Ziel
Costs associated with the repair and strengthening of underwater members on an offshore structure are high. In addition, whilst a number of repair techniques have previously been used or considered, all options have limitations in terms of either the subsea operations involved or the structural integrity of the repair.
The utlimate aim of the project has been to develop a novel low cost repair/strengthening method (The 'grouted stud/strap connection') which is compact, lightweight, quick to install and relatively maintenance-free when compared with existing alternative repair techniques. To achieve this aim, the following objectives needed to be satisfied : verification of the performance of the stud/strap shear mechanism in underwater grouted connections; derivation of agreed design procedures for the repair/strengthening method; approval of the technique by Certifying Authorities. and detailed design of a semi-automatic machine for rapid installation of studs/straps.
The viability of the stud/strap connection for repair and strenghtening of steel offshore structures has been demonstrated.
When compared with currently available alternative techniques, it has been shown that the stud/strap connection offers benefits in terms of weight reduction, shorter installation time and less onerous inspection requirements.
Traditionally, one of the most widely used methods for repair and strenghtening of steel structures below water level has been the stressed grouted clamp. Two design comparisons used this as the base case to assess weight reduction. In the first, a typical grouted clamp for strenghtening the leg of a Northern North Sea platform was considered: the stud connection was found to be less than 30% of the weight of the stressed system. In the second, the comparison took an actual damaged jacket structure where load needed to be shed into adjacent piles and three clamps were involved: here, the stud/strap configurations were approximately halfthe weight of the traditional designs. The major savings in both cases are attributable to the absence of heavy top plates, side plates and stiffeners in the stud grouted connection.
In most repairs, the largest cost items are either vessels or diving works associated with installation of the repair. The reduced weight and physical size of the studded system provide savings in terms of ease of installation. In addition, since the stressed grouted clamp requires the studbolts to be tensioned once the grout has gained adequate strength, the stud connection takes approximately 2-3 days less than the traditional hardware to install.
The inspection requirements of stressed grouted clamp revolve around the need to ensure that the stress, applied to the studbolts after installation, is maintained. Whilst methods have been developed to quickly assess the stress in studbolts, e.g. the Rotobolt system, the only sure way of accurately determining the stress is to restress the bolts and record the stress level at which the nut becomes loose. Due to the high stresses involved, this cannot be achieved using manual tools and requires the use of subsea hydraulic tensioning equipment. As the stress in the bolts determines the connection strength, it can be seen that regular inspection is essential. In contrast, the bolts used in the stud grouted clamp are of secondary importance as the strength of the connection is controlled by the shear collars. Thus the inspection criteria are reduced. Furthermore, the integrity of the bolts could be easily and quicky checked by a diver using a hand torque wrench. Consequently, there is a much reduced requirement for inspection with the stud grouted clamp than the stressed connection.
Stud connections combine two proven technologies: grouted pile-sleeve connections and stud friction welding. The repair/strengthening method employs a split sleeve which is deployed around the damaged tubular. The grouted annulus can accommodate any surface imperfections on the tubular and, hence, eliminates the need for an accurate dimensional survey.
The grout forms the load path between the sleeve and the tubular. Considerable interaction between the grout and the two tubular components is needed if a large load-carrying capacity is required. This interaction is created partly by the bond between the grout and steel surfaces, but mainly by the attachment of shear connectors to the inner face of the sleeve and the outer face of the tubular. The sleeve would be manufactured onshore and shear connectors in the form of continuous weld beads can be adopted, as with traditional pile-sleeve connections. However, weld beads cannot be attached satisfactorily underwater. By friction welding studs through a pre-drilled strap (or collar), a shear key with the same form and characteristics as a weld bead can be formed subsea. This is the basis of the grouted stud/strap repair technique.
Recent developments in friction welding have produced a relatively simple and reliable system for underwater attachment of studs.
The development project for the stud/strap repair technique was carried out in five phases as follows :
(1) Definition of design requirements.
(2) Derivation of connection design formulae.
(3) Evaluation of stud welding systems.
(4) Welding trials.
(5) Marketing.
Phase 1 commenced with a review of potential areas of application for the repair techniques, as well as an investigation into alternative stud attachment methods. With due consideration of the possible need for underwater jacket-member repairs, tubular-joint repairs and addition of new bracing to offshore structures, it was concluded that friction welding offered the greatest scope for further development.
During Phase 2, it was established that the friction welding apparatus could be used to weld a stud "through" a pre-drilled strap, firmly attaching the stud and strap to a tubular member. The physical modelling originally intended for Phase 2 was replaced by confirmatory tests to gain certification for the use of the system offshore. A preliminary design procedure wa also postulated in Phase 2.
Evaluation/verification of friction stud welding techniques in the application of the repair method were carried out in Phase 3. This led to the design development of a system suitable for rapid and accurate semi-automatic attachment of studded shear straps underwater; the scope of Phase 3 was considerably extended to encompass detailed design of all the necessary components of this system.
Phase 4 was incorporated into the confirmatory test programme, established to gain Certifying Authority (CA) approval from Lloyd's Register of Shipping for the stud/strap repair concept.
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