Phase II Combined Cycle Power Plant

Service Provided: Industrial Construction

Since industrial plant expansion typically involves a number of diverse site activities, it is advantageous to the owner, in terms of both overall schedule time and cost, to partner with a company with a proven reputation in a wide range of industrial and geotechnical construction methods. Moretrench is such a company. Our Tampa office has the resources, manpower and capability to undertake everything from preliminary site dewatering to large-diameter concrete pipe installation to final site restoration and seeding. At a West Central Florida power station these requirements, and more, were met under a single contract.

Moretrench was subcontracted to the project General Contractor for installation of 2,300 lf and 1,500 lf respectively of 108-in. and 42-in. diameter, concrete-lined steel pressure pipe; installation of 2,500 lf of precast concrete electrical trenches between the cooling water reservoir pump locations and the power block; and ancillary works including storm drainage, dredging, riprap embankment, 12 acres of grading, asphalt roads, concrete pavement, rock surfacing, and grass restoration.

Pipeline Challenges

The most complex aspect of the overall work was the installation of the 2,300 lf of 108-in. diameter pressure pipe, which consisted of a double-barrel run within the power block area, splitting into single runs approximately midway, with one heading north to the cooling water reservoir intake structure and one to the east to the discharge structure. Installation presented a number of significant challenges:

Alignment Obstructions: Pipeline alignment crossing beneath a 12-in. diameter, active natural gas line with limited above-ground clearance; beneath a cluster of high voltage transmission mains approximately 50 ft above grade; and above existing large-bore circulation water piping requiring constant operation to feed the plant.

Component Handling: Pipe sections approximately 16 ft in length and weighing as much as 80,000 lbs. each, precluding standard pay-loader handling.

Restricted Access: Very tight working conditions in the power block area and inadequate clearance beneath the transmission lines for large crane work.

LiDar Scanning & Modeling

Given the weight of the joints and pipe sections, together with the space limitations, extensive planning and engineering ahead of mobilization was crucial to the successful execution of the work. Moretrench utilized its remote LiDAR laser scanning system to produce highly detailed and accurate 3-D models of the pipe run alignments and utilities. Data gathered enabled detailed crane and rigging plans specific to each problem area to be developed.

Wellpoint Dewatering

With groundwater at 5 ft below grade, and excavation depths of up to 20 ft, dewatering was critical to maintaining the stability of the trench, particularly where the 108-in. pipeline would pass through a sheetpile cofferdam constructed for large water pump installation at the cooling water reservoir intake and discharge locations. Moretrench developed a wellpoint system along both sides of the trench alignment, supplemented by deep wells at isolated locations, to draw the groundwater down below excavation subgrade and ensure dry working conditions.

Of particular importance at these locations was that the intake and discharge structures would not be completed until after the pipelines were laid. Moretrench therefore engineered temporary reinforced steel plate plugs. However, with the pipeline depth at close to 20 ft below the surface of typical Florida sands and less than 200 ft from the cooling water reservoir, both the excavations and the terminations would be subject to very large soil and hydraulic loads, with potentially disastrous implications if a failure occurred. Moretrench consulted with a local dam expert for additional recommendations on the hydraulic stability of the soils between the excavations and the reservoir, and modi ed its dewatering alignment to eliminate any risk.

Production Pipe Work

For the 108-inch pipe, the initial 250 lf of trenching within the power block area was accomplished by a combination of large custom trench boxes and sheetpiles, with the remaining 2000+ ft of pipeline laid by means of an open excavation.

Natural Gas Line Crossing: Where the double-barrel run passed beneath the active gas line, the excavation was sloped, with cross-sectional widths of 90 ft at the top and 30 ft at the bottom and a depth of almost 20 ft. The utility as well as the corridor in this area was laser scanned, with modeling determining that the installed pipeline would cross beneath the gas line with a clearance of 3 ft. Special attention was paid to the actual location of pipe joints and the rigging required to lift the pipe, resulting in a flawless installation operation. Excavated material was used to backfill the trench up to 5 ft below the gas line, with flowable fill placed up to final grade.

High Voltage Transmission Mains Crossing: The powerlines were approximately 50 ft above grade. Laser scanning of both the towers and powerlines in relation to the pipeline alignment enabled Moretrench to develop a detailed earthwork and installation plan in accordance with OSHA clearance requirements for crane work in the vicinity of powerlines. All crane locations, pipe joints and material staging areas were modeled. As a result, a larger crane was positioned at grade to pick up staged pipe sections and feed them to a smaller crane that was able to travel and work safely within the carefully enlarged excavation, well within clearance requirements.

Large Bore pipe Crossing: The 108-in. piping alignment crossed over the top of existing large bore circulation water piping that could not be taken out of operation. The new pipeline therefore needed to be partially above ground surface at this location. The challenge lay in achieving the transition from normal trench subgrade at 20 ft below the surface to approximately 2 ft below grade and back down, utilizing four elbows. Since Florida soils and bedding rock are not stable at a 45° angle, Moretrench developed a very detailed rigging and installation plan to lift and support each of the 80,000 lb. pipe section separate of each other until the bedding could be substantially completed. This required multiple cranes with whip lines and specialty rigging for pipe joint positioning as well as assistance from multiple large excavators. The crossing was achieved safely and without incident.

Installation of the 1,500 lf of 42-in. pressure pipe also presented several obstacles that were overcome in a similar manner.

Teamwork and Safety were Key

The complicated nature of the pipeline installation and the unique challenges it presented were overcome with forethought and thorough pre-planning. Streamlining of all the elements of the contract for the most efficient work plan, while at all times paying the highest level of attention to safety within the active power plant, was carefully coordinated with the General Contractor, the Owner, Engineer, and plant operations personnel. As a result, the work was executed flawlessly.

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