Guadalcanal Deep Wreck Mixed Gas Project

The mission objective was to be the first free swimming divers to dive the American cruiser USS Atlanta, sunk in one of the many savage naval battles that raged around Guadalcanal in 1942. Although we were diving with Dive Solomon's, we planned on using Island Dive Services' new submersible vehicle that was just beginning operations in Honiara to both locate and do a dry reconnaissance of the Atlanta prior to our wet dives. The dive team consisted of Rob Cason, the IANTD Director for Australasia and myself, an IANTD Trimix Dive Supervisor, along with my wife Mirja who would act as surface support diver and photographer. Unfortunately the submersible was wrecked in a storm just prior to our arrival in Honiara, making the USS Atlanta an unfeasible target as the ship in not buoyed and is in an area of strong currents and low visibility.

Continuing heavy tropical rain and the outpouring of the many rivers in the area made searching for her where she lay in the murky depths (85/280 to 100msw/330fsw+) impracticable, given the lack of sounders on the dive boats and minimal topside personnel available to support the dive team. With the abundance of other deep wrecks both American (USS John Penn) and Japanese, (the Asumasan Maru or "Deep Ruinui" and an unknown Japanese transport called "ridiculous Ruinui" because the bow is not reached until 65msw/215fsw) we switched our sights onto the Asumasan Maru for our first push on Trimix. Having been assured that the stern of the Asumasan Maru rested in 120msw/395fsw, we selected a target depth of 110msw/360fsw and a mix that would both reduce the END and bring down the PO2 to well below the acceptable extended range diving level of 1.4 ATA. The mix we chose was to be 10% oxygen, 57% Helium and 33% Nitrogen. Not only would this give us a PO2 of 1.2 ATA at our target depth (and a PO2 of only 1.3 ATA if we actually went to the supposed bottom), our END would be only 40msw/131fsw at our target depth.

Because we had only a finite amount of Helium available to us in the Solomons, it would also allow us to top up what would be our more than half-full primary tanks after this dive with air and still have a relatively good mix for a shallower dive depth the following day.

To accomplish this dive SAFELY (a key word in all extended range diving plans!) we needed to generate mission specific custom decompression tables and also calculate the necessary amount of decompression gas required given our individual SCR. Having access to three of the four advanced recreational (!?) Trimix decompression computer programs available in the world today, we had plenty of cross checking and referencing to do to assure ourselves that our calculations were correct. First we needed to mix and analyze both our bottom mix and our decompression gas so the exact percentages could be entered into the program to run the decompression calculations. All the necessary mixing and analyzing of the multitude of gas mixes we needed for this expedition would be done using Alternate Diving Services' portable mixed gas panel and oxygen analyzing equipment brought to the Solomons specifically for this mission.

After extensive dry dives on the laptop computers and working within the parameters of maximum fill pressures we could achieve filling from the oxygen storage cylinders (which are filled to only 160Bar/2350psi) we selected the following mixes to use as decompression gas. First we would switch to EAN 32 at its maximum operating range of 39msw/128fsw (if we reached 110msw/360fsw our first decompression stops would start at 48msw/158fsw and we chose to do those on our bottom mix so as not to have to carry any more cylinders than already necessary with us on the dive) and then switch to EAN 65 from 9msw/30fsw to the surface. We would have preferred to switch to our standard deco mix of EAN 80 from 9msw/30fsw, but owing to the low pressures in the oxygen storage cylinders (160Bar/2350psi), could not fill our stage bottles with oxygen to the necessary volume/pressure to achieve this mix.

Using EAN 65 to release or "wash out" the excess nitrogen accumulated in our bodies from the deep exposure would not give the optimum off gassing gradient of EAN 80, hence lengthening our decompression time, but we had little choice without the aid of a booster pump. The beauty of using the custom computer generated deco tables rather than hard tables is not only that any mix of bottom gas and deco gas can be entered and subsequent tables calculated, but an additional individual safety factor percentage can be programmed into the calculations.

