CASE STUDY
Ormonde was the first major offshore wind farm that FoundOcean has grouted, but since then they have completed grouting operations at two more offshore wind farms in the North Sea.
The company is due to start grouting operations at two more offshore wind farms in the North Sea. The company is due to start grouting operations for four more in 2012, in addition to their thriving global oil and gas operations.
FoundOcean
www.foundocean.com e = See enhanced entry online
GROUTING MATERIAL The progression towards jacket foundations has opened up the market for standard cement-based grout: foundation designers can turn to Ordinary Portland Cement (OPC) for the grouting material.
The grout connectors were located on the jacket transition deck, with the lines running down the inside of the legs. Oil and gas jackets tend to have these located below the water line. Having the grout connectors located top-side made for speedier hose connections and disconnections when moving between jackets.
The 31 four-legged steel jackets were secured to the sea bed by their four main legs which were inserted into pre-driven piles. The connection between the leg and the pre-driven piles was made by injecting an OPC cement grout into the annulus around each pile and stab-in leg. Each leg had a primary and secondary grout line through which grout could be pumped into the annulus. Grout overflow was observed at the top of each pre-driven pile. A minimum overage of 10 per cent in excess of the theoretical annulus volume was pumped to the annulus after good quality grout returns were confirmed at the top of the pre-driven pile.
GROUT TESTING Grout strengths are tested by manufacturing a series of grout cubes at intervals during the mixing process. They are stored in a temperature-controlled water bath, and then crushed at age 24 hours, 7 days, and 28 days to monitor their compressive strength build up. Grout properties are defined by their 28 day strength, but 24 hour strengths are important to know how resistant a structure is to freak waves or collisions in the early stage of the curing process.
FoundOcean undertook two types of material testing for Ormonde: grout strength testing and grout slurry density. The grout slurry density was monitored by densitometers mounted on the mixer units to provide continuous monitoring of the slurry density. The slurry density was also measured at intervals using a pressurised slurry density balance.
Three samples were taken during the grouting of each annulus: one after approximately 1.5m3 had been pumped, one approximately halfway through grouting, and one towards the end of the grouting process. A total of 1,116 grout cubes were manufactured for testing, each weighing approximately 850g. Each cube was marked to identify the platform, leg number, time and date of casting and the specific gravity. The samples were cured underwater at 80C ± 0.50C until testing.
www.windenergynetwork.co.uk
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