Sleipner A
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Transcript of Sleipner A
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SLEIPNER A NORTH SEA OILPLATFORM COLLAPSE
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Background
The original Sleipner A (SLA-1)
platform was to be used for oil andnatural gas drilling in the Sleipner gaseld in the North Sea.
t was built in the fashion of t!pi"al
#ondeep platforms but san$ during a"ontrolled ballast test.
t was found that a "ombination ofpoor geometr! and inade%uate designwere the "auses of the platform failure.
The failure me"hanism was "on"ludedto be a shear failure that split opense&eral walls in one of the platformshafts' whi"h led to rapid inta$e ofwater.
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Typical CondeepPla!or"
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Typical CondeepPla!or" einfor"ed "on"rete stru"tures meant to
oat in water up to *++ meters deep. #onsists of a number of buo!an"! "ells
that ser&e as the oating me"hanism.
,ater ballast is pumped in and out of the
buo!an"! "ells to "ontrol the depth of thelower portion of the stru"ture (ra&it!ase Stru"ture or S) in the water.
Se&eral of the buo!an"! "ells e/tendupward to pro&ide stru"tural support tothe de"$' whi"h "ontains all the buildings'ma"hiner! and amenities needed forse&eral hundred people to li&e on theplatform and drill for natural gas.
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Typical
CondeepPlatform
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Con#rucion Proce##
Starts on a dr! do"$' where all
of the buo!an"! "ells andshafts are "ast withouten"losing the "ell tops.
The do"$ is then ooded' and
the "onglomeration ofbuo!an"! "ells is oated out toa sheltered lo"ation with deepwater (0ford0)' where the
buo!an"! "ells that will notsupport shafts are "apped o2'and the S is slipformedupward.
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Con#rucion Proce##
As more of the stru"ture is
"ast' water ballast is pumpedinto the buo!an"! "ells to sin$the stru"ture and $eep the"onstru"tion "lose to the
water. 3n"e the S is "ompleted'
the platform stru"ture mustundergo a "ontrolled ballast
test to test the me"hani"ale%uipment under ser&i"e loadsand "he"$ for minor lea$s.
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Con#rucion Proce##
A "ontrolled ballast test re%uires
the platform to be sun$ until theS is submerged' as it wouldbe during de"$-mating' whi"hwould follow at the end of the
test. 4uring de"$-mating' the S is
sun$ until submerged on"emore' and the 55'+++ ton de"$is oated abo&e the S6
ballast is released from thebuo!an"! "ells' and the Slifts the de"$ into the air. The"ompleted platform "an then be
oated to its nal lo"ation.
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Partially completed buoyancy cells to be floated to a fjord
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T$e Sleipner A%&
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T$e Sleipner A%&
einfor"ed #onstru"tion for SLA-1 started
in 7ul! of 1898' ust under : !ears beforethe failure of the S.
The SLA-1 platform had :; buo!an"!"ells' four of whi"h e/tended into the
shafts that supported the de"$. Two of the shafts ser&ed as 0drill shafts0
while the remaining two ser&ed as riserand utilit! shafts.
The S was 11+ meters tall' anddesigned to operate in 9: meters ofwater.
The de"$ that would be mated to the SLA-1 S weighed appro/imatel! 5
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T$e Sleipner A%&
The "riti"al design detail in the SLA-1
platform was the "onne"tion of thebuo!an"! "ells to one another' "alled a0tri-"ell=.
The redu"ed thi"$ness of the walls in
SLA-1 tri-"ells as "ompared to a t!pi"alplatform is attributed to an e2ort b! thedesigners to optimi>e the "on"rete&olume and tri"ell geometr!.
The initial "ost to build the SLA-1 Swas appro/imatel! ?19+ million (@S).
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Tri-cell geometry of a
typical platform comparedto the geometry of SLA-1.
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Tri-cell joint reinforcement detailing and shaft wallreinforcement detailing.
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T$e Acciden
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T$e Acciden
Sleipner A-1 was s"heduled to
undergo de"$-mating on1September 1881. A se"ond"ontrolled ballast test wasadministered on the morning of
rida!' :* August 1881 in theandsford near Sta&anger(southwest "oastal Norwa!' inthe North Sea) after theplatform underwent repairs forminor lea$s dis"o&ered duringthe rst test.
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T$e Acciden
The platform began ta$ing on
water un"ontrollabl! at adepth of 8
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Su""ary o! Condiion#
The initial inta$e of water was denoted
with a &er! 0deep bang-li$esound0 from one of the drill shafts' 4*.
An e!ewitness a""ount indi"ated thatwater ingress "ould be seen in the
lower portion of the shaft' at anele&ation of ;8 meters or less.
The estimated si>e of the openingre%uired to sin$ the SLA-1 platform in
the obser&ed time was appro/imatel!+.; s%uare meters' whi"h translates toa "al"ulated "ra"$ length of about < to9 meters.
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Su""ary o! Condiion#
At the time of failure' the 4* shaft wall was
e/perien"ing a di2erential water pressure ofabout B< meters water head.
@ndersea probes were released after thefailure to e/plore the sea oor and obserðe wre"$age. The debris of the SLA-1platform was s"attered o&er an area ofroughl! 8+'+++ s%uare meters.
The top of one of the shafts "ould beidentied in the debris. 18: pie"es of SLA-1
were mapped a"ross the sea bed' indi"atinga total "ollapse of the "on"rete S.
As a result of the "omplete demolition ofSLA-1' no ph!si"al e&iden"e "ould be usedto indi"ate the "ause of the failure.
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Likely Cau#e#
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Likely Cau#e#
t was "on"luded that the tri-"ell walls
and supports at the "ell oints werethe wea$est points in the platform.
#al"ulations based on nal designgeometr! showed that the load on
these parts was at or &er! "lose totheir ma/imum "apa"it!.
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Likely Cau#e#
The in&estigators from N# "on"luded
that 0nal failure was belie&ed to ta$epla"e as "rushing of the "on"rete'presumabl! at the interse"tionbetween the tri-"ell wall and the "ell
oint.= The in&estigators wor$ing for Statoil
agreed and elaborated on this"on"lusion' reasoning that 0shear
failure o""urred due to la"$ oftrans&erse reinfor"ement.=
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T$e Ne' Sleipner APla!or" (SLA%)*
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T$e Ne' Sleipner A%)
The new Sleipner platform (SLA-:) was
designed entirel! using hand"al"ulations. This was the result of ade"ision b! Statoil to "ontinue throughwith the design while in&estigators
"ontinued troubleshooting the softwareused to design all of the N# platformsin the North Sea.
The stipulation behind the SLA-:
design was that' in addition to hand"al"ulation and design' all of thebuo!an"! "ells in the platform weremodelled and anal!>ed before the! left
the dr! do"$ for further "onstru"tion.
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T$e Ne' Sleipner A%)
The designers of SLA-: "hanged the
"ell wall geometr! to minimi>e stress"on"entrations.
The "riteria for the new designin"luded a 1+C higher load fa"tor
when a""ounting for water pressure.
The formula for shear "apa"it! wasre&ised as well.
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T$e Ne' Sleipner A%)
3&erall' the total time spent on
&eri"ation of the design was%uadrupled in "omparison to thedesign of the SLA-1 S.
The SLA-: platform was up and
operate in 3"tober of 188*' morethan : !ears after the initial SLA-1S "ollapse.
The o&erall estimated "ost of theSLA-1 S "ollapse totals to ?
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Tri-cell geometry of
SLA-2 compared to thegeometry of SLA-1
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