A SHIP WITHOUT A RUDDER

      The saga of the New Carissa on Oregon's beaches over the winter provided an interesting diversion from the usual mid-winter news stories about rain, rain, and more rain. We found the story particularly interesting because we had just completed a project involving a disabled ship; in this case, however, the ship didn't end up on a beach or at the bottom of an ocean somewhere.

      The ship was a 725-foot long bulk carrier that had been built in 1995. During a recent ocean crossing, the ship suddenly lost all steering. A quick inspection revealed a rather serious problem: The rudder had fractured, and the bottom three-quarters had fallen off and was presumably on its way to the bottom of the ocean! Fortunately, the loss occurred in relatively calm seas, and a tug was able to reach the ship and hook up before any damage occurred to the ship.

      The ship was towed to Portland, which was the nearest port with major ship repair facilities, so a new rudder could be built and installed. We were retained by the ship's owners to conduct a failure analysis and establish the cause for the rudder loss.

      Interestingly, this wasn't the first time this particular ship had lost its rudder! When the ship was less than two years old, the rudder shaft had fractured, resulting in the complete loss of the original rudder. A replacement rudder had been fabricated and installed, and the ship had been in service for about two years since then.

      The rudder was hollow, with an "airfoil"-type shape and was about 4 feet wide, 16 feet long, and 30 feet tall. With internal steel ribs and a 1-inch thick steel skin, it was designed to be a water tight chamber fabricated by welding.

      An examination of the remaining stub of the rudder quickly revealed the presence of two large fatigue cracks on the fracture surface, one on each side of the rudder. These cracks had propagated through the full thickness of the rudder shell, then advanced toward both the front and rear of the rudder simultaneously. Eventually, the fatigue cracks became so large that the remaining cross section at the front of the rudder could not support the rudder's weight, at which point, the front of the rudder fractured by overload; this caused the rudder to pivot down and back, tearing and twisting off the portion of the rudder below the crack.

     In this case, the age of the fatigue crack was fairly easy to establish. We knew it wasn't any older than about two years, because that's when the rudder had been installed. We also knew the crack wasn't brand new, because the fracture surface was covered with thick layers of corrosion oxides, and there was a well-established colony of barnacles growing on the surface of the fatigue crack! By measuring the thickness of the oxide layer, we estimated that the rudder had probably cracked through the full shell thickness within a matter of weeks of being placed in service.

      So, what caused the rapid formation of the fatigue cracks? Well, let's digress here for a moment for a brief lesson on how you go about attaching a 30-foot tall, 35-ton rudder to a 725-foot long ship. How, you ask? With a gigantic nut, of course!

      In this case, the rudder was suspended beneath the ship by a large steel shaft. The bottom end of the shaft was threaded and was inserted into the top of the rudder, which was secured to the end of the shaft with a 38-inch diameter hydraulic nut (see drawing at bottom left).

The construction plans called for installing the rudder, then inserting and tightening the nut inside the rudder cavity via two access doors in either side of the rudder. After the nut had been secured, the plans called for welding shut the two access doors to provide a watertight compartment for the rudder.

     Unfortunately, the door closure welds were the weak link in the plan; the weld quality was absolutely terrible! Weld penetration was only half-way through the plate, and the roots of the welds had massive slag and incomplete fusion welding defects. These acted as severe stress concentrations and served as initiation sites for the fatigue cracks.

     The fatigue cracks quickly grew through the thickness of the rudder skin, flooding the rudder compartment with sea water. The corrosive action of the sea water then accelerated the propagation of the fatigue cracks.

     Now, many projects would have ended at about this stage with the recommendation that appropriate quality control/inspection plans be implemented to assure the welds on the new rudder be made to the specified quality level. But in this instance, the client needed additional assurances that this would be sufficient to solve the problem.

     Why the added concern? Well, although the loss of the first rudder had been attributed to metallurgical defects in the shaft, the client was bothered by the "coincidence factor." That is, he had just lost two rudders in a period of only 4 years; although the causes appeared to be unrelated to one another, he wanted to evaluate the problem in more detail to be sure.

     To further evaluate the cause of the rudder loss, we proposed a two-pronged approach: First, we instrumented the brand new rudder being built in Portland with strain gages and accelerometers so that the actual service stresses and accelerations could be monitored. Second, we retained John Rodgers, PE of Engineering Analysis Services to conduct a finite element design review of the rudder life, based on the service stresses measured with our strain gages. The strain gages revealed only moderate levels of stress in the rudder, even under severe maneuvering conditions designed to simulate worst-case service conditions.

     John's design review showed that the original design of the rudder was adequate for the intended service and that if appropriated weld quality were maintained during fabrication, the rudder could be expected to last the life of the ship. Additionally, John's design review confirmed the results of our failure analysis by showing that in the presence of major welding defects, an extremely short service life was expected due to fatigue cracking.