A note on our dive procedures is also in order here; as IANTD trained Trimix Divers we are both trained to (and personally choose to) carry all our decompression gases with us throughout the whole dive rather than leave our deco gases clipped to the anchor line! This method, along with carrying a special wreck reel with up to 100 meters (330ft) of line and a heavy duty lift bag, gave us self-sufficiency in the event of not being able to return to or find the anchor upline. In an emergency, by deploying the lift bag, not only would we have a stable upline to do our lengthy decompression on, it would also alert the topside support personnel that they have a diver doing a blue water decompression and can follow the lift bag accordingly. (Also with us on site was an E size oxygen cylinder containing 125cf of 02, 15m/49ft of hose and an AGA Full Face Mask to be used in the event of an emergency requiring IN-WATER recompression!)

For this particular dive exposure I carried two steel 65cf cylinders full of EAN 65 slung on special harnesses under one arm and a steel 80 cf cylinders full of EAN 65 slung on special harnesses under one arm and a steel 80 cf cylinder with EAN 32 slung under the other. This would give me the necessary amount of deco gas, given even a worst case deco scenario, even breathing at a higher rate than calculated from my SCR level. My dive companion Rob Cason would sling two steel 65's under each arm, one side containing EAN 65 and the other EAN 32. On our backs were twin 150 cf steel cylinders of bottom mix (300 cf total each diver) connected with isolation manifolds. We believe in rigging like this for self-sufficiency and self-reliance and our philosophy is that if you are not comfortable doing the dive SOLO then you should not be doing the dive period! Your buddy is there to share the experience, not as a crutch. Swimming with the above setup is not as difficult as it may seem but scooters sure would have made it easier! All of our equipment was our own personal deep diving gear freighted up from Australia.

Now while diving with this amount of equipment is awkward from a regular size dive boat, in the Solomon's it becomes an exercise of epic proportions when diving out of what are basically long slender (about 1.25m/4ft wide) motorized aluminum canoes. Rigging up while sitting on the seats and running through a final pre-dive visualization, we then had to be literally lifted to our feet by two of the Dive Solomon's personnel and pushed over the side! Once in the water the equation changes somewhat and we were once again semi-weightless. If any of you have done your sums you will have realized that our bottom mix is hypoxic on the surface because of the low oxygen content (10%) so a travel mix to a depth of at least 6msw/20fsw (where the PO2 will be 0.16 ATA and able to support life) is needed. Because we had to make a horizontal surface swim while descending to a buoy at about 8msw/26fsw, rather than use my EAN 32 mix with its regulator securely bagged and hose strapped to the stage tank (this system leaves no chance of swapping to the wrong reg/mix, which could prove fatal at an incorrect depth; this is high PO2 deco gas, remember!) and then have to restow it on further descent, I chose to drop to about 8msw/25fsw using a small Spare Air system and then hand it off to the safety diver, while Rob chose to drop on his EAN 32.

Dropping through the first 30 metres (100ft) the visibility was low due to the heavy rains and river outpourings, then out of the gloom a shape materialized. Suddenly we burst through into clearer water and in front of us lay a huge ghostly cargo vessel. Hovering next to a coral encrusted four or five inch bow gun at about 42msw/138 fsw, we adjusted our buoyancy, rechecked our run time and started the long swim down the wreck into the depths. Looming above us like huge goal posts were the first of four giant gantrys, its center filled with schools of tightly-packed, circling pelagic fish. A slight current was running from the stern diagonally across the ship from starboard to port, making swimming with five tanks even more difficult.

As we passed the second cavernous hold with our helmet mounted lights peering into the gloom, a fast swimming school of large, silvery jacks raced up over the side from the darker depths and spiraled around the second gantry in a mesmerizing procession. Slowly the bridge came into view, a twisted layer of buckled metal (apparently the bridge was originally intact but has deteriorated and collapsed over time). We passed the bridge at 60msw/198fsw and the third gantry loomed into view at about 70msw/230fsw. Looking at my watch I realized that it was taking an inordinately long time to get deeper against the current and wondered if we would make the bottom within our run time schedule. This wreck was HUGE! As we swam over the fourth hold and past the final gantry, the vessel suddenly ended in a mass of twisted wreckage!

