Analysis of Coupled Hull: Cold Water Pipe Mooring for Offshore Desalination Plant

This paper presents the experimental study on the hydrodynamic behavior of a jacket spar platform designed for 10 MLD (million liters per day) desalination plant in the water depth of 375 m with the pay load of 50×103 kN. The surge, heave and pitch natural periods from free decay tests are presented and validated with the numerical results obtained using NAOS (Non-linear Analysis of Offshore Structures) developed in IIT Madras. The jacket spar is modeled using sixty beam elements and each mooring lines by ten beam elements with axial prestress to model the pretension. The beam elements with axial prestress do not take any compressive load. The surge, heave and pitch response and mooring line tension of the jacket spar platform for three wave height range of 2.6m to 8.75m in the wave period range of 7.82s to 19.00s are presented. The maximum surge of 0.5% of water depth, the maximum heave RAO of 0.1 and maximum pitch of 1.54° are within the operational limits. The maximum tension in mooring line is only 10% of breaking strength.

The characters of tension in the mooring lines of a single HDPE gravity cylindrical cage were studied by experimental tests.The proto type of cage had a perimeter of 40 m and a height of 10 m.Based on the Dixon rule of gear tests,two scales were chosen for the experimental model,i.e.λ= 1/10 and λ'= 1/2.The experiments were carried out in Ocean Engineering Basin of Institute of Industrial Science of the University of Tokyo.The grid mooring system used in the experiments was geometrically similar to the proto type.The horizontal projections of the front mooring lines were in the same direction of current or incident waves.Uniform current was produced in the whole basin at a velocity of 0 cm/s,10 cm/s and 15 cm/s.Regular waves with a period of the range from 1 s to 2.2 s and a height varying from 10 cm to 30 cm were generated.Tension in the mooring lines wwa measured.The following conclusions were made from the analysis of measured data:(1) Under the actions of current only,the tension in the front mooring lines was greater than that in rear mooring lines.(2) Under the actions of waves only,the tension in the rear mooring lines was greater than that in the front mooring lines.An empirical formula for the maximum tension(Tmax) in the rear mooring lines was deduced.It turned out that the maximum tension was line-arly related to the wave height(H),i.e.Tmax=C1H+C2(C1 and C2 are constants).(3) Under the combining effects of waves and current,the tension of the front anchor line was again greater than that in the rear one.Another empiri-cal formula was deduced.It showed that the maximum tension in the front mooring lines was exponentially in-creased with the wave height,i.e.(C3 and C4 are constants).

Effects of mooring failure on the dynamic behavior of the power capture platforms

The oil and gas industry has moved towards the offshore deep water regions due to depletion of these resources in shallow and intermediate water depths. Conventional fixed jacket type platforms and bottom supported compliant platforms have been found to be inefficient and uneconomical for exploring these resources in deep water regions. In view of deep water conditions, Spar platforms have been seen to be the most economical and suitable alternative offshore platforms. Several operational Spar platforms such as SB-1, Shell’s ESSCO, Brent Spar, Oryx Neptune Spar, Chevron Genesis Spar and Exxon’s Diana Spar in the Gulf of Mexico and North Sea have shown the effectiveness and success of such platforms in deep-ocean. In deep water conditions, the severity of sea states has substantial effects on the spar platform. The mooring lines contribute significant inertia and damping because of their longer lengths, larger sizes, and heavier weights. Precise motion investigation of platforms should consider these actions in deep waters. However, proper dynamics cannot be assessed by the commonly used decoupled quasi-static method that ignores all or part of the interaction effects between the mooring lines and platform. Coupled analysis, which includes the platform and mooring lines in a single model, is the only way to capture the damping from mooring lines in a consistent manner. In the present study, coupled analysis of integrated Spar-mooring system has been performed. Cylindrical spar hull is treated as a rigid beam element and catenary mooring line as hybrid beam element. Nonlinear dynamic responses have been evaluated under several severe sea states of dissimilar wave heights and wave periods. Damping due to mooring lines has been assessed. An automatic Newmark-β time incremental approach has been implemented to conduct the analysis in time domain. Wave induced spar hull motion in surge, heave and pitch direction along with maximum tension in mooring line has been assessed for different wave conditions with and without current in 1018 m water depth. The time histories of spar responses follow substantial alteration for larger wave heights and wave periods. Maximum tensions in mooring line are very sensitive with momentous value for extreme sea loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines.

