Mooring Loads Research Articles

Basin Testing of Wave Energy Converters in Trondheim: Investigation of Mooring Loads and Implications for Wider Research

This paper describes the physical model testing of an array of wave energy devices undertaken in the NTNU (Norwegian University of Science and Technology) Trondheim basin between 8 and 20 October 2008 funded under the EU Hydralabs III initiative, and provides an analysis of the extreme mooring loads. Tests were completed at 1/20 scale on a single oscillating water column device and on close-packed arrays of three and five devices following calibration of instrumentation and the wave and current test environment. One wave energy converter (WEC) was fully instrumented with mooring line load cells, optical motion tracker and accelerometers and tested in regular waves, short- and long-crested irregular waves and current. The wave and current test regimes were measured by six wave probes and a current meter. Arrays of three and five similar WECs, with identical mooring systems, were tested under similar environmental loading with partial monitoring of mooring forces and motions. The majority of loads on the mooring lines appeared to be broadly consistent with both logistic and normal distribution; whilst the right tail appeared to conform to the extreme value distribution. Comparison of the loads at different configurations of WEC arrays suggests that the results are broadly consistent with the hypothesis that the mooring loads should differ. In particular; the results from the tests in short crested seas conditions give an indication that peak loads in a multi WEC array may be considerably higher than in 1-WEC configuration. The test campaign has contributed essential data to the development of Simulink™ and Orcaflex™ models of devices, which include mooring system interactions, and data have also been obtained for inter-tank comparisons, studies of scale effects and validation of mooring system numerical models. It is hoped that this paper will help to draw the attention of a wider scientific community to the dataset freely available from the Marintek website.

東北地方太平洋沖地震津波来襲時の桟橋係留大型石炭船の挙動再現について

The 2011 Tohoku earthquake and tsunami at 14:46 JST on March 11, 2011 affected large areas of the eastern and northern part of Japan, and many sea disasters against vessels moored along terminal inside harbor basins occurred. The authors conducted surveys upon sea disasters of a Very Large Crude Carriers (VLCC) at S-Port and an Aframax tanker at O-Port. There was also a severe disaster of large coal carrier at H-Port, which encountered a tsunami of tsunami wave height beyond 10 meters and damaged mooring equipment such as fenders and mooring lines, and unloader of coal, entirely. In this paper, the authors have also tried to reproduce the ship's behavior and analyze the mooring loads during the tsunami through a numerical simulation of tsunami and ship motion.

Dynamics of a Floating Platform Mounting a Hydrokinetic Turbine

AbstractA two-dimensional mathematical model was developed to predict the dynamic response of a moored, floating platform mounting a tidal turbine in current and waves. The model calculates heave, pitch, and surge response to collinear waves and current. Waves may be single frequency or a random sea with a specified spectrum. The mooring consists of a fixed anchor, heavy chain (forming a catenary), a lightweight elastic line, and a mooring ball tethered to the platform. The equations of motion and mooring equations are solved using a marching solution approach implemented using MATLAB. The model was applied to a 10.7-m twin-hulled platform used to deploy a 0.86-m shrouded, in-line horizontal axis turbine. Added mass and damping coefficients were obtained empirically using a 1/9 scale physical model in tank experiments. Full-scale tests were used to specify drag coefficients for the turbine and platform. The computer model was then used to calculate full-scale mooring loads, turbine forces, and platform motion in preparation for a full-scale test of the tidal turbine in Muskeget Channel, Massachusetts, which runs north-south between Martha’s Vineyard and Nantucket Island. During the field experiments, wave, current, and platform motion were recorded. The field measurements were used to compute response amplitude operators (RAOs), essentially normalized amplitudes or frequency responses for heave, pitch, and surge. The measured RAOs were compared with those calculated using the model. The very good agreement indicates that the model can serve as a useful design tool for larger test and commercial platforms.

Full-scale measurement investigation of the hydrodynamics of a turret-moored FPSO in a typhoon and deduction of its mooring loads

As a type of offshore structure, an FPSO unit is typically permanently moored to a designated position. These structures experience very complex sea states during which the motion response of the FPSO is enhanced, resulting in extreme loads on the mooring system. Full-scale measurement of the motion characteristics of an FPSO unit was carried out by the Shanghai Jiaotong University and the China National Offshore Oil Corporation from 2007 to 2009, lasting 25 months (Hu Z. 2010. Report for the full-scale measurement on the motion response of FENJIN FPSO in South China Sea and investigations on the deduction of the mooring line loads. Shanghai: State Key Lab of Ocean Engineering, Shanghai Jiao Tong University). We analysed the FPSO unit's motion responses during the passage of the typhoon ‘Koppu’ and its hydrodynamic performance. The loads acting on the mooring system were deduced through a decoupled analysis with the measurement data as input. The ability of the mooring system to hold station and the performance of the vessel in actual sea states are also presented to contribute important input to the future designs of FPSO systems.

