide, while the other six failures are thought to be attributed to metocean loads (i.e., wind, wave and current) exceeding the capacity of the structures. The seven destroyed platforms are from data provided by the MMS. Note that additional platforms may have been later decommissioned by the operator as a result of damage sustained from Ivan. There were also a number of other fixed platforms that sustained varyi ng degrees of damage during Ivan. Some ofthe damage and failures were not considered a surprise, since the most of the platforms that failed or sustained major damage were olde r facilities. These platforms were generally
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Assessment of Fixed Offshore Platform Performance in Hurricane IvanPuskar, F.J., Spong, R.E. and Ku, A., Energo Engineering; Gilbert, R.B. and Choi, Y.J., The University of Texas at Austin
Copyright 2006, Offshore Technology Conference
This paper was prepared for presentation at the 2006 Offshore Technology Conference held inHouston, Texas, U.S.A., 1–4 May 2006.
This paper was selected for presentation by an OTC Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Offshore Technology Conference and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Offshore Technology Conference, its officers, or members. Papers presented atOTC are subject to publication review by Sponsor Society Committees of the OffshoreTechnology Conference. Electronic reproduction, distribution, or storage of any part of thispaper for commercial purposes without the written consent of the Offshore TechnologyConference is prohibited. Permission to reproduce in print is restricted to an abstract of notmore than 300 words; illustrations may not be copied. The abstract must contain conspicuous
acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.
AbstractHurricane Ivan is one of several hurricanes that have
damaged or destroyed fixed offshore platforms in the Gulf of
Mexico in recent years. These events provide a unique
opportunity to determine the effectiveness of structural design
standards and regulations and develop recommendations for
changes, if needed. Specifically, Ivan provided an opportunity
to evaluate the API RP 2A (RP 2A) design process for fixed
platforms to ensure that it provides for well designed
structures.
The first part of this paper describes the general impact of
Ivan on fixed platforms in terms of survival, damage or
destruction. Specific findings and trends are reported related
to global platform performance as well as component
performance. The second part describes a quantitative
assessment to determine the adequacy of the RP 2A design
process. The approach uses a probabilistic based process that
compares analytically predicted platform damage and survival
to that actually observed during Ivan. The result is a Bias
Factor that reflects how well RP 2A predicts platform
behavior under hurricane loads. The work was funded by the
Minerals Management Service (MMS).
Introduction
Ivan was one of several hurricanes in the last dozen yearsthat have significantly damaged or destroyed fixed offshore
platforms. Prior hurricanes are Andrew in 1992 and Lili in
2002. Katrina and Rita in 2005 also caused significant
platform damage and destruction. These types of events
provide an opportunity to determine how fixed platforms in
the Gulf of Mexico perform in hurricanes on both a qualitative
and quantitative basis. The qualitative basis includes a review
of the typical types of damage to topsides and jacket, as well
as the general trends observed, such as the number and type of
platforms with wave-in-deck damage. The quantitative basis
involves the comparison of the observed damage with what
would have been predicted by RP 2A which is the basis for
design of fixed platforms in the Gulf of Mexico. This
provides a quantified assessment of the accuracy of RP 2A
and if it is adequate for design. This paper describes these
assessments for Ivan based upon an in-depth study performed
for the MMS focusing on fixed platforms (no caissons)
[Energo Engineering, 2006]. These types of assessments have
been performed previously for Andrew and Lili [Puskar, et
al., 1994 and 2004]. Recent hurricanes Katrina and Rita in
2005 provide similar opportunities, but have yet to be studied.
Ivan CharacteristicsIvan developed off the west coast of Africa in late August
2004. By September 5th it was a hurricane about 1,100 miles
east of the southern Windward Islands. The hurricane
strengthened running south of the Dominican Republic and
passed within about 20 miles of Grand Cayman on the 12th
By the late afternoon on September 15th, Ivan was in the east-
central Gulf of Mexico approaching the deepwater offshore oi
and gas facilities. During this time, the hurricane was a
Category 4 storm on the Saffir-Simpson scale, with maximum
sustained wind speeds of more than135 mph.
Figure 1 shows the storm track through the key offshore
oil and gas blocks. Also shown in the figure are the fixedplatforms that were destroyed during the hurricane. Ivan
tracked North over the deepwater facilities in the Mississippi
Canyon blocks and up into the Viosca Knoll (VK) and Main
Pass (MP) block areas. The majority of the destroyed o
damaged fixed platforms resided in the VK and MP block
areas. Ivan continued its Northerly track through the eastern
edge of the Mobile block area, making landfall as a major
hurricane with maximum winds of 130 mph on the early
morning of September 16th
just west of Gulf Shores, Alabama.
