SSK Soryu's Collision Off Cape Ashizuri : Curse of the Kuroshio?

 

The damaged SSK Soryu at Kochi Port 
9th Feb 2021. Photo : Asahi Shimbun 

Collision At Sea

Collisions at sea involving submarines thankfully do not happen everyday. Even when such unfortunate incidents occur, they tend to take place at remote waters where there are not much media coverage. In any case the need to maintain operational security would ensure very little information whatsoever will be made available to the public on these mishaps.

On 8th Feb 2021, the Japanese attack submarine JS Soryu collided with the Hong Kong registered bulk carrier the Ocean Artemis while it was surfacing during a training deployment in international waters southeast of Cape Ashizuri, Kochi Prefecture. The collision occurred at periscope depth and left the Soryu with extensive damage but fortunately no fatalities.

What was unusual about this incident was the large amount of high resolution images and video footage taken of the damaged submarine after the accident. Once the word was out the Japanese news media even had reporters and cameramen flown out to sea in helicopters to film the stricken submarine, made possible also because of the accident location's proximity to land.

Many theories had been proposed by new agencies and defense web portals as to how and why the accident happened, from poorly trained sonar technicians to equipment failure to thermal layers and even professional negligence. Frequently the opinions of serving or ex-submariners were sought and while they provide a good overview of the matter, none mentioned anything about a local maritime phenomenon that could have played a significant role in causing this incident : the fast moving warm Kuroshio Current that flows northward and eastward off the Pacific coast of Japan.

The Kuroshio Current off the coast of Japan
Here for real time map by earth.nullschool.net

Cape Ashizuri, approximate collision site
and current path of the Kuroshio in red.

The Submarine Soryu

The Soryu is one of the largest and most advanced diesel attack submarine in the world. It is equipped with an air-independent propulsion system. It is also relatively new, being launched as the first-in-class in Dec 2007 and commissioned in March 2009. It has a length of 84m with a displacement of 2900 tonnes surfaced. Its sonar suite comprises of a bow array, flank arrays and a towed array. 

It could make 13 knots on the surface and 20 knots submerged. Its maximum diving depth, though classified, is believed to be between 600m to 800m.

The Soryu is assigned to the 5th Submarine Squadron, 1st Submarine Flotilla operating out of Kure. Her commanding officer is Commander Koji Keisuke ( 恒次啓介 ). The submarine had recently spent considerable time in the ship yard on routine maintenance. She left her home port on 6th Feb 2021 with 90 crew members on a training deployment, most likely for re-certification of her operational readiness.

Soryu-class submarine on surfaced transit.
Photo : JMSDF

The Bulk carrier Ocean Artemis 

The Ocean Artemis is a bulk carrier with a gross tonnage of 51208 tonnes and a dead weight of 93103 tonnes. It was built in 2011. It has an overall length of 229m with a beam of 38m. Its maximum speed is said to be 13.8 knots. It was transporting iron ore from the Chinese port of Qingdao and was heading for the Japanese port of Mizushima, Kurashiki City, Okayama Prefecture when it collided with the Soryu. AIS data suggested that the Ocean Artemis had a draught of 14.9m on the day of the collision.   

Map of Shikoku Island and Kuroshio

The seabed 50km off Shikoku is 
between 1000m to 1500m deep.
Map : Geological Survey of Japan 

Reconstructing The Collision

Based on satellite, oceanographic, meteorological and maritime traffic data, we know that it was a clear but windy day out at sea 50km southeast of Cape Ashizuri on 8th Feb 2021. The wind was gusting at 25 knots from the northwest and whipping up swells with significant wave heights of 1.59m, probably the equivalent of Sea State 4.

The collision site is located over the continental shelf of Shikoku Island and lies over waters about 1000m deep. It straddles not only the main shipping lanes south of Shikoku but also the main path of the warm Kuroshio Current. Despite being winter, the sea surface temperature was about 21C, only dropping to 15C at the 200m depth. The current is strong with a flow speed of about 2 knots and an axis of 070 degrees. 

The Ocean Artemis, laden with ore and riding low in the water, was east bound for the port of Mizushima in the Seto Inland Sea. It would likely be maintaining a course of approximately 060 degrees as she navigates around Shikoku to enter the Inland Sea from the east via the Naruto Straits. 

