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Collaborating Authors
Results
VIM Tow Model Test of TLP with Round Columns
Kou, Yufeng (Shanghai Jiao Tong University) | Lu, Haining (CISSE - Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Yin, Hanjun (Shanghai Jiao Tong University) | Cai, Yuanlang (CISSE - Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration)
Abstract Vortex Induced Motion (VIM) of Tension Leg Platform (TLP) can cause fatigue stresses on the risers and tendons and must be taken into account in design. The VIM response of a TLP hull with 4 round columns is investigated by tow model test, which was conducted by State Key Laboratory of Ocean Engineering in Shanghai Jiao Tong University (SKLOE, SJTU) from August to September of 2015. A 1:50 TLP hull model is built accurately in geometry, including the appurtenances such as anodes, tendon porches, fenders, supports, caissons and other pipe-like structures running on the hull. However, the topsides are neglected and replaced by an aluminum deck which is strong enough to support the air bearing system and horizontal mooring system. The air bearing system composed of six air bearings and a smooth flat-plate is applied to simulate the vertical load of tendons and TTRs, which allows matching the same mass ratio with prototype in model test. The horizontal restoring force and moment of initial tendon and TTR system are modeled by 4-spring horizontal mooring system mounted above water, ensuring the correct natural periods for surge, sway and yaw motion of the TLP. The current is simulated by carriage speed in towing tank. Screening tests for 16 current headings with the interval of 22.5 show different VIM response in every heading. The comparatively larger VIM amplitude in transverse direction, about half of column diameter, occurs in 90 and 337.5 current headings. The lock-in range of reduced velocity between 6 and 8 is also observed in 90 and 337.5 current headings. Introduction Due to the increasing draft, VIM of multi-column floating platforms under sea current may become notable, and cause fatigue stresses on the risers and mooring systems. The interference among columns makes more complex VIM phenomenon than Spars and mono-columns. Liu et al (2015a) experimentally investigated the flow and force on fixed four-square-cylinder array, revealed that the downstream cylinders experience smaller mean drag force but higher fluctuating force than the upstream ones, and that the interaction was highly related to current heading and spacing ratio. Gonçalves et al (2012a, 2012b, 2013) studied a lot on VIM of Semi-submersible with four square columns by tow model test. It was found that not only VIM in transverse direction but also considerable yaw motion oscillation occurred. The largest transverse amplitudes can reach 40% of column width for 30 and 45 incidences. However, the maximum yaw motions were found about 4.5 for 0 incidence. Comparatively, the largest in-line motions were no more than 15% of column width. It was also found that the hull appurtenances had some influence on the VIM response of Semi-submersible, especially the pipes located at columns. The external damping was proven another critical factor to determine the VIM performance, 20% extra damping can decrease 50% of VIM amplitude for some specific response frequencies. Gonçalves et al (2015) also discovered the different behavior of round and square-rounded columns in terms of transverse and yaw response for different incidence angles. Bai et al (2013) studied the VIM of new type Deep Draft Semi-submersible (DDS) with four rectangular columns by tow model test and 2D numerical simulation. The lock-in range of reduced velocity between 6 and 8 was obtained for 135 incidence. Because of over-simplified model, the numerical simulation predicted broader lock-in range and larger response amplitude. Actually, even the 3D CFD method is hard to accurately predict the VIM performance of multi-column floaters with consideration of the appendages, because of the great difference in size between the hull and attached structures. The model test is the preferred solution to investigate the VIM response of a new platform. The effect of current velocity and heading, hull appurtenance, and external damping should be taken into account in model test, and the transverse and yaw motion must be mainly focused on.
A Preliminary Analysis on the Statistics of about One-Year Air Gap Measurement for a Semi-submersible in South China Sea
Ge, Xiaona (Shanghai Jiao Tong University) | Tian, Xinliang (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Yang, Jianmin (Shanghai Jiao Tong University) | Kou, Yufeng (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration)
Abstract Air gap performance has been a key issue in the design of semisubmersible platform. In this paper, a comprehensive study on air-gap is carried out based on the field measurement data of a semisubmersible in South China Sea using statistical analysis. The analysis shows that the energy extremes of air gap mainly appear in the wave frequency range. By fitting the probability density and cumulative probability distribution curve, the air gap extreme can be predicted. And the air gap performance of the platform is relatively stable in different months because the curves in different months appear very close. Moreover, the correlation between air gap and vertical motion of the platform is analyzed, which indicates that parts of the lowfrequency data are unreliable, and a method to evaluate the reliability of the field measurements is proposed. Introduction Semi-submersible offshore drilling platform has drawn a wide range of attention in the offshore community because of its large deck area, large displacement, strong wind resistance and excellent motion performance. One of the key issues regarding the performance of the semi-submersible platform is the air gap which is defined as the vertical distance between the lower deck of platform and the wave surface. Sufficient air gap should be ensured to reduce the possibility of the damage from the wave impact on the lower deck. On the other hand, a larger air gap would result in a great change in draft and gravity center of the platform, which may directly affect the overall design and cost of the platform. Thus, the air gap is often determined based on the compromise among those concerns. Air gap is closely related to two aspects, the wave elevation at the specific position of the platform and the vertical motion response of the platform. Currently most requirement and restrictions of the air gap are on the minimum value during the service life. In fact, its characteristics and probability distribution are also important in addition to extreme values. Simply elevating deck of the platform to reduce the wave impacts on the deck is neither a good idea nor economical. Sweetman (2004) proposed a method to reduce the cost. The principle of the method is to reinforce the structures which are more likely to be impacted by the waves.