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Meng, Xun (Ocean University of China, Shandong Provincial Key Laboratory of Ocean Engineering) | Liu, Meng (Ocean University of China) | Huang, Weiping (Ocean University of China, Shandong Provincial Key Laboratory of Ocean Engineering) | Fu, Qiang (Ocean University of China, CIMC Offshore Engineering Institute Company Limited)
This paper studies on the typical design load cases that dominate the characteristics of structural stress distributions. The OC4-DeepCwind conceptual semi-submersible substructure with the 5.0MW floating offshore wind turbine (FOWT) is adopted as the target structure. Parametric finite element method (FEM) is employed for idealized numerical modelling. Different environmental load cases controlled by random variable parameters such as wave directions, phases, heights and periods are imported into static probabilistic design system (PDS) as samples. Core areas with localized stress concentration based on probability statistics and corresponding typical design load cases are summarized. This study presents a method of effectively simplifying the complicated dynamic strength analysis procedures and would serve as a reference of reasonable optimization of main dimensions of the semi-submersible FOWTs.
Due to the depletion reserves and negative environmental influences of fossil fuels, human beings have been forced to seeking for alternative energy sources. According to the report of Intergovernmental Panel on Climate Change (IPCC), nearly 80 percent of the world’s energy supply could be provided by renewable energy resources in 2050, and wind energy would make up one of the largest contributions to the energy system by then (Sun, Huang and Wu, 2012). Nowadays wind energy industry has moved its interest offshore. Reference shows that offshore wind power will cover 14% of European electricity demand by 2030 (Athanasia, Anne-Bénédicte, and Jacopo, 2012). In the first half of 2017, developers have totally installed about 6.1GW of capacity, including 1.3GW in Europe. The activity in the offshore market is 2.6 times higher than for the first half of 2016 (WindEurope, 2017).
Most offshore wind farms so far are installed and operating in shallow waters (<30m), where bottom-fixed foundations with simplified structure concepts such as monopile and gravity concrete caisson are widely used (Failla and Arena, 2015). At water depths between 30m and 60m, multi-foot foundations such as tripod or jacket support are considered (Lozano-Mjinguez, Kolios and Brennan, 2011). For the benefits of relatively unrestricted space, lower social impacts and rich wind resources, wind farms are pushed into deeper waters. For cost-effective solutions, floating offshore wind turbines (FOWTs) become feasible options to extract energy (Meng, Lou and Shi, 2014).
In order to study vortex-induced motions (VIM) response of floating cylinders, a model experiment was conducted in a water flume at reduced velocity from 1.3 to 10.2. Considering the aspects of response amplitude, vortex shedding frequency, force analysis, and motion trajectories, the VIM characteristics of cylinders with and without helical strakes were analyzed. According to the results, the ratio of vortex shedding frequency to natural frequency increased linearly with increasing reduced velocity before and after the lock-in district, with St≈0.18 before lock-in district and St≈0.14 after it. Helical strakes change the motion trajectories and frequencies very obviously and deliver remarkable VIM suppression effects.
For a long time, the phenomena of vortex-induced vibrations (VIV) has drawn the attention of many researchers. In the field of ocean engineering, many studies have been conducted on the vortex-induced vibrations of flexible structures that have extremely high aspect ratio, such as submarine oil pipelines and risers.
Since the 1990s when Spar platform was used in 588m deep water of the Gulf of Mexico for the first time, low-frequency and large-amplitude motions caused by the action of the Gulf Stream have become a focus of industrial research (Kokkinis et al.,2004). On the one hand, the main structure of the Spar platform is a deep draft column, which will also cause vortex shedding at a certain flow velocity. Therefore, the Spar platform performs the corresponding motion characteristics which is similar to the theory of VIV. On the other hand, the platform structure is rigid, has a relatively low aspect ratio and floats on the surface of water, altering its motion significantly from the predictions of VIV theory. To distinguish the two, researchers refer to the motion of floating structures as vortex-induced motions (VIM) (Mehernosh et al.,2008).
Large-amplitude vortex-induced motions (VIM) will lead to fatigue damage to risers and anchor chains, shorten fatigue lifetime. On engineering, helical strakes are often used to reduce the amplitude of VIM. Irani (2005) conducted model tests to study the effectiveness of Truss Spar helical strakes. The tests simulated several important parameters, including the geometrical shape and quality of the platform hull and truss, the mooring system characteristics, and the different shapes of strakes. Wang (2010) designed vortex suppression helical strakes for a new concept Cell-Truss Spar platform and obtained an optimal parameter combination with a combination of model experiments and numerical simulations. With orthogonal experimental designs and numerical simulation, Wei (2013) studied the influence of pitch, strake height, current velocity and other parameters on the vortex suppression offered by helical strakes. Wei also performed range analysis and variance analysis on the data to observe the effects of strake parameters and interactions on the efficiency of the VIM suppression.
