Abstract
Triceratops is one of the new generations of offshore compliant platforms suitable for ultra-deepwater applications. Apart from environmental loads, the offshore structures are also susceptible to accidental loads. Due to the increase in the risk of collision between ships and offshore platforms, the accurate prediction of structural response under impact loads becomes necessary. This paper presents the numerical investigations of the impact response of the buoyant leg of triceratops usually designed as an orthogonally stiffened cylindrical shell with stringers and ring frames. The impact analysis of buoyant leg with a rectangularly shaped indenter is carried out using ANSYS explicit analysis solver under different impact load cases. The results show that the shell deformation increases with the increase in impact load, and the ring stiffeners hinder the shell damage from spreading in the longitudinal direction. The response of triceratops is then obtained through hydrodynamic response analysis carried out using ANSYS AQWA. From the results, it is observed that the impact load on single buoyant leg causes periodic vibration in the deck in the surge and pitch degrees of freedom. Since the impact response of the structure is highly affected by the geometric and material properties, numerical studies are also carried out by varying the strain rate, and the location of the indenter and the results are discussed.
Key Words
impact analysis; stiffened cylindrical shell; ring frames; stringers; triceratops
Address
Srinivasan Chandrasekaranand R. Nagavinothini: Department of Ocean Engineering, IIT Madras, India
Abstract
Herein, we present numerical simulation based model to study the use of a \'Tuned Mass Damper (TMD)\' - particularly spring mass systems - to control the displacements at the deck level under seismic and ice loads for an offshore jacket structure. Jacket is a fixed structure and seismic loads can cause it to vibrate in the horizontal directions. These motions can disintegrate the structure and lead to potential failures causing extensive damage including environmental hazards and risking the lives of workers on the jacket. Hence, it is important to control the motion of jacket because of earthquake and ice loads. We analyze an offshore jacket platform with a tuned mass damper under the earthquake and ice loads and explore different locations to place the TMD. Through, selected parametric variations a suitable location for the placement of TMD for the jacket structure is arrived and this implies the design applicability of the present research. The ANSYS*TM mechanical APDL software has been used for the numerical modeling and analysis of the jacket structure. The dynamic response is obtained under dynamic seismic and ice loadings, and the model is attached with a TMD. Parameters of the TMD are studied based on the \'Principle of Absorption (PoA)\' to reduce the displacement of the deck level in the jacket structure. Finally, in our results, the proper mass ratio and damping ratios are obtained for various earthquake and ice loads.
Key Words
anti-vibration device; parametric study; vibration control effect; seismic time history; dynamic ice force
Address
R.K. Sharma: L&T-Valdel Engineering Limited, India
V. Domala: RIMSE, CADIT Lab, Seoul National University, Republic of Korea
V. Domala: RIMSE, CADIT Lab, Seoul National University, Republic of Korea
R. Sharma: Design and Simulation Laboratory, Department of Ocean Engineering, IIT Madras, India
Abstract
Tracking the location (position) of a surface or underwater marine vehicle is important as part of guidance and navigation. While the Global Positioning System (GPS) works well in an open sea environment but its use is limited whenever testing scaled-down models of such vehicles in the laboratory environment. This paper presents the design, development and implementation of a low energy ultrasonic augmented single beacon-based localization technique suitable for such requirements. The strategy consists of applying Extended Kalman Filter (EKF) to achieve location tracking from basic dynamic distance measurements of the moving model from a fixed beacon, while on-board motion sensor measures heading angle and velocity. Iterative application of the Extended Kalman Filter yields x and y co-ordinate positions of the moving model. Tests performed on a free-running ship model in a wave basin facility of dimension 30 m by 30 m by 3 m water depth validate the proposed model. The test results show quick convergence with an error of few centimeters in the estimated position of the ship model. The proposed technique has application in the real field scenario by replacing the ultrasonic sensor with industrial grade long range acoustic modem. As compared with the existing systems such as LBL, SBL, USBL and others localization techniques, the proposed technique can save deployment cost and also cut the cost on number of acoustic modems involved.