At 10:58am local time, the Soryu was conducting a surfacing drill when, rising from the deep, she failed to detect the presence of the bulk carrier until she was at periscope depth. It was already too late to take evasive action by the time the surface vessel was seen through the periscope and a collision was inevitable. 

The bow of the bulk carrier struck the starboard side of the submarine's conning tower, denting it and damaging the anechoic tiles covering that area. It also caused severe damage to the starboard hydroplane and the sensor masts, taking out the communications antennae. Three submariners suffered from light injuries like bruises and none required evacuation.

Since the damage occurred mainly on the starboard side of the submarine, it can be deduced that it would have been a glancing blow had the submarine been on a course similar to the bulk carrier or if it were on a reciprocal course. Conversely it would have been a head-on impact had the submarine been maintaining a southerly bearing.

We would later learn that the Ocean Artemis did not report detecting any noise or vibration from the collision and most likely continued with her voyage without even realizing that a collision had taken place.

The loss of communications rendered the Soryu incapable of reporting the incident to the 1st Submarine Flotilla headquarters. Fortunately the Soryu could still move under her own power on the surface and she eventually navigated to an area with cellphone reception. The Ministry of Defense would eventually learn of the accident by 2:20pm. 

Transiting on the surface, the Soryu made it to the port city of Kochi by nightfall. Meanwhile the JCG diverted the Ocean Artemis to Kobe the next day for investigations. There, divers discovered that the impact left multiple scratch marks with black paint residue that presumably matched those from the Soryu on the bulbous bow of the Ocean Artemis. A dent and a 20cm crack corresponding to the point of impact was also found on the bow. This crack had caused some sea water seepage into the vessel.

The black paint residue were collected and had been dispatched to the JCG Research Centre in Tachikawa City, Tokyo, for further analysis and confirmation that they originated from the Soryu.   

Underway enroute to Kochi.
Photo : 5th Regional HQ JCG

Damage to the hydroplane, sensor masts and conning tower 
of the JMSDF submarine Soryu. Photo : Mainichi Shimbun

Moored off Kochi, 9th Feb 2021
Photo : Mainichi Shimbun

Moored off Kochi, 9th Feb 2021
Photo : Mainichi Shimbun

Moored off Kochi, 9th Feb 2021
Photo : Mainichi Shimbun

Moored off Kochi, 9th Feb 2021
Photo : Mainichi Shimbun

The damaged Soryu at Kochi on 9th Feb 2021

Ocean Artemis off Kobe 9th Feb 2021
Photo : Mainichi Shimbun

Divers inspect the bow of the Ocean Artemis
Photo : Asahi Shimbun

Scratch marks on the bow of the Ocean Artemis
Photo : Jiji Press

What Caused The Collision?

Surfacing drills are among the most hazardous submarine operations undertaken during peace time. Even with modern sonar equipment, collisions with surface vessels are always a possibility especially around busy shipping lanes.

Every submarine's forward mounted sonar has blind spots known as the baffles, an area directly behind the submarine where sound detection is not possible. In order to surface safely, a submarine has first to perform a manoeuvre known as clearing the baffles to ensure there are no vessels hiding in the blind spot. It requires the boat to change its course slightly so that the previously hidden areas could now be scanned. Once the baffle is cleared, the submarine will have to execute the ascend to the surface within a specified time of not more than 10 to 15 minutes, any longer and the traffic pattern could have changed.

There is a possibility that the Soryu did not perform a proper baffle clearing or it could have taken too long to ascend after that. Her skipper Cmdr Koji Keisuke had commanded the JS Takashio ( SS-597 ) before assuming command of the Soryu on 15th Oct 2019, so he is not new to his post. However, the Soryu has recently spent considerable yard time on regular maintenance and had just resumed operational training after a long lapse. Perhaps crew proficiency may not be at the peak level and mistakes might be more common.

Clearing the baffle ( sonar blind spot )
Image : Asahi Shimbun

The same trend of thought tells us that the likelihood of the sonar specialists not reporting a surface contact as they should is also possible and could have been made more likely by their long absence from regular training. Having said that, due to the hazardous nature of operating submarines, only the best of the best gets selected to train as submariners, and MSDF personnel have to undergo rigorous training of high standards for their qualification process. So again, professional negligence, though unlikely, cannot be completely ruled out. 