Numerical investigation on the hydrodynamic difference between internal and external turret-moored FLNG

Motion responses of the floating liquefied natural gas (FLNG) hull and the mooring loads in a 100-year return environmental condition are predicted with the help of the well known coupled dynamic analysis code DeepC. A ship-shaped turret-moored FLNG moored by 4×3 chain-polyester-chain lines in 1.5 km depth of water is studied. Two types of turrets such as internal and external turrets, resulting from different locations of the turrets, are adopted respectively in the numerical simulations. Motion responses of the FLNG hull and forces of the mooring lines obtained from the internal turret case and external turret case are compared with each other. Significant differences are obtained. Statistic analysis is also used to analyze the comparison results, and effects of the turret location on the FLNG hydrodynamic characteristics are summed up. The conclusion regarding the hydrodynamic differences between internal and external turret-moored FLNG systems would provide help for design of the FLNG system.

Initial attack of large-scaled tsunami on ship motions and mooring loads

In this paper, we propose a numerical simulation procedure of moored ship motions due to initial attack of large-scaled tsunamis and investigate the effects on the motions and mooring loads. The effect of methodology on selection of tsunami wave components and of the drag forces are then considered by using the numerical simulation method, applying to several case studies for LNG-carrier. Large ship motions and excessive mooring loads beyond the safe working loads are induced by the resonant tsunami wave components in the sway and surge motions and drag forces.

Mooring loads analysis of submersible aquaculture cage system using finite element method

The expansion of near shore aquaculture is feasibility of moving aquaculture facilities into the open ocean. Numerical modeling technique using finite element method was used to enable the optimum design and evaluation of submersible aquaculture cage system. The characteristics of mooring loads response in mooring lines under waves and current and their response amplitude operators were calculated for single and three point mooring configuration at the surface condition and submerged one. The static mooring loads without wave and current loading were similar for both the surface and submerged configuration. It was calculated that three point mooring was more adequate than single point mooring for the mooring configuration of submersible aquaculture cage system. The wave induced response amplitude operators for the single point mooring configuration with the influence of currents were identical to those without the influence of currents.

Mooring load of a ship single-point moored in a steady current

The Technical Committee I.2 of the 14th International Ship and Offshore Structures Congress 2000 identified the occurrence of highly peaked loads in the mooring hawser of single-point moored (SPM) ships as an important issue. Industry, research laboratories and universities were requested to contribute to a systematic study investigating the mooring load of two single-point moored tankers subject to a steady current. Contributions were received from three industrial participants, one research laboratory, and three universities. Comparative simulated time histories of the mooring load as well as the corresponding horizontal motion response of the SPM tankers were analyzed. A relatively large spread characterized the predicted results.

Response-based extreme value analysis of moored offshore structures due to wave, wind, and current

Floating moored offshore structures have a significant future in offshore operations as an attractive economic alternative to fixed structures in deep waters and/or in areas where there is no existing infrastructure. This paper describes an analysis procedure based on the structure variable approach to estimate load and response values of a moored offshore platform at a given return period by taking into account the joint occurrence of wave, wind, and current. The results show that the most severe mooring loads may not occur when wind, wave, and current are collinear and are at their maximum design values, i.e., the 50- or 100-year case. It is recommended that the extreme mooring design loads for moored offshore systems should be determined through a range of physical or numerical simulations where wave, wind, and current are noncollinear and act with less severe magnitudes than the 50- or 100-year case. This recommendation has also been adopted in the ITTC/Ocean Engineering Committee recommendations to the ITTC Conference held in September 1996.

Handbook of port and harbour engineering: geotechnical and structural aspects

Port and harbour engineering encompasses a wide range of subject areas and, over the last few decades, has involved approaches that are becoming increasingly sophisticated. Although an extensive literature has developed on many aspects of port and harbour engineering, there are relatively few books that provide a comprehensive coverage of this subject for use by practicing engineers. This handbook represents an attempt at providing such a contribution. Thus, the preface states: ithis book has been written... to provide port designers... with stateof-the-art information and guidelines to the design and construction of the basic types of marine structures.i At the outset, it should be stated that such an objective may be overly ambitious for a single volume, since a port engineer needs to rely on a vast amount of information, often scattered through many sources, in order to undertake the design of ports and harbours. The foreword to the book provides a concise summary of what maritime structural engineering is, as well as a good commentary on the crisis in understanding, which has been created by the ability to overanalyze using computers o often beyond the limits of the information that is available. This book, which is about 1000 pages long, has ten sections or chapters. Chapter 1 relates to the marine environment and covers topics such as water level variations, weather, wind, currents, waves, and ice. The second chapter focusses on design principles, and includes a classification of ports, the influence of ship characteristics on port design, and the design principles relating to navigation channels, harbour entrances, inner harbour areas, offshore installations, breakwaters, shore protection schemes, navigation aids, and mooring and fender systems. The third chapter treats design loads, including environmental loads associated with wind, currents, and waves; mooring loads; loads from cargo and equipment; ship impact loads; ice loads; seismic loads; and finally load combinations. The fourth chapter deals with geotechnical aspects of port and harbour engineering. The next four chapters relate to specific categories of structure: gravity-type quay walls, sheet-pile bulkheads, piled waterfront structures, and offshore deep

