Perils of the deep

A RISKY PROFESSION

And things do go wrong. According to the US Center for Disease Control (the most recent study to include commercial diving was released in 1998), “sat diving” was the third most dangerous job (behind logging and fishing) for “occupational fatalities.” Commercial diving is an insular community, so accurate figures for annual diving fatalities are hard to come by, but they do occur with depressing regularity. However, new technologies are being introduced to manage risks. “Apart from the coffee maker, pretty much everything aboard a Dive Support Vessel has one purpose,” says Gylseth. “To deliver a man to a subsea pipeline or installation, and return him safely back to the surface.”

Unlike shallow water dives, preparations for deep sea diving begin in a “dive system” either installed into a DSV or mounted on a rig. A dive system looks a bit like a small space station and usually has four main compartments: living chambers, which resembles a berth of a submarine, small compartments that contain a toilet, a sink, and a showerhead, diving bell(s) and hyperbaric rescue chamber(s) – a capsule which has enough breathing mixture to last the crew for three (or more) days. Dive systems come in different sizes, with a capacity ranging from six to 24 divers.

UNDER PRESSURE

Once the divers enter the dive system, the inner environmental pressure is increased until it matches the pressure at the job’s working depth – this generally takes less than 24 hours. In addition, the breathing mixture inside the complex is also adjusted correspondingly – the deeper the job, the more helium will be added to the breathing mixture. “Helium is easier to breathe under pressure because of its low density and helps divers avoid nitrogen narcosis,” explains Gylseth. “Breathing nitrogen at depth is like being drunk, and represents a genuine occupational hazard.” Indeed, some divers refer to nitrogen narcosis as the “Martini Effect”, calculating that every ten meters of depth is the equivalent of drinking one martini.

MAN VS. MACHINE

Once pressurized, the divers can get to work. Two or three-man teams enter the diving bell, which is lowered to the work site on the seabed via a “moon pool” cut into the bottom of the DSV. The bell man stays in the diving bell while one or two divers swim to their worksite, which can be anything from subsea installation to risers and pipelines. While Gylseth notes that some of this work can be undertaken by Remote Operated Vehicles (ROVs), divers are necessary for more complex jobs. “So far, no technical solution has been developed that can compete with an experienced diver,” he says. “The ability of a diver to process information and utilise the dexterity of his hands cannot be easily replaced with a machine.”

Gylseth says that underwater working conditions are often very difficult. “Below 60 meters, there is no natural light, and despite dive suits insulated with warm water, it can get chilly,” he says. “Also, because sound is transmitted easily below the surface, the noise from engines and thrusters is very loud – a bit like working at a engine factory. With senses limited to sight and touch, it can be very  disorientating.”

SEAFOOD

Currents can make navigating around complex structures difficult, especially since divers are tethered to the diving bell by a thick umbilical cable, which carries breathing gas, warm water and electronic systems and sensors. “In some ways, divers are a bit like astronauts performing a spacewalk, although astronauts don’t run into large fish with big teeth,” he says. “It’s hard enough to concentrate underwater as it is without having to worry about barracudas, 100 kilo groupers, wolf eels and manta rays that may linger around subsea structures.”

Gylseth adds that communications with the diving supervisor topside are complicated by the helium in the breathing gas. “Divers sound like Donald Duck, or like kids who have inhaled helium from balloons at a birthday party,” he says. “It may sound comical, but the risk of misunderstandings can result in mistakes. Some control panels are equipped with ‘Helium Speech Unscramblers’, a device that slows down the speed of the divers’ voices to improve coherency and thereby safety.”

WORKING IN SHIFTS

Dive crews can work without interruption in consecutive eight-hour shifts, making possible twenty-four hours of continuous work, with three different dive crews rotating into the bell from the dive system topside. Indeed, some of the larger, more advanced DSVs are now equipped with two diving bells, which can ferry divers to the worksite. A second dive bell also helps avoid downtime in the event of a system failure in the first bell, or can serve as a rescue bell if the first bell is compromised.

While divers rely on a broad range of systems aboard a DSV to get to and from work safely, ensuring the vessel remains in place (station-keeping) during dive operations is perhaps the most challenging. As a recognised pioneer in the development of DP systems, Kongsberg Maritime is a market leader in supplying DP systems to operators of DSVs. Unlike most offshore vessels, DSVs often operate in open water, without easily visible reference points that can be detected and processed by sensors that feed information into a Dynamic Positioning (DP) system. Furthermore, station-keeping for a DSV is further complicated when the bell is deployed.

According to Nils Albert Jenssen, Business Development Manager for Kongsberg Maritime, a deployed diving bell acts as a sea anchor, subjecting the vessel to subsea currents not detectable on the surface that can impact a vessel’s ability to stay in place. “In the world of dynamic positioning, the two greatest challenges for reliable station-keeping are deepwater drilling and DSVs,” he says. “However, while a system failure for a deepwater drill rig might result in downtime, a DP systems failure on a DSV could result in a fatality – there is little margin for error.”