Operation Lisbon Subway

OPERATION LISBON SUBWAY- Mixing Technical Diving and Commercial Diving Techniques

By João Neves

There was a time when only commercial diving existed. Then, came military diving and rebreathers. They learned for a long time from each other. Then, open circuit scuba was introduced and recreational diving was born. The three types lived fairly apart for a while, following parallel paths, sorting out different procedures, until recreational diving started learning mixed gas from the other two. From that learning it went on to adapt and develop its very own techniques and a new breed appeared, technical diving. These days, commercial and military diving are starting to learn from technical diving. Is there a new species of diving about to be born? The following is an example of how hybrid techniques are being used a bit here and there, around the world.

Note: Due to being a sensitive event in the eyes of the authorities, a 15-year non-disclosure agreement was imposed on the dive team, thus the delay in disclosing the operation to the public.

Going home from work, seated on the subway train, have you ever wondered what it would be like to dive the miles-long tunnels under the city? Well, I have dreamed about it on occasions. Only I could not imagine I would actually come to do it one day. Here is the whole story of how that became possible.

The incident

On the morning of the 9th of June 2000, a team of workers were drilling bore holes, 15 centimeters in diameter, through the bottom of the concrete wall of a newly built subway tunnel in downtown Lisbon, Portugal. Here a 600-meter long section of tunnel runs under the soft sediment bottom of the Tagus River. The boreholes would allow the high-pressure injection of cement to consolidate the ground around the tunnel where a station was to be built over the next few months. Unlike the other holes, the last one drilled that morning was leaking a tiny amount of water when the team left for the surface on their lunch break. Small leakage is likely to occur in such underground construction sites so the workers did not give it much thought. The return, one hour later, would greet them with a chilling surprise. The leaking borehole was now a huge geyser on top of a growing cone of sediment. The team rushed to plug the hole but soon other holes were leaking too. They had no more plugs to control the flood and by the time they become available, water was rushing in through the joints of the wall structure ring elements as well. One could have used the scene to shoot footage for a sinking WW II U-Boat movie. The nightmare had begun. Once the surface was informed, scores of workers with heavy equipment entered the tunnel but it was too late. Engineers could not believe their eyes, witnessing the destruction of the tunnel they worked so hard to build, frustrated and powerless to stop the flood. The incoming water was carrying tons of sediment from the outside, leaving a water filled chamber behind the tunnel north wall. The tunnel was becoming unstable from losing the outside ground support and was soon developing cracks. The order came to abandon it for fear of impending collapse. Everybody ran for their lives. The first part of the nightmare ended when everybody safely returned to the surface. Sometime later that day, the underground void left by the sediment entering the tunnel caved in, leaving a huge crater on the roads and sidewalks of the river side. No one knew what could have happened to the tunnel itself 27m underneath.

Other problems were then called to everybody’s attention. This 3km long subway branch is “u” shaped, the accident location being at the deepest portion, 500m from on e end. In order to prevent any more sediment from being transferred to the inside of the tunnel and further collapsing, endangering the historical buildings nearby, the water ingress had to be stopped. This could only be achieved by totally flooding the tunnel, thus reducing the hydrostatic gradient. This, however, posed two major problems. At one end, was a dead end, where the tunneling machine worth millions lie, waiting to be taken apart and winched back to surface to be used elsewhere. They could not allow it to flood. The other end of the tunnel connects to the busy Lisbon subway network, used by hundreds of thousands of people each day, and they definitely could not allow it to get flooded either! Powerful water pumps were installed to slow down the rising water and buy some time. Luckily, there was an access well between the accident area and the tunneling machine that allowed the pouring of concrete to create a dam. On the other end, on another well, 500m from the accident area, a concrete wall was raised, leaving however a flanged door in it for future access to the tunnel. The tunnel was then flooded by pumping river water in. At the surface, over and around the collapsed crater, sensor instruments were installed to monitor ground movement, and after a few days the readings showed it had stabilized.

ROV vs. Divers

One could again take a deep breath and pause to think things over. Now what? Before taking any action, engineers needed to know exactly how much damage the tunnel hadBack stage of operation lisbon suffered. Pumping it dry for inspection was out of the question. The inspection would have to be done underwater.

Two options were considered: ROV or divers. Now, let’s just evaluate what the operation was all about. They had a flooded 8m diameter tunnel full of debris and abandoned machinery, piled rails, coiled wires, wood lumbers, tools, dumper trucks, you name it – they had it in there. The accident zone was 500m from the only possible entrance, and at a depth of 28m from the surface with the real possibility it had, or was about to, collapse. Water had a visibility of about 40cm at best, contaminated by sewer seepage and alkaline with a pH of around 10.