Equipment failure is a remote possibility but Japan's fleet of submarine is young by other navy's standards. Until recently, JMSDF used to retire its submarines after an average of just 20 years! We also know that the Japanese tend to maintain their equipment in tip top condition, and that the Soryu had just completed its regular yard maintenance, so the scenario where there is a catastrophic failure of the sonar system leading to a collision has a low probability. In fact, Chief of the Maritime Staff, Adm Yamamura Hiroshi had already categorically denied any reports of a periscope or sonar malfunction. 

The size and draft of the bulk carrier brings up another possibility to the cause of the accident - the bow-null effect. Simply put, the enormous length of the surface vessel approaching head-on effectively acts as a shield to greatly diminish the acoustic signature from the engine and propellers which are a couple of hundred meters behind the bow. This reduced sound level could have been misinterpreted to be emitted from a source much further away along the same bearing, leading to a potentially disastrous situation. Even veteran submariners have had close shaves due to the bow-null effect. It has also been postulated to be the cause of many collisions between whales and large surface vessels.

Apart from human errors and equipment failures, environmental and geographical factors frequently have a contributory if not causative role to play in such accidents. For submarines, water temperature, depth and to a lesser extend, salinity, will have a huge impact on sound transmission. The existence of shallow thermal layers for example, might create environments with positive sound velocity profile gradients, causing some surface generated sound waves to be deflected upwards. It would result in shadow zones below the layer and make vessel detection from beneath all but impossible. The Kuroshio could have contributed to the creation of such thermal layers.

The Black Tide

The collision took place on a winter's morning in an area known to have high maritime traffic, over the continental shelf extension of Shikoku with depths of approximately 1000m. February is typically the coldest month of the year in the northern hemisphere and being located in the temperate zone one could logically expect the sea surface temperature around Japanese waters to be cold as well, causing thermal layers commonly encountered in warmer months to be greatly diminished or even dissipate altogether. 

That would well be the case if not for the powerful Kuroshio Current which brings warm water from the tropics to the Japanese archipelago all year round. Kuroshio ( 黒潮 ) means black tide in Japanese. It is named because of its nutrient poor, deep blue, clear waters. It is one of the world's major oceanic currents comparable to the Gulf Stream of the Atlantic. The impact of this warm current on Japanese climate and maritime related industries is so great that the Japan Coast Guard monitors its activities constantly and issues daily reports.

JCG Quick Bulletin of Ocean Conditions
Path of Kuroshio & Tsushima Currents
Flow speed is 2 knots south of Cape
Ashizuri on 8th Feb 2021

Sea Surface Temperature off Cape
Ashizuri is about 21C on 8th Feb 2021

Because of the Kuroshio Current, the sea surface temperature south of Cape Ashizuri where the collision occurred remains at 20C to 22C in winter and between 26C to 28C in summer. So even in winter a significant layer of warmer water will exist above the colder water deeper below. How deep is warm layer gets depends on factors such as the current's flow rate, mixing from colder layers below and even eddies that form when the current is deflected by geological features or when it meets another ocean current, like the southward flowing cold Oyashio current from the Kuril Islands. 

It is a well known fact that the salinity and water temperatures can vary considerably especially at the boundaries of the tide, making navigation with sonar extremely tricky. To complicate matters, the path of the black tide is not always fixed and can sometimes shift quite drastically in what is known as the " Great Meandering " ( 黒潮大蛇行 Kuroshio Daidako ) especially off the Kii Peninsula east of Shikoku Island.

Sea Surface Temperature 20.6C
Flow Direction 070
Flow Speed 0.94m/s ( 1.88 knots )

Significant Wave Height 1.59m
Peak Wave Period 5.2s
Direction 305 deg

Surface Wind 25.5 knots
Direction 310 deg

The waters around the collision site is 
approximately 1000m deep.
Bathymetry from Japan's Submission to
the Commission on the Limits of the
Continental Shelf.

Maritime traffic density 2012
Image : shipmap.org

The Aftermath

The JMSDF has not been all forth coming with information on the collision. There were no mention of the incident on its website and social media accounts. The Minster for Defense and the Chief Cabinet Secretary did have press conferences and the MOD had a press release, but that was it. Some of the images and videos came from the JCG but mostly they were from the news media. Investigations are ongoing and hopefully the findings will be released soon. 

