Silently Blasting the Sea Bed
Silence and tranquillity aboard a seagoing monster machine that uses a more than one metre wide drill head to blast into sub sea rocks: it seems an illusion. Yet, this is what Loggers’ shock, vibration and noise control achieves with automated air suspension. Comfort and healthy working conditions for the crew aboard such heavy machinery provide competitive benefits for ship owners.
“We have the technology to eliminate all vibration in the deck house. This starts as low- frequency as 1.5 Hz: meaning even very slow pulsing motions that have a rhythm as slow as 90 beats or motions per minute, will not be felt in the accommodations and working areas of a vessel.” Ruud van Wijngaarden, manager of Loggers’ Center of Excellence is sure about the results he can achieve with the specialised company know-how. “We provide vibration absorbing and sound-isolating installations to almost any specification. Before choosing the type of dampening and setting up a plan for placement of isolators, we calculate the level of vibration control and sound isolation.
Clients can ask us to design vibration and noise control solutions to specification. When we know the frequencies of the most important on-board equipment and engines, we can calculate the lowest frequencies we need to eliminate. For different frequencies, different solutions are required. We only choose the dampening solution after we have determined the sources of noise and vibration and the frequencies they induce. This, together with the total weight of the construction that needs to be isolated and its surface in the wind considering different sea conditions, is the basis for our noise and vibration isolation plan. The vibration mounts used in our solutions have been tested on more than one occasion on strength and characteristics, by independent organisations like TNO and DNV and every time they concluded that the dampening facilities we installed, perform to the specifications we had foreseen in the stage of design. Spot on. We know from experience and from advanced calculation which specifications individual elements of a dampening installation need to have to perform to specification in the framework of the whole installation.”
Engineering vibration control systems and being able to predict its performance in detail is the specialisation of Loggers, evolved though their systematic approach. The company is the first in the world to install a dampening system aboard a seagoing vessel, completely isolating the whole accommodation from vibrations using pressurised air.
This patented technology enables offshore dredging companies to compete in the market. As the dredging business sees a trend towards more offshore contracts and work in remote areas, providing healthy and safe working conditions for the crew, manifests as a growing problem. Vibrations aboard are immense, especially on cutter dredgers that drive a heavy cutter arm in slow rotation with five or six heavy teeth rows on the cutting end, hammering into rock. This results in a pulsating 3 to 4 Hz vibration that is felt everywhere on the vessel. In the control room, on the bridge, in the mess, in the galley, on the decks and in the cabins where the crew should find their rest. Constant hammering tires them down.
It is impossible to work long shifts and stay aboard in between shifts if a vessel vibrates this much. Ship owners and commissioning companies that want their pipelines installed in the sea bed, will understand the need to find a day’s off board rest after working the shift. Yet, this inflicts great costs in remote areas or miles off shore. In a 24 hour working programme, the whole crew needs to be replaced three times a day, transferred to hotels with crew supply ships if the work is not too far offshore but with helicopters if the work is really remote. Having comfortable and vibration-free accommodation aboard is an immense cost-saver. Dredging companies that employ this kind of advanced vessels can perform offshore jobs with efficient crewing and be strong competitors in the market.
The deck house aboard Athena, the self-propelled cutter suction dredger built at IHC Merwede as commissioned by Van Oord, is the first seagoing vessel to have the complete accommodation isolated from vibrations with air suspension. Operating offshore or in remote areas, Van Oord now has the on-board accommodation where the crew is really able to rest. The crew stays aboard in between shifts. The air suspension control system automatically takes different positions and different measures of inflation depending on sea state and movement of the accommodation. The deck house foundation air suspension is divided into three groups, acting together as an automated tripod.
“Benefits of vibration control are numerous”, Loggers’ sales director Peter Berting adds. “Not only can the crew rest aboard when off duty, they are also less fatigued during shifts and communication is much easier when noise is silenced. Another great benefit is the enhanced life span and service interval for bridge equipment and instruments. Not only humans, also machines and structures suffer from continuous vibration. Metal fatigue and malfunctioning instruments will occur a lot less on bridges that are vibration dampened.”
Loggers has built up experience with sound and vibration control for 30 years. The company’s involvement started with a project for German submarines. These vessels need to be exceptionally silent, in order not to be spotted by enemy ships. “We designed the suspension in which the engine was mounted, so no engine vibration would transfer into the hull”, key account manager Dirk-Jan van Ramshorst tells about the company history. “As it often goes, innovation starts in military applications and is then adopted in civil engineering.” The process at Loggers has been likewise. The big breakthrough of vibration control in maritime applications came with the construction of the Veerhaven push boats.
“When push boats navigating the Dutch rivers got permission to push rigs consisting of six barges, instead of the maximum four barges before, this opportunity was immediately embraced by ship owners. However, this caused higher engine loads as the push boats designed for four barges now had 150 percent of the load. Engines were constantly working at high load to keep the large rigs going. This caused a lot of vibration throughout the boats. Not only engine vibrations, also the propeller cavitation was a source of vibrations. It became a hard and enticing job to steer the boat at the bridge. Life in the on-board accommodation got noisy and shaky. We were asked for a solution by the yard of Gebroeders Kooiman. Following our calculations, we promised the skipper at Veerhaven: after we are finished, you can place a guilder (pre-Euro Dutch currency, ed.) coin on its edge on the kitchen table and it will stand up straight, even when engines are running. As a matter of fact, the guilder did not fall over.”
On a tour through the Loggers warehouse and assembly hall, the crates of seemingly identical rubber vibration absorbers come into view. In a small laboratory, two craftsmen test a series of rubber dampers. “We test every piece of suspension on its characteristics and mark them with the measured values”, Berting clarifies. “Assembling vibration control that performs exactly to calculated specification, begins with knowing the characteristics of each part of the assembly. We know from experience in general the specifications of wire rope suspension, rubber suspension and air suspension. High frequency vibrations can effectively be eliminated by rubber suspension. Low frequencies less than three 3 or four 4 Hz can only be eliminated by air suspension. Still, every piece of suspension has its own specific characteristics, which we need to know exactly for the right set-up of our vibration control systems.”
Not every vessel type suffers the intense vibrations a cutter dredger experiences. Aboard every vessel however, the engine exhaust is a source of noise and vibrations. “With every engine rotation, the number of cylinders each puff a volume of compressed exhaust gas into a hollow chamber”, Van Ramshorst explains. “This constant motion is often transferred into the hull, as the exhaust pipes need to be routed through the vessels hull towards the chimney.” Besides their customised engineering for specialised craft, Loggers provides the LEXSYS-modular exhaust piping system in a range of diameters. The system consists of thin concentric aluminium tubes, with a layer of isolation in between. It isolates heat and is very lightweight, saving overall vessel weight. “Lightweight in the case of motion, vibration being motion, means that there is less energy in the motion. Vibrations that occur in the isolated exhaust piping, can be silenced with lighter and less complex suspension because there is little energy in the vibration.”
Loggers also has experience with impact and shock dampening, again experience from projects for military applications. In the civil market, vibration absorbing wheels for inner city trams are provided by Loggers. However, their expertise in the maritime market is a business branch of growing importance.