Experimental Investigation vs Numerical Simulation of the Dynamic Response of a Moored Floating Structure to Waves

A combined numerical/theoretical investigation of a moored floating structure response to incoming waves is presented. The floating structure consists of three bodies, equipped with fenders, joined by elastic cables. The system is also moored to the seabed with eight mooring lines. This corresponds to an actual configuration of a floating structure used as a multipurpose platform for hosting wind-turbines, aquaculture farms or wave-energy converters. The dynamic wave response is investigated with numerical simulations in regular and irregular waves, showing a good agreement with experiments in terms of time histories of pitch, heave and surge motions as well as of the mooring line forces. To highlight the dynamical behavior of this complex configuration, the proper orthogonal decomposition is used for extracting the principal modes by which the moored structure oscillates in waves giving further insights about the way waves excites the structure.

Harmonic Analysis on Low Quality Tidal Data

The aim of the study is to devise a method to conservatively predict a tidal power generation based on relatively short current profile measurement data sets. Harmonic analysis on a low quality tidal current profile measurement data set only allowed for the reliable estimation of a limited number of constituents leading to a poor prediction of tidal energy yield. Two novel, but very different approaches were taken: firstly a quasi response function is formulated which combines the currents profiles into a single current. Secondly, a three dimensional vectorial tidal forcing model was developed aiming to support the harmonic analysis with upfront knowledge of the actual constituents. The response based approach allowed for a reasonable prediction. The vectorial tidal forcing model proved to be a viable start for a full featuring numerical model; even in its initial simplified form it could provide more insight than the conventional tidal potential models.

Model Tests With the HVS Semisubmersible for Dry Tree Application

The global hull motion performance of the HVS semisubmersible for dry tree application is investigated with model tests. The HVS semisubmersible, which has been validated for low heave motion and VIM (Vortex Induced Motion) response, was modified for dry tree application. As a base case, the modification includes a keel plate with riser keel guides at the level of the pontoons. The keel plate is optimally designed to increase the hull heave period to compensate for the heave period reduction in the HVS semisubmersible due to the riser tensioners for the dry tree application. The plate also provides additional viscous damping that decreases the heave response at the heave natural period. The model tests were performed to investigate the in-place hull motion performance for the Gulf of Mexico environmental conditions. The pneumatic riser tensioners were modeled using a spring with dual stiffness. Because of the water depth limit in the wave basin, a truncated mooring was used to simulate the full scale prototype mooring system. An alternate modification to the HVS semisubmersible that includes pontoon plates was also tested and the measured response was compared to the response of the base case. The measured hull responses were correlated with MLTSIM, a Technip in-house nonlinear time-domain 6-DOF motion analysis program.

Trajectory Tracking of Underactuated Surface Vessels Based on Hierarchical Sliding Mode

Trajectory tracking is an importance practice in ship motion control field. It attracts more attention recently due to its difficulties. Trajectory tracking requires the ship to arrive pinpoint location at exact time. It is a underactuated system because the degrees of freedom of control inputs are fewer than the degrees of freedom that needed to be controlled. In this paper, a hierarchical sliding mode controller and a common sliding mode controller are proposed to deal with the trajectory tracking problem of underactuated surface vessels. Simulation results validate the tracking performance of the proposed controllers. The closed-loop stability is testified by the Lyapunov stability theorem.

Thruster and FPSO Hull Interaction

A hybrid method which couples a Vortex-Lattice Method (VLM) solver and a Reynolds-Averaged Navier-Stokes (RANS) solver is applied to simulate the interaction between a Dynamic Positioning (DP) thruster and an FPSO hull. The hybrid method could significantly reduce the number of cells to fifth of that in a full blown RANS simulation and thus greatly enhance the computational efficiency. The numerical results are first validated with available experimental data, and then used to assess the significance of the thruster/hull interaction in DP systems.