A method based on Reynolds number similarity, of the vortex-induced vibration (VIV) of circle cylinders is proposed to achieve VIV similarity between prototype and tested model. Because the mode of vortex shedding highly depends on Reynolds number, the VIV response of a circle cylinder is closely related to Reynolds number and reduced velocity. However, the scaled model test of circle cylinder’s VIV is nowadays designed based on Froude number similarity but Reynolds number not similar under the same fluid for both model and prototype. Therefore, the VIV response of tested model is not similar to that of the prototype because they have different vortex shedding modes. It means that the test results can not be used to predict the VIV response of the prototype according to the scaling law based on Froude number similarity. In this paper, the relationship between prototype and model based on Reynolds number similarity has been deduced. The prototype and three scaled models with different similarity schemes have been simulated using CFD to validate the method. The results show that the similarity between prototype and model is satisfied by the Reynolds number similarity and both Froude number and Reynolds number similarity. But the similarity between prototype and model is not satisfied by Froude number similarity.
In this paper, a new concept of ETLP has been proposed. It is composed of four square columns and a ring pontoon which is consisted of four box beams. The new platform has lesser blocks and welds compared to ETLP, so it can be built at a lower cost and in a shorter construction period. Meanwhile, the pontoon extensions in the new platform is part of pontoons, therefore, the fatigue problem in the welds at the root of extensions in ETLP is solved. A hydrodynamic analysis is conducted to prove the structure’s dynamically stabilities. The results showed the new design has a reasonable hydrodynamic characteristic.
The User-Defined Function (UDF)and dynamic mesh technique are used to simulate the circular cylinder motion. An Elastically supported cylinder placed in uniform flow and also in combination of uniform and oscillating flows at low Reynolds number values are simulated. The uniform and oscillating flows are both in the x direction. SIMPLE algorithm is used to solve the Navier-Stokes equations. In the paper, we compute the lift and oscillating motion amplitude as functions of Reynolds number and time. Simulations show the vortex induced vibration process of an elastically supported circular cylinder in uniform flow and compare the oscillating motion amplitude and amplitude of lift coefficient in uniform flow and combination of uniform and oscillating flow.
Steel Catenary Risers (SCRs) became the preferred riser systems with the development of deepwater oil and gas exploration. A new model for the dynamic analysis of SCRs is proposed in the paper, the rigid motion, which is the rigid rotation about the axis from the hanging point to the touch down point (TDP), is simulated in the new model. The rigid motion of SCRs is coupled with bending vibration model as inertial force and hydrodynamic damping, and then the out-of-plane motion of SCRs with both bending vibration and rigid motion are simulated. The case studies show that the rigid motion affects the dynamic response of SCRs greatly and can not be ignored in their dynamic analysis.
Deng, Yue (Department of Ocean Engineering, Ocean University of China) | Huang, Weiping (Department of Ocean Engineering, Ocean University of China) | Zhao, Jingli (Marine Technology Center, Shandong Marine Fisheries Research Institute)
This paper presents a new type of Spar platform named S-Spar. Its midsection is a cylinder with the same diameter as the centre well. And the centre well and midsection is designed as an integrated structure. Heave plates are attached appropriately along the connection section. With the unique midsection, S-Spar is suitable for operating at the special oceanic environment and ultra-deep water depth in South China Sea. The paper will discuss both structural design and hydrodynamic analysis. Detail motion analysis results show that the platform offers excellent motion characteristics, and optimizing to carry large payloads in ultra-deep water. Finally, the effect of potential and viscous damping in different region has been analyzed.
As the offshore oil exploitation activities expending to deep water and even ultra-deep water, many new types of floating structures suitable for this depth are being developed concomitantly. Compared with other floating structures, Spar platform has excellent stability, benign motion behavior and adaptation to wide range of water depth. Besides, the rigid risers and dry tree system can be used in Spar platforms. Due to so many advantages depicted above, Spar platform is then regarded as an attractive design solution for regions of deep water. Since the first generation of Classic Spar has come to use, the Spar platform has been evolved into the second generation of Truss Spar and the latest Cell Spar (Finn, Maher and Gupta, 2003). With the rapid development of deep-water oil industry, many new technologies and innovative Spar concepts are frequently being proposed to adapt to new operation conditions and ultra-deep water depth. Nowadays, the South China Sea has focused the attention due to the rich storage of oil and natural gas. So the exploitation of the South China Sea is a hot and attractive task and has broad prospects.
Tang, Shizhen (Shandong Key Laboratory of Ocean Engineering, Ocean University of China) | Huang, Weiping (Shandong Key Laboratory of Ocean Engineering, Ocean University of China) | Deng, Yue (Shandong Key Laboratory of Ocean Engineering, Ocean University of China) | Liu, Jianjun (Shandong Key Laboratory of Ocean Engineering, Ocean University of China)