Failure Knowledge Database / 100 Selected Cases

Capsize of Offshore Oil Drilling Platform
March 27th, 1980, Ekofisk oilfield in the North Sea
KITSUNAI, Yoshio (Japan Crane Association)
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
On Marc h 27th, 1980, t he s emi-submersible pl atform Alexander K ielland su ddenly caps ized during astorm in t he North Sea, because one of its five vertical columns supporting the platform was broken off.
123 workers among the 212 people on board were killed in the accident. The investigation showed that afatigue crack had propagated fro m the d ouble fi llet w eld near the hydrophone mounted t o t he t ubular
bracing D6. As a result, the five other tubular bracings connecting to the vertical column D broke off due tooverload, a nd t he colum n D be came separat ed from the pla tform. Cons equently, t he platform becam e
unbalanced a nd capsized. After theaccident, t he offshore de sign rul es we re revisedand som ecountermeasures were added to maintain a reserve of buoyancy and stability for a platform under a storm.
1. ComponentSemi-submersible oil drilling platform
The offshore oil drilling rig, Alexander L. Kielland, is a semi-submersible platform supported by fivecolumns standing on five 22 meter diameter pontoons (Fig. 1). The dimensions of the platform are 103 x 99
meters in pla n and 10,105 tons in weight. A 40 m eter high drilling derrick and accommodations for 34 8people w ere installed o n the platform. The f ive 8 .5 meter d iameter c olumns on th e pontoons w ere
interconnected by a netw ork of horiz ontal a nd diagonal bracings. The d iameters of the h orizontal a nd
diagonal bracings are 2.6 m and 2.2 m, respectively.
2. Event
At aro und 6. 30 pm on M arch 2 7th, 1 980, t he sem i-submersible oil drilling pl atform "A lexander
Kielland" capsized near the Norwegian Ekofisk oil field located at a latitude 56 degrees 28 minutes northand a longitude of 3 d egrees 7 minutes (Fig. 2), i n a storm w ith w indow ve locities fro m 16 to 20 m /s,temperatures of 4 to 6 C, and wave heights of 6 to 10 m, because the platform's columns broke off. Within
seconds, the platform tilted between 35 and 45 degrees. After 30 minutes, the platform turned upside down
(Fig. 3).
3. Course
The Alexander L. Kielland was designed as an oil drilling platform with a pentagon structure by France,
and it w as constructed f rom 1969

through 1 977 by CFEM (Com pagnie Francaise d' Enterprises

Metalliques). The platform was delivered to Norway in July 1976. In 1978, the accommodation capacity of1

Failure Knowledge Database / 100 Selected Cases

80 was increased to 348. Annual inspections were mainly carried out for the columns and pontoons, and theinspection in September 1979 had passed. However the D6 bracing had not been included in the inspection.
4. Cause(1) Fracture features
A circular h ole w as intr oduced to the underside of t he D 6 brac ing, an d a p ipe, w hich is ca lled ahydrophone, was mounted i nto t he c ircular h ole by welding (Fig. 4). Th e hydrophone w as 325 mm indiameter with a 26 mm wall thickness. The hydrophone was welded using a double fillet weld with a weld
throat thickness of 6 mm. A drain of the bracing D6 had to be installed at a location 270 mm away from thehydrophone (Fig. 5).
As a result of examination of the welds of the D6 bracing, some cracks related to lamellar tearing were
found in the heat affected zone (HAZ) of the weld around the hydrophone. However, no weld defects were
found at any other location. Traces of paint coinciding with the paint used on the platform were recognizedon the fracture surface of the fillet weld around the hydrophone in the bracing D6. These paint traces showthat the cracks were already formed before the D6 bracing was painted. Examination of the fracture surface
also showe d that t he fat igue cracks pr opagated fro m two in itiation si tes near th e fi llet we ld of t hehydrophone to t he dire ction c ircumferential t o the D 6 bra cing. M oreover, the f atigue fract ure surfac e
occupied more than 60% of the circumference of the D6 bracing (Fig. 6), and beach marks were formed on
the fracture surface, which was about 60 to 100 mm away from the hydrophone. Striations with spacing of
0.25E-3 to 1.0 E-3 mm were observed in patches on the fracture surface of the D6 bracing.
(2) Characteristics of the welds of the hydrophone
Considering of the importance of the strength of the D6 bracing, welding of the drain into the bracing
was carried out carefully according t o the design rules. In the cas e of the installation of t he hydrophone,
however, a c ircular hole was made on t he D6 bracing by gas cut ting, and the surface of t he hole was nottreated by some process, s uch as a grinding. After cutting, a pipe, which was made by cold bending andwelding using a pla te with 20 mm thickness, was mounted into the hole of t he bracing, and the pipe wasattached by welded around the hole by double fillet welding with a throat thickness of 6 mm (Fig. 7).
When the hydrophone was installed by welding, welding defects, such as incomplete penetration, slaginclusion, a nd root crac k, were in troduced i n the w elds, b ecause of the poor g as cut ting and w elding.
Moreover, la mellar teari ng relat ed to i nclusions i n the material us ed was found near

