Floating Marine Structures Research Articles

On the study of a body force model for a jetting type breakwater

The introduction of jetting-type floating breakwaters can significantly enhance the performance of large floating marine structures. Especially, this novel technology may effectively handle traditional breakwaters' bottleneck, namely, weak wave dissipation performance under large period waves. In this paper, a new method is established using body force model, in conjunction with a computational fluid dynamics solver and dynamic overlapping mesh technology, to simulate jetting-type floating breakwaters. The mathematical model is elucidated in the context of fluid mechanics, propellers body force model, and floating breakwaters’ geometric aspects. The method is proved to be compact and flexible in terms of implementation, offering advantages for the simulation of floating breakwaters employing hydraulic systems. Moreover, the new method can resolve low efficiency during computation, normally caused by fine local flows when simulating long-range jetting-type floating breakwaters: By controlling parameters precisely, it simulates accelerating local flows, high-speed water-jets, and it offers efficient visualization of the water-jets and their effects on floating breakwaters. Results show that the novel jetting-type floating breakwater significantly enhances wave dissipation. A comparison is performed between the novel breakwater against the typical example of square box floating breakwater with same dimensions and under wave incidence, thus the accuracy of the method was verified likewise. This research provides a new numerical method for jetting technologies applied to floating breakwaters and porous structures, also offering valuable insights for new types of wave dissipation methods.

A tool to select offshore renewable energy facilities. The case of study of shipyards and ports in Spain

The objective of this work is to create a tool that serves to know the possibilities of manufacturing, storage, assembly, and maintenance of floating marine structures trying to take advantage of the ports and shipyards of Spain. This would help reduce costs and could make this type of energy more profitable. To this end, all the Spanish ports and shipyards have been analyzed, considering the characteristics and possibilities of each of them. In the study, those that did not meet certain essential conditions for the manufacture, storage, assembly, and maintenance of power generation equipment have been discarded.

Numerical and experimental investigation on deterministic prediction of ocean surface wave and wave excitation force

Floating marine structures implement real-time wave excitation force prediction to address optimal control issues. The accuracy of force prediction relies on adequate wave forecasting. This paper presents a comprehensive analysis of deterministic wave forecasting by considering various wave steepnesses and directional spreads. In addition, we introduce new methods for predicting wave excitation forces acting on the floating body of interest. The methods are based on a set of frequency coefficients of wave excitation forces, which are generated in conjunction with wave amplitude parameters optimized in the data assimilation and frequency response functions obtained from boundary element method tools. These approaches offer the advantage of streamlining the calculation process, eliminating the need for simulating wave surfaces through wave propagation. Moreover, for the first time, we study a prediction zone for wave excitation forces by comparing predicted forces with theoretical forces. Lastly, the force prediction is validated against experiments conducted on a captive platform model in both unidirectional and multidirectional sea states.

A review of research on tether-type submerged floating tunnels

Submerged floating tunnels (SFTs) offer many advantages over bridges and immersed tunnels, and they are regarded as the most promising form of architecture for future transportation across bodies of water. However, an SFT has not been built because there are many critical technical difficulties in its design and construction. The tether-type SFT is one of the forms with the potential for transportation applications. This paper provides a review of the progress of research on tethered SFTs and discusses the critical issues and solutions involved in their design, dynamic characteristics, safety evaluation and vibration control. Although the design and the safety evaluation of SFTs can partially refer to guidelines for floating marine structures and pipes, it is essential to establish guidelines specifically for SFTs as soon as possible. The hydrodynamic and dynamic characteristics of SFTs are usually investigated based on the segmental model, which ignores longitudinal effects. To study the complete dynamic behaviour of the SFT in the longitudinal direction, it is important to establish an overall model that considers the effects of segment joints. Tests on large-scale models of SFTs under the combined effects of waves, currents and earthquakes are rare, and much is unknown about the progressive collapse and failure mechanisms of these structures. Vibration control of an SFT could be achieved by hydrodynamic methods and mechanical methods. To further improve safety, it is essential to propose an adaptive control system for the vibration control of SFTs under the combined effects of various external loads.

A Study on the Application Criteria of Domestic Regulations for Floating Marine Structures

A Study on the Application Criteria of Domestic Regulations for Floating Marine Structures

Verification and validation of CFD simulations for semi-submersible floating offshore wind turbine under pitch free-decay motion

In this paper a detailed verification and validation study is performed to evaluate the accuracy of CFD simulations for a semi-submersible floating offshore wind turbine in pitch free-decay motion. The simulations are carried out with both linear and dynamic mooring models. The linear and quadratic damping coefficients of the semi-submersible are derived from the pitch motion data. In the verification study, a least-squares method using the observed order of convergence in combination with a data quality measure for different spatial and temporal refinement is applied to estimate the discretization errors. The numerical uncertainty is quantified by applying a safety factor to the summation of numerical errors. The numerical solutions are validated against the experimental measurements obtained from wave tank test of OC5-DeepCWind semi-submersible. Validation is achieved in the pitch natural period and the linear damping coefficient within the level of validation uncertainty. While the numerical simulation under-predicts the quadratic damping coefficient. A guideline is provided for verification and validation study of a CFD code application for floating marine structures under decay tests.