      As the project in our lead story demonstrates, the loss of a key part (like a ship's rudder!) occasionally results in merely an expensive inconvenience, rather than complete disaster. Over the years, we have worked on quite a few parts-falling-off projects on ships and aircraft where the loss of a key part had serious and sometimes even fatal consequences.

      Probably the most dramatic of these cases was one in which the wings came off a small single engine airplane. In that project, the pilot had apparently lost control of the aircraft, resulting in an uncontrolled dive. When he attempted to regain control and pull out of the dive, the resulting stresses overloaded the airframe, causing the complete separation of the wings from the fuselage.

      Another project involved a single engine private aircraft which was speeding down the runway under full power and had just reached takeoff speed when one blade of the two-blade propeller fractured due to a fatigue crack and flew off. Fortunately, the pilot was able to stop the airplane before he ran out of runway, and at the same time, he was able to shut down the engine before the vibrations shook it apart.

      Another project involved the loss of a small retaining ring. That doesn't sound too serious, you say? Well, ordinarily it might not be, but in this case, the retaining ring was in an engine of an aircraft that had just departed on a charter flight along the Kona coast in Hawaii. The loss of the retaining ring resulted in the loss of a rocker arm shaft, which caused a complete loss of power in the engine. The pilot was unable to make it back to the airport and was forced to ditch the aircraft at sea. Fortunately, he survived, as did his passengers.

      Another project involved an Alaskan fishing vessel that was making a late season run through the Bering Straight when the propeller shaft fractured, causing the complete loss of the propeller.
In this case, the shaft had been improperly welded, resulting in very high residual stresses, which caused the formation of multiple fatigue cracks. Although the vessel was able to secure a tow and make it safely back to port, the loss of the propeller had the potential for disaster.

      In our Winter ‘99 newsletter, we told you about how we were studying crystal formation in catalyst beds last summer with our Apprenticeships for Science and Engineering intern student, David Blindheim. David's project, which was being done in cooperation with General Chemical in Anacortes, had revealed the presence of some "mystery crystals" downstream from a catalyst bed.

Following an investigation of the chemical and physical properties of the mystery crystals and a review of the scientific literature, we were able to determine that the crystals were "stuffed" tridymite, a form of silica in which the crystalline lattice is packed, or stuffed, with other elements.

      We then followed up with an investigation of how the crystals might have formed at that particular location in the reactor. The mystery we left you with last time was how did silica evaporate and recrystallize at the temperatures in the reactor (about 1,000oF) when it is well known that silica doesn't even melt, let alone evaporate, at that temperature?

      Toward that end, we ran some experiments to simulate the conditions in the catalyst bed, to see if we could make silica volatilize and then recrystallize at a relatively low temperature. For this, we set up the following experiment.

      First, we cut a 1-foot long piece of steel pipe in half lengthwise, placed a pellet of catalyst in the middle, and clamped the two halves back together. Then, after installing a thermocouple in the mid-section of the pipe to monitor the temperature, we connected the pipe to a nitrogen tank and allowed nitrogen to flow slowly through the pipe while we heated the mid section to about 1150ºF with Bunsen gas burners for five hours. Afterward, we disassembled the apparatus and carefully examined the inside of the pipe.

      Sure enough, we found small bits of white material several inches downstream from the catalyst. Although they were not crystalline, they did have the appearance of being a condensate of some sort.

      We examined them by scanning electron microscopy to study their morphology and energy dispersive spectroscopy to study their elemental composition. We found they were primarily composed of iron and sulfur, with lesser amounts of silicon.

      At this stage, we stopped and reevaluated our working hypothesis that the phenomena we were studying was due to vaporization and recrystallization. The composition and structure of the deposits, in combination with the knowledge that the temperatures were far less than the vaporization temperatures of silica, suggested that we were dealing with a chemically-driven transport phenomena rather than one involving vaporization and recrystallization. That is, the evidence suggested that the silica wasn't being vaporized per se, but instead, was simply being transported in the gas phase by pyrosulfate salts in the catalyst when heated.

      This was an excellent project for an aspiring young scientist like David to be involved in because, unlike school where the answers are already known and usually presented in a canned format, it demonstrated how real world science works.

      So what do real world scientists do when confronted with observations that contradict their original hypothesis? Why, they hypothesize anew about what might be happening to explain the observed facts.

      In this case, we speculated that an unstable, ring-shaped molecule, perhaps something like the one shown below, is formed within the catalyst and then transports the silica, molecule by molecule downstream.
 

Of course, many more rigorous experiments would be needed to provide any publishable data elucidating details of this transport mechanism.
 

      Mark Habel, engineering technician, passed the American Society of Nondestructive Testing Level III examinations in Magnetic Particle Testing and Liquid Penetrant Testing. As we told you in our last newsletter, Mark recently passed the Level III examination in Radiographic Testing, and he now holds Level III certification in three of the ten disciplines available through ASNT.

      Mark, who holds an Associate of Applied Science degree in Nondestructive Testing from Southeast Community College, plans on taking Level III examinations in additional disciplines in the upcoming year.

      Good job, Mark!