Bir açık deniz gözlem dubasına gelen dalga yüklerinin sayısal hesabı
Numerical analysis of wave loads on offshore observation buoys
Yazar: OKTAY EREN TÜREYEN
Danışman: PROF. DR. HAKAN AKYILDIZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Gemi ve Deniz Teknoloji Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Deniz Bilimleri = Marine Science ; Gemi Mühendisliği = Marine Engineering
Dizin: Açık deniz platformları = Offshore platforms

Özet:
Açık deniz yüzer gözlem dubaları, deniz suyunun fiziksel ve kimyasal özelliklerinin periyodik aralıklarla ölçülmesi, meteorolojik verilerin düzenli olarak kayıt altına alınması, pasif örnekleyiciler ve biyogöstergelerle deniz ortamının izlenmesi ve bunların yanında deniz ortamında zararlı malzemelerin yayılması gibi acil durumların tespitinde kullanılan yapılardır. Bu yapıların belirli bir alana bırakılması, demirlenmesi ve efektif çalışma şartlarının sağlanması için, yapı üzerine gelen hidrodinamik yüklerin hesaplanması, bu yükler etkisinde yapının hareket denkleminin çözülmesi ve yapı hareket genliklerinin hassasiyetle hesaplanması gerekmektedir. Bu çalışmada, bu amaç doğrultusunda, bir açık deniz gözlem dubası üzerine gelen yükler ve bu dalga yüklerinin etkisindeki hareketleri birim dalga için panel metoduyla sayısal olarak WAMIT ile hesaplanmıştır. Dört farklı gözlem dubası geometrisi hazırlanarak, farklı geometrilerin maruz kaldıkları dalga yükleri ve yapıların hareket genlikleri karşılaştırılmış, bu yolla en uygun geometrinin belirlenmesi amaçlanmıştır. İncelenen gözlem dubalarının su üstünde kalan kısımları birebir aynı olarak seçilmiş ve su altında kalan geometrileri farklı çene hattı eğimleriyle tasarlanmıştır. Çene hattı eğiminin hidrodinamik kuvvetleri ve yapı hareketlerini nasıl etkilediği incelenmiştir. Marmara Denizi'nde yer alan Adalar bölgesi için maksimum ve ortalama rüzgar hızı verileri Meteoroloji Genel Müdürlüğü'nden temin edilmiş ve bu istasyon için Pierson-Moskowitz dalga enerji spektrumları oluşturulmuştur.  Birim dalga için hesaplanan yapıların frekans karakteristikleri ve Adalar bölgesi için elde edilmiş dalga enerji spektrumları ile bu bölgeye koyulması planlanan bir açık deniz gözlem dubası için nihai hareket spektrumları hesaplanmıştır. Nihai hareket spektrumları ile yapıların herbir hareket modu için belirgin hareket genlikleri elde edilmiştir. Çalışma sonucunda çene hattı eğiminin yapı hareketlerini doğrudan etkilediği, artan çene hattı eğimiyle yapıların dalıp çıkma hareketi genliklerinin arttığı, bunun yanında yalpa ve sürüklenme hareketi genliklerinin azaldığı görülmüştür. Fakat yapı hareketlerinin gerçek deniz ortamındaki incelemeleri göstermiştir ki, yapıların frekans karakteristikleri tek başına en uygun geometri seçimi için yeterli değildir. Herhangi bir hareket modu için yapıların doğal frekansının, seçilmiş bir açık deniz bölgesinde dalga enerjilerinin yoğunlaştığı frekans aralığına denk gelmemesi yapıların büyük hareket genliklerine sahip olmaması için ve en önemlisi rezonansa girmemeleri için gereklidir. Bu sebeple yapıların frekans karakteristiklerinden ziyade nihai hareket spektrumlarının dikkate alınması gerekmektedir.  Bu analizlerin sonucunda elde edilen veriler, bu yapıların deniz tabanına bağlama sistemlerinin geliştirilmesi, mukavemet hesapları, yapı için doğru malzeme seçiminin sağlanması ve yapının çalışma ömrünün optimum hale getirilmesi için kullanılacaktır.

Abstract:
Floating offshore observation buoys are utilized for several purposes such as the long-term measurements of the physical and chemical properties of the seawater, recording meteorological data regularly, monitoring activities by using passive samplers and bio indicators and also for the detection of any emergency case like oil spilling. 