Enclosure aquaculture is a healthy and ecological aquaculture pattern developed in recent years to relieve the pressure due to the wild fish stock decline and water pollution. The object of this paper was a floating rope enclosure, which mainly consisted of floaters, mooring lines, sinkers and a net. In order to optimize mooring design factors, the hydrodynamic responses of the floating rope enclosure with different mooring systems in combined wave-current were investigated by experimental and numerical methods. Physical model experiments with a model scale of 1:50 were performed to investigate the hydrodynamic characteristics of a floating rope enclosure with 12 mooring lines. Based on the lumped mass method, the numerical model was established to investigate the effects of mooring design factors on the mooring line tension, force acting on the bottom, and the volume retention of the floating rope enclosure. Through the analysis of numerical and experimental results, it was found that the maximum mooring line tension of the floating rope enclosure occurs on both sides of the windward. Increasing the number of mooring lines on the windward side is helpful to reduce the maximum mooring line tension. Waves and current both have an influence on the mooring line tension; in contrast, currents have a more obvious effect on the mooring line tension than waves. However, the influence of the wave period on the maximum mooring line tension is small. The force endured by the bottom of the floating rope enclosure also changes periodically with the wave period. Yet, the maximum force endured by the bottom of floating rope enclosure occurred at the windward and leeward of the structure. The volume retention of the floating rope enclosure increased with the increasing amount of mooring lines.

This paper presents dynamic behaviors of a deep water utilization system composed of a 2 point moored barge equipped with machineries and a cold water pipe (CWP) attached to the barge. Dynamic behaviors of the floating barge of the present scale and the CWP are mutually affected by each other. The dynamic behaviors were investigated in both numerical calculations and full scale experiments.A full scale system was tested at Toyama bay in Japan Sea of about 300 m water depth in the summer season of 1989 and 1990. Measurements were performed for motions of the barge, tensions of mooring lines and CWP behaviors.Motions of the floating barge in waves are calculated including the effects of CWP. Dynamic behaviors of the CWP in the real sea are calculated using the discretized Lagrange's equation in the time domain. It is assumed that the CWP is excited only by the floating barge. The present numerical scheme for the CWP can be applied to the dynamic calculations of various types of line structures.

To investigate the dynamics of gravity cage, the finite element program Aqua-FE™, was applied to gravity cages with two different meshes, square-mesh net and diamond-mesh net. Two case studies are chosen to compare the dynamics of cages based on the numerical modeling techniques. The numerical models were developed in Aqua-FE™ to simulate the effects of waves and current. They were validated by comparison with water tank results. The comparison showed good agreement. In both case studies, we consider several loading conditions consisting of different uniform currents and monochromatic waves. Assuming that the system can be modeled as a linear system, we investigated the motion response (heave, surge and pitch) characteristics of the fish cages with square-mesh net and diamond-mesh net, their deformation and the resultant tension in mooring lines. For different wave conditions, we study the storm response of the two cages based on the response amplitude operators (RAOs). In particular, the length of the grid line has significantly effect on the motion of the cage and the tension in mooring lines in wave. In additional, the effect of the mesh shape on the motion of the cage and tension in mooring lines is also analyzed. The comparison results show that the surge motion of the fish cage with square-mesh net is significantly larger than that with diamond-mesh net.

The effects of hydrodynamic forces acting on the cold water pipe and mooring lines on motions of the OTEC platform are investigated experimentally and theoretically. The hydrodynamic forces acting on the cold water pipe and mooring chains oscillating in water are obtained by using mode methods. The computer time needed for the calculations is very short; therefore, the methods proposed in this paper are of great practical use. Calculated results of the bending moment and shearing force of the cold water pipe at its upper end and the horizontal and vertical components of the dynamic tension of the mooring chain at the mooring point are compared with those of experiments. They are in good agreement. Experiments and theoretical calculations on the motions of the moored OTEC platform show that the viscous drag forces acting on the cold water pipe and mooring chains have a great effect on the motion at its resonant period. They act as the viscous damping force for the motion of the platform at its resonant period, and reduce its magnification factor.

On dynamic coupling effects between a spar and its mooring lines

The structural hydrodynamic response of longline aquaculture facilities under the influence of waves and currents is complex. Studying the hydrodynamic characteristics of this aquaculture structure in complex sea environments can contribute to sustainable offshore aquaculture solutions. Thus, we established a numerical model using AquaSim2.18, a proven and effective finite element hydrodynamic software for analyzing the maximum tension in mooring lines and main lines, the displacement of the main lines, and the forces on the lantern nets under waves and currents. The results showed that positioning the system in the direction of incidence of waves and currents minimizes tension in both mooring and main lines, making a downstream arrangement optimal; compared with a single row, the maximum reduction in the tension of the mooring lines is 3.3% and 1.8% for five-row and row-row lines, respectively, and the shadow effects reduced the downstream mooring force. Additionally, line tension increased with wave height and current velocity, whereas wave periods had variable effects due to the period range; the lantern net forces increased with wave height and decreased with wave period. Wave height was also shown to influence the horizontal displacement of main lines. The findings can provide a reference for the hydrodynamic characteristics of different components of the structure.

