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Repairing and strengthening of ageing offshore structures

21st February 2013


Structural integrity management is an increasingly important element of the oil operator's offshore engineer's role. H W Lee and R S Miles report.

The number of ageing offshore platforms has increased globally and a substantial number are facing operation beyond their intended design life.

Asset management programs must perform an annual assessment of their offshore structures and should any significant change of the structural integrity of the structure be suspect, then implementation of an appropriate repair or strengthening plan must be implemented sooner rather than later.

Ninety per cent of all offshore revenue utilises the requirement of offshore structures, and any oil operator that thinks that the asset of an offshore structure is failure free is adopting that well known ostrich factor of burying ones head in the sand and hoping it will not happen to this company.

ULO Systems has been actively involved in structural repair projects since 1983. Initial works related to schemes that were implemented as a direct result of collision, impact, storms and fatigue. Recently computer re-analyses has been used to identify the need for additional strengthening. As a result of ULO Systems extensive experience a range of services to restore the structural integrity and life, utilising specialist knowledge, and best practice methods to ensure effective long term results. ULO Systems have developed a range of specialised high performance grouts, epoxy mortars and light weight sodium-silicate based grouts that may be used to meet the often demanding requirements.

A broad range of services in repair and strengthening are available that encompass specialist consultancy, design, project management, experienced field engineers for supervision, together with the specialist grouting services, personnel, materials and equipment.

Strengthening techniques

The most common industry adopted method is welded and grouted repairs however, whilst a welded repair may typically restore a structure to its initial condition, if the cause of the damage was fatigue loading this will not have been eliminated, the damage is likely to reappear.

Grouted repairs are therefore widely used to provide additional strength and typical applications include:

- Filling members to prevent propagation of a dent or buckle or, to enhance the capacity.

- Sleeved repairs, filling annulus between sleeve and member to strengthen a thin wall or to bridge over local damage.

- Leg strengthening, eg filling main pile annuli for increased stiffness.

- Clamped repairs for load transfer from existing structure into strengthening steelwork, at joints, or as part of sleeves to structural members.

- Leak sealing and plugging of redundant pipe-work, using hot tap techniques.

- Reinstatement of damaged concrete, well-head strengthening and under base grouting of gravity structures, for void filling.

Technical approach

While the repair design will evolve to meet the parameters of the project, it is often important to design a solution that has minimum weight, or which occupies minimum space on the structure. In these cases, a design, which offers the maximum structural efficiency, should be adopted.

There is considerable advantage in designing repairs that require minimal in-service inspection and maintenance. The 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.

Execution

ULO Systems adopt 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. The conductor guide frame had suffered severe fatigue damage from the effects of wave slam and vertical wave load, which caused cracks in the frame members where they met the jacket's main horizontal tubular members. The 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 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 chord can were determined. The repair took the form of a three-dimensional framework surrounding the node, which was connected to the adjacent braces or the cord using stressed grouted clamps. The 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. The analysis concluded that strengthening would be best achieved by installing grouted, mechanical clamps around the node, temporarily secured using spacer bolts. Specially constructed geotextile seals were positioned at the open ends of the clamp and filled with grout to seal 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.

4. At two different platform locations, strengthening was required to the main legs and for different reasons. At the first, whilst the original design was main piles with welded crown shims at the top of the jacket, shortly after installation, the structure developed a 'shimmy'. To overcome this and increase the leg stiffness, the solution implemented was to pressure grout the annulus with a high performance cement. At the second location it was necessary to strengthen the legs to permit elevation of the deck. The 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 used. Grout filling the legs provided increased resistance to buckling, resistance to the post tensioning forces developed by the clamp (sleeve) bolts and improved ultimate load capacity.

Common to all projects, ULO Systems services undertake the preparation of detailed procedures to ensure complete safety for the installation of the repair, QA/QC, design and testing of specialised grouting materials, procurement and supply, through to offshore operations. ULO Systems strength lies in their wealth of experience and knowledge that enables them to engineer the optimum solution for a given situation, applying particular skills and engineering approaches to a repair scheme as appropriate.o

Enter 25 or XX at www.engineerlive.com/iog

H W Lee and R S Miles are with ULO Systems LLC (HFZ), Sharjah, UAE. www.ulosystems.com









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