“You said divers? No way, we’ll try the goddamned machine first”. If you didn’t know much about ROV’s you too would have gone for the ROV option first. The engineers in charge felt inclined to choose this approach because it meant
no risk for human lives, but they left an open door for the divers, just in case…

A five-day ROV operation was scheduled and an inspection class vehicle brought in, complete with navigation and profiling sonar, inertia and Doppler positioning and low light cameras. Did any of it work? You guessed it! No! After 15 days of a long list of “unfortunate” equipment and telemetry failures, repeatedly stuck umbilical occurrences, spiced with illegal, irresponsible and suicidal covert retrieval by divers of opportunity, unaware of the dangers involved, the ROV operation was ordered to be stopped, just in time – before having to deal with a serious accident. Solution to the new situation, go to option 2: formal diving operations.

Commercial or Technical?

Commercial diving operations in this type of scenario do not come around every day. In fact, you can probably count with fingers on one hand those that actually took place on a
worldwide basis. Commercial diving techniques rely, as a rule, on the use of a surface supply umbilical for gas and real time communications. In this case that would be nearly impossible to do, not with a 500m long umbilical. Usually, commercial divers do not have the training required to safely handle 500m penetrations on scuba in a very hostile overhead environment such as this one either. That is why many commercial diving companies declined to do the job after carefully considering the risks. The right techniques can only be found in cave diving, developed in hundreds of thousands of dives and refined by careful accident analyses.

Being a cave diver myself, with commercial dive training, I was summoned to a meeting of the “engineers crises committee” to give a cave diver’s expert opinion about the feasibility of such a manned diving operation. From a procedural point of view it could be done but it certainly involved a fair amount of risk. This could not be a couple of cave divers operation. It required being done by a well-prepared well-equipped commercial diving company and plenty of resources. I was assigned the task of finding one fit for the job. That was not difficult, in fact the managing director of AtlanticoSub, a well established Portuguese commercial diving contractor, the very one I would choose anyway, was standing right beside me at that very moment. Right there we decided on the joint venture. AtlanticoSub would provide the means and logistics and I would train the divers and supervise the whole operation. Operation Lisbon
Subway was on.

Operation Lisbon Subway hells gateThe Operation Logistics

Getting such an urgent operation running is no easy job. In just two weeks you have to plan every detail, design, and build custom equipment, make choices on equipment to buy, adapt existing equipment, select the best divers and train them in cave diving. Training was done following the TDI Cave Diving course and practice dives were conducted in three different caves, under conditions as similar as possible to those foreseen to exist in the tunnel. Only when you have excellent divers can you achieve this task in such a limited time frame, and excellent divers they were.the Bell

When mobilization day came, it took only three days to set everything up on site. On the main access well, a 1600mm diameter, “L”shaped steel tube was mated to the flanged door previously built into the concrete dam wall. The steel tube stretched to surface level and constituted the only entrance to the tunnel. We named this “hell’s gate”. As the water only reached half way to the top, a custom built, half circle section, two stage wet bell was introduced in the tube suspended from a 30 ton crane, and used as a safe means of lowering and retrieving divers from water level, 12m below. The wet bell, that traveled all the way down to the flanged door, also served as staged deco and as a safe and comfortable refuge for the safety diver (bellman) on his long waits for the push team to return. The bell was equipped with an umbilical for pressurization, emergency gas supply, communications and video. A separate basket, spare winch, and power supply allowed for diver retrieval in case of main bell malfunction or power failure.The diving operation gas stock included three racks of sixteen 50L, 200bar tanks of Nitrox40 used for staged decompression and bell operation. Two similar racks of medical grade oxygen were used for partial pressure blending of nitrox and chamber BIBS supply. Hyperbaric chamber pressurization would be done with one rack of air. A large number of 50L, 200bar tanks of EAN50 premix would allow mixing nitrox in the diving tanks. EAN40 and EAN36 were the most used for diving. Two TDI certified gas blender/service technicians and two HP compressors worked full time to keep diving tanks filled and regulators serviced and tuned, ready to be used the next day. During each bell run, an on-site twin lock hyperbaric chamber and mixingcrew were kept ready. An ambulance stationed just next to the chamber would transport any emergency, not treatable on site, to the Portuguese Navy Hospital Hyperbaric Center, 30 minutes away.

Due to the poor quality of the tunnel water, chemical and biological analyses had to be performed and each possible action was taken accordingly, including vaccines and drugs, to assure divers stayed in good health. After surfacing, divers would plunge into a decontamination pool and equipment was scrubbed with a disinfectant. A doctor also checked divers on a regular basis for signs of ear infection and other contaminated water related problems.

Last but not least, the control room featured the bell gas and control panel, communications, video, other instrumentation, computers, etc. All bell runs and communications were videotaped.

