Gemi makinelerinin yataklanması ve titreşim etüdü
Marine engine foundation and study of vibration
Yazar: AKİLE NEŞE HALİLBEŞE
Danışman: PROF. DR. OSMAN AZMİ ÖZSOYSAL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Gemi İnşaatı ve Gemi Makineleri Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Gemi Mühendisliği = Marine Engineering

Özet:
Değişen ve gelişen endüstriyel aşamalarla kullanılan birçok makine hem boyut hem
de güç olarak büyümektedir. Gemi tasarımındaki son gelişmeler, daha büyük stroklu
ve daha güçlü dizel motorların kullanıldığı, daha büyük boyutlarda, daha hafif, daha
esnek gemilerin yapılmasına yol açmaktadır.Yapıyla beraber büyüyen statik ve
dinamik kuvvetlerden kaynaklı gürültü ve titreşim gibi problemler ve bunların hava,
akışkan ve yapı ile iletimi bu gelişmelerle büyüyen problemlerden bir kısmıdır. Yapı
kaynaklı gürültü ve titreşimde ana makine ya da diğer makinelerden kaynaklı titreşim
makine yatağına, yataktan ise tüm gemi yapısına iletilmektedir. Meydana gelen bu
titreşimler yolcu konforu, mürettebat yaşamını ve hareketli aksamların çalışma
performansını etkileyip makina ve cihazlarda bozulmalara neden olmakla birlikte
yapısal elemanlarda da yorulma hasarına neden olup kontrol edilmesi gerekmektedir.
Ancak gemi çalışmaya başladıktan sonra alınacak önlemlerin uygulanması hem daha
zor hem de maliyeti yüksek olması sebebiyle gemi inşa esnasında teorik ve deneysel
çalışmalar yapılmalıdır. Ana makine yatağı titreşimlerinin önüne geçilmesi için ilk
olarak tasarım aşamasında henüz ayrıntılar geliştirilmeden önlem
alınmalıdır.Yataklarda tekrar eden şekilde ortaya çıkan ve deneylerle sabit olan
titreşim problemlerinin çözüm kaynağının erken safhadaki tasarım aşaması olduğu
açık olup, sonradan yapılacak olan düzeltmelerin çok ağır maliyetler gerektirdiği
bilinmektedir. Rezonansı önlemek ve ikaz kuvvetlerini azaltmak amacıyla detaylı
hesaplamalar ve deneysel çalışmalar gerekmektedir. Bu sebeple gemi ana makinesi
ve diğer makinelerin titreşim hesaplamaları çok önemlidir ve makine yatağı tasarımı
yaparken yapıyı gereksiz yere ağırlaştırmadan titreşim genliklerini minimum
seviyede tutmak amaçlanmalıdır. Gemi ana makine titreşim hesaplamaları yapılırken
kullanılan bir teknik ise tüm sistemi çift kütle yay sistemi ile modellemektir.
Bu tez çalışmasında dinamik analizler ve gemi makine yatağının duyarlılık analizleri
sistemi iki serbestlik dereceli sönümlü zorlanmış titreşim denklemleri ile
modelleyerek çözülmüştür. İlk aşamada sistem statik açıdan incelenmiş, dayanıklılık
hesaplamaları yapılmıştır. Bulunan dinamik kuvvetler (yanma odasında piston kafası
üzerine etkiyen maksimum gaz basıncı) ve dinamik yükler (motorun sabit ve
hareketli parçaları, hareketli parça kütleleri, ataletleri, dönen ve doğrusal hareket
eden kütlelerin oluşturduğu kuvvetler) hesaplanmıştır. Statik ve dinamik kuvvetlere
karşı yatağın nasıl cevap verdiği göz önünde bulundurarak ön yatak tasarımı
yapılmıştır. Yatak tasarımının doğru olması çok önemlidir çünkü doğru tasarım
motorun baş-kıç vurma, öteleme ve maksimum yer değiştirmesini engelleyecektir.
İkinci adımda ise, analitik model oluşturulmuştur. Ana makine ve yatağı; bir çift yay
ve kütle sistemi ile modellenmiştir. Zeminin yay gibi davrandığı kabul edilmiştir.
Motor test yatağının dinamik dayanım ve titreşim hesaplamalarında kullanılan
datalar Deniz Kuvvetlerine ait gemilerdeki dizel motorlardan seçilmiştir. Bunun
yanında, olası düşey yönde aşağı yukarı hareket, ağırlık merkezi etrafında dönerek
baş – kıç vurma ve ileri geri doğrusal öteleme hareketleri incelenmiştir. Hesaplanan
yer değiştirme ve dönmeler tablolarda verilmiş olup, ayrıca sonuçlar frekans – genlik
grafiklerinde de gösterilmiştir. Sonuçların izin verilen sınırlar arasında olup
olmadıkları kontrol edilmiştir. Son aşamada, sistemin matematik modelinin
çözümüne geçilmiş, elde edilen sayısal sonuçlar değerlendirilmiş, tartışmalar ve
yorumlar sonuç kısmında verilmiştir.

Abstract:
An unwanted side effect of building faster and lighter ships is the increasing noise
and vibration in ships. In order to retain the full benefit of building faster ships
without compromising the ride comfort and safety, effective noise and vibration
control needs to be implemented to ship structures. Most severe damage to ship
structures, however, is caused by large deformation and high dynamic stress
concentration from low frequency vibration. The low frequency noise and vibrations
also contribute most to discomfort onboard ships.
