CODAG İle Tahrik Edilen Askeri Bir Geminin Sevk Sistemi Eksenel Ve Burulma Titreşim Analizi

Abstract

Gemi tasarımındaki son gelişmeler, daha büyük stroklu daha güçlü dizel motorların daha büyük boyutlarda, daha hafif, daha esnek teknelerin yapılmasına yol açmıştır. Bu yapıların daha esnek olması ihtiyacı titreşim problemlerini de beraberinde getirmektedir. Gemi titreşimleri, yerel yapısal elemanlar ve önemli makina aksamlarında yorulmalara ve nihayetinde kırılmalara neden olabilir. Bunun yanında gemide bulunan, geminin güvenli seyri için hayati öneme sahip ekipmanların performanslarını olumsuz yönde etkiler ve bakım masraflarını arttırır. Ayrıca yolcu ve mürettebatın yaşam kalitesini düşürür. Dolayısıyla bu tür aşırı gemi titreşimlerinin kontrol altına alınması gerekmektedir. Ancak gemi çalışmaya başladıktan sonra titreşimi engelleyecek bir takım düzeltmelerin yapılması hem çok zordur hem de yüksek maliyetler gerektirmektedirler. Bu nedenle gemilerde titreşim problemlerinin çözüm kaynağının tasarım aşamasında belirlenmesi için çeşitli çalışmalar yapılmaktadır. Gemi titreşimleri; iç kaynaklar ve dış kaynaklar olmak üzere iki grupta incelenebilir. İç kaynaklar; ana ve yardımcı makinalar ve sevk sistemi elemanları, dış kaynaklar ise; gemi hareketleri veya dış etkilere bağlı oluşan hidrodinamik yük etkileridir. Gemilerde ortaya çıkan aşırı titreşimler ilgili yapısal elemanlarda büyük hasarlara neden olabilir. Literatürde yapılan çalışmalarda gemi titreşimlerinin neden olduğu hasar maliyetlerinin üçte birinin gemide bulunan makina grubundan kaynaklandığı ortaya çıkmıştır. Makina grubu içerisinde ise hasarların kaynağı olarak ana makina ve sevk sistemi elemanları ön plana çıkmıştır. Gemi sevk sistemleri genel olarak üç çeşit titreşime maruz kalırlar. Bunlar; eksenel, burulmalı ve lateral titreşimlerdir. Ancak, en kritik titreşim modlarına eksenel ve burulmalı titreşimler sahiptirler. Genel olarak, sevk sistemi elemanlarından en büyük titreşim kaynakları dizel motor ve pervanelerdir. Gaz türbinleri dizel motorlara göre daha düşük etki kuvvetleri oluşturmaktadır. Dizel motor çalışır durumdayken bünyesinde ortaya çıkan titreşim oluşturucu tahrik kuvvetleri olarak, motor yatağına üç periyodik kuvvet bileşeni ve üç periyodik moment bileşeninin etki ettiği kabul edilir. Aslında motor ekseni boyunca etkiyen periyodik kuvvet bileşenlerinin titreşime etkisi yoktur ve motorun karakteristik özelliklerine bağlı olarak diğer bazı bileşenler de dengelenip sıfıra eşitlenebilir. İçten yanmalı pistonlu motorlar iki farklı, karakteristik kuvvet ile ilişkilendirilebilir.Bunlar: (a) yanma esnasında oluşan gaz basıncı kuvvetleri (guide force couples) ve (b) pistonlu ve dönen motor aksamlarının oluşturduğu atalet kuvvetleri (external forces). Diğer taraftan pervane zorlayıcı kuvvetleri ve momentleri çok çeşitli titreşim hareketleri oluşturabilir. Şaft hattının eksenel ve burulma titreşim hesabı için, itme ve tork dalgalanmaları göz önüne alınmıştır. Bu çalışmada, Codag sevk sistemi ile tahrik edilen askeri bir geminin ana makina ve sevk sisteminin eksenel ve burulma titreşim hareketleri incelenmiştir. Bu kapsamda titreşim analizi gerçekleştirilirken ilk önce sistem için kütle, yay, sönüm elemanlarından oluşan uygun bir fiziksel model geliştirilmiştir. Fiziksel modele bağlı olarak her bir kütle için hareket denklemleri elde edilmiştir. Hareket denklemlerinin matris yaklaşımla çözülmesiyle eksenel, burulma ve eksenel-burulma durumları için ayrı ayrı sistemin dinamik davranışları elde edilmiştir. Simülasyon kısmında Matlab/Simulink programı kullanılarak sistemin modeli kurulmuş, sisteme etkiyen iç ve dış kuvvetler yüzünden meydana gelen eksenel ve burulma titreşimleri analiz edilerek modelin sönümsüz ve farklı sönüm oranları karşısında nasıl cevap verdiği grafiklerle gösterilmiş ve sistemin titreşim hareketlerini başarılı bir şekilde sönümleyebilecek eksenel ve burulma sönüm oranları tespit edilmiştir.