	Before offshore observation buoys, observations and records were obtained by ships which were using major transportation routes of oceans and seas. However, for obvious reasons, only the safest routes of transportation were used by those ships and all the oceanographic and meteorological data coming from those ships were belong to those safe routes. All the other parts of the oceans were ignored until the offshore observation buoys and satellites became the major source of information and data of oceans. Satellites are the most technological and improved systems for observations and they are used as a main tool for any kind of physical or chemical measurements. On the other hand, the systems and methods used by those satellites requires a ground checking and calibration which are still provided by offshore observation buoys. Once they are calibrated and ready to use, they can perform very accurately however new sensors and instruments are developed day by day and all these systems are needed to be calibrated and checked by a ground observation system.
	Observation buoys are designed according to their intended purposes and the wave conditions of their intended locations. Once the location of the deployment of the buoy is clear, wave conditions of the location should be analyzed by establishing the wave energy spectra which can be obtained by several methods such as Pierson-Moskowitz wave energy spectrum and JONSWAP wave energy spectrum. A wave energy spectrum, which is based on Fourier Series, is a concept that allows the scientist to express the wave conditions of region with the distribution of wave energies of a wave group with different frequencies and wave lengths. In this study, Adalar region which is on The Marmara Sea, is chosen as a deployment region and wave energy spectrum of this region was obtained by Pierson-Moskowitz wave energy spectrum method. Wind speed records can be used the estimate the wave conditions of a specific area according to the Pierson-Moskowitz method. Wind speed and direction records of the Adalar region were obtained from Turkish State Meteorological Service. Maximum and average wind speed data were used to developed the wave energy spectrum of the area and used for the hydrodynamic analysis of the observation buoys.  
	When the deployment region of the offshore observation buoy is certain and the wave energy spectrum of the region is known, the frequencies that the wave energies are concentrated on can be determined. The aim of the observation buoy design is to avoid to have a natural frequency around the frequency interval which wave energies are concentrated on the spectrum. Motion spectrum of the buoy are developed by superposition of the wave energy spectrums and the response amplitude operators of the buoy in given motion mode. If the natural frequency of the observation buoy coincides the critical frequency interval of the wave energy spectrum, resonance situation can occur and structural damage can be observed.  
	Four offshore observation buoy geometry were designed in this study for comparative hydrodynamic analysis to determine the most suitable buoy geometry which is assumed to conduct its activities on the Adalar station. Above the free surface all the superstructures of the buoys are identical. On the other hand, four different jaw-line slope were chosen for the underwater parts of the buoys to analyze the effect of the jaw-line slope on the hydrodynamic forces acting upon the structures and the motions of the buoys.  Jaw-lines with 45°, 60°, 75° slopes and the sharp perpendicular jaw-line were chosen to conduct analyzes. The offshore observation buoys were named according to their angle with the bottom line as Buoy45, Buoy60, Buoy75 and Buoy90. All the geometries were symmetrical about the axis x and axis y. The draught of the observation buoys was chosen one meter. The total volume of the underwater parts of the buoy and obviously the total masses of the buoys were different however the mass matrixes were insignificant because all the instruments and sensors which will be the main parts of the payloads, were unknown. The mass distributions of the buoys are not clear at this moment, therefore radii of gyrations of the buoys are also not certain, that is why, the radii of gyrations of the buoys were chosen arbitrarily assuming that the masses of buoys were concentrated on the hull of the structures. 
	In this study, the wave forces and the motion characteristics of free-floating offshore observation buoys were investigated by using the panel method package program WAMIT for unit wave. Because of the body symmetries of the buoys about axis x and axis y, surge and sway motions are identical. Besides pitch and roll motions are also expressing the same type of motion. Therefore, in addition to heave motions of the bodies only the surge and pitch motions were investigated in this study. Results of the hydrodynamics calculations indicated that yaw motion amplitudes of the buoys were insignificantly low so their results were not given in this study. WAMIT is a radiation and diffraction panel program developed for analyzes of the interaction of any kind of floating and submerged offshore structures and surface waves. The basic application of the WAMIT has two steps, preparation of the input files and running the program. Input files which consist parameters such as water depth, wave periods, wave heading angles and geometric data are prepared by user.
	The geometry of the structure which will be analyzed in WAMIT, are represented with quadrilateral panels. The solutions of the velocity potentials are approximated by piecewise constant on each panel. The Cartesian coordinates of each vertex of each panel must be given in an order in the geometric data file. The geometric data file includes the total number of panels, body axis of symmetries, type of unit system and all the coordinates of the vertexes of the panels. Vertex coordinates should be given in order of counter-clockwise. Each panel represented by four vertex coordinates and if two coordinates of the panel coincide, that means that the panel is a triangle. The geometric data file has a unique format and the preparation of the geometric data file is not an easy task with the common CAD programs. That is why a special CAD program called MultiSurf is also developed by the developers of the WAMIT software for the purpose of creating a geometric data file in desired format. Although this CAD program is an essential tool for the hydrodynamic analyzes of the complex geometries in WAMIT, for simple geometries like the geometries analyzed in this study, some other methods can be used for the preparation of the geometric data file. The geometries analyzed in this study are cylindrical and frusto-conical therefore they can be expressed mathematically and the vertex coordinates of the each panel were calculated by using the Fortran PowerStation. The Buoy45 was represented with the total number of 2772 panel, the Buoy60 was represented with the total number of 2592 panel, the Buoy75 was represented with the total number of 2880 panel and the Buoy90 was represented with the total number of 3456 panel. 