General Platform DamageA total of seven fixed platforms were destroyed in Ivan as
shown in Table 1. Figure 1 shows the location of theplatforms destroyed. One of the seven (MC 20A) was toppled
by a mudslide, while the other six failures are thought to be
attributed to metocean loads (i.e., wind, wave and current)
exceeding the capacity of the structures. The seven destroyed
platforms are from data provided by the MMS. Note tha
additional platforms may have been later decommissioned by
the operator as a result of damage sustained from Ivan.
There were also a number of other fixed platforms tha
sustained varying degrees of damage during Ivan. Some o
the damage and failures were not considered a surprise, since
the most of the platforms that failed or sustained major
damage were older facilities. These platforms were generally
approach can be found in the associated references [Puskar, et.
al., 1994 & 2004] and are not repeated here. The Andrew and
Lili studies show that there is about 15% conservatism
inherent in RP2A once all known factors of safety are
removed.
The approach results in what is known as a “Bias Factor,”
which indicates the ratio of the actual capacity of the platform
determined by observation to its analytical capacitydetermined using RP 2A. If a platform survives after a
hurricane, while RP 2A analyses predicted it should have been
destroyed, this platform has a Bias Factor greater than 1.0. In
this case it would imply that the RP 2A analysis recipe is
conservative. The Bias Factor is computed with all known
factors of safety (FS) in RP 2A removed (i.e., the bias is in
addition to the normal RP 2A FS).
The prior work for Andrew and Lili resulted in Bias
Factors of approximately 1.1 and 1.25 respectively. The bias
is approximately 1.15 when Andrew and Lili are combined.
These results imply that RP2A is doing a good job in terms of
fixed platform design, with an inherent conservatism of about
15%.
For this study, the Bias Factor was recomputed considering
Ivan, based upon six platforms – 2 destroyed, 3 damaged and
1 survived. The results are shown in Figure 11. The resulting
Bias Factor for Ivan is 1.0, which means the prediction
matches the observation almost exactly. The Bias Factor for
Ivan was then combined with Andrew and Lili to determine a
combined Bias Factor of 1.10 considering all three hurricanes.
Note that the combined Bias Factor was calculated through a
complicated probabilistic process and is not obtained by
simply averaging the three individual Bias Factors.
The Ivan Bias Factor is lower than for Andrew and Lili.
The lower Ivan results may be explained by the particular
selection of the specific platforms used to determine the Bias
Factor, mostly damaged or destroyed. The inclusion of moreplatforms that survived Ivan, would increase the Bias Factor,
but there was little information on survived platforms
available to this study (most operators study damaged
platforms and not those that survive). There is also a
possibility that some of the damaged platforms had prior
unknown existing damage that was not taken into account in
the assessment. Hence the Ivan Bias Factor is believed to be
conservative.
Another factor for the lower Ivan Bias Factor may be the
large number of wave-in-deck damaged and destroyed
platforms and the associated uncertainties, such as wave crest
elevation and the associated wave-in-deck loads. As noted
previously, wave-in-deck issues should be investigatedfurther.
Overall, the Quantitative Assessment for Ivan indicates
that the RP 2A fixed platform design approach has a Bias
Factor of about 1.0. When combined with Andrew and Lili,
the Bias Factor increases to 1.10. These results indicate that
RP 2A is doing a conservative job.
ConclusionsIvan provided an opportunity to evaluate fixed platform
performance in extreme storms. The results were generally as
expected, with most of the destroyed or damaged platforms of
older design. As in hurricane Andrew and Lili, most of the
destroyed platforms were thought to be the result of wave-in-
deck loads.
There were a significant amount of platforms with wave-
in-deck damage and wind damage that caused the platforms to
be shut-in for an extended period while repairs were made
There was significantly more wave-in-deck damage for Ivan
than for Lili and Andrew, indicating very large waves for
Ivan. In particular, the wave crest elevations as determinedfrom observed deck damage were exceptionally high.
