Case study in T&D construction: Koolau-Pukele structures replacement project

By Clinton Y. Char, Hawaiian Electric Co. & Jerry L. Lembke, Valmont

Nov. 26, 2002 — One of the key substations on Hawaiian Electric Co.’s (HECO) grid in Oahu is the Pukele Substation. This substation services approximately 51 percent of the total load on the island, including the key business and vacation area of Waikiki and the densely populated area of east Oahu. The Pukele Substation is serviced by two parallel 138kV transmission lines-the Koolau-Pukele 138kV #1 line (KP #1) and the Koolau-Pukele 138kV #2 line (KP #2).

The two most critical structures in this section of these lines are located on the top of the ridge: structure #9 on the Koolau-Pukele 138kV #1 line and structure #2 on the Koolau-Pukele 138kV line #2. The cumulative effects of both normal and severe weather in their 40-plus years of service caused some deterioration of the strength carrying capabilities of structures #2 and #9.

An inspection of structures #2 and #9 in 1998 revealed that both were showing signs of distress. As a result of this inspection, and because of the importance of these two lines, a tower replacement program was quickly initiated. Since these two lines are the only two lines servicing the Pukele substation, one line would have to remain in service at all times.

Design and engineering

Due to easement limitations, the new structures would need to be constructed and erected in the same location as the existing structures. Plus, the structures would need to be capable of being erected by a helicopter with a maximum lift capacity of 3,500 pounds.

Also, since these structures were located on State of Hawaii conservation land, HECO needed to obtain State of Hawaii, Department of Land and Natural Resources (DLNR) approval for the project, which required that HECO perform a botanical survey of the project site to determine if any endangered floral species existed in the area and design the structure to minimize the aesthetic impacts to the ridge area. Because the structures are situated on a ridgeline where they are highly visible from many areas of Honolulu, aesthetics was a key consideration in the project. HECO made a conscious effort to try to provide an aesthetically pleasing design.

The individual members of the H-frame were kept as slender as practical. Also, structure color was selected with the assistance of a color consultant. The consultant viewed the project site from a variety of locations under differing light conditions and helped develop paint samples for field-testing. The result of this effort was the selection of a paint color that helped blend the structure into the background colors under most light conditions.

Once the DLNR requirements were met, the botanical report and the paint sample were submitted to the DLNR where approval was given. Surveyors were subsequently flown to the project site to survey and develop a topographic map of the area. Upon completion of the maps, the surveyors sent the data to the HECO engineers, who proceeded to develop a 3-D PLS-CADD model of the line.

In the next phase of the project, HECO engineers met with engineers of its alliance partner, Valmont Industries, to discuss the project and the proposed construction methodology. HECO and Valmont engineers worked closely with HECO construction people to identify the specific assembly plans and added facilities to the design to increase safety during construction.

In the field

Erecting the structures was a unique challenge for the HECO Construction crews. The ridgeline where the structures are located is typically at or above the cloud level. At this elevation, the weather conditions are highly variable. The site is frequently clouded over with winds in the range of 15 to 30 knots. Because all men and equipment had to be flown up to the site, these highly variable site conditions made each day a challenge.

Foundation construction commenced approximately 6 weeks prior to the arrival of the structures. Due to the remote location, the excavations for these foundations had to be hand dug with jackhammers. The soil at the site was solid rock; therefore, the excavation was exceedingly slow with progress averaging approximately one foot per day.

Once the foundation excavations were completed, the reinforcing steel and anchor bolt cages were flown up and installed. The subsequent concrete pour involved staging concrete trucks at the base of the mountain. The concrete trucks poured the concrete into a 3/4-yard hopper, which was suspended below a helicopter. When the hopper was filled, the helicopter flew the concrete to the structure site where it was placed in the foundation. Due to the size of the foundation and the limited amount of concrete that could be transported at one time, the concrete pouring operation took approximately five hours for each foundation.

The erection of the structure proceeded as planned. Highly skilled helicopter pilots were used during the structure erection phase. The structures consisted of several sections, which had to be bolted together. The HECO crews would take a section and connect it with the partially completed structure on the ridge. Precise flying was required in difficult conditions to properly align the sections and match the bolt holes of the flanges.

After the vertical legs of a structure were completely assembled, the shield wires were transferred from the old structure to the tops of these legs. This took the load off of the top portion of the lattice tower and allowed for the removal of the lattice tower top section. Next the phase conductor cross arm was lifted and installed, along with the shield wire arm.

Once both arms were in place, the shield wires were transferred from the top of the vertical legs to positions on the shield wire arm. The 138kV conductors were subsequently transferred from the old structure to the new 138kV arm. After the wire transfers were accomplished the remainder of the old structure was removed and the new towers were officially in place.

Char is a civil/structural engineer with HECO. He can be reached at cchar@hei.com.

Lembke is a structural engineer with Valmont Industries. He can be reached at jll@valmont.com.

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