Recent advances in ship design has led to build greater in size and more flexible vessels employ more powerful diesel engines with larger strokes. Flexibility in these structures brings about variety of vibration problems. Vibration aboard ships may result in fatigue failure of local structural members or major machinery components, adversly effect the performance of vital shipboard equipment, increase maintenance costs and greatly increase discomfort or annoyyance to passengers and crew. These all are why excessive ship vibrations must be taken under control. However, when ship is sailing, corrections on the vessel to decrease vibration problems is too difficult and couses great costs. Therefore, it is very important to identify and make corrections to minimize vibration problems in design stage. The main sources of ship vibration can be classified into two major groups; internal and external sources. Internal sources are main & auxilary machines and propulsion system components and the external sources are hydrodinamic loadings by direct action or induced by the ship motions. Ship vibrations can lead to huge damages to the related local regions. According to a study, one third of ship problems by cost is related to machinery. Main engine and propulsion systems claims stand on the first place and are 75% of the machinery claims. So vibratory modes of main engine and propulsion vibrations considered to be most critical and must be taken into control during design stage. Ship propulsion vibration analysis look at the power train as a collection of masses, springs, dampers and exciting loada in a variety of forms; torsional (rotational), axial (fore-and-aft) and lateral (shaft bending). But the vibratory modes considered to be most critical are the axial and torsional. In general, tha major vibration sources of ship propulsion systems are diesel main engine and the propeller. Gas turbines are generaly considered to give less excitation than diesel engines. Diesel engine vibratory excitation can be considered as composed of three periodic force components and three periodic moment components acting on the engine foundation. Actually, the periodic force component along the axis of the engine is inherently zero and some other components usually balance to zero depending on particular engine characteristics. Two distinctly different types of forces can be associated with the diesel engine. These are: (a) gas presure forces due to the combustion processes(guide force couples and (b) inertia forces produced by the accelerations of the reciprocating and rotating engine parts(external forces). On the other hand, propeller bearing forces and moments can excite various modes of vibration. For computations of axial and torsional vibrations of the shaft line, fluctiations in thrust and torque are to taken into account. Bending vibrations of theshaft line influenced by transverse forces in horizontal and vertical directions as well as by bending moments about the corresponding axes. Axial vibration originating in the propulsion system can result in excessive vibration of the hul, deckhouse and sıuperstructure; produce serious local vibration of diesel motor, gas turbine and shafting system. In most cases the excitation originates at the propeller and is magnified by resonances within the total shafting system. Torsional vibration of the propulsion system may be excited by the alternating torque produced by the propeller and the engine harmonics in a diesel drive system. Ordinarily torsional resonances within the shafting system does not produce serious vibration problems in the ship's structure although they can produce damaging effects in reduction gear drives, particularly under adverse sea conditions. In diesel engine drive system of all types, torque reactions can be a major structural vibration concern. Additionally, torsional resonances can be damaging to system components. The vibration of main propulsion machinery tends to be severe because of the excitation from the propellers. The machinery of primary concern is to be the main propulsion machinery, principally in longitudinal vibration at propeller blade rate frequency. The vibration criteria for the main propulsion machinery are to be provided from manufacturers. Otherwise, when the data on the vibration criteria are not available, the following criteria are recommended as a reference. ANSI S2.27 and SNAME T&R 2-29A provide comprehensive guidelines on the vibration limits for the main propulsion machinery. The vibration limits are provided in terms of broadband rms values with multi-frequency components (nominally from 1 to 1000 Hz). The longitudinal vibration (rms, free route) at thrust bearing (and bull gear hub for geared turbine drives) is to be less than 5 mm/s rms. For other propulsion machinery components exclusive of engines, propellers and shafting aft of the thrust bearing, the longitudinal vibration is to be less than 13 mm/s rms. For stern tube and line shaft bearing, the lateral vibration is to be less than 7 mm/sec rms. For direct diesel engines (over 1000 HP, slow and medium speed diesels connected to the shafting), the vibration limits are 13 mm/sec at the bearings and 18mm/sec on the engine tops, in all three directions. For high-speed diesel engines (less than 1000 HP), the vibration is to be less than 13 mm/sec at the bearings and engine tops, in all directions. Ship vibration avoidance process must begin early at the concept design stage. It is clear that if the vibration problems are adressed at the earliest design stage, where there has been no development of details, ultimately serious problems involving great cost in correction efforts, may be avoided. If as much as possible can be done in concept design with the simple tools and rules of thumb available at that level, it will help to avoid major vibration problems. The major potentional problems may often be present in the crude concept design definition. Just identifying and adressing those potential problems in terms of the minimal technology available at the concept design stage is considered very important to the success of ship design. Several operations can be done at the ship design stage to reduce the excessive ship vibrations, such as reducing exciting forces amplitude, increasing stifness, avoid values of frequency ratio near resonant condition and increasing damping ratios. Avoiding vibrations by increasing damping ratios is the main objective of this study. In this thesis, axial and torsional vibrations of the main engine and propulsion system of a naval vessel driven by Codag propulsion is analyzed. Codag propulsion system consists of diesel engine for cruising and gas turbines that can be switched on for high-speed transits. Within vibration analysis of the system firstly; phsycal model of the naval ship’s propulsion system consist of mass, spring and damping elements developed properly. Afterwards, equation of motions derived for each mass owing to Newton’s second law and consequently by solving these equations, employing matrix approach, natural frequencies and modes of the axial, torsional, axial-torsional vibration systems obtained. Finally, a model for propulsion system formed in Matlab/Simulink program. The axial and torsional vibrations coused by internal and external forces that excite the system examined and illustrated with graphs. Model respond in the face of undamped and under different axial and torsional damping ratios that can absorb vibrational motions of the system successfully are determined.