Project To Upright The Alexander L. Kielland

Abstract The Alexander L Kielland is a drilling rig of Pentagone type. It was being used as an accommodation unit when it capsized in the North Sea in 1980 as a result of a structural failure. In June 1980 the owners invited tenders to upright the rig. The physical size and geometry of the rig made the prospective uprighting a novel undertaking. The method devised was based on the simple concept of rotating the rig underwater with the minimum amount of nett positive buoyancy that was consistent with safety. Adopting such a method would reduce the applied loads to acceptable levels. However, it required a detailed knowledge of the response of the rig to the applied loads, and the structural stresses involved. It was therefore decided that, in addition to the engineering systems, a computer programme should be developed to simulate the motion of the rig and that comprehensive computing capability should be deployed on site. Its purpose was to carry out the necessary analytical investigations and to work interactively with the monitoring system that had been designed for the project. Introduction The initial estimates, made immediately prior to the accident, indicated that the displacement of the rig was in excess of 17000 tonnes. The loss of D leg, its associated bracings and other structural damage was estimated to account for approximately 1400 tonnes. However the final condition of the rig in the inverted position was not known accurately. Many of the tank fittings had been damaged during the capsize and water had flooded into a number of compartments. It had also been necessary to pump air into several compartments as a preventative measure to stop the rig from sinking. The basic uprighting method involved the transfer operation between nominated tanks and hence it was imperative to ascertain the status of each tank and from that information assess the mass and structural integrity of the rig. The basic philosophy had to be developed into operating systems that would maintain control of the structure throughout the rotation. Safety of personnel and the structure were a top priority. The development of the systems and the computer software were done under line project conditions and a limited timescale. Philosophy An invitation to bid for the salvage of the rig was received in June 1980. In the document, the owners stated their intention to have the rig repaired and recommissioned as a drilling unit. They also indicated their desire to have the salvage operation completed by the end of September. This would avoid the difficulties associated with a winter towage operation to a repair yard. This implied that the contract for uprighting and preserving the rig should be completed in approximately twelve weeks. The owners also drew our attention to the fact that there was a Commission of Enquiry and Police Investigation concerned with the rig. Having given due consideration to the nature of the accident and these requirements, it was concluded that any salvage operation should satisfy the following basic criteria.