Dynamic Analysis for the Design of CALM System in Shallow and Deep Waters

Abstract This paper presents a time domain analysis approach to evaluate the dynamic behavior of the catenary anchor leg mooring (CALM) system under the maximum operational condition when a tanker is moored to the terminal, and in the survival condition when the terminal is not occupied by a tanker. An analytical model, integrating tanker, hawser, buoy, and mooring lines, is developed to dynamically predict the extreme mooring loads and buoy orbital motions, when responding to the effect of wind, current, wave frequency, and wave drift response. Numerical results describing the dynamic behaviors of the CALM system in both shallow and deepwater situations are presented and discussed. The importance of the line dynamics and hawser coupled buoy-tanker dynamics is demonstrated by comparing the present dynamic analysis with catenary calculation approach. Results of the analysis are compared with model test data to validate the mathematical model presented.

Computer Simulation of Moored Ship Behavior

Mathematical simulation models are of growing importance in the design stage and for the operation of mooring facilities at exposed locations. In this paper such a model is described for the prediction of motions and mooring loads of ships and other floating structures moored in irregular waves, wind, and current. The mathematical model is based on the equations of motion in the time domain. This approach allow nonlinear and asymmetric mooring loads and arbitrarily in time varying excitation of the vessel. Input variables are: environmental conditions; mooring layout; elasticity characteristics of mooring lines and fenders; and geometry of the vessel. Output information includes: time histories; spectra and statistical values of motions, and mooring loads. Results are presented of an extensive validation study on the behavior of a 200,000 TDW tanker moored to an offshore jetty in shallow water. From the results it may be concluded that there is good qualitative and quantitative agreement between the resul...

Mean- and Low-Frequency Wave Forces on Semisubmersibles

Abstract Mean- and low-frequency wave drift forces on moored structures are important with respect to low-frequency motions and peak mooring loads. This paper addresses prediction of these forces on semisubmersible-type structures by use of computations based on three-dimensional (3D) potential theory. The discussion includes a computational method based on direct integration of pressure on the wetted part of the hull of arbitrarily shaped structures. Results of computations of horizontal drift forces on a six-column semisubmersible are compared with model tests in regular and irregular waves. The mean vertical drift forces on a submerged horizontal cylinder obtained from model tests also are compared with results of computations. On the basis of these comparisons, we conclude that wave drift forces on semisubmersible-type structures in conditions of waves without current can be predicted with reasonable accuracy by means of computations based on potential theory.

Design Criteria for Floating Tire Breakwaters

Wave-transmission and peak-mooring force design curves were generated for the Goodyear and Wave-Guard floating tire breakwaters from measurements on ⅛-scale and ¼-scale models using regular and irregular waves, and found to be in good agreement with available full-scale data. These curves may be used to determine the breakwater size needed to attenuate a given regular design wave to an acceptable level, and also to determine the maximum mooring load associated with this. A simple semi-empirical energy-dissipation model was developed and found to simulate measured wave-transmission characteristics with sufficient precision to be useful in many engineering applications; e.g., the ratio of transmitted to incident wave height is stated as an exponential function of wave steepness, ratio of wavelength to breakwater-beam-size, breakwater porosity, and a drag coefficient. An empirical relationship for the peak mooring force was obtained.

Applications of wave diffraction theory

AbstractA computer program, originally developed to predict wave loads on large‐diameter fixed offshore structures, has been modified to compute added masses, damping coefficients and response motions of free‐floating and moored rigid systems. It is now being extended to predict multi‐degree‐of‐freedom responses of articulated structures, particularly wave energy absorbing devices. The structure is modelled by surface singularities (point sources) in potential flow. The boundary conditions, fluid and structure equations of motion are linearized. Engineering applications here include the predictions, fluid and structure equations of motion are linearized. Engineering applications here include the prediction of wave loads on a gravity platform, response motions of a moored barge, and comparisons with laboratory data on the mooring loads and response of a tethered buoyant platform, and on the wave loading, response and efficiency of two wave energy absorbing devices.

Design Criteria for Single-Point Mooring Terminals

Single-point moorings (SPMs) have come to be accepted as a logical choice for offshore berths for large tankers. The type most commonly used is that in which a single-buoy mooring (SBM) is anchored to the seabed with long catenaries of chain. Site selection for SPMs is governed above all by safety of operation of the tanker in maneuvering and mooring. Site conditions and marine environment play a major part in judging this safety. Detailed information is required on bottom conditions, sea levels, currents, winds, waves, and visibility. In the design of a specific SBM system, the main parameter is the mooring load. Determination of this load requires model tests, since reliable design formulas are not yet available. SBM can be operated without causing pollution to the surrounding coastline and without detrimental effect on the local marine environment.