Tekil kazık etrafında dalga ve akıntı etkisi altında oyulma ve geri dolma olaylarının incelenmesi
The investigation of scour and backfilling mechanism around a single slender pile in steady current and waves
Yazar: BERRAK DOĞAN
Danışman: DOÇ. DR. VEYSEL ŞADAN ÖZGÜR KIRCA ; DOÇ. DR. MEHMET SİNAN ÖZEREN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kıyı Bilimleri ve Mühendisliği Ana Bilim Dalı / Kıyı Bilimleri ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Su dalgaları = Water waves ; Tekil kazık = Single pile

Özet:
Temiz enerji kaynaklarının kullanımının yaygınlaşmasıyla birlikte, açık denizde kurulan rüzgar çiftliklerinin sayısı epey artmıştır. Bu yapılar genellikle tekil kazık üzerinde inşa edilen ağır yapılar olduğundan, oyulma ve geri dolma mekanizmasının anlaşılması son derece önemlidir. Çünkü oyulma, yapının stabilitesine önemli bir tehdittir. Bu olaylar, yapıların yorulma analizleri yapılırken dikkate alınabileceği gibi, yapıların tasarımında aşırı emniyetli tarafta kalınmasının da önüne geçebilir.
Deniz ortamına yerleştirilen bir yapı, akıntının, dalganın veya dalga ve akıntının birlikte etkisine maruz kalır. Yapıyla etkileşime giren akım değişime uğrayarak, yapı dolaylarındaki katı madde taşınım kapasitesinin artmasına neden olur ve oyulma meydana gelir. Dalga ve akıntı etkisinde meydana gelen oyulma ve geri dolma olaylarını yöneten esas faktörler birbirinden farklı olduğundan, birbirinden ayrı olarak incelenmelidir.
Oyulmanın meydana gelmesinde ana unsur akıntı iken, geri dolma mekanizmasını dalgalar kontrol etmektedir. Eğer incelenen andaki denge derinliği başlangıçtaki derinlikten daha büyükse oyulma gerçekleşir. Öte yandan denge derinliğinin anlık oyuk derinliğinden daha sığ olması durumu geri dolma olayının gerçekleşmesi için yeterli değildir. Mevcut oyulmanın geri dolma mekanizmasını kontrol eden olay dalgalar olduğundan, dalgaların dominant etkisinde hareketli taban durumunun da mevcut olması gerekmektedir.
Bu çalışmada, başka bir çalışmadan temin edilen su derinliği, dalga ve akıntı zaman serileri, oyulma ve geri dolma olaylarının incelenmesi üzerine kurulan modelde girdi parametreleri olarak kullanılmak üzere, dijitalleştirilmiştir. İkinci bir test yürütmek için aynı çalışmadan dalga, akıntı ve su derinliği gibi veriler alınmıştır. Yürütülen her iki test için dalga ve akıntı parametreleri sırayla belirlenmiştir. Hesap edilen bu parametreler oyulmanın denge derinliği ve zaman ölçeğini belirlemekte kullanılmıştır. Oyulmanın denge derinliği ve zaman ölçeği kullanılarak yapı etrafındaki oyulma derinliğinin zamana bağlı değişimi araştırılmış ve verilerin sağlandığı çalışmadaki bulgularla kıyaslanmıştır. Elde edilen sonuçlara göre, oluşan oyulmanın geri dolma zaman ölçeği oyulmaya kıyasla çok daha yavaş olmaktadır.
Akabinde, sadece dalga koşulları altında gerçekleşen oyulma ve geri dolma olayının incelendiği parametrik bir çalışma yapılmıştır. Bu çalışmalarda uzun dönem istatistiksel özellikleri Weibull dağılımına uyan rastgele bir dalga zaman serisi elde edilmiştir. Aynı dalgaların büyükten küçüğe ve küçükten büyüğe sıralandığı durumlar ile gelişigüzel olduğu halde gerçekleşen oyulma ve geri dolma olayları incelenmiştir. Böylelikle, farklı dalga yüksekliklerinin etkime sırasının değişmesi halinde oyulma ve geri dolma olaylarının nasıl etkileneceği araştırılmıştır. Bu çalışmalar sonucunda, büyük periyodlu dalgalar etkisinde oyulma oluşurken, nispeten küçük periyodlu dalgaların etkisinde geri dolma meydana geldiği gözlenmiştir.

Abstract:
In recent years number of offshore windparks has increased because of a tendency to use of renewable energy. Monopile is the most used foundation type for this kind of structures. Therefore, it is extremely important to understand scour and backfilling mechanisms. Designers may pay attention to these incidents while performing fatigue analysis of structures and may be assisted to avoid overdesign of the structures.
When a structure is located in a marine environment, it is exposed to currents, waves and combined currents and waves. As flow interacts with the structure, the existence of the structure causes a change in flow characteristics and leads to scour. The time when the scour process stops, it means that scour reaches its equilibrium stage. Since main factors governing the scour mechanism under steady current and wave conditions are different from each other, these phenomena should be taken into consideration seperately.
