PROBABILISTIC SAFETY OF ESTUARY VESSELS BASED ON NONLINEAR ROLLING IN WIND AND WAVES

The author was previously involved in the development of the risk-based stability analysis which is now further extended, and used for the safety assessment of estuary container vessels subjected to stochastic action of beam wind and irregular waves. The study was motivated by the new set of safety regulations for estuary vessels issued by Belgian authorities in cooperation with Lloyd’s Register. These regulations introduce very innovative probabilistic ideas to ship stability regulations, and therefore present a significant step forward compared to the classical approach. Still, they do not account properly some important influences, such as wind gusts and motion nonlinearities, so considerably simplify the problem. The present investigation models the vessel motion much more realistically, analyzes the influence of beam wind and beam waves on the probability of a stability failure, and argues whether simplifications proposed by the regulations were justified. It is believed that presented method is not limited to the safety of estuary vessels only, but also gives important guidelines for a more general investigation of ship safety in wind and waves.

Numerical study on deployment of subsea template using coupled and uncoupled model

This study compares deployment of a subsea template simulated as a coupled model and as an uncoupled model in the time domain simulation software Orcaflex. Defining vessel motion as prescribed simplifies the model and will therefore also decrease the simulation time. Models with predefined vessel motions are called uncoupled models. Vessel motion in a coupled model is a continuously calculated reaction to the forces acting on the vessel. Some software might struggle to run coupled models. The deployment simulations are narrowed down to focus on the incident where the template crosses the splash zone when lifted with an offshore construction vessel. Noticeable differences between the allowable sea state results are observed from the two different simulation methods. Running the time domain simulation as an uncoupled model gives lower allowable sea states than the results from the coupled time domain simulation model.

Adaptively robust filtering algorithm for maritime celestial navigation

Celestial navigation is an important means of maritime navigation; it can automatically achieve inertially referenced positioning and orientation after a long period of development. However, the impact of different accuracy of observations and the influence of nonstationary states, such as ship speed change and steering, are not taken into account in existing algorithms. To solve this problem, this paper proposes an adaptively robust maritime celestial navigation algorithm, in which each observation value is given an equivalent weight according to the robust estimation theory, and the dynamic balance between astronomical observation and prediction values of vessel motion is adjusted by applying the adaptive factor. With this system, compared with the frequently used least square method and extended Kalman filter algorithm, not only are the real-time and high-precision navigation parameters, such as position, course, and speed for the vessel, calculated simultaneously, but also the influence of abnormal observation and vessel motion status change could be well suppressed.

Numerical Study on the Tank Heel Determination Using Smoothed Particle Hydrodynamics

Tank heel minimization is a significant issue in the design of LNG fuel tanks because it is associated with stable suction pump operation and thermal shock requirements during LNG bunkering. This study examined how the LNG tank heel is minimized, maintaining a suction pump fully submerged in LNG during dynamic vessel motion. The study assumed two LNG fuel tanks mounted on the forward deck of a 50,000 deadweight class oil product carrier. Information on the dimensions and shape of the LNG fuel tank was determined from Wartsila’s brochure, and the specifications of Vanzetti’s suction pump were referred to. The LNG fuel tank and LNG heel were modeled as rigid elements and hydrodynamically smoothed-particles, respectively. The number of particles could be determined by performing even keel analyzes by adding or subtracting particles until the target head was satisfied under the gravity load. To simulate the motion of the LNG fuel tank, the pitch and roll periods and amplitudes of the ship were calculated using the DNV classification rules. Visual observations of the dynamic flow during the pitch and roll motions with respect to the ship’s center of mass showed that the roll motion was more critical from the viewpoint of the LNG heel than the pitch motion. After performing the simulations for three cycles of roll and pitch motions, the suction pump submergence was reviewed in the last cycle. Under the conditions assumed in this study, a filling ratio of 15% was determined as the minimum LNG tank heel. Although the LNG heel has customarily been determined, the LNG heel needs to be determined through hydrodynamic analyses of each vessel because it depends on the shape of the fuel tank and the vessel motion characteristics.

Autonomous Obstacle Avoidance Algorithm for Unmanned Surface Vehicles Based on an Improved Velocity Obstacle Method

Focusing on the collision avoidance problem for Unmanned Surface Vehicles (USVs) in the scenario of multi-vessel encounters, a USV autonomous obstacle avoidance algorithm based on the improved velocity obstacle method is proposed. The algorithm is composed of two parts: a multi-vessel encounter collision detection model and a path re-planning algorithm. The multi-vessel encounter collision detection model draws on the idea of the velocity obstacle method through the integration of characteristics such as the USV dynamic model in the marine environment, the encountering vessel motion model, and the International Regulations for Preventing Collisions at Sea (COLREGS) to obtain the velocity obstacle region in the scenario of USV and multi-vessel encounters. On this basis, two constraint conditions for the motion state space of USV obstacle avoidance behavior and the velocity obstacle region are added to the dynamic window algorithm to complete a USV collision risk assessment and generate a collision avoidance strategy set. The path re-planning algorithm is based on the premise of the minimum resource cost and uses an improved particle swarm algorithm to obtain the optimal USV control strategy in the collision avoidance strategy set and complete USV path re-planning. Simulation results show that the algorithm can enable USVs to safely evade multiple short-range dynamic targets under COLREGS.

On Vessel Motion Induced Vortex-Induced Vibrations of a Steel Lazy Wave Riser

Deepwater steel lazy wave risers (SLWR) subject to vessel motion will be exposed to time-varying oscillatory flow, vortices could be generated and the cyclic vortex shedding force causes the structure vibrate, such fluid-structure interaction is called vortex-induced vibrations (VIV). To investigate VIV on a riser with non-linear structures under vessel motion and oscillatory flows, time domain approaches are needed. In this study, a time-domain approach is used to simulate a full-scale SLWR. Two cases with simplified riser top motions are simulated numerically. By using default input parameters to the time domain approach, the key oscillatory flow induced VIV response characteristics such as response frequency, curvature and displacements are examined and discussed. More accurate VIV prediction could be achieved by using realistic hydrodynamic inputs into the time domain model.