Four involve use of grouted repair clamps to restore structural integrity, while the fifth calls for rectification of movements in a platform by addressing seabed scour.
The first platform processes oil and associated gas from several fields. Its corroding firewater caisson is being repaired by fitting two grouted clamps connected by four new structural members around the caisson. A routine Portland cement and seawater mix is to be injected into the annulus for structural integrity.
Off the northeast coast of the Shetlands, another platform with a corroded caisson is being fitted with a grouted clamp over the damaged areas. Once the grout plug is set, FoundOcean will continue to fill the rest of the clamp/caisson annulus.
The clamp has been designed with bolt-together sections and is equipped with primary and secondary grout inlets. It will be attached to an adjacent brace member by a stressed mechanical clamp to restrict lateral movement.
The third platform receives and processes oil and gas from various North Sea fields. A four-sleeve repair clamp has been installed on the jacket’s drill cuttings caisson. FoundOcean grouted sleeves one, three and four in 2007, leaving sleeve two which could not be completed before the end of the planned maintenance period.
A bladder is to be inserted into the caisson and inflated, ensuring grout does not enter the caisson. A grout plug will be set at the bottom of the clamp; grout then will be injected into the annulus between the clamp and the caisson, using a stinger until good grout returns are seen at the outlet.
The fourth platform repair is 312 km (194 mi) east of Dundee, eastern Scotland. Here the jacket is being fitted with two stressed clamp configurations. The first is an “X-Node” clamp assembly and the second a “K-Node” clamp assembly.
Each configuration comprises bolt-together sections equipped with primary and secondary grout connections. The annuli between the clamps and the members will be grouted to ensure structural integrity, using CEM I Type 52.5 Ferrocrete cement to achieve a specific gravity of 1.98.
The fifth platform is in the southern North Sea, in 24 m (78.7 ft) water depth. Localized scour has eroded the seabed from under the platform leg mudmats and is exerting too much pressure on the crown shim welded leg-to-pile connections at the top of the legs. This is causing movement in the platform which could reduce the structure’s life.
Movement will be stopped by rectifying the scour and strengthening the leg-to-pile connection with grout. Donut-shaped grout bags will be placed around and under the circumference of each mudmat.
The bags will be filled to form a seal between the underside of the mudmat and the seabed. The void under the mudmat will then be filled with grout to form a solid base and plug the bottom of the leg. Finally, a grout inlet will be drilled into each leg and the annulus between the outside of the pile and the inside of the jacket leg filled with grout.