Feasibility Stage Assessment of Side-By-Side LNG Offloading Operation

2007 ◽  
Author(s):  
Jun Wong ◽  
Colin Paton ◽  
Cedric Morandini ◽  
Timothy Withall ◽  
Andrew Kilner
Keyword(s):  
Numerical Simulation ◽  
Time Domain ◽  
Model Testing ◽  
Environmental Data ◽  
Mooring Line ◽  
High Fidelity ◽  
Sea State ◽  
Mooring Lines ◽  
Starting Point ◽  
Relative Motions

A key driver in assessing the economic viability of floating LNG terminals is the marine offloading operations uptime. Marine offloading operations uptime is the percentage of time on site for which weather conditions are such as to permit offloading operations to be undertaken. Physical model testing or time domain numerical simulation techniques can model these marine offloading operations to a very high level of fidelity. However it is not practical for reasons of time and cost to apply such high fidelity modeling to the long duration data sets necessary to make reliable uptime estimates. Simpler solution methods, which can be used to carry out rapid what if studies as well as provide uptime assessment based on very long data records are therefore required. This paper illustrates that a reliable and fast numerical approach based on frequency domain analysis can be developed and used as a pre-screening tool to identify key marine operations uptime drivers. In this method the process of determining the marine offloading operations uptime involves the following steps: 1. Collect and collate site-specific environmental data. The typical starting point for an uptime analysis will be 5 to 10 years of hindcast environmental data, consisting of records of the average wind, wave and current amplitude over successive 3-hour sea states. 2. Evaluate the expected vessel heading in each successive 3-hour sea state throughout the hindcast record. 3. For each 3-hour sea state, estimate the relative motions between the FPSO and LNGC, at the previously determined vessel heading. From the relative motions estimate the envelope of motions of the loading arms and the maximum tensions in the mooring lines between the FPSO and LNGC. 4. For each 3-hour sea state compare the estimated loading arm motion envelopes and maximum mooring line tensions with the maximum acceptable design values to determine whether offloading would be feasible in this 3-hour sea state. 5. Identify times when there are sufficient consecutive 3-hour sea states to allow the offloading operation to be completed (weather window). Determine the percentage uptime from the ratio of the total of these periods to the total environmental data length. A range of sensitivity analysis can also be performed using this methodology, thereby allowing critical cases to be identified for further examination using the high fidelity model testing or time domain numerical simulation programs.

Evaluation of Conventional Methods of Establishing Extreme Mooring Design Loads

2017 ◽  
Author(s):  
Dunja Stanisic ◽  
Michalakis Efthymiou ◽  
Mehrdad Kimiaei ◽  
Wenhua Zhao
Keyword(s):  
Time Domain ◽  
Coupled Model ◽  
Gumbel Distribution ◽  
Extreme Condition ◽  
Mooring Line ◽  
Sea State ◽  
Mooring Lines ◽  
Line Load ◽  
Design Loads ◽  
The Time Domain

A key aspect in the design of a mooring system for a floating production unit is the estimation of the extreme mooring line loads for a specified short-term sea state of typical duration equal to 3 hours. Commonly used design approaches today are based on time-domain simulations whereby each 3 hour sea state is run a number of times (typically 10–30 times) to represent the randomness of the sea. A maximum response is recorded from each simulation. Particular statistic of the maxima data (e.g. mean, most probable maximum or a percentile) is used to represent the extreme mooring load for which the lines are designed. This paper studies and assesses the accuracy of obtaining design value from a population of maxima with reference to the mooring line load of a large ship-shaped floating production vessel. A coupled model, including all mooring lines and risers, has been developed, validated and used to generate responses for 100yr extreme condition and 10,000yr survival condition. To establish an accurate benchmark against which the results are compared, the time-domain analyses (duration 3 hours) are repeated 170 times, for each sea state, to represent different random realisations of each environment. It is examined how the accuracy of predicting the design mooring line load, from a sample of response maxima, improves as the number of simulations is increased progressively from 10 through to 170. The assessment is performed across different statistics of maxima that are usually chosen to represent the design response. Besides the mooring line load, other response parameters such as heave and turret excursion, are examined in this paper. The paper examines whether the severity of the response (100yr vs 10,000yr storm) or the response variable affect the number of maxima required to achieve statistical stability. The results indicate fitting a Gumbel distribution to the maxima from about 30–40 simulations can yield results that are statistically stable and accurate and are recommended as preferred methods of estimating the design response.

Optimized Mooring Line Simulation Using a Hybrid Method Time Domain Scheme

2014 ◽  
Author(s):  
Niels Hørbye Christiansen ◽  
Per Erlend Torbergsen Voie ◽  
Jan Høgsberg ◽  
Nils Sødahl
Keyword(s):  
Hybrid Method ◽  
Time Domain ◽  
Computation Time ◽  
Mooring Line ◽  
Mooring Lines ◽  
Fem Model ◽  
Input Variables ◽  
The Time Domain ◽  
The Cost ◽  
Short Time

Dynamic analyses of slender marine structures are computationally expensive. Recently it has been shown how a hybrid method which combines FEM models and artificial neural networks (ANN) can be used to reduce the computation time spend on the time domain simulations associated with fatigue analysis of mooring lines by two orders of magnitude. The present study shows how an ANN trained to perform nonlinear dynamic response simulation can be optimized using a method known as optimal brain damage (OBD) and thereby be used to rank the importance of all analysis input. Both the training and the optimization of the ANN are based on one short time domain simulation sequence generated by a FEM model of the structure. This means that it is possible to evaluate the importance of input parameters based on this single simulation only. The method is tested on a numerical model of mooring lines on a floating off-shore installation. It is shown that it is possible to estimate the cost of ignoring one or more input variables in an analysis.

