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Alexander L Kielland accommodation platform

Case Study

Summary Details

  • Failed structure: Pentagone type semi-submersible rig
  • Date: 27 March 1980
  • Place: Ekofisk field, North Sea
  • Conditions: Bad weather, ~60-75km/hr wind speeds, ~6-8mm wave height
  • Failure mode: Fatigue failure followed by brittle fracture in one brace and ductile overload in remaining adjacent braces
  • Cause: Fatigue crack growth from a weld defect
  • Consequences: Loss of 123 lives and platform

Background

Fig.1. Alexander L Kielland accommodation platform (Reproduced courtesy of the Norwegian Ministry of Justice)
Fig.1. Alexander L Kielland accommodation platform (Reproduced courtesy of the Norwegian Ministry of Justice)

On the evening of 27 March 1980, one of the five columns of the 'Alexander L Kielland' accommodation platform anchored in the North Sea, broke off (see Fig. 1). (The five columns were the principal buoyancy elements of the platform). The platform immediately heeled over to an angle of 30-35° and then continued to heel and sink slowly. Twenty minutes after the loss of column D, the platform capsized. Of the seven lifeboats on board, only two were launched successfully albeit with great difficulty in part due to bad weather conditions (one landed upside down in the water). Some inflatable rafts launched themselves due to the listing of the platform. A massive international air and sea rescue operation was undertaken. Of the 212 men on board the platform when it failed, 123 died.

The Alexander L Kielland was a semi-submersible mobile rig of the Pentagone type, a design which had been developed in France. The rig was built between 1973 and 1976 in France for an American operator. Although it was designed as a drilling rig, it was only ever operated as an accommodation platform during its four years in service.

The platform had five columns, of overall height of 35.6m, mounted on 22m diameter pontoons. The columns were positioned at the apexes of a pentagon with braces running between adjacent columns and the deck or hull. Accommodation units and a drilling tower were mounted on the deck.

Causes of Failure

Following the accident, the platform and the separated column D remained afloat. Column D was towed to Stavanger and divers removed all the fracture faces from the capsized platform for investigation (see Fig. 2).

The Commission responsible for the inquiry into the disaster concluded that the structural failure had occurred in the following stages:

  1. Fatigue crack growth in brace D6 initiating from pre-existing cracks in the fillet welds between a hydrophone support and the brace
  2. Final, mainly ductile, fracture of brace D6
  3. Subsequent failure of five remaining braces joining the column to the structure by plastic collapse

Fig.2. Alexander L Kielland accommodation platform (fracture face)
Fig.2. Alexander L Kielland accommodation platform (fracture face)

Brace D6 and the hydrophone support were both made from a C-Mn structural steel (equivalent to a Lloyds' ship steel Grade EH) with a minimum specified yield strength of 355N/mm 2. The brace was 2.6m in diameter with a wall thickness of 26mm. The hydrophone support was 20mm thick with a diameter of 325mm and was set-through the brace. It was attached to the brace by two fillet welds, one on the outside of the brace and the other on the inside. Examination of these fillet welds revealed poor penetration into the hydrophone tube material and an unsatisfactory weld bead shape. Significant cracking was also found which was dated to the time of fabrication by the presence of paint on the fracture surfaces.

Fatigue crack growth in brace D6 originated at the hydrophone support weld and extended, in the latter stages partly by ductile tearing, around approximately 2/3 of the circumference of the brace until final failure took place by brittle fracture.

The chemical compositions of the brace and hydrophone material were within specification, as were the Charpy and in-plane tensile properties. The through-thickness ductility of the hydrophone material (which was not specified) was, however, poor. This, combined with its through-thickness tensile strength being lower than the in-plane strength of the brace material and with sub-standard welding, led to partial cracking of the fillet weld during fabrication.

Lessons learnt

Although material properties and welding quality played a significant part in this disaster, rig design was also a critical factor. Apart from the stability and buoyancy aspects which were inadequate, the design did not consider attachments to highly stressed braces such as D6 as important. The fatigue performance of the hydrophone attachment and its effect on the fatigue life of the brace were tragically overlooked.

This is a case history taken from Report 632/1998 . For further case histories, Industrial Members may consult the full report.