Settling lightly onto the sand amongst this wreckage I continued to film with my video camera while looking incredulously at my digital depth gauge, 88msw/290fsw! Where was the 100/330 to 120msw/395fsw we were ASSURED the stern rested in? Still, we had bottomed out the wreck and not being one to swim out onto the sand just to attain a deeper depth reading (for every five minutes longer spent at even 90msw/295fsw, forty-five minutes will be added to an already long decompression schedule) we decided to modify the dive plan accordingly. Wreckage littered the now relatively flat bottom on all sides and behind where we were lay what appeared to be the wreckage of the stern, apparently blown off in the attack, or possibly broken off as the wreck hit the bottom. Off to one side of the vessel a huge propeller lay partially buried in the sand. (The Asumasan Maru, along with two other large Japanese transports were attacked and sunk off Tassafaronga Point by a flight of American B17's operating out of Espiritu Santu on October 15, 1942).

Sadly enough our long swim down the vessel had used up most of our projected bottom time anyway, so after another look around we signaled each other that it was time to go. Although weighted down with more than 80kgs/176lbs (when empty of steel tanks plus other assorted equipment, and under the weight of almost ten atmospheres, lifting off the bottom was relatively easy, even with only one of the redundant dual wings style BCD's I was wearing inflated. Swimming forward and upwards was much easier assisted now by the light current, so we had time to have a closer look over a set of twin anti-aircraft guns mounted near the bridge that were still pointed skyward in a final act of defiance. In one of the dark holds lay a motorbike and again schools of tightly packed fish swirled around twisted wreckage. Slowly, in the distance, a flashing light came faintly into view, a strobe we had left attached to the upline to aid in ease of finding the line in case of a drop in visibility. Finally the bow gun materialized, pointing forlornly back towards the bridge as if in a last hari-kari gesture. Mirja, our safety diver, was waiting on the line and came back down at the appointed time, hovering just above the wreck to check that we were OK and to take the video camera. As is standard in all our operations we had prepared a multitude of custom decompression tables for different depths (90, 100, 110 & 120msw/295, 330, 360 & 395fsw) with a range of bottom times (10, 15, 20 & 25 minutes) for each of the depths concerned. (As a further backup we carried hard copy IANTD/UAC waterproof Trimix tables just in case the others were lost!) It is then a simple matter of selecting the corresponding deco schedule for the exposure time concerned and proceed with the appropriate decompression schedule.

Starting our first decompression stop at 39msw/128fsw we switched to EAN 32 and didn't switch gases again until we went onto our 65% mix at 9msw/30fsw. By now a rather strong surface current had sprung up which had us flying horizontally off the deco line. More than an hour and a half after starting our decompression stops we surfaced to a gray sky and glassy conditions. Getting back into the boat was much easier as we could unhook the stage bottles and pass them into the boat and then slip off our inflated wings supporting the main cylinders and have them dragged out.

Although disappointed that we had mentally prepared for and expected to reach a deeper depth (110/360 to 120msw/395fsw) and having carried all the necessary gas and equipment to safely reach and return from that depth, we were elated to have bottomed out the wreck! We had visited a piece of history a relative untouched deep wartime grave that was sunk with extreme prejudice, on which very few people with the exception of the crazy, early commercial salvage divers, had ever been much deeper than the bridge! Along with our dives done on the USS John Penn, the series of Trimix dives we did on the Asumasan Maru were the highlights of our trip to the Solomon Islands. TO AIR IS HUMAN, TO TRIMIX WAS DEVINE!

The members of the dive team would like to thank IANTD and Tom Mount in particular (for the extensive training) and the following parties for their help in the successful completion of this expedition. Alternate Diving Services, Action Unlimited Photographics, Fun Dive Center, Solomon Airlines, Dive Solomon's Island Dive Services, CIG Solomon's (Oxygen), Linde Gas Australia (Helium), Mark Owens (special helium mixing tables), Kevin Gurr (ProPlanner Trimix software), Corey Berggren (DPA program) and the late Sheck Exley for inspiration (and his DrX software's gas mixing & technical information).