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which occupies minimum space on the structure. In these cases, a design, which offers the maximum structural efficiency, should be adopted. Tere is considerable advantage in designing
repairs that require minimal in-service inspection and maintenance. Te optimum solution is arrived at by a carefully balanced assessment of the relative importance of these factors. Installation costs are often the major element in
the overall cost of the repair, and hence must always be minimised. Furthermore, it is possible to adopt approaches to the structural design, where there is a trade-off between the engineering efforts required and the final cost of the repair. Options include: design for full member capacity; design for full member loads, and load sharing design. For ‘fast-track’ projects, one of the first two options may be adopted but where the project schedule permits, the load sharing design approach would usually provide the most economical overall solution.
attracted to other nodes sufficiently to avoid the need to place clamps around them.
2 During a routine underwater inspection of a platform, cracking in some of the welds connecting five brace members to the node joint were determined. Te repair took the form of a three-dimensional framework surrounding the node, which was connected to the adjacent braces or the node using stressed grouted clamps. Te new brace members of the framework were shop welded to the clamps or where site connections were necessary, a conventional pile/sleeve type connection was used taking advantage of weld bead shear connectors, selected to provide the maximum tolerance on final positioning and thereby facilitate installation and subsequent grouting.
3 A routine structural analysis to a platform found it was necessary to strengthen the jacket structure at a particular node connecting one of the legs to a diagonal brace. Te analysis concluded that strengthening would be best achieved by installing grouted, mechanical clamps around the node, temporarily secured using spacer bolts. Specially constructed seals were installed at the open ends of the clamp and when activated, sealed the clamp annulus prior to filling with a high performance grout. When the grout had gained strength, the spacer bolts were withdrawn and post-stress applied to the clamp by means of the external stressing bolts.
Fig. 2. T-Node stress grouted clamp.
Execution ULO Systems LLC adopts a staged approach to the development of a repair scheme from assessment, and design through to installation. To illustrate how this is implemented, a summary overview of different strengthening projects follows:
1 At a platform. Te conductor guide frame had suffered severe fatigue damage from the effects of wave slam and vertical wave load component, which caused cracks in the frame members where they met the jacket’s main horizontal tubular members. Te design took account of the interplay between the original structure and the clamps placed upon it. By increasing the stiffness of certain of the horizontal stressed grouted clamps it was possible to reduce the loads
4 At two different platform locations, strengthening was required to the main legs and for different reasons. At the first, while the original design was main piles with welded crown shims at the top of the jacket, shortly after installation, the structure developed a ‘shimmy’ or, ‘wobble’. To overcome this and increase the leg stiffness, the solution implemented was to pressure grout the annulus with high performance cement. At the second location it was necessary to strengthen the legs to permit elevation of the deck. Te leg-strengthening scheme incorporated external sleeving with a small annulus for subsequent grouting. Due to the large volume of infill material needed for the legs, a specialised low heat cement grout was to minimise heat of hydration. Grout filling the legs provided increased resistance to buckling and also resistance to the post tensioning forces developed by the clamp (sleeve) bolts and improved ultimate load capacity.
Ultra high performance grouts More recently, ULO Systems LLC in association with Contec ApS with have identified the Secutec range of ultra high performance grouts (UHPG’s) that may
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