Incorrect Data, No Structural Review... Ulsan Power Plant Collapse Ruled Human Error

Ministry of Land, Infrastructure and Transport and accident investigation committee: Last year's Ulsan Thermal Power Plant demolition collapse
"Client and prime contractor also fell short in construction and project management"

The collapse during demolition work at the Ulsan Thermal Power Plant last year was caused by the construction scope being determined on the basis of incorrect data during the preparatory phase, an investigation found. The structure’s weight was underestimated, and no structural review was conducted. The investigation concluded that the accident was caused by human error, including the failure of the client and prime contractor to fulfill their responsibilities.

The Ministry of Land, Infrastructure and Transport and the accident investigation committee announced the findings on October 30. In November last year, while work was being carried out on Boiler Tower No. 5 at the demolition site for Units 4 through 6 of the Ulsan Thermal Power Plant, the tower collapsed, killing seven people and injuring two.

During the pre-weakening work, the columns were cut beyond the limit of what they could withstand. Pre-weakening is the process of cutting some members of a structure’s columns or beams to reduce its ability to withstand forces, so that it falls in the intended direction when blasted. The committee concluded that the columns could not support the weight above them, causing the steel plates to buckle and fracture (local buckling), after which the entire structure collapsed in a chain reaction.

Heavy machinery is being used to dismantle the boiler tower structure at the site of the Ulsan Thermal Power Plant boiler tower collapse on the 3rd, in an effort to find the last person trapped beneath the debris. Photo provided by Yonhap News.

The columns were weakened because incorrect information about the structure was used in the structural review, alongside several technical shortcomings. The amount of material that could safely be cut away was miscalculated. The steel plates in the columns became thinner at higher levels, but their thickness was measured at only one ground-level location and that measurement was applied uniformly to all the upper columns, leading to an overestimation of their load-bearing capacity. At the first point of collapse, the steel plate was actually 15 mm thick, but the structural review assumed it was 24 mm thick.

Conversely, the weight of the boiler tower was underestimated, leading to an underestimation of the force the columns had to withstand. Its actual weight was 3,526 tons, but the review used 3,006 tons. Part of the column cross-section had been cut away, leaving a thinner section that required an assessment of local buckling, but no such assessment was made. A safety margin should also have been allowed for uncertainties such as damage during cutting, but this was not done either. A detailed structural analysis by the committee using actual information predicted that the structure would fail at a similar location.

Construction and project management were also inadequate. HJ Shipbuilding & Construction, the prime contractor, relied on the specialist contractor to make decisions on the pre-weakening and blasting work and did not thoroughly check whether the final construction drawings matched the conditions at the site. The client, Korea East-West Power, supervised the work directly without a separate construction manager, but failed to systematically oversee the work performed by each party involved. As a result, errors in the data and technical shortcomings were not identified in advance.

The site of the boiler tower collapse at the Ulsan Thermal Power Plant last November. Yonhap News Agency.

As measures to prevent a recurrence, the committee recommended strengthening site investigations when planning demolition work and establishing technical standards. It also said that design and construction management for high-risk demolition projects should be strengthened, along with the roles and responsibilities of clients. “I hope the findings of this investigation will help prevent similar accidents,” said Lee Kyung-gu, chair of the committee. “We will organize and supplement the findings and submit the final report to the Ministry of Land, Infrastructure and Transport next month.”

Meanwhile, the ministry said it would develop measures to improve the demolition work system by combining the findings of this investigation with those of the Seosomun Overpass demolition collapse investigation, which is scheduled to be completed by the end of next month. The findings will be reported to the relevant agencies, which will take measures such as administrative sanctions or investigations.

This content was produced with the assistance of AI translation services.