In cases where there is a demand for high structure-borne noise attenuation a twostage
mounting system (also called as raft mounting) is employed. The solution to
reduce the structure-borne sound due to diesel excitations is usually to attempt to
isolate the engine or the generating set from the surrounding structure by interposing
elastic elements. The simplest isolation mounting arrangement normally used is to
interpose a spring, often in the form of a rubber mount, between the vibrating diesel
and the underlying hull structure. Another conventional but a more attractive
arrangement for isolation is the so-called two-stage mounting system where an
intermediate mass is attached to both the diesel and hull structure by springs. It has
been proven that, in nearly all cases, a two-stage mounting system affords superior
vibration isolation at high frequency to a simpler single stage mounting. The aim is
to reduce vibration levels from machinery to foundation, and thereby to reduce
radiation noise levels from ship hull.
Theories for vibration isolation and the attenuation of vibration using resilient
mounts have been investigated by many researchers. Comprehensive reviews of the
literature concerning many aspects of vibration isolation can be found. The design
criteria and guidelines for vibration isolation are available in many design handbooks
for the car, ship, and airplane industries.
In the prediction of vibration transmission from a diesel via the mounting system to
the ship's hull, one important issue is the determination of the diesel excitation level
and property. This is also true in the design of a resilient mounting system because
the selection of appropriate mounting elements is highly constrained by excitations.
The exciting sources of a marine diesel are due to rotational imbalance and
reciprocating masses. Combustion, inertia of a reciprocating piston, rotational inertia
of the connecting rod and crankshaft, and the impact between the piston and
cylindrical liner result in the shaking force and moments on the engine block.
In the traditional isolation design of a diesel-mount system, only piston-crank inertia
loads are taken into account in determining the excitation level and source frequency,
while piston-slap induced exciting components of force and moment are of much less
concern in order to simplify the analysis and design. It has been demonstrated both
analytically and experimentally that piston-slap is a major excitation source of airborne
noise from an internal combustion piston engine, especially from turboxx
charged diesels. The role that piston-slap plays in the excitation of a vibratory diesel
is still questionable when analyzing the engine vibration transmission via resilient
mounts to the ship's hull and consequent underwater radiated noise. Exploring the
answer to this question is one part of this work.
Furthermore, initial studies in design of diesel-mount systems have focused on
keeping the natural frequencies of the system away from the undesirable engine
operation frequency range. Often, when choosing isolators only the mass and
operational frequency of the engine to be isolated are considered, i.e. chosen only for
a force acting on the isolation in one direction. This classical only one DOF of
motion or only the translational DOFs of motion vibration isolation prediction will
not suffice for diesel-mount systems where there exist a combined force and moment
excitation and a multiple-mounts system between engine and foundation. Previous
researchers have also studied vibration isolation including more than one DOF of
motion, often including only the translational stiffness, or they are considered only
for rigid body motion. The shortcoming of modeling the engine source and
foundation as a rigid body can be improved by including their dynamic
characteristics which can be represented by their mobility or blocked impedance .
One more important aspect is devoted to the effects of rotational stiffness of resilient
mounts on vibration transmission.
Analysis of vibration transmission from a diesel engine to the ship's hull via a
resilient mounting system is very complicated, since the transmitted vibration is
characterized by a large number of parameters that in some cases cannot be directly
compared. Including both force and moment excitations and considering coupled
multiple-DOF transmission largely increases the complexity of the problem. On the
analysis of vibration transmission from a combined force and moment excited source
to a flexible receiver via a coupled multiple-DOF mounting system, a number of
publications can be referred to in open literature. From the standpoint of ease of
interpreting the results, an analytical method of studying the isolation problem is
more advantageous than a numerical method by finite element analysis (FEA).
However, to perform the vibration transmission calculation, solutions of mobilities or
impedances of exciting source, resilient mounts and receiving structure must be
known in advance. Owing to the complexity of construction, it is nearly impossible
to obtain analytical solution of mobilities or impedances of such a complex vibration
receiver as a ship's hull unless some biased assumptions are made. FEA is an
efficient numerical tool to achieve the target of vibration transmission prediction of a
complex source-mount-receive system.
This is done by placing the engine on anti-vibration mounts which are mounted on a
foundation. This foundation is again supported on the inner bottom shell through
another spring (inner bottom shell acts like a naturel spring) which act as the second
layer mounts. The inner bottom shell may be treated as the fixed support. Thus the
engine-mount-foundation system can be modelled as a two degree freedom system
with certain assumptions. This change from single stage mounting to double stage
mounting, results in reducing the transmissibility of forces to the foundation.
Resilient mounting systems of engines also provide a powerful means of isolating
structure borne sound on its path from the engine to the foundation. Improvement of
the mounting system may be achieved by changing from a conventional single stage
to a double stage mounting system.
This study consists of four main steps. At first, the engine foundation is focused, and
its general characteristics, dimensions, sectional views, concrete properties, gravity
centers, ground characteristics and shell properties are evaluated. So a statical
endurance capability and strength calculations are overcome. Secondly, the
dynamical forces (maximum gas pressure inside the combustion bowl and acting
onto the piston crown area) and dynamical loads (fixed and moveable parts of the
engine, their masses, inertia, rotating and linear moving mass forces) are evaluated.
Vibrations model is utilized by two degrees of freedom. Solution of the mathematical
model about the system (engine + mount +foundation+inner bottom shell) is
discussed and concluded at final step. At further step, analytical model was formed.
The all system is then modelled by a pair of spring and mass system. The behaviour
of inner bottom shell is represented as a spring. The vertical , rocking or transverse
type of vibrations of the system are assumed to be most predominant. We will
analyze effects of changes in various dimension of the foundation. It is aimed to
observe the behavious of the system by analyzing foundation.
Displacement and rotations are calculated and checked if they are in the permissible
limits or not according to the frequency – amplitude charts. Results are shown in
graphics and discussion, comments and conclusions are made supported by charts.
Suggestions for any academic studies in future are also noted at the end of the thesis.