The paper is concerned with the analysis and repair of damage to a corner leg of the Heather ‘A’ platform in the British sector of the North Sea. This damage was caused by a foundation pile which was dropped during installation. Stereo photography and mechanical templating techniques were used to verify deformations to the fractured shells. The successful use of a mechanical solution carried out in the winter and spring periods is outlined in comparison with other methods. The case history also reviews the interface with commissioning and module installation activities.
The Heather field is located in Block 2/5 of the U.K. sector of the North Sea as shown on Figure 1. The Heather platform is located at 0°56'22.53" longitude and 60°57'13.51" latitude, lying approximately 100km (64mi) east of the northern end of the Shetland Islands in 143m (470 ft) of water. The oil field was discovered in December 1973 and a steel pile supported platform selected to develop the discovery. With the availability of larger pile driving hammers the decision was made in the fall of 1975 to drive 1.52m (60in) diameter piles. Each of the 24 piles 96m (315 ft) × 64mm (2 ¼ in) and weight 2353KN (240 tons) is made from a 64m lead section with a 32m extension welded offshore. During the offshore piling operations in 1977 when one of the piles had been joined and was being held by the derrick barge in preparation for lowering to the seabed one of the shackles linking two sections of slings broke. The pile, latterly restrained in the guides, then dropped more than 40m (130ft) in almost free vertical fall before hitting the jacket at the location shown in Figure 2. The extent of the damage discovered in late summer posed a potentially serious setback to the 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 October 1977 when the repair operation commenced from the semi-submersible vessel Treasure Hunter. The first phase of the repair was completed by mid January 1978 and the second phase from mid April to the end of May finalized the repair. The paper traces the stages of design, repair and the techniques developed and appraises the interface with the remainder of the development.
Initial impact of the pile was at the cone shaped section of the ‘bottle’ where the leg diameter increases from 2.5m (8ft) to 5.7m (18.5ft). The pile glanced off the cone buckling the 40mm(1.625in) thick plate and punching a hole (approx 1.25 × 1 m) 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 bent downwards. As the pile fell it continued to dent the cylindrical shell up to 200mm (10in) radially and also caused further damage to a second lower ring stifferner.
