sleipner a

7/23/2019 Sleipner A

1/28

SLEIPNER A NORTH SEA OILPLATFORM COLLAPSE


2/28

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.


3/28

Typical CondeepPla!or"


4/28

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.


5/28

Typical

CondeepPlatform


6/28

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.


7/28

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.


8/28

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.


9/28

Partially completed buoyancy cells to be floated to a fjord


10/28

T$e Sleipner A%&


11/28

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


12/28

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).


13/28

Tri-cell geometry of a

typical platform comparedto the geometry of SLA-1.


14/28

Tri-cell joint reinforcement detailing and shaft wallreinforcement detailing.


15/28

T$e Acciden


16/28

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.


17/28

T$e Acciden

The platform began ta$ing on

water un"ontrollabl! at adepth of 8


18/28

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.


19/28

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 obser&ethe 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.


20/28

Likely Cau#e#


21/28

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!.


22/28

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.=


23/28

T$e Ne' Sleipner APla!or" (SLA%)*


24/28

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.


25/28


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.


26/28


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 ?


27/28

Tri-cell geometry of

SLA-2 compared to thegeometry of SLA-1


28/28

sleipner a

Documents