A Parametric Study of Low-Frequency Damping of Mooring System

Mooring line damping plays an important role to the body motion of moored floating platforms. Meanwhile, it can also make contributions to optimize the mooring line system. Accurate assessment of mooring line damping is thus an essential issue for offshore structure design. However, it is difficult to determine the mooring line damping based on theoretical methods. This study considers the parameters which have impact on mooring-induced damping. In the paper, applying Morison formula to calculate the drag and initial force on the mooring line, its dynamic response is computed in the time domain. The energy dissipation of the mooring line due to the viscosity was used to calculate mooring-induced damping. A mooring line is performed with low-frequency oscillation only, the low-frequency oscillation superimposed with regular and irregular wave-frequency motions. In addition, the influences of current velocity, mooring line pretension and different water depths are taken into account.

Heave Induced Internal Flow Fluctuations in Vertical Transport Systems for Deep Ocean Mining

Deep ocean mining systems will have to operate often in harsh weather conditions with heavy sea states. A typical mining system consists of a Mining Support Vessel (MSV) with a Vertical Transport System (VTS) attached to it. The transport system is a pump pipeline system using centrifugal pumps. The heave motions of the ship are transferred to the pump system due to the riser-ship coupling. Ship motions thus will have a significant influence on the internal flow in the VTS. In this paper, the influence of heave motions on the internal flow in the VTS for a typical mining system for Seafloor Massive Sulfide (SMS) deposits in Papua New Guinea is analyzed. Data on the wave climate in the PNG region is used to compute the ship motions of a coupled MSV-VTS. The ship motions then are translated into forces acting on the internal flow in order to compute fluctuations in the internal flow. In this way, the workability of the mining system with respect to the system’s production can be assessed. Based on a detailed analysis of the internal flow in relation to ship motions, the relevance of a coupled analysis for the design of VTS is made clear. This paper provides a method for performing such analyses.

Influence of the Waterline Integral on the Solution of the Frequency-Domain Forward-Speed Radiation-Diffraction Problem of a Ship Advancing in Waves

The radiation-diffraction potential of a ship advancing in waves is studied using the three-dimensional frequency-domain forward-speed free-surface Green function (Brard 1948) and the forward-speed Green integral equation (Hong 2000). Numerical solutions are obtained by making use of a second-order inner collocation boundary element method which makes it possible to take account of the line integral along the waterline in a rigorous manner (Hong et al. 2008). The present forward-speed Green integral equation includes not only the usual free surface condition for the potential but also the adjoint free surface condition for the forward-speed free-surface Green function as indicated by Brard (1972). Comparison of the present numerical results of the heave-heave wave damping coefficients and the experimental results for the Wigley ship models I, II and III (Journee 1992) has been presented. These coefficients are compared with those calculated without taking into account of the line integral along the waterline in order to show the forward speed effect represented by the waterline integral when it is properly included in the free-surface Green integral equation. Comparison of the present numerical results and the equivalent time-domain results (Hong et al. 2013) has also been presented.

Numerical Study of Two Vessels Seakeeping in Waves

This paper presents a numerical study of seakeeping in regular waves for two vessels in close proximity using commercial seakeeping software HydroStar and an in-house code MOTSIM. The objective was to study the possible sheltering effect of the larger vessel (FPSO) on the smaller one (OSV) during personnel transfer between the two vessels, where one vessel was at some angle relative to the other vessel and there was no connection line between them. The study mainly focused on the OSV motion resulting from the interaction of the FPSO when the OSV was at different headings and wave directions. Initially the OSV motions close to the FPSO (and parallel) were compared with those for the OSV alone. For an un-parallel position of the two vessels, an objective function based on the OSV RAOs motion in roll, pitch and heave directions was used to optimize the OSV position. Finally comparisons between HydroStar and MOTSIM results are provided. The main conclusions are: 1) When the FPSO and OSV are located in parallel, the OSV motions in sway, roll and yaw are larger than the single OSV motions in head waves while surge, heave and pitch are almost the same. The OSV motions in most of the six degrees of freedom are smaller than the single OSV motions when the waves are from other directions (always on the port side of the FPSO), which means that there is a sheltering effect. 2) The simulation results from different OSV rotation angles show that the hydrodynamic interaction between the FPSO and OSV e.g. the sheltering effect is related to the OSV angle and the wave heading. The objective function in roll, pitch and heave RAOs indicates that the OSV should maintain a close to parallel position with the FPSO to minimize motion when the waves come from the port side of the FPSO from 180 to 240 degrees. When the wave direction is around 240 degrees the OSV should have relatively small motion in waves for any OSV rotation angle. 3) A comparison of HydroStar and MOTSIM results shows that the MOTSIM results of a single vessel seakeeping simulation is in a good agreement with HydroStar. In two vessels situation more validation work needs to be done.