Since the Kuroshio Current dominates over almost the entire Pacific coast of Japan, I am convinced that the collision has something to do with it, barring other factors such as human error. Its high flow volume of very warm water creates a perpetual summer-like condition to Kyushu, Shikoku and Western Japan extending eastwards to the Kanto region at times. Its ever changing flow axis with major deviations and meandering and its multitude of eddies and countercurrents where there are deflections by land masses and confluences with other currents means that it is as unpredictable as can be, creating treacherous acoustic environments for submarine navigation. 

As usual, most accidents are caused by cumulative errors committed by all parties involved, but sometimes it can boil down to just pure bad luck, like being at the wrong place at the wrong time. The Soryu unfortunately has quite a long history of mishaps .... from electrocution and burns at the shipyard to attempted suicide and the accidental drowning of three petty officers within the flooded areas of the sail during routine operations in 2012. 

At the time of writing, the Soryu is back in Kobe where the Mitsubishi Heavy Industries Shipyard is located, presumably for damage assessment and repairs. Even though her pressure hull wasn't damaged, it could still possibly be out of active duty for several months and the repairs could cost several tens of millions of dollars.

Cmdr Koji will likely be relieved of his command regardless of the cause of the collision and the outcome of the investigations. His career as a sub-driver or even his future in the JMSDF could already be jeopardised. 

Meanwhile, the MOD decided that a 3.5 hour delay in incident reporting due to communications failure is utterly unacceptable and the JMSDF has pledged to ensure the availability of satellite phones on their vessels and boats.

Cmdr Koji Keisuke
Photo : JMSDF

Foot Note : Cape Ashizuri

It would not be right to end this article without elaborating on Cape Ashizuri. Known in Japanese as Ashizuri Misaki ( 足摺岬 ), this is the southernmost point of Shikoku Island. It is located in the remote western part of Kochi Prefecture where the pristine Shimanto River lies, with rugged coasts and beautiful night skies. 

Cape Ashizuri Lighthouse
Photo : JNTO

I had visited Cape Ashizuri in Feb 2017, after completing the Kochi Ryoma Marathon. Accessibility was an issue as the rail network did not extend to that region and even driving along the winding coastal roads was tedious and time consuming. But the effort was well worth it.

The Cape lies within the Ashuzuri-Uwakai National Park and it has majestic sea cliffs that towers above the vast expense of the Pacific Ocean. I remembered that it was extremely windy when I visited the Cape Ashizuri Lighthouse and walked through the tunnel of camellia which was in full bloom.

Cape Ashizuri is the southernmost
point of Shikoku Island. Statue
of John Manjiro in the Background.
Photo : Author

Another attraction at Cape Ashizuri is the statue of John Manjiro ( 中浜万次郎 ), a Japanese whose remarkable story I first learnt only during that visit. Manjiro had hailed from Nakanohama, an area known today as Tosashimizu, not far from Cape Ashizuri. 

Manjiro came from a poor family and lost his father at a young age. In 1841 when Manjiro was fourteen years old, he left home to work on a fishing vessel to help make ends meet. Unfortunately, due to inclement weather, he was shipwrecked on an uninhabited island off the coast of Japan for six months. Manjiro was eventually saved by the captain of a passing American whaler who named him John and brought him back to his hometown of Fairhaven, Massachusetts, so that he could be educated. 

Statue of Nakahama John Manjiro
at Cape Ashizuri. Photo : Wikipedia

Manjiro stayed in America for ten years, learning English and the science of navigation among other things, becoming the first Japanese to achieve long term residency in America. All that during an era when the ordinary Japanese were forbidden to travel or interact with foreigners! He would later join a whaling ship and work his way up to become the first mate and even participated in the California Gold Rush, earning enough money to fund his journey back to Japan. 

Upon his return, instead of meting punishment for violating the travel ban, the Shogunate government found his knowledge in English and foreign ways valuable and made him an advisor. He was also the translator for Japan's first trans-Pacific delegation to San Francisco and was appointed a lecturer at the Kaisei Institute dedicated to the teaching of foreign languages, science and advanced western technologies, paving the way for the Meiji Restoration and the industrialization of Japan which followed shortly. 

John Manjiro's story is worthy of an article all by itself but fortunately all the details can be found here.