Theoretical Study on Hydroelastic Responses of Very Large Floating Structures Near Islands and Reefs

In order to explore the fishery, oil and gas, and tourism resources in the ocean, Very Large Floating Structures (VLFS) can be deployed near islands and reefs as a logistic base with various functions such as a floating harbor, accommodation, fishery processing, oil and gas exploration, environment surveillance, airplane landing and taking off, etc. However, in addition to the complicated hydroelastic coupling effects between the hydrodynamic loads and structural dynamic responses, when tackling the hydroelastic problems of floating structures deployed near islands and reefs, several other environmental effects and numerical techniques should be taken into account: 1) The influences of the non-uniform incident waves (multi-directions, different wave frequencies); 2) Complex seabed profile and its impact on the incident waves; 3) Nonlinear second order wave exciting forces in the complex mooring system, shallow water and coral reef geological conditions; 4) Parallel computing technology and fast solving methods for the large scale linear equations, accounting for the influence of dramatic increase of number of meshes to the computation efforts and efficiency. In the present paper the theoretical investigation on the hydroelastic responses of VLFS deployed near islands and reefs has been presented. In addition, based on the pulsating source Green function, the high performance parallel fast computing techniques and other numerical methods, in solving large scale linear equations, have been introduced in the three-dimensional hydroelastic analysis package THAFTS. The motions, wave loads, distortions and stresses can be calculated using the present theoretical model and the results can be used in the design and safety assessment of VLFS.

From Single Point Gauge to Spatio-Temporal Measurement of Ocean Waves: Prospects and Perspectives

With the recent advancement of spatial measurements of ocean waves, we are clearly facing new challenges regarding how to handle an expanded new data system when it becomes widely available. In this paper we wish to present a preliminary attempt at confronting these prospects. Because the data is still very limited at present and also conceptually new, it’s a new, unfamiliar, and unrelenting world to pursue. We need a paradigm shift away from our familiar single-point conceptualization in order to effective approach the new world of truly spatial ocean waves.

The Development of Meteorological and Oceanographic Data Collection and Recording System Operating on Training Ship

The observation at sea for marine meteorology is achieved by weather reports from merchant ship’s crew every 3 or 6 hours, mainly. However, the number of available observation data is insufficient for weather forecasting and marine environmental simulation, compared with landside. Nowadays, the special data collection function is required if the automatic observation data collection system is installed on ship. But, it is difficult to install special equipment onto general merchant ship. Therefore, we develop a prototype marine observation system, which can be installed various types of ships easily without any special data collection function for improving data collection source and/or period of the observation at sea. In this paper, a) the configuration of high reliability and durability marine observation system by using general-purpose PC and general meteorological / oceanographic sensors, b) outlook of utilizing the data, which collected by this system, are described.

Spar Platform Installation: Tank Flooding Simulation

Spars are towed to installation site horizontally and upended by progressive flooding of tanks. It is common practice to perform a dynamic time domain simulation for a self upending classic spar to determine hydrostatic pressures on compartments. There are many different flooding scenarios that create challenges in modeling and simulation during the design phase. In one particular scenario, the spar upending is initiated by opening valves that allow water to flood into the skirt tank. The skirt tank will progressively fill, based on the differential hydrostatic pressure at valves, and cause the spar to upend. Flooding into keel tanks will commence once respective openings become submerged. Several openings from the skirt tank into the keel tanks reduce the differential pressure experienced in the keel tanks during upending. Simulation of the transfer of water between tanks cannot be modeled with ease using the standard tank flooding options available within the software suite. This particular compartment flooding problem is solved by utilizing a scheme in which the time domain simulation was performed iteratively for a specified time interval. For every iteration the amount of water transferred between the skirt and keel tanks are calculated. The amount of water transferred is calculated using a custom modeling technique. The openings from the skirt tank into the keel tanks are not modeled as a typical hole or valve into a compartment, but the location of these holes are modeled. The amount of water flowing through these openings is determined by the water level in the skirt tank, friction through the opening, and pressure inside the keel tanks. This paper will describe in detail the scheme developed, the tank modeling requirements, and the results obtained.

Numerical Simulation of Short Duration Hydrodynamic Impact

Numerical simulations of wedge impact experiments, undertaken by the Naval Surface Warfare Center, Carderock Division, NSWCCD, and more recently by the United States Naval Academy, USNA, Hydromechanics Laboratory, were performed using the computational fluid dynamics code Numerical Flow Analysis, NFA, to assess its capabilities in simulating the short duration hydrodynamic loading associated with free-surface impact. NSWCCD performed experiments using drop heights of 15.24 cm (6 in) and 25.4 cm (10 in), while the Naval Academy used drop heights of: 7.94, 12.7, 15.88, 25.4, 31.75, 38.1, and 50.8 cm (3.125, 5.0, 6.25, 10.0, 12.5, 15.0, and 20.0 in), measured from the keel of the wedge to the calm water surface. Simulations and comparisons were made at heights of 15.24 cm (6 in) and 25.4 cm (10 in) with the NSWCCD data, and 12.5 inches for the USNA data providing for a detailed examination of NFA’s ability to simulate and predict short duration hydrodynamic impacts.