중형 석유화학제품 운반선의 추진축계 안정성 평가에 관한 연구
- 중형 석유화학제품 운반선의 추진축계 안정성 평가에 관한 연구
- Publication Year
- 한국해양대학교 대학원
- As the ship has high output and large size with the development of
shipbuilding and steel technologies, the shaft stiffness increases, but it is
the situation that the hull is deformed much more easily than before due to
using high-strength steel plate. Therefore, deep experience and high
attention of the designer are required as the propeller shaft cannot endure
the reaction force change coming from the deformation of the hull, if the
calculation of shaft alignment is done without considering the deformation
of the hull.
It can be said that the other area that is very closely connected with
shaft alignment is the lateral vibration of propulsion shaft system. So, it
should be considered in the safety assessment of shafting. It is better that
the distance between centers of supporting bearings of the shaft system is
longer in terms of shaft system alignment, but in terms of lateral vibration,
the natural frequency becomes lower, so there’s a chance that resonance
occurs in the range of engine operating speed.
The research related to lateral vibration still remains as a problem to be
solved due to unclear elements such as supporting bearing’s stiffness in
shaft system, oil film’s stiffness, propeller’s exciting force, etc.
Until now, it only ensures whether there’s a sufficient margin to avoid
the natural frequency of 1st order propeller blades to be within ±20% of
the engine nominal speed in Classification Society, international standards,
etc. Therefore, when considering such a situation, it is necessary to verify
the calculation result of the natural frequency in the lateral vibration with
the actual measurement.
In the shaft system of a ship, the increase of local load in the stern tube
bearing which supports a propeller shaft occurs prominently due to the
influence of the propeller weight at the shaft end, similar to the case of
the cantilever beam. Especially, the after stern tube bearing is likely to
have a concentrated load in the bottom of aft side while the forward stern
tube bearing does on the bottom of forward side. While such magnitude
and distribution of local load are determined by the relative inclination
angle between the shaft and bearing, the bottom of aft stern tube bearing is
most affected among them. Such local load can deflect significantly toward
the aft end of aft stern tube bearing in case that the shaft sags down,
when the eccentric thrust force acts downward due to the propeller force in
the hydrodynamic transient status.
Case studies by some authors have presented the real-time dynamic
behavior analysis of the shaft system in going-straight and turning by using
the telemetry system. While the impact analysis of the shaft system in
going-straight and turning of ship was carried out by domestic researchers
recently, it was difficult to find the case which analyzed real-time dynamic
behavior so far, so that it was considered meaningful to review the shaft
behavior’s impact on the shaft system through this study.
50,000 DWT oil/chemical tanker is a type of ship emerging recently as a
highly efficient eco-friendly ship and it lowered the engine speed by
applying de-rating technology. It reduced fuel consumption significantly
compared to similar ships and its feature is to maximize propulsion
efficiency through applying the propeller of increased diameter.
Therefore, some negative changes in terms of shaft alignment should be
compared to similar ships, as the change in aft structure and increased
weight of the propeller affect the deformation of the hull. Also, as the
forward stern tube bearing is skipped, the natural frequency of lateral
vibration becomes lower, so that the possibility of resonance in the
operating speed range is expected to be slightly increased.
After a review of previous researches, it is considered that there’s no
comprehensive case study reported yet, which is related to the hull
deformation, the lateral vibration and acceleration of the vessel and the
shaft behavior in going-straight for 50,000 DWT oil/chemical tanker.
Therefore, a theoretical review and analysis of measured data were
performed in this study by using finite element analysis, strain gage method
and reverse calculation method. And then the results are reported as follows
after reviewing in detail the stability of the propeller shaft system of the
The finite element analysis result expects that the shaft is placed right
down compared to the design value when it moves from light loading to
full loading due to the hull deformation, and the reaction force of each
bearing satisfying allowable values even under deformation. Also, the effect
of hull deformation acts as a little positive factor increasing stability of the
shaft system by relieving the relative angle of inclination of the aft stern
While the hull deformation which is analyzed by using the strain gage
method, is expected -2mm from the intermediate shaft bearing and about
-4mm from the main engine bearing and it is a little bigger compared to
the existing 47,000 DWT class, the increased weight of the propeller and
main engine and the aft change due to the increase in propeller diameter
are considered as main causes.
The reaction force of the bearing supporting shaft system met allowable
value like in the finite element analysis result, also in the full deformation
and the cross validation result of bearing force obtained by the strain gage
method, jack up method, and the shaft alignment program showed good
correlations in most conditions so that the reliability of the analysis was
able to be confirmed.
The calculation result of lateral vibration’s natural frequency showed that
resonant revolution speed was located in the area of more than 163.8%
compared to MCR, so that it was above the limit value(±20%) and it was
confirmed that there was no notable resonance point also in measurement
In case of 1st order component of lateral vibration, it showed the constant
response of bending stress value regardless of rpm generally, it was
considered because of run-out value. Furthermore, the measured bending
stress was only 10% level of the provided measurement result, so that a
negative influence by the lateral vibration was not expected to occur.
The review result of the trajectory during the full laden draft operation
showed that slight partial friction phenomenon was estimated with 25% of
engine load in strain gage position #7. Therefore, it would be necessary to
be careful on the long-time operation at 25% engine load to ensure the
stability of shaft.
strain gage position #5, it was considered acceptable phenomenon which
could occur due to different anisotropy in vertical and horizontal stiffness
in the intermediate shaft bearing. And while the hit and bounce friction
phenomena was suspected at NCR condition (83 rpm) and it was expected
to be stabilized after running-in operation. However, periodical monitoring
was required to be continued through the shaft open-up survey when
Knowing the fact that the shaft direction in strain gage position #7 which
was installed at the closest to propeller, moved toward left down direction
with increasing engine load at both conditions regarding the shaft behavior.
It was determined that the shaft direction in propeller location moved to
the right upper direction which was the opposite to the moving direction of
the strain gage installed location, and its effectiveness could be confirmed
based on the previous study results obtained with direct measurements.
Although the method above couldn’t determine the exact displacement
component at the center of shaft, it could provide the moving direction’s
pattern of propeller shaft by the engine load during operation, so that it
was confirmed that it was significant and practical as an alternative method
to the direct measurement one in the position of the propeller. Also, the
propeller force during going-straight acted as a force lifting the shaft from
the aft stern tube bearing and it reduced the possibility of damage to the
aft stern tube bearing. Therefore, it was considered to contribute to improve
the reliability of the shaft system.
If the results obtained in this study are applied to similar type of ships
for the shaft alignment and lateral vibration calculation, it is considered that
it will help ensure the stability of the shaft system and prevent damages
not only in quasi-static but also in dynamic conditions.
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