As a multifunction floating platform, Floating Drilling, Production, Storage and Offloading (FDPSO) combining the well-known Floating Production, Storage and Offloading (FPSO) with a drilling unit. For the environment condition of deep-water oilfield is very severe, the motion response and mooring line tension of FDPSO is a worthy topic of studying. In this study, the numerical time-domain coupled prediction method for the mooring line tension and motion response of FDPSO system is constructed by ANSYS AQWA software. Furthermore, the results of a model test conducted in Harbin Engimeering University are used to investigate the feasibility and validity of the commercial simulation. The effect of mooring line pre-tension on the response of FDPSO is studied by varying the pre-tension of mooring line during the calculation. The time series curve of the mooring line tension and motion response, and the comparison of motion spectrum and mooring line tension spectrum are provided in this article.

Nonlinear mooring system for a ‘Sharp-Eagle’ wave energy converter

Station-keeping is one of the important factors in the design of offshore platforms. Some offshore platforms, such as Spar, Semi-submersible and FPSO, use mooring lines as a mean for station-keeping. Tensions in the mooring lines are one of the key factors in station-keeping. The design of an offshore platform and its mooring lines is based on computed motions of the platform and associated mooring line tensions from numerical simulations using a software code on the basis of metocean criteria. This paper presents an Artificial Neural Network (ANN) model for the prediction of mooring line tensions based on the motions of the platform. This ANN model is trained with time histories of vessel motions and corresponding mooring line tensions for a range of sea states from the results of numerical simulations. After the model is trained, it can reproduce with great fidelity and very fast the mooring line tensions. In addition, it can generate accurate mooring line tensions for sea states that were not included in the training, and this demonstrates that the model has captured the knowledge for the underlying physics between vessel motions and mooring line tensions. The paper presents an example of the training and the validation of the model for a semi-submersible offshore platform for a range of sea states. The training of the ANN model employed a back-propagation learning algorithm. In this algorithm the computed output error is back-propagated through the neural network to modify the connection weights between neurons. The training started with a small number of hidden neurons, and the model grew adaptively by adding hidden neurons until either the target output convergence is achieved or a maximum number of additional hidden neurons is reached. The ANN model discovers nonlinear relationships between the input and output variables during training. The paper presents comparison of time series of mooring line tensions for sea states that were and were not included in the training between those from the numerical simulations and those computed by the trained ANN model. Fatigue assessment is also used to quantitatively measure the accuracy of the ANN model prediction of the time series of mooring line tensions. The paper presents the results of fatigue assessment using various stages of the ANN models with different number of hidden neurons. This shows that the additional hidden neurons improve the prediction of the ANN model of the mooring line tensions for sea states that were and were not included in the training. This approach of prediction of mooring line tensions based on vessel motions using ANN model paves the way to the development of an ANN-based monitoring system. Also, this ANN study demonstrates a great potential for the use of a more general and comprehensive ANN model to help monitor the dynamic behavior of floating systems and forecast problems before they occur by detecting deviations in historic patterns.

Mooring lines systems for point absorber, wave energy converters (WECs) devices, must be engineered to withstand the cyclical loads and motions generated by the interaction of the wave load, as well as the WEC's motions in the random elevation of the sea surface. A simulation model was developed and validated concerning the simulated of point absorber motioned in a previous study; this study compares tension methodologies for point absorber with moorings lines using simulation approach. The purpose of this paper is to propose a mooring lines simulation approach that is suited for a one-body and two-body point absorber thorough examination of the heave motion response and, tension analysis of mooring lines coupled on the device and on seabed. The case study is a cylindrical, floating point absorber with one, two, three and four distributed mooring lines with catenary and taut configurations. The dynamics of the point absorber were simulated and analysed in the time domain using different configurations of mooring lines using Computational Fluid Dynamic (CFD) simulation approaches were compared. A thorough response analysis is undertaken using a Malaysia low wave condition in shallow water. The results were compared to the assessed for both one and two-body point absorber methods. The simulation process employing a two-body with four mooring lines recommended as the preferred method for capturing the interaction between the WEC system's components, which has been demonstrated to be critical for heaving response evaluation. The average maximum tension on four mooring lines with catenary configuration is preferred to have less tension with high heaving response of the device compared to others. The findings showed that a two-body point absorber with four catenary mooring lines average maximum tension of 700 N is more efficient at absorbing low wave heights than a single body.

A series of physical model experiments was performed to investigate the hydrodynamic responses of a semi-submersible offshore fish farm in waves. The structural configuration of the fish farm primarily refers to that of the world’s first offshore fish farm, Ocean Farm 1, developed by SalMar in Norway. The mooring line tension and motion response of the fish farm were measured at three draughts. The study indicated that the tension on the windward mooring line is greater than that on the leeward mooring line. As the wave height increases, the mooring line tension and motion responses including the heave, surge, and pitch exhibit an upward trend. The windward mooring line tension decreased slightly with increasing draught. The existence of net resulted in approximately 42% reduction in mooring line tension and approximately 51% reduction in surge motion. However, the heave and pitch of the fish farm increased slightly with the existence of net. It was found that the wave parameters, draught, and net have noticeable effect on the hydrodynamic response. Thus, these factors are suggested to be considered in structural designs and optimization to guarantee the ability of the fish farm to resist destruction and ensure safety of workers during intense waves.