Reaching new heights4 October 2002
Innovative repairs at a Scottish hydroelectric plant avoided costly delays to day-to-day operations
THE 65MW Locheabre station in Scotland is owned by Alcan Primary Metal and is used to power local plant at the company's aluminium smelting works. When a 30cm ductile iron scour pipe ruptured within the hydro plant on Ben Nevis, strengthening of the three remaining lines to prevent further catastrophic failure became paramount.
A collaboration between Furmanite International and DML known as the FD Alliance (FDA) was able to provide an engineered solution that combined advanced composites technology with bespoke clamps.
Since hot work is not required for the application of composites the need to interrupt production (both for the smelting works and the hydro power station), or the risk of debris finding its way into turbines, was avoided with significant savings to the customer.
Used to remove trapped gravel and pebbles from sumps positioned on the main flow lines to Alcan's hydroelectric generating station, the four lines were found to be suffering severe corrosion and erosion, causing one to rupture. Their position (some 200m along a service tunnel in the mountain side, encased in rock with the lower sections emerging into an underground chamber just 4m in height) meant that replacing the lines would be extremely difficult without shutdown. However, to shut off the water supply would also affect the Alcan aluminium smelting works, incurring further costs in the region of US$36M since the electrolytic process would be interrupted.
Composites materials are able to cope with confined access and are easily manhandled, requiring no pre-fabrication. This was particularly key in this instance since materials had to be carried the 200m into the small tunnel to the application site.
Further benefits of these materials include multi-axial strength that can be up to ten times that of steel and twice as stiff, yet with less than one quarter the density, enabling full structural strength and pressure integrity to be restored without significantly increasing the size or weight of the existing pipeline.
With the failed line temporarily plugged using a large offshore mooring buoy (to be replaced by a more permanent Furmanite-designed solution later), the remaining lines were wrapped using layers of carbon fibre hand-impregnated with epoxy resin to form a 5.6mm thick repair.
FDA composites manager, Paul Smith, explains: 'To ensure that the finished repair meets requirements and can be fully warranted, the resin matrix and ultimate thickness of the repair is carefully calculated at the design stage. In this instance a lesser thickness might have been sufficient to contain the 12 barg maximum pressure and 12kN axial load (caused through self-weight), and to cope with the operating temperatures of between 5°C and 15°C. However the standards to which we work specify a minimum of five layers to ensure long-term reliability.'
In every composites application preparation is key to achieving the optimum bond strength. The substrate must be clean, achieving SA2.5, and meet the required 75 micron surface roughness. Dry abrasive blasting is generally used, however in this case, due to the extreme nature of the wall thinning (the remaining wall thickness was just 3.25mm), the lines were hand abraded to avoid through-wall damage occurring. A glass tie coat is then applied, preceding the carbon fibre layers and providing a sound bond surface, as well as insulation to avoid galvanic reaction.
Once preparation is complete, layers of composites are applied using a wrapping technique. Application must begin within four hours of shot blasting to avoid oxidisation weakening the bond. For this application the protruding 2m of each line was composites-wrapped.
Since the lines disappear into the rock above, specialist clamps were engineered to ensure that there could be no future breakage at the intersection between the composites wrap and the rock. A clamp was designed and manufactured to strengthen the section of pipe as it enters the 'ceiling' of the tunnel, featuring supporting tripod legs that rest on the tunnel floor. Once the clamp was in place the composite repair was extended up over it to ensure pressure containment across the interface and also to ensure structural integrity of the whole section of pipe. The clamp was then injected with a fast setting resin to seal any uneven space between clamp and roof.