 |
Cases
Techno Fysica BV is a Dutch-based engineering company, highly specialised in solving all kinds of problems related to the dynamic loading of machinery and installations. For this, several measuring and analysis techniques are used. In a series of articles, we will give examples of what can go wrong in the engineering and operating of vessels, and what errors lie at the root of the damages observed.
|
| Example: Reluctance of ship owners to invest in extensive measurements |
| Techno Fysica B.V. has the experience that the amount of measurements that is performed during sea trials is restricted to the absolute minimum, requested by the ship owner in the building specification. A lot of potential, future failures that could be detected by more extensive measurements remain unnoticed. These slumbering faults often lead to failures (usually just after the warranty period has ended) with consequential cost and down time. The ship owner does not request additional measurements because he does not have the necessary knowledge, the ship yard does not perform additional measurements because these involve extra cost and time which is unwanted in a very competitive market. A RoRo Ferry was equipped with four engines, two gearboxes and two controllable pitch propellers. For reasons of comfort to passengers and crew, the engines were resiliently mounted. Consequently. all pipes of the cooling-, fuel, exhaust- and lubrication system are connected to the engines by means of flexible connections. Because, in the speed range between idle speed and nominal speed, natural frequencies of the engines on the flexible mountings are excited, considerable deflections occur between engines and piping which necessitates high flexibility of the connections. During tests, the rubber connections that have the necessary flexibility, in the lubrication oil piping failed. The yard decided to replace the rubber bellows by stainless steel bellows. However, this type of bellow has a much lower flexibility, especially in directions perpendicular to the piping, than rubber bellows. |
|
|
 |
 |
|
Cracked Stainless Steel bellows in a lubrication oil piping system
|
|
During the sea trials no attention was paid to the dynamic behaviour of
the piping systems. When the ship was in service for some time, the
bellows in the lubrication oil piping cracked several times.
This resulted in large oil spills in the engine room and the risk of
engine room fires.
These failures were
blamed on pressure pulsations
in the lubrication oil system that over-stressed the bellows.
It
was decided to install pulsation dampers to lower the pulsation level in
the piping.
However, the effect was zero and bellows kept on cracking.
Metallurgical investigation and a failure analysis
of a cracked stainless steel
bellows and measurements on board confirmed that the dynamic deflections
between engine and piping exceeded the allowable levels by far.
It
was also found that the vibration level of the pipes was much too high (up
to 100 mm/s) due to a lack of brackets in the right places and in the
right direction and fixation of brackets to relatively weak parts of the
hull which makes them ineffective.
At
that stage, the yard decided to build more flexibility in the piping
system by adding intermediate piping pieces that were supported by coil
springs through (weak) brackets and connected to the piping by additional
rubber bellows
|
|
|
|
|
|
Original piping and pulsation damper (left) and additional piping with rubber bellows and spring supports (right) |
| The result was that the brackets that support these pieces cracked, resulting in cracking of the stainless steel bellows and another oil spill into the engine room. The advise was to replace the stainless steel bellows for a type that can cope with the movements of the engine during passing of critical speeds, to carefully review the correct location and direction of brackets and to connect these brackets to stiff parts of the ship. By taking more extensive measurements, carried out by a competent party with knowledge of critical areas, this problem would have been noticed much earlier, corrective measures could have been taken and oil spills could have been prevented. In this case, it was only a piece of cheap piping, but failure of this piping can cause engine room fires which makes it a very important piece of piping. |
| For more info, please contact techno Fysica BV in the Netherlands at: Techno Fysica BV P.O. box 351 2990 AJ Barendrecht The Netherlands Tel. +31 180 620211 Fax. +31 180 620705 info@technofysica.nl www.technofysica.nl |
