During reassembly of an upgraded turbine in the 41.7-MW Powerhouse 2 at Puget Sound Energy’s 69.7-MW Snoqualmie Falls project, the crane cable detached from the winch drum. The cable and a 150-pound draft tube liner segment dropped onto the work platform, where four PSE employees were working. Fortunately, the crew suffered only minor cuts and bruises during this incident, which occurred in July 2004.
PSE reviewed the incident and determined that the cause was a combination of improper fittings on the cable ends and the crane hook being extended beyond its lower limit. For safe crane operation, two wraps of cable are required to remain on the winch drum during operation. The operator did not realize there were fewer than two wraps of cable left on the drum, which put undue tension on the cable ends. The cable ends were compromised, resulting in the release of the cable from the winch drum.
To remedy the situation, Puget Sound Energy replaced the cable end fittings and painted the cable yellow from the point where only five wraps remain on the drum and red from the point where only three wraps remain. The latter measure allows the crane operator to instantly see when the hook is reaching its operating limit and disconnect the power source for the winch motor.
The crane incident
During reassembly of a turbine after an upgrade in July 2004, a four-man PSE crew was beginning to trial-fit the first of the draft tube liner’s three transition segments. The four men were positioned around the inner edge of the draft tube to receive a 150-pound liner segment. This segment was being lowered into position via the powerhouse’s bridge crane auxiliary 20-ton hook with a shackle and 6-foot lifting sling.
The crane operator lifted the segment over the top of the generator stator housing and began lowering the segment through the center of the machine, which was empty because the rotor and turbine had been removed. When the segment was about 1 inch from its final position, the two crew members who were touching the draft tube insert section felt it begin to fall to the work platform, about 4 feet below. All four crew members heard a loud whirring sound, after which the auxiliary hook fell about 10 feet to the work platform, followed by the cable.
Thankfully, the members of the installation crew immediately pressed themselves against the inside surface of the draft tube to avoid the falling objects. Due to their quick thinking, the crew members received only minor cuts and bruises.
All four crew members exited the area through the draft tube access door. Other crew members activated the emergency response system, which resulted in dispatch of an ambulance and the police. After the emergency medical technicians assessed each of the four crew members, PSE held a post-accident tailgate meeting to determine what the parties involved had observed.
Investigating the cause
The crane in Powerhouse 2 is a Shepard Niles double girder top-running bridge crane. The crane and a single 50-ton main hook were installed in 1910. In 1955, PSE added a second, 20-ton hook and trolley supplied by the crane manufacturer. PSE replaced the 7/8- and 5/8-inch-diameter cables for both the main and auxiliary lifting hooks in 1988.
The crane undergoes annual inspections. In addition, formal load tests are standard procedure before any critical work is performed. In June 2004, before disassembling the turbine unit, the crane was load-tested by an independent crane consultant.
The day after the incident, representatives from PSE, the Washington Department of Labor and Industries, the original load-testing consultant, and a second crane consulting company gathered at the powerhouse to conduct an investigation.
The group’s inspection revealed that the auxiliary trolley, winch-drum brake mechanism, and auxiliary lifting hook were in suitable condition.
The investigation then focused on the lifting cable and its termination within the lifting drum. An inspection revealed that the ends of the lifting cable were unraveled, with tool marks suggesting the use of a wedge-type ferrule. A search of the powerhouse located the wedge-type ferrules that were used to secure the cable ends inside the winch drum.
The group then measured the length of the auxiliary lifting cable, to verify the lowest elevation to which the auxiliary hook could be lowered while still maintaining a minimum of two complete wraps of cable on the winch drum. This measurement revealed that the auxiliary hook’s elevation was past the minimum hook elevation at the time of the incident. With less than two wraps of cable on the drum, the tension resulting from the combined weight of the suspended load was increasingly transferred directly to the cable terminations within the winch drum.
Overall, the assessment revealed that the following factors contributed to the accident:
– The auxiliary hook was extended beyond the lower operating limit authorized by the Occupational Safety and Health Administration (OSHA), which requires a minimum of two wraps of cable on the winch drum.
– Wedge-type ferrules were used as the cable terminations within the winch drum. These fittings require constant cable tension to remain tight. Over the lifetime of this cable, the tension may have relaxed during the cycles of normal crane operation, compromising the wedge ferrules.
– The failure of the wedge ferrules resulted in the uncontrolled release of the cable. The loud whirring sound heard during the accident resulted as the cable unspooled/stripped off the winch drum and through the associated pulleys.
– The cable terminations within the auxiliary winch drum are not accessible with the trolley fully assembled. Thus, although previous annual inspections focused on the overall condition of the bridge crane, the cable terminations were not reviewed during these inspections.
Developing a solution
As a result of this specific incident and the subsequent investigation, PSE made several improvements to the auxiliary hoist equipment at the Snoqualmie Falls plant. These include:
– Placing swaged end fittings on the ends of the new crane cable. Swaged end fittings are hydraulically compressed onto the cable, thus forming a very strong cable end termination. The swaged ends of the cable were then secured inside the winch drum in a manner that restored the cable-to-drum connection to the manufacturer’s design.
– Using crane installation professionals to install the new cable on the auxiliary hoist drum and pass it through all the pulleys on the auxiliary hook load block, rather than PSE personnel.
– Installing additional lighting on the auxiliary trolley to better illuminate the number of wraps on the hoist drum.
– Placing mechanical-limit switches on both sides of the winch drum to trip and automatically disconnect the motor’s power source if the cable reaches the two-wrap minimum.
– Painting the cable “safety yellow” starting at the point where five wraps remain on the hoist drum, then switching to “safety red” for the last three wraps of cable. The yellow and red cable sections are visible to the crane operator and provide a visual warning that the auxiliary hook is reaching the end of its operating limit.
To prevent a similar failure situation occurring at any of its other hydro projects, PSE immediately conducted inspections of the cable terminations on all bridge cranes. One crane was found to have similar wedge-type ferrules for the end fittings, and this crane was immediately outfitted with new cable and swaged fittings by crane professionals. Furthermore, PSE has implemented a site- and crane-specific program to review crane safety, including improved lighting of the winch drum, installing mechanical limit switches, and painting the crane cable.
Changing policies and practices
As a result of the incident and the findings of the investigation, PSE’s policy now mandates that cable terminations within the winch drum be included in its annual crane inspections. Additionally, the findings from this crane mishap were incorporated into PSE’s crane safety protocols and training procedures. PSE’s revised training emphasizes crane operating limits as well as the operating procedures specific to each crane.
– By Edward Cassady, P.E., license implementation manager, Puget Sound Energy