	The hydrodynamic forces and the body motions of the buoys were calculated in the wave frequency interval of 0.1 rad/s – 6 rad/s with the step size of 0.1 rad/s for the frequency domain analyzes by using the WAMIT software. Unsteady hydrodynamic pressure, wave loads on the bodies and the motion of the bodies were evaluated in WAMIT. For this evaluation the boundary value problems are solved by using the Green's theorem to derive integral equations for the radiation and diffraction velocity potentials on the body boundary. Once the diffraction and radiation velocity potential were calculated, the wave exciting forces on the bodies and the hydrodynamic added masses and the damping coefficient can be calculated. In the lights of these calculations equation of the motions of the observation buoys were solved and the response amplitude operators of heave, surge and pitch motions of the buoys were calculated. According to these calculations the observation buoy with the greatest heave motion response amplitude operator is Buoy90 and the buoy with the lowest heave motion response amplitude is Buoy45. The buoy with the greatest pitch motion response amplitude operator is Buoy45 and the buoy with the lowest pitch motion response amplitude is Buoy90. The buoy with the greatest surge motion response amplitude operator is Buoy90 and buoy with the lowest surge motion response amplitude is Buoy45. Offshore observation buoys have their greatest response amplitudes at their natural frequencies of the given motion mode. Therefore the motions of the buoys are not only signified by the maximum response amplitudes but also with their natural frequencies. The motions of the buoys are evaluated by the motion spectrums which present the relationship between the response amplitude operators of the buoys and the wave energy spectrum of the deployment region.
	Motions spectrum of the buoys were obtained by the superposition of the response amplitude operators of the given motion mode and the wave energy spectrum of the given region. Motions spectrums are also called response spectrums or motion energy spectrums which indicate the frequency intervals where motion energy is concentrated in the given region. The moments of the motions spectrums are used to calculate the significant motion amplitudes of the structures average periods of the motions. In this study, wave energy spectrums of the Adalar region were calculated for the maximum and the average wind speed records. Therefore the motion spectrums of the buoys were obtained by using the wave energy spectrums and the response amplitude operators. Heave motion spectrums of the buoys showed that at the maximum wind speed conditions, all the buoys have the same motion spectra because the wave energies are concentrated in very low frequencies and in the low frequencies all the buoys have the same heave response amplitude operators and at the average wind speed conditions, the Buoy90 has the greatest and the Buoy45 has the lowest significant heave motion amplitude. 
	Pitch motion spectrums of the buoys showed that at the maximum wind speed conditions, the Buoy75 have the greatest significant pitch motion amplitude even though the Buoy45 has the greatest pitch response amplitude operator. This situation occurs as a result of the coincidence of the natural frequency of the Buoy75 and the critical frequency of the wave energy spectra. The lowest significant pitch motion amplitude belongs to the Buoy90. At the average wind speed conditions the maximum significant pitch motion amplitude belongs to the Buoy45 and the minimum significant amplitude belongs to the Buoy90.
	The greatest significant surge motion amplitude was obtained for the Buoy45 with the both maximum and average wind speed conditions. At maximum wind speed conditions, the other buoys have the same significant surge motion amplitude while at the average wind speed conditions the Buoy90 has the lowest significant surge motion amplitude. 
	In conclusion, the results of the study shows that even with the wave energy spectrum and the frequency characteristics of the buoys, the choice of the most suitable observation buoy geometry is not a simple task. The measurement techniques, instruments and projected service time of the buoy should also be under consideration for the choice of the geometry. None of the buoy geometries have the smallest amplitudes for all modes of motions as it was highlighted by the hydrodynamic analyzes. Therefore the motion modes of the buoys which effects the functions of the buoys most negatively should be investigated carefully and the most suitable geometry must be chosen according to this approach. The effect of the jaw-line slope of an observation buoy was investigated in this study and results showed that as the slope of the jaw-line gets higher, the motion amplitude of the heave motion gets higher while the amplitudes of the surge and pitch motions gets lower. The experimental comparison of this numerical analysis should be conducted in the future for the verification of the numerical results and the better understanding of the motions of the offshore observation buoys.