Structural Degradation of Oil Tanker Operating in Indonesian Waterways

Corrosion is one of the significant factors for decreasing strength in floating marine structures. In general, rules of classification society use the North Atlantic Ocean located in the sub-tropical region as a reference area in the formulation of the structural strength. If the sub-tropical area compare with tropical region such as Indonesia waterway, Indonesia waterway has high salinity and humidity. This makes marine environment in Indonesia more corrosive. One example of the effects of corrosion is the reduction in plate thickness. This leads to decrease in ship strength. Using measured thickness data collected from 25-thickness measurement reports from 24 Oil Tankers that sailing in Indonesian waterways, the diminution of ship structural thickness has been analysed. The age of Oil Tankers, whose data was taken, ranges from 5 to 23 years. Data analysis was carried out by using statistical approach based on Weibull distribution. Using 95% cumulative probability, it was found that the thickness diminution of Oil Tanker with design life of 20 years is 2,07 mm for deck plate, the side plate is 2,06 mm, the bottom plate is 1,81 mm and the inner bottom plate is 1,53 mm.

Wave crest height distribution during the tropical cyclone period

The wave crest height plays a vital role in designing both the fixed and floating marine structures. The present study examines the measured wave crest data at 18 m water depth off Visakhapatnam in the northwestern Bay of Bengal during the tropical cyclone (TC) Hudhud. The maximum significant wave height (Hs) measured during the TC is 7.59 m on 12 October 2014 at 1030 h; the maximum wave height is 10.75 m and the crest height is 6.81 m. The average ratio of the crest height of the wave to the wave height is 0.4–0.7 during the TC. For waves with Hs more than 6 m, the most extreme wave height is 1.46–1.83 times the Hs and the crest height of the wave is 0.81–1.47 times the Hs of the same 30-min record. The study shows that during the high wave events, the Weibull distribution agrees with the crest height distribution of observed data better than the Rayleigh distribution.

Biocontrol of fouling pests: Effect of diversity, identity and density of control agents

Augmentative biocontrol, using native natural enemies, has been suggested as a promising tool to control marine biofouling pests on artificial structures. However, there are still important knowledge gaps to be addressed before biocontrol can be considered as a management tool. In a field experiment on floating marine structures we examined intra- and interspecific consumer interactions among biocontrol agents on different surface orientations. We tested the effect of identity, density and diversity of three invertebrates (the 11-arm seastar Coscinasterias muricata, the sea urchin Evechinus chloroticus and the gastropod Cook's turban Cookia sulcata) to reduce established biofouling and to prevent fouling growth on defouled surfaces. High densities of biocontrol agents were not more effective at fouling control (cover and biomass) than low densities. Nor did multi-species treatments function more effectively than mono-specific ones. However, biocontrol agent identity was important, with the 11-arm seastar and Cook's turban being the most effective at fouling reduction and prevention, respectively. Surface orientation had a strong effect on the effectiveness of control agents, with the best results obtained on vertical compared to diagonal and underside surfaces. This study confirmed the potential of biocontrol as a management tool for marine pest, indicating that identity is more important than richness and density of control agents. It also highlighted the limitations of this approach on diagonal and underside surfaces, where control agents have limited retention ability.

Fast computation of large scale marginal extremes with multi-dimensional covariates

Safe and reliable design and operation of fixed and floating marine structures often located in remote and hostile environments is challenging. Rigorous extreme value analysis of meteorological and oceanographic data can greatly aid the design of such structures. Extreme value analysis is typically undertaken for single spatial locations or for small neighbourhoods; moreover, non-stationary effects of covariates on extreme values are typically accommodated in an ad-hoc manner. The objective of the work summarised here is to improve design practice by estimating environmental design conditions (such as return values for extreme waves, winds and currents) for a whole ocean basin, including additional covariate effects (such as storm direction) as necessary, in a consistent manner. Whole-basin non-stationary extreme value modelling is computationally complex, requiring inter-alia the estimation of tail functions, the parameters of which vary with respect to multi-dimensional covariates characterised by us using tensor products of penalised B-splines. We outline two technical contributions which make whole-basin non-stationary analysis feasible. Firstly, we adopt generalised linear array methods to reduce the computational burden of matrix manipulations. Secondly, using high-performance computing, we develop a parallel implementation of maximum likelihood estimation for the generalised Pareto distribution. Together, these innovations allow estimation of rigorous whole-basin extreme value models in reasonable time. We evaluate the new approach in application to marginal extreme value modelling of storm peak significant wave heights in two ocean basins, accommodating spatial and directional covariate effects.