All aspects related to diver safety and comfort were addressed in order to keep the best possible operational efficiency. Due to summer high temperatures, divers were sprinkled with a water hose from the moment they donned their drysuits to the moment of lowering the bell. The bell itself had to be constantly sprinkled to keep it from broiling the divers until submerged. It got so hot at times that operations had to be shifted to night hours to avoid the sunlight. A catering service permitted serving meals and beverages without having to leave the site. This kept everyone on hand and focused on his duties at all times.

The Diving

Diving was exciting but not exactly fun. Each bell run took down two push divers and a support diver, for a bottom time of 80 to 90 minutes. Three to four bell runs were made per day with different teams. The support diver would help the divers out the bell, across the flanged door and see them off. He would then return to the bell and report to surface control. Then he would switch to the bell surface supply regulator, to save his gas for emergency intervention use if needed.

Rinsing bellmanBecause of the long duration expected for the operation and number of dives involved, the plan included staging 15L tanks every 25 meters along the guideline for safety. The push divers would usually each take a twin 15L 250bar back-mounted, and two side-mounted stage tanks, to be left at their assigned locations. The rule of thirds was used, staged tanks being strictly intended for very last resort.

The line was marked with plastic arrows displaying direction and distance to tunnel exit. For inspection away from the main line, divers tied off separate reels, following conventional cave diving procedures.

The routine went on until, around 450m into the tunnel, significant cracking was reported along the walls. We knew the accident zone was nearing. The divers started a video survey of each ring element of the walls. Each ring had a number painted near the tunnel floor that allowed the engineers, back at the surface, to identify them in the construction plans. However, the sediment that had entered the tunnel soon covered the numbers, making it very difficult for the divers to identify each ring. From that point on, ring identification would be achieved using a sloter locator, a VLF transmitter/receiver developed by a friend and used by us in cave diving survey. The transmitter would be placed under the ring to be identified, switched on, and the receiver antenna walked at the surface until signal would indicate the transmitter vertical was reached. A conventional surface survey would place the spot on the tunnel plan and the ring number was identified.

Ground Zero

Things got worse with every meter surveyed. By the time the 500 meter mark was reached, half of the tunnel section was filled with sediment. At “ground zero”, the ring elements showed heavy damage and some were about to fall off. The first diver to get there said, “I would hold my breath every time I passed under a hanging ring element. I feared the exhaust bubbles would shake one loose and the whole tunnel would collapse on me.” Another diver reported, ”It was scary. You could actually see some of the protruding ring elements vibrating, probably from the traffic passing in the vicinity. On some, only a couple of screws were preventing them falling. The screws were under a tremendous strain, stretched to a fraction of the original diameter.” City traffic was then diverted from the surrounding area and all possible measures taken to prevent vibration from being transmitted to the tunnel structure. Time spent at “ground zero” was kept to the very minimum necessary.

going-downDue to daily exposure to potentially dangerous situations, divers had to deal with a considerable degree of stress. However, morale was high and the team proved to be top class. This is an invaluable asset to have in any such diving operation. When the video survey was completed, everybody felt a sense of relief and achievement. But it was not over yet. All equipment had still to be removed from the tunnel. The operation went through the phase where relaxation can become dangerous and accidents may happen. That is when safety rules have to be reinforced and agreed procedures strictly adhered to. But in the end it all went smooth.

The diving operation had a duration of 5 weeks. A total of 302 man dives were performed without a single incident. The data collected permitted engineers to understand what happened and decide on the corrective actions that would save the project.

In this example of a commercial diving operation, the combination of standard commercial diving techniques and technical diving procedures proved to be possible to implement in a safe and successful way. Conversely, the same blend of techniques have been used in technical diving operations, to the point where one has some difficulties in classifying them as commercial or technical type. There could be an entirely new type of diving being bred out there.

João Neves is a long time cave diver with many explored caves in Portugal and other European countries as well as in Mexico. He is a certified HSE commercial diver, hyperbaric chamber technician, ROV pilot, TDI Trimix Instructor Trainer, Cave Diving Instructor, Rebreather Instructor Trainer and head of the TDI Portugal HQ. He is also owner and managing director of Submate, Lda. , a diving equipment supplier company, which distributes Poseidon, and DiveRite, among other brands.

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1 reply
  1. Will
    Will says:

    Wow. Just wow. I’m fourteen, doing open water in a month and hoping to be ready to do Helitrox and deco procedures when I turn eighteen. Badasses like you are the reason I dive. One question though, where you taking stage tanks and attaching them to your harness, classic tech style for quicker access if shit really hit the fan, as well as clipping them to the guide wire, or where you strictly staging them along the tunnel, for use in emergency. Regular dive planning tables/charts for 21% oxygen give me a headache, so props to you for planning this. Must have been fucking terrifying with all that wreckage, cause a piece of the reinforcement could pierce your suit.

    On a side note, please tell me that the divers who went in to get the ROV where not being as stupid as that sentence makes it sound?


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