the HAZ of t hehydrophone. The stress concentration factor, Kt, of the fillet weld of the hydrophone was in the range of 2.5
to 3. 0, whi ch is hi gher than the a verage va lue of Kt of 1 .6 for a f illet weld performed un der n ormalconditions.
(3) Chemical composition and mechanical properties of materials
Specifications and results of analyses of the chemical composition of the fractured materials are shownin Table 1. T he chemical composition of t he m aterials was f ound t o be with in t he spe cified l imits. A
comparison of the m echanical properties betwe en t he specification and the tes t results f or th e fracture dmaterials is shown in Table 2. The yield strength of the D6 bracing in the longitudinal direction is slightly

2Failure Knowledge Database / 100 Selected Cases

lower than t he specified minimum values. In case of the hydrophone, the C harpy impact energy is low erthan t he req uired v alue of 39 J a t -40 C. More over, the re duction of area of the hydrophone for t hethrough-thickness direction is markedly reduced because of the large amount of inclusions.
(4) Stress on the D6 bracing
Considering t he wi nd a nd wave data before t he ac cident, t he s tress amplitude o n the D6 bracing wa s
estimated to be in the range of 13 1 to173 MPa. T his result shows that the stress l evels of the D6 bracing
were relatively high as compared to the other horizontal bracing in the platform. The fatigue life of the D6
bracing with the hydrophone was calculated to be in the range of 0.7 to 5 years.
5. Immediate Action(1) Although the D6 bracing was one of primary components of the platform, little attention was given tothe installation of the hydrophone into the bracing. Hence, a crack with a length of about 70 mm wasintroduced in the fillet weld around the hydrophone, before the D6 bracing was painted.
(2) Fatigue cracks prop agated from tw o init iation sit es n ear th e fil let weld of th e hydrophone in t hedirection circumferential to the D6 bracing at the early stage of the life of the platform.
(3) The five other bracings connected to the column D broke off due to overload, and the column D w as
separated from the platform. Consequently, the platform became unbalanced and capsized.
(4) Inspection of the D6 bracing had not been carried out.
6. Countermeasure
Based on the accident report, redundancies of stability and structural strength, and lifesaving equipment
for the offshore oil dri lling pla tforms were o bligated by the N orwegian M aritime Direct orate (NMD).
Amendment of th e MODU (Mo bile Of fshore Drillin g Units) Co de was carried ou t by the In ternational
Maritime Or ganization, a nd sta ndards for stab ility, motion c haracteristics, m aneuverability, waterti ght
doors, and structural strength of the oil drilling platforms were strengthened.
7. Knowledge
Installation of att achments, su ch as th e hy drophone, o n a stress ed c omponent by w elding oft en
introduces a cause of fatigue failure. In order to improve the fatigue resilience of structures, it is important
to avoid u nnecessary w elding a nd at tachments. A ttachments ca n redu ce stres sed c omponents to thelowest design class.
8. Hearing from Parties Concerned
A process e ngineer said that he heard two bangs while he was watching a fi lm in the c inema. Within
seconds, the platform tilted between 35 and 40 degrees, and all the lights went out. People in the cinema,
which was filled to its capacity of about 40, were hurled across the room, and some of them were injured.
All peo ple ru shed outside, but the s urvival sui ts an d li fesaving b oats equi pped i n the p latform could n ot
used because of the severe degree of tilt.