The quantitative assessment indicated once again that RP
2A does a good job of predicting platform performance, with a
Bias Factor of 1.0 for Ivan and 1.10 (or about 10 percent
conservatism) when combined with prior results for Andrew
and Lili. The lower Ivan value is thought to be a combination
of the platforms selected for the assessment, which were
mostly destroyed or damaged and are thought to provide a
conservative estimate (i.e., lower Bias Factor than actual).
AcknowledgementsThe authors wish to thank their respective organizations
for the opportunity to publish this paper. We also wish to
thank the MMS for sponsoring the effort.
ReferencesABS Consulting, “ Hurricane Lili’s Impact on Fixed Platforms”
Final Report to the Minerals Management Service, June, 2004.API, " Hurricane Readiness and Recovery Conference," Sponsored by
the American Petroleum Institute, Houston, Texas, July 26-272005.
API, Recommended Practice for Planning, Designing and
Constructing Fixed Offshore Platforms, API RP 2A, Twenty
First Edition, 2nd Supplement American Petroleum Institute(API), Washington, D.C., October, 2005.
Energo Engineering, “ Assessment of Fixed Offshore Platform
Performance in Hurricanes Ivan Lili and Andrew,” Final Repor
to the Minerals Management Service, Report Number 549January, 2006.
Laurendine, T. “ Hurricane Ivan Impact to Offshore Facilities and
Status on Section 17 Assessments”, Presentation given during2005 Hurricane Readiness and Recovery Conference –Production Facilities Break-out Session, Houston, Texas, July
27, 2005.MMS, “ Impact Assessment of Offshore Facilities from Hurricanes
Katrina and Rita”, News Release #3418, January 19, 2006.Oceanweather Inc., Hindcast Study of Hurricane Ivan (2004)
Offshore Northern Gulf of Mexico, December 2004.O’Conner, P. “Observations from Pompano and Nakika”
Presentation given during Hurricane Readiness and RecoveryConference – Production Facilities Break-out Session
Presentation on Hurricane Ivan Damage Observations, Houston
Texas, July 27, 2005.Puskar, F. J., Aggarwal, R. K., Cornell, C. A., Moses, F. and
Petrauskas, C., “ A Comparison of Analytical Predicted Platform
Damage During Hurricane Andrew”, Proceedings, 26thOffshore Technology Conference, OTC No. 7473, May 1994.
Puskar, F.J., Ku, A. and Sheppard, R.E., “ Hurricane Lili’s Impact on
Fixed Platforms and Calibration of Platform Performance to
API RP 2A,” Proceedings, Offshore Technology ConferenceOTC No. 16802, May 2004.
Wisch, D. “Some Observations - Petronius and VK 900”
Presentation given during Hurricane Readiness and RecoveryConference – Production Facilities Break-out Session, HoustonTexas, July 27, 2005.
Figure 4 - Deck Heights of Platforms in the Path of Andrew Compared to API RP 2A Minimum Deck Elevation RequirementsThe deck heights were taken form operated supplied elevations to the MMS. As indicated, some of the deck elevations above 55’ may be
inaccurate since few platforms have decks this high.
20
30
40
50
60
70
80
0 50 100 150 200 250 300 350 400
Water Depth (ft)
I v a n H i n d c a s t W a v e H e i g h t ( f t )
Section 2 - L1
Section 2 - L2
Section 17 - L1 - DL
Section 17 - L2 - DL
Destroyed Platforms
Major Damaged Platforms
Minor Damaged Platforms
Section 2 - L1
Section 2 - L2
Section 17 - L1
Design Level
Section 17 - L2
Design Level
Figure 5 - Hindcast Maximum Wave Heights at Locations of Platforms in the Path of Andrew Compared to API RP 2A Wave Height Criteria
(typically between-25’ to -40’ feet).This is theconductor framingelevation most
susceptible todamage.
Waterline Elevation
Typical boatlanding structure
ConductorsPumpCaisson
Legs
Figure 10 - Example of Conductor Guide Framing Damage from IvanThe figure on the right shows the typical location of conductor guide framing located near the water line that is prone to this type of damage.
Figure 5Bias Factor Comparison
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.6 0.8 1.0 1.2 1.4 1.6 1.8
Bias Factor for Jacket Strength
P r o b a
b i l i t y D e n s i t y
Combined
Andrew
Lilly
Ivan
Ivan, mean=1.00
Lilly, mean=1.24
Combined, mean=1.10
Andrew, mean=1.09
Figure 11 - Comparison of Bias Factors Determined for Ivan, Lili and Andrew