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In the spring of 2004, construction began for a $2.2 billion expansion of the Oak Creek generating station, south of Milwaukee Wisconsin, on the shore of Lake Michigan. A primary component of this project is a water intake system to provide cooling water in excess of 2 billion gal/day to both the new coal fired plant as well as the existing plant.

This system consisted of four 12’ diameter intake shafts, located 1.5 miles offshore at the end of a 9200 ft. long, 27’-4” diameter TBM mined rock tunnel. The shafts were spaced 128’ apart in 45’ of water. Overall drilling depth from lake bed to top of tunnel was 110 to 120 ft.

It is locally known that only the 5 to 6 months of summer and early fall are suitable for continuous construction work on the waters of Lake Michigan before the winter storms begin. Therefore, when Case signed on as part of the design/build team, it assumed an added challenge of insuring shaft completion within just two of the three seasons allotted for the entire system. Follow-on water work would require all of the third season, in order to meet the targeted “flood date”.

Early year preparations began with design and fabrication of the 12’ diameter steel liner assemblies and also the conductor casings and drill casings needed for shaft installation. This work would be ongoing over the next two years. At the start of the construction season, two reusable steel-framed drilling platforms were fabricated at a shore facility and taken by barge to the offshore site. A steel jacket/subframe was set at two alternately spaced locations (i.e. 256’ apart) to allow for service barge clearances in-between. Six 36” diameter pipe piles were driven through each jacket leg to refusal below the lake bed. The platforms were then fixed in position 10’ above lake level to be clear of wave action. These platforms would serve as both a template for shaft drilling and a support for the rock drilling rig.

With two platforms in place and barges equipped and mobilized at site, work proceeded in the following sequence:

• Conductor Casing Installation: The lower end of a 17’-6” diameter by 65’ ft. steel casing was imbedded 5’ to 10’ below lake bed with its upper portion extending above, thru open water, to the underside of the platform.


• Rock Investigation: A soil sampling rig was set up on the platform and proceeded to establish top of rock elevation and then took rock samples to determine its quality.


• Overburden Drilling: A crane-mounted drill rig, mounted on a barge, augered a 16’ diameter shaft through 50’ of lake bed soils to the rock surface. A 15’ diameter “drill casing” was then set and twisted into the rock to a depth, as needed, to create an effective seal.


• Rock Socket Drilling: In order to provide adequate clearance for the (final) steel liner assembly, a socket diameter of 14’-8” would be required. This excavation was accomplished by using a full face roller bit – the largest ever used in North America – mounted on Wirth PBA 933 drill rig. As the rotating action of the 185,000 lb bottom hole assembly (BHA), crushed the hard limestone, the reverse circulation of 2700 to 3000 gal/min removed the cuttings and discharged them into an adjacent scow barge. Depth of sockets varied between 55 and 65 ft.


• Shaft Survey: Gyro-compass and side-scan sonar techniques were employed to determine depth and verticality of the socket plus the relative position of the upper casings and to provide bottom co-ordinates for the follow-up tunnel-to-shaft connections.


• Riser Assembly Placement: The riser assembly, designed for an overall length of 110’, was fabricated in 3 segments to facilitate handling and setting. Internal pressure domes and relief valves were placed in the upper and lower segments to permit future shaft dewatering, connection to the tunnel and its subsequent flooding. The bolt flange connections were double gasketed and instrumented for field pressure testing.

A barge-mounted 250 ton crane lowered each successive segment into the shaft, hair-pinning the segments at platform level to facilitate bolting of flanges and connection of the external grout pipes. The last/upper segment was welded to the top of the 15’ diameter drill casing, assuring stability of the liner and fixing the top at its designed position at lake bed elevation.

Grouting: A barge mounted grout plant conducted a 3 stage grouting operation, filling the annulus void within the socket and up into the drill casing above. This was accomplished through external grout tubes welded to the liner assembly which were then connected to soft hoses extending to the platform deck.

At the conclusion of the first construction season, only two shafts had been drilled and the on-set of late summer storms precluded liner setting. Therefore, the two platforms were removed and stored on shore in preparation for resumption of work the following spring. The second construction season saw drilling of the last two shafts and setting of all 4 liners – the last being in mid October – as severe weather started closing in.

During the third construction season, a marine contractor dredged a trench in the lake bed, then set and connected 9’ diameter horizontal manifold pipes on each side of the liner assemblies. These also included 24 specially constructed 8’ foot diameter cylindrical screens to protect the fish habitat. Concurrently, the tunnel contractor drilled and blasted from the crown of the tunnel up to the steel shaft liner and made the permanent connections to the tunnel. After filling the tunnel and releasing the air pressure in the vertical shafts, divers removed the pressure domes thereby completing the entire intake system.

Case’s successful and timely installation is especially noteworthy considering that:

• Only one port facility was available, 10 miles away in South Milwaukee and that availability of large marine equipment on Lake Michigan was very limited.
• Tight fabrication tolerances, rigid inspection procedures and the shear size of the liners presented an ongoing challenge to meet fabrication, delivery and installation schedules.
• Frequent storms and high waves caused extensive and disruptive delays throughout the two construction seasons.

This project now represents the nation’s largest cooling water intake system utilizing multiple offshore shafts. In addition, both the size of the liner assemblies and the rock socket diameters are the largest of their kind ever used in this application.