Coupling of Heave and Roll for High-Speed Planing Hulls

For planing hulls, dynamic lift reduces the submergence of the hull, allowing small motions to result in large changes in hydrodynamic forces and moments. The dynamic lift forces acting on the bottom of a planing hull dominate the hydrodynamics and these lift forces are known to depend on speed and wetted surface. As a planing boat rolls the wetted surface changes, which affects the dynamic lift. A series of tests using a wooden prismatic planing hull model with a constant deadrise of 20 degrees were done at static heel and heave positions as well as oscillating heave conditions. This paper presents the results from these experiments, primarily looking at the hydrodynamic coefficients in heave as a function of heel angle and exploring the coupling between these motions for a prismatic high-speed planing hull.

Numerical Study on Coastal Wave and Near-Shore Current Interaction

Coastal waves and near-shore currents have been investigated by many researchers. This paper developed a two-dimensional numerical model of near-shore waves and currents to study breaking wave induced current. In the model, near-shore water wave was simulated by a parabolic mild slope equation incorporating current effect and wave energy dissipation due to breaking, and current was simulated by a nonlinear shallow water equation incorporating wave exerted radiation stress. Wave radiation stress was calculated based on complex wave amplitude in the parabolic mild slope equation, and this result in an effective method for calculating wave radiation stress using an intrinsic wave propagation angle that differs from the ones of using explicit wave propagation angle. Wave and current interactions were considered by cycling the wave and current equation to a steady state. The model was used to study waves and wave-induced longshore currents at the Obaköy coastal water which is located at the Mediterranean coast of Turkey. The numerical results for water wave induced longshore current were validated by measured data to demonstrate the efficiency of the numerical model, and water waves and longshore currents were analyzed based on the numerical results.

Hyperbaric Testing of an Alternative Approach to Remove Carbon Dioxide From Underwater Life Support Equipment

Traditional CO2 absorption methods for underwater life support equipment use alkali metal hydroxide chemical beds — mostly calcium hydroxide — that have been shown to have poor absorption efficiencies at cold temperatures, and must be replaced at considerable trouble and expense on a frequent basis. With chemical utilizations as low as 20% in water temperatures of 2°C, these hydroxides do not lend themselves to applications requiring extended durations in cold water due to the inability to carry sufficient quantities of expendables. A joint research effort between Duke University and the University of Bath has verified the feasibility in laboratory trials of an alternative carbon dioxide removal method that intimately mixes seawater with breathing circuit gases within a packed bed of Dixon rings. Based on the results of these laboratory trials, two multi-path scrubber prototypes were designed and fabricated for unmanned testing. In March 2013, the hyperbaric performance of these prototype scrubbers was characterized over a wide range of gas and water flow rates when operating the scrubbers in counter-current (water flowing in the opposite direction as gas flow) and co-current (water flowing in the same direction as gas flow) fashion. Significant findings from these tests included the following: • Both scrubber prototypes were found to be capable of delivering exit CO2 levels below 0.5 vol% (surface equivalent) at respiratory rates up to 22.5 liters per minute and at depths ranging from 0 to 40 meters of seawater (MSW). • Negligible collateral O2 absorption was observed at surface pressure (exit O2 levels were typically above 20.2 vol%), and exit O2 levels were typically above 18.4% during testing at 10 MSW. • At surface pressure, both prototypes had significantly lower breathing resistances than design goals established by the U. S. Navy.

Hydroelastic Analysis for Extended-Column of Extended Draft Semi-Submersible

This paper presents a simplified model for assessing the hydroelasticity of the extended-column (e-column) for an extended-draft semi-submersible (E-SEMI) by using commercially available software. The E-SEMI comprises a second tier pontoon (STP) connected to a conventional semi-submersible by using extended-column (e-column). The purpose of attaching the STP is to increase the heave added mass, and as a result, could reduce the heaving motion and shift the natural period further away from the wave spectrum by increasing the natural period. One of the challenges of the E-SEMI is to obtain the hydroelastic response of the e-column when subjected to wave and current loadings. The simplified model models the e-column and STP using beam-column theory that has similar vibration frequencies and modes shapes of the complex model. The potential wave theory is used to model the waves and the Morrison equation is use to obtain the drag and inertia force acting on the e-column due to current loading. Hydroelasticity shall be performed on this simplified model to assess the strength performance as well as the deflection of the e-column. The bending moment obtained from the hydroelastic analysis is also significant in the connector design that keeps the e-column in place.

Hydrodynamic and Hydroacoustic Phenomena in the Propeller Wake-Rudder Interaction

The present paper describes the major mechanisms underlying the hydroacoustic and hydrodynamic perturbations in a rudder operating in the wake of a free running marine propeller. The study was based on a holistic approach which concerned time resolved visualizations and detailed flow measurements around the rudder as well as wall-pressure fluctuation measurements over the rudder surface, at different deflection angles.