세장체의 파랑중 거동에 대한 실험에 관한 고찰

The exploration areas for maritime resources such as oil and natural gas have gradually moved to deep sea areas. It has become difficult to use existing fixed marine structures, which are very costly to build, because that have reached the uppermost economic limit. Therefore, floating marine structures and flexible marine structures are preferred. In particular, slender bodies such as risers and pipes are important parts of ocean depth marine structures. These slender bodies have more flexible structural characteristics in deep water areas because their overall length becomes longer and thediameter/length slenderness ratio gets smaller. In addition, the dynamic behavior of slender bodies becomes complicated as external forces such as tides and waves act on it directly. In this study, in order to solve these problems, we performed model tests in a 2-D wave basin using flexible slender bodies with different modulus of elasticity values. As a result, we compiled statistics and compared the behaviors of flexible slender bodies with respect to the effect of the modulus of elasticity. We expect that the results could be used as reference data for the design of structures with flexible elements.

Crest-height distribution of nonlinear random waves

The wave crest is an important factor for the design of both fixed and floating marine structures. Wave crest height is a dominant parameter in assessing the likelihood of wave-in-deck impact and resultant severe damage. Many empirical and theoretical distribution functions for wave crest heights have been proposed, but there is a lack of agreement between them. It is of significance to develop a better new nonlinear wave crest height distribution model. The progress in the research of wave crest heights is reviewed in this paper. Based on Stokes’ wave theory, an approximate nonlinear wave crest-height distribution formula with simple parameters is derived. Two sets of measured data are presented and compared with various theoretical distributions of wave crests obtained from nonlinear wave models and analysis of the comparison is given in detail. The new crest-height distribution model agrees well with observations. Also, the new theoretical distribution is more accurate than the other methods cited in this paper and has a greater range of applications.

Station-keeping of moored vessels by reliability-based optimization

Floating marine structures are frequently kept in position by means of mooring lines in addition to a thruster system. Various types of control schemes for the thruster system are first investigated based on a simplified response model. In particular, the role of structural reliability criteria applied to the mooring system is investigated. Subsequently, a more refined control algorithm based on such reliability criteria is introduced. The performance of this control system is demonstrated by numerical simulations.

Dynamic Positioning of Marine Vessels based on Structural Reliability Criteria

Floating marine structures are considered which are kept in position by means of mooring lines and a thruster system. Various types of control schemes for the thruster system are first investigated based on a simplified response model. In particular, the role of structural reliability criteria applied to the mooring system is investigated. Subsequently, a more refined control algorithm based on such reliability criteria is introduced. The performance of this control system is demonstrated by numerical simulations.

Optimal setpoint chasing in dynamic positioning of deep‐water drilling and intervention vessels

Conventional controller designs for dynamic positioning of ships and floating marine structures have so far been based on the principle on automatic positioning in the horizontal-plane about desired position and heading co-ordinates defined by the operator. A three degrees-of-freedom multivariable controller either of linear or nonlinear type, normally with feedback signals from surge, sway and yaw position and velocities, has been regarded as adequate for the control objective. For floating structures with small waterplane area such as semi-submersibles, feedback from roll and pitch angular rotation velocity may also be included to avoid thrust-induced roll and pitch motions that are caused by the hydrodynamic and the geometrical couplings between the horizontal and vertical planes. However, for certain marine operations this control philosophy may not be the most appropriate approach ensuring safety and cost effectiveness. For drilling and work-over operations the main positioning objective is to minimize the bending stresses along the riser and the riser angle magnitudes at the well head on the subsea structure, and at the top joint as well. A positioning control strategy solely based on manual setting of the desired position co-ordinates may not be the most optimal solution for these applications. In this paper a new hybrid dynamic positioning controller, that also accounts for riser angle offsets and bending stresses is proposed. It is shown that a significant reduction in riser angle magnitude can be achieved. Simulations with a drilling semi-submersible demonstrate the effect of the proposed control strategy. Copyright © 2001 John Wiley & Sons, Ltd.

