The weekend read: Don’t throw caution to the wind

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From pv magazine 02/2020

In the wake of Japan’s last typhoon season, floating PV companies are best advised to walk across a stone bridge after hitting it. It’s a tongue-in-cheek Japanese proverb to mock cowardly or overly cautious behavior, but with a double meaning, because it also has the positive connotation of erring on the side of caution.

The night of Sept. 9, 2019, saw typhoon No. 15 sweep through Japan’s Chiba prefecture. And the country’s largest floating PV installation – Kyocera TCL Solar’s 13.7 MW Yamakura Dam array near Ichihara City – was in its path.

The storm came in at an average wind speed of 41 m/s, with the nearby Chiba weather station measuring top wind gusts of 57.5 m/s, or 207 km/h. The storm triggered a series of events at Yamakura Dam that resulted in two-thirds of the plant being destroyed and parts of it catching fire.

As with any civil engineering structure, floating PV plants must comply with regional codes prescribing design wind loads. In Japan, JIS C 8955 sets the code for all PV installations, whether they are floating or not, and for the region where sites are located, the code demands a minimum wind speed of 38 m/s that must be sustained. Despite nature throwing quite a lot more at the site, owners, designers, and regulators are currently investigating the exact failure mode, to prevent the recurrence of such events, as typhoon No. 15 was not the strongest storm ever recorded in Japan.

In fact, only two weeks later, typhoon No. 17 hit the Kyushu region at an average wind speed of 40 m/s, destroying the 2.4 MW Shintaku Tameike floating PV installation, among other damage.

What we know

METI has published a preliminary investigation into the Yamakura Dam incident, with a final report announced for release in the first quarter of this year.

Hitherto, the ministry has concluded that the leading cause of the issue was anchor failure. The floating island was held in place by 420 anchors, which connected to a total of 823 mooring lines. During the storm, seven anchors came loose on the central southern end. The exact reason why the anchors came loose is still under investigation, according to floating racking supplier Ciel et Terre.

Meanwhile, in its preliminary report, METI investigated a possible connection between water level and mooring wire tension. On the day of the typhoon, 108 mm of rain per day, or 28.5 mm, was pouring down. The maximum water level of the lake is 37.5m, and on Sept. 9, the water level was measured at 37.3m. “The [mooring] lines are installed under tension at high water level, while usually water level does not reach high water level, the lines have slack,” a Ciel et Terre representative told pv magazine. The tension on the lines allowed the forces from the oscillating movement caused by wind and waves to dissipate to the anchors more directly than if there was slack. However, METI did not draw a conclusion on the matter.

The shape of the plant also played a role. Anchors and mooring are attached around the perimeter of the island, where mooring wires connect only to the outermost floats. The floats behind the fixed row are attached with resin bolts. Each row picks up wind forces, which accumulate into the first bolt.

The Yamakura Dam array had 112 anchors on the northern end, 107 on the western side, and 133 on the eastern edge, but only 68 anchors on the southern perimeter, where the anchors failed. When asked why that happened, a Kyocera spokesperson said that “one of the reasons is that typhoon No. 15 was the strongest class of typhoon ever to hit the Kanto region. As such, it exceeded the standard technical assumptions set out when the site was designed.” Nonetheless, the investigation into the exact cause of the anchor failure is still underway.

Bolts and uplift

After the anchors failed, the resin bolts started to collapse, because the wind loads were now dissipated more unevenly. After each collapsed bolt, the loads on the adjacent ones increased, causing a chain reaction. In this way, the array was ripped into three parts. The southern parts remained in their normal position, while the northern half was blown away.

The Shintaku Tameike floating PV station was destroyed in a similar fashion. Winds collapsed the bolts, leading to one part of the installation being blown to the shoreline.

The outermost floaters of floating PV installations are usually ballasted with water, to prevent uplift in windy conditions – a phenomenon that has been observed in the past. When the array was ripped apart, the wind-facing edge did not have any ballasted floats anymore. The installation then began to curl up, and mangled modules and other equipment started to short circuit, causing the electrical fire.

Ciel et Terre has rolled out 350 MW of floating capacity across 30 countries, with 150 MW deployed in Japan. The company’s spokesperson said that it is already developing countermeasures to prevent such incidents from happening again. The company will assess the degree to which retrofits in other installations might be necessary.

Kyocera said that “17 [floating PV] plants are in operation […] Possible revamping of other plants shall be determined based on the recurrence prevention measures established after completion of the investigation of the cause of the incident at the Yamakura Dam.”

METI’s report touched upon another aspect in this case – there is no floating-specific structural code. On the water, wind speeds can increase compared to land, because water, especially lakes, cause very little resistance. In typhoon No. 15 and No. 17, wind speeds exceeded design wind loads only marginally. Over lakes, however, the wind speed was probably well above the 41 m/s measured on land.

Until the causes and effects of wind on floating PV installations are known in greater detail, floating system suppliers are best advised to err on the side of caution. Ciel et Terre says it will hit some stone bridges before walking on them, as it “has increased the safety margin for all current and future designs.”