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Failure Knowledge Database / 100 Selected Cases

9. Information Source(1) Norwegian Public Reports, The "Alexander L. Kielland" Accident, November, 1981, pp.1-472.
(2) A,Almar-Naess, P. J. Haag ensen, B. Lian, T. Moan, and T. Simonsen, Trans. ASME, Vol.106, March,
1984, pp.24-31.
(3) S.Ishida, J. Naval Architecture and ocean Eng., No.734, 1990, pp.13-17, (in Japanese).
10. Primary Scenario
01. Ignorance
02. Insufficient Knowledge
03. Poor Understanding
04. Production
05. Hardware Production
06. Shipping and Maritime
07. Offshore Oil Rig
08. Column
09. Bracing
10. Hydrophone
11. Failure
12. Fracture/Damage
13. Metal Fatigue
14. Welding and Join
15. Fracture of Welds
16. Usage
17. Maintenance/Repair
18. Inspection
19. Lacked Inspection
20. Crack Propagation
21. Failure
22. Large-Scale Damage
23. Capsize of Oil Rig

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Failure Knowledge Database / 100 Selected Cases

Table 1 Chemical composition of D6 bracing and hydrophone（wt.%）

Specifications

D6 bracing

HydrophoneCSi

Max

Max

0.18

0.18

0.17

0.32

0.16

0.29

MnAlEW

SP

NB

Max

Max

Max

1~1.5

0.03

0.03

0.42

1.34

0.015

0.026

0.056

0.025

0.41

1.37

0.019

0.021

0.044

0.029

0.41

1.35

0.020

0.017

0.0380.023

0.40

Table 2 Mechanical properties of D6 bracing and hydrophone
ThickSpecifications

D6 Bracing

Yield strength

TensileReduction ofCharpystrength(MPa)area (%)

energy (J)ness(MPa)(mm)L< 40Min

490

MinMin353~6082239.225

Hydrophone20

Hydrophone

20T

LT

L

T

343

301

506

501

30

29~353~351~518~523~34~33

405

516
398

30

L

T

40

46

16

17

4.8

Z-direction
Ｌ：Longitudinal direction,Ｔ：Transverse direction,

5Charpy test temperature−40℃

Failure Knowledge Database / 100 Selected Cases

Fig. 1Alexander L. Kielland.

Fig. 2 Location of the accident.

6Failure Knowledge Database / 100 Selected Cases

Fig. 3

Process of capsize of Alexander L. Kielland.

Fracture
Fracture

ColumnEColumn
DD-6
Fracture
Fracture

Hydrophone
Fig. 4

Crack

Drain

Position of hydrophone and bracings connected columns1)2)

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Failure Knowledge Database / 100 Selected Cases

Main fracture surface

Web

35
160

20~21325

370

Hydrophone

Butt welding joint

Drain

Crack

870

270

1945
Fig. 5Main fracture surface near hydrophone1)2).

350

400

450

300

500

Oppression part
250

-220°
TensileFracture

200550600

Fatigue
Fracture

-300°

150

650

Oppression part

Origin10070050
Origin788750Hydrophone

Fig. 6 Fracture feature of D6 bracing2).

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Failure Knowledge Database / 100 Selected Cases

Fig. 7 Dimensions of hydrophone1)2).

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