The Heather Platform Leg Repair
- J.M. Thompson (Unionoil Co. of Great Britain) | A.R. White (Halcrow Ewbank Offshore and Petroleum Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- April 1980
- Document Type
- Journal Paper
- 569 - 579
- 1980. Society of Petroleum Engineers
- 6.2.8 Ergonomics, 7.5.3 Professional Registration/Cetification, 4.1.2 Separation and Treating, 4.5 Offshore Facilities and Subsea Systems, 1.10 Drilling Equipment, 4.3.1 Hydrates, 4.2.3 Materials and Corrosion, 1.6 Drilling Operations
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This case history reviews the analysis and repair of a corner leg of the Heather A platform; damage was caused by a foundation pile dropped during installation. The paper describes the stereophotographic and mechanical templating techniques used to verify shell deformations. The choice of a mechanical solution and the interface with module installation and commissioning are reviewed.
The Heather field is located in Block 2/5 of the U.K. sector of the North Sea (Fig. 1). The Heather A platform is located at 0 degrees 56'22.53" longitude and 60 degrees 57'13.51 " latitude, approximately 100 km (64 miles) east of the northern end of the Shetland Islands in 143 m (470 ft) of water. The oil field was discovered in Dec. 1973, and a steel-pile-supported platform was selected to develop the discovery. With the availability of larger pile-driving hammers, the decision was made in the fall of 1975 to drive 1.52-m (60-in.) diameter piles. Each of the 24 piles [96 m long x 64 mm thick (315 ft x 2.25 in.) and 218 Mg (240 tons)] is made from a 64-m (210-ft) lead section with a 32-m (105-ft) extension welded offshore. During the offshore piling operations in the summer of 1977, one of the piles had been joined and was being held by the derrick barge in preparation for lowering to the seabed, then one of the shackles linking two sections of slings broke. The pile, laterally restrained in the guides, dropped more than 40 m (130 ft) in almost free vertical fall before hitting the jacket at the location shown in Fig. 2. The extent of the damage posed a potentially serious setback to field development. With the reduction of load-carrying capability of the platform, the complete module installation program was reanalyzed. The selection of the repair method was judged in line with performing the repair during the winter period. A limited number of modules were installed on the platform by mid-Oct. 1977, when the repair operation commenced from the semisubmersible vessel Treasure Hunter. The first phase of the repair was completed by mid-Jan. 1978, and the second phase from mid-April to the end of May finalized the repair. This paper traces the stages of design, repair, and techniques developed and appraises the interface with the remainder of the development.
Description of Damage
Initial impact of the pile was at the cone-shaped section of the "bottle" where the leg diameter increases from 2.5 to 5.7 m (8 to 18.5 ft). The pile glanced off the cone, buckling the 40-mm (1.625-in.) thick plate and punching a hole [approximately 1.25 x 1 m (4 x 3.5 ft)] in the cylindrical shell. At the junction of the cone and shell is a critical ring stiffener designed to contain the hoop stresses at this change in section. This ring stiffener was torn in two places, and the tongue section in between was bent downward. As the pile fell, it continued to dent the cylindrical shell up to 200 mm (10 in.) radially and caused further damage to a second lower ring stiffener. Denting of the vertical shell was caused over some 4.5 m (14.5 ft).
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