In steady currents, the main factor that leads the scour process is the horseshoe vortex while the scour process is mainly governed by KC number in waves. As the boundary layer thickness, δ and Reynolds number increase, the size of the horseshoe vortex will be larger and it will cause the bed shear stress to increase as much as 5 times larger than its undisturbed value. This will lead to increase in Shields parameter which mainly governs the coastal sediment transport.
Backfilling can take place under waves and combined-waves-and-current condition if actual scour depth is deeper than the equilibrium scour depth and live bed condition exists. On the other hand, scour occurs if the actual scour depth is shallower than the equilibrium depth.
This study consists of two stages. In the first stage, two tests were performed and time series of tide, current velocity, wave height and wave period were provided from another study. These data have been collected from the field measurements and have been digitized via GetData Graph Digitizer Software. After obtaining digit numbers, they have been interpolated. In this way, each parameter was received in regular time intervals. These data were used for the second test. Data for the first test, on the other hand, were also obtained from the same study, but the test condition was a little simpler. Furthermore, test conditions were obtained from the same study so that the results could be compatible with each other. In the second stage, parametric studies were carried out and time scales of scour and backfilling process were investigated under waves alone case. Random wave time-series was generated by fitting its long-term wave climate characteristics to a Weibull distribution. It was sorted in ascending and descending order, thus it was analyzed how the scour and backfilling incidents were affected by different wave heights acting in different order.
In the current study, the calculation method for scour depth is as following: First, the scour depth was calculated at the end of Δt as if initial scour depth was equal to 0. Second, it was checked if calculated scour depth was lower or higher than initial scour depth. After determining whether scour or backfilling would occur, S(t+Δt) was calculated by a suitable equation for each step. In this study Hs was converted to Hrms values. In addition, a different approach was followed for the backfilling calculations.
Accordingly, in Test 1 it was investigated that scour mechanism in steady current and backfilling by waves. Input parameters were as following: D=0.5 m, h=4.6 m, d50= 0.5 mm, s=2.65 and Vc=2.0 m/s. After passing 30 minutes, the following parameters were changed Vc=0 m/s, Hs=2.0 m and Tp=6.4 s.
Wave and current parameters were calculated regarding to input parameters. Calculated values were as following: Wave length, L=39.74 m, maximum value of orbital velocity at the bed, Um=0.8717 m/s, wave friction factor, fw=0.0084, current friction velocity, Ufc=0.0749, wave friction velocity, Ufw=0.0566, Kaulegan-Carpenter number, KC=11.2, Shields parameter under current condition, θ=0.6927, critical Shields parameter under current condition, θcr=0.0423, Shields parameter under wave condition, θ=0.3956, critical Shields parameter under wave condition, θcr=0.0397. The equilibrium scour depth normalized by pile diameter, S/D was found as 1.2874 in current while it was found as 0.1864 in waves.
As a result, in this model it was found that the hole was backfilled and reached its equilibrium depth approximately 4 times slower than the original study. This was an expected result because, in the original study it was known that backfilling process would be slower than scour process, however, due to the limited knowledge an assumption had to be made that backfilling would be 10 times slower than scour process. On the other hand, in the current study empirical equations were used for calculating the time scale of backfilling. Therefore, it can be said that the results obtained for the time scale of backfilling in this study is much more trustable.
In Test 2, input parameters were as following: D=4 m, h=11 m + tide, d50= 0.5 mm and s=2.65, respectively. According to time series of the waves and current the first 2000 hours and the last 2500 hours waves were more effective in the marine environment while the current dominated the rest of the time. As a result, obtained pattern for time-varying scour depth were almost completely different from each other.
While waves were dominant for the first 2000 hours, small amount of the scour occurred and this result was compatible with the original study. However, there were constantly scour and backfilling process due to the misleading assumption in the original study. Since the time scale of backfilling predicted faster than it should be, the scour could reach its initial flat bed condition quickly.
In the parametric studies, input parameters were as following: D=1 m, h=10 m, d50= 0.13 mm, s=2.65 and Vc=0 m/s, respectively. The maximum KC number for the time-series was obtained as 14.4.
According to results of parametric studies the maximum scour depth was 0.29 m and it was reached at t=5050 in random waves. The process of scour and backfilling took place alternately and while there was backfilling, it was not able to reach its equilibrium value due to the slow process of backfilling. Under condition of waves in ascending order, instant scour depth was appeared to overlap with the equilibrium scour depth which meant that scour managed to reach its equilibrium depth in every step. While waves were in descending order, scour took only 12 minutes to reach its equilibrium stage. As the wave heights decreased, backfilling took place and had continued until t=80 hr. In this moment KC number was calculated as 6.6. After that, the backfilling process became much slower and it came to an end at t=1950 when the scour depth was equal to 0.015 m and θ was 1.3 times larger than its critical value.
According to studies it can be said that currents are much more effective on scour process than waves. Currents can lead to deeper scour depth in a short of time when proper flow conditions are occurred. On the other hand, waves govern the backfilling process, yet it takes such a long time to reach its equilibrium stage. When it comes to scour in waves, scour occurs during periods with high wave activity while backfilling takes place during periods with a bit smaller waves.