Optimal setpoint chasing in dynamic positioning of deep‐water drilling and intervention vessels

AbstractConventional controller designs for dynamic positioning of ships and floating marine structures have so far been based on the principle on automatic positioning in the horizontal‐plane about desired position and heading co‐ordinates defined by the operator. A three degrees‐of‐freedom multivariable controller either of linear or nonlinear type, normally with feedback signals from surge, sway and yaw position and velocities, has been regarded as adequate for the control objective. For floating structures with small waterplane area such as semi‐submersibles, feedback from roll and pitch angular rotation velocity may also be included to avoid thrust‐induced roll and pitch motions that are caused by the hydrodynamic and the geometrical couplings between the horizontal and vertical planes. However, for certain marine operations this control philosophy may not be the most appropriate approach ensuring safety and cost effectiveness. For drilling and work‐over operations the main positioning objective is to minimize the bending stresses along the riser and the riser angle magnitudes at the well head on the subsea structure, and at the top joint as well. A positioning control strategy solely based on manual setting of the desired position co‐ordinates may not be the most optimal solution for these applications. In this paper a new hybrid dynamic positioning controller, that also accounts for riser angle offsets and bending stresses is proposed. It is shown that a significant reduction in riser angle magnitude can be achieved. Simulations with a drilling semi‐submersible demonstrate the effect of the proposed control strategy. Copyright © 2001 John Wiley & Sons, Ltd.

Modelling and Control of Riser Angles and Stresses in Dynamic Positioning

Conventional controller designs for dynamic positioning of ships and floating marine structures have so far been based on the principle on automatic positioning in the horizontal-plane about some desired position and heading co-ordinates. A three degrees of freedom multivariable controller either of linear or nonlinear type, normally with feedback signals from surge, sway and yaw position and velocities, has been regarded as adequate for the control objective. For semi-submersibles feedback from roll and pitch angular rotation velocity may also be included. However, for certain marine operations this control philosophy may not be the most appropriate approach ensuring a safe and cost effective operation. For drilling and work over operations the main positioning objective is to minimise the bending stresses along the riser and the riser angle magnitudes at the well head on the subsea structure and at the top joint as well. A positioning strategy solely based on position and velocity feedback may not be the most optimal solution for these applications. In this paper a new hybrid dynamic positioning controller, that also accounts for riser angle offsets and bending stresses is proposed. It is shown that a significant reduction in riser angle magnitude can be achieved. Simulations with a drilling semi-submersible demonstrate the effect of the proposed control strategy.

On motion reduction characters of floating body utilizing TMD

Several troubles will occur because that floating marine structures are shaken by waves. So, the reduction mechanism for motions of the floating body has been developed. This mechanism was adopted TMD (Tuned Mass Damper), and experiments has been done to estimate reduction effects for this mechanism. As a result, it is obtained that reduction effects are influenced by TMD natural period which is based on the relation between springs and weights, and heaving and pitching motion on a specific period can be reduced by this mechanism.

A Study of Motion and Strength of Floating Marine Structure in Waves

Many floating marine structures of various kinds have been constructed for varied purposes ; however, researches and technical experiences for such structures, some of which are quite different from conventional ships, are not so exhaustive and comprehensive, as those for the ships, and many problems still remain unsolved.Taking an opportunity of construction of newly developed iron ore loading station with submersible, column stabilized, catamaran hulls, a systematic study was made on the motion and strength of the structure in waves. Theoretical analysis of the motion and strength of the structure was carried out first. Secondly, the theoretical results of its motion in waves were compared with that of model tank test. Thirdly, in order to check the theoretical calculation the stress measurements were made at the time of launching. Finally, during the ocean towage of the structure from Japan to India, comprehensive measurements of motion of and stress in the structure in waves were conducted simultaneously with the accurate measurements of waves by means of a clover-leaf buoy.Conclusions of the present study are summerized as follows : 1) Motions of floating marine structure with catamaran hulls in irregular waves can be estimated with sufficient accuracy for engineering purpose by the spectral computations of responses utilizing the linear superposition principle using the response functions which are obtained by the strip method usually used for ship motions in oblique waves and are corrected by the results of model tank test.2) Longitudinal stress of the structure can be estimated with fairly good accuracy, by the finite element method using the response function of longitudinal bending moment in waves which is also calculated by the strip method.

Wave Action on Groups of Vertical Circular Cylinders

Researches on the motions in waves have very important meanings for floating marine structures like offshore drilling rig, since they must be stationary platforms in the sea.The theoretical calculations of hydrodynamical forces upon such structures moving in waves are necessary for correct theoretical predictions of their motions. As a first step, in this paper is developed a method to calculate the wave forces acting upon groups of vertical circular cylinders, simple models of floating rig composed of several columns, fixed in waves with finite draft and infinite draft. This method takes into account the hydrodynamical interaction effect, which has ever been neglected, between cylinders due to the diffraction waves by each cylinder.Numerical results by this method, the wave forces on 2 or 3 cylinders, show that the interaction effect upon surging forces is not necessarily small especially in not large wave length range compared with the cylinder diameter. But in this wave length heaving force itself upon the structures is small and therefore the effect does not appeared clearly in this force. It must be investigated what an influence the effect has on their heaving or rolling motion in waves.The numerical results are in good agreement with measured ones obtained by comparatively simple experiment.