Key Words
Low Energy Ultrasonic based Beacon (LEUB), Extended Kalman Filter (EKF), Autonomous Surface Vehicle/ Underwater Vehicle (ASV/UV)
Address
Awanish C. Dubey and V. Anantha Subramanian: Department of Ocean Engineering, Indian Institute of Technology, Madras, Chennai, India
V. Jagadeesh Kumar: Department of Electrical Engineering, Indian Institute of Technology, Madras, Chennai, India
Abstract
Sloshing is a phenomenon which may lead to dynamic stability and damages on the local structure of the tank. Hence, several anti-sloshing devices are introduced in order to reduce the impact pressure and free surface elevation of liquid. A fixed baffle is the most prevailing anti-sloshing mechanism compared to the other methods. However, the additional of the baffle as the internal structure of the LNG tank can lead to frequent damages in long-term usage as this structure absorbs the sloshing loads and thus increases the maintenance cost and downtime. In this paper, a novel type of floating baffle is proposed to suppress the sloshing effect in LNG tank without the need for reconstructing the tank. The sloshing phenomenon in a membrane type LNG tank model was excited under sway motion with 30% and 50% filling condition in the model test. A regular motion by a linear actuator was applied to the tank model at different amplitudes and constant period at 1.1 seconds. Three pressure sensors were installed on the tank wall to measure the impact pressure, and a high-speed camera was utilized to record the sloshing motion. The floater baffle was modeled on the basis of uniform-discretization of domain and tested based on parametric variations. Data of pressure sensors were collected for cases without- and with-floating baffle. The results indicated successful reduction of surface run-up and impulsive pressure by using a floating baffle. The findings are expected to bring significant impacts towards safer sea transportation of LNG.
Key Words
sloshing; floating baffle; membrane tank; LNG
Address
Hooi-Siang Kang: Marine Technology Centre, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia;
School of Mechanical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia
Ummul Ghafir Md Arif: School of Mechanical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia
Kyung-Sung Kim: School of Naval Architecture and Ocean Engineering, Tongmyong University, Busan, Republic of Korea
Moo-Hyun Kim and Yu-Jie Liu: Department of Ocean Engineering, Texas A&M University, College Station, Texas, USA
Kee-Quen Lee: Department of Mechanical Precision Engineering, Malaysia-Japan International Institute of Technology,
Kuala Lumpur, Malaysia
Yun-Ta Wu: Department of Hydraulic and Ocean Engineering, National Cheng Kung University, Tainan, Taiwan (R.O.C)
Abstract
Very Large Floating Structures (VLFS) are one among the solution to pursue an environmentally friendly and sustainable technology in birthing land from the sea. VLFS are extra-large in size and mostly extra-long in span. VLFS may be classified into two broad categories, namely the pontoon type and semi-submersible type. The pontoon-type VLFS is a flat box structure floating on the sea surface and suitable in regions with lower sea state. The semi-submersible VLFS has a deck raised above the sea level and supported by columns which are connected to submerged pontoons and are subjected to less wave forces. These structures are very flexible compared to other kinds of offshore structures, and its elastic deformations are more important than their rigid body motions. This paper presents hydroelastic analysis carried out on an innovative VLFS called truss pontoon Mobile Offshore Base (MOB) platform concept proposed by Srinivasan and Sundaravadivelu (2013). The truss pontoon MOB is modelled and hydroelastic analysis is carried out using HYDRAN-XR for regular 0 waves heading angle. Results are presented for variation of added mass and damping coefficients, diffraction and wave excitation forces, RAOs for translational, rotation and deformational modes and vertical displacement at salient sections with respect to wave periods.
Key Words
Hydroelasticity; Mobile Offshore Base (MOB); Hydrodynamic coefficient; fluid forces; Response amplitude operator; vertical displacement; longitudinal and bending stresses
Address
S. Somansundar and R. Panneer Selvam: Department of Ocean Engineering, Indian Institute of Technology Madras,
Chennai – 600036, Tamil Nadu, India
D. Karmakar: Department of Applied Mechanics and Hydraulics, National Institute of Technology Karnataka,
Surathkal, Mangalore – 575025, India
