중형 석유화학제품 운반선의 추진축계 안정성 평가에 관한 연구

Abstract

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

target vessel.

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

tube bearing.

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

analysis results.

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

docking.

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.