Docking and Undocking Considerations for Floatover Analyses and Operations

2013 ◽  
Author(s):  
Jillian Duquesnay ◽  
Jason Baldwin ◽  
James Wesley Rains
Keyword(s):  
Numerical Simulations ◽  
Time Domain ◽  
Cost Savings ◽  
Model Testing ◽  
Offshore Platform ◽  
Mooring Lines ◽  
High Profile ◽  
Weight Transfer ◽  
Proper Analysis

An alternative to lifted installation of topsides by a derrick barge is installation of single, integrated offshore platform topsides by floatover method. Floatover installation reduces hook-up and commissioning, which results in overall schedule and cost savings. Numerous papers were written recently to describe many aspects of the floatover operations. Nature of the floatover is such that it requires detailed engineering analyses, numerical simulations, model testing, and planning to evaluate all phases of the operation [Ref 5], [Ref 6]. Proper analysis of floatover requires numerical simulations using time-domain methodology to evaluate the system non-linearities inherent in the floatover hardware, fendering, mooring lines. Normally, weight transfer stages are given a high profile however it is found that the docking and undocking stages are equally as important. These sensitive stages of the floatover operation occur when the barge is entering the jacket slot prior to the floatover and exiting the jacket slot afterwards. The operation is sensitive to the prevailing weather and the number of simulations to make sure the operations can be performed safely is significant. Results of the docking and undocking analyses usually determine the weather standby and thus workability. This paper will address the docking and undocking stages of floatover for a barge that does not have its own propulsion. The paper shall include a concurrent investigation on effects of weather criteria. Stiffness of the hardware, mating lines/cross lines, mooring lines and the effect they have on the system will be discussed.

Calibration of Mooring Line Damping for Taut-Leg FPSOs Model Testing

2008 ◽  
Author(s):  
Antonio C. Fernandes ◽  
Allan Carre´ de Oliveira ◽  
Fabio P. S. Mineiro ◽  
Joel S. Sales ◽  
Andre´ Luis Rosa ◽  
...  
Keyword(s):  
Numerical Experiments ◽  
Model Testing ◽  
Model Tests ◽  
Reduced Model ◽  
Mooring Line ◽  
Mooring Lines ◽  
Stiffness And Damping

The paper discusses alternatives to represent the MLD (Mooring Line Damping) in models tests with truncated mooring lines. The work has performed both numerical experiments and reduced model tests. The results for stiffness and damping have been compared. This allows further considerations for future designs.

Coupled Dynamics Modelling of a Floating Wind Farm With Shared Mooring Lines

2018 ◽  
Author(s):  
Matthew Hall ◽  
Patrick Connolly
Keyword(s):  
Wind Farm ◽  
Numerical Models ◽  
Wind Farms ◽  
Offshore Wind ◽  
Irregular Waves ◽  
Mooring Line ◽  
Sea State ◽  
Mooring Lines ◽  
Offshore Wind Farms ◽  
Phase Relationships

This work presents a coupling of numerical models to allow simulation of a farm of floating wind turbines in which some mooring lines are shared between platforms. This shared mooring approach has potential to reduce mooring costs for floating offshore wind farms, but introduces additional complexity in system behaviour and design considerations for which new simulation capabilities are needed. Multiple instances of the FAST floating wind turbine simulator are coupled modularly to the MoorDyn mooring system simulator to achieve a coupled simulation of a full shared-mooring floating wind farm. The model is demonstrated on a square-shaped four-turbine shared mooring farm configuration in the presence of irregular waves and turbulent winds. Results show reasonable behaviour of the platform motions, with surge displacements under wind and wave loading that reflect the complex restoring properties of the shared mooring arrangement. Varying phase relationships in the platforms’ motions arising from their spatial offsets in the sea state show that the shared mooring lines will see different excitation at either end. Fluctuations in the mooring line tensions bear out this fact, and also show the importance of line dynamics in these shared mooring arrangements. In particular, the shared mooring lines show a greater tendency for resonance due to the absence of seabed contact.

scholarly journals A SLACK MOORING FOR A LARGE SHIP UNDER BEAM SEAS IN PORT

2018 ◽  
pp. 57
Author(s):  
Shigeki Sakakibara ◽  
Shunji Sunahara ◽  
Kaku Ito
Keyword(s):  
Numerical Simulation ◽  
Simulation Method ◽  
Mooring Line ◽  
Ship Motions ◽  
Physical Model Test ◽  
Mooring Lines ◽  
Beam Seas ◽  
Large Ship ◽  
Small Ship ◽  
Slack Mooring

As an empirical and traditional ship mooring method, a pretension mooring (a hard mooring) or slack mooring (without pretension in mooring line) is performed for large ship or small ship, respectively. However, the slack mooring is applied to a large ship under beam seas in port to avoid large roll and sway motions, and excessive loads in mooring lines and fenders. In this study, the mechanism and validity of the slack mooring for large ship is investigated by a physical model test and a numerical simulation method of the moored ship motions and mooring loads under beam seas.

scholarly journals Fatigue Analysis of Deepwater Hybrid Mooring Line Under Corrosion Effect

2014 ◽  
Vol 21 (3) ◽  
pp. 68-76 ◽  
Author(s):  
Dongsheng Qiao ◽  
Jun Yan ◽  
Jinping Ou
Keyword(s):  
Fatigue Damage ◽  
Oil And Gas ◽  
Line Tension ◽  
Chain Structure ◽  
Mooring Line ◽  
Mooring System ◽  
Load Spectrum ◽  
Sea State ◽  
Mooring Lines

Abstract In the deepwater exploitation of oil and gas, replacing the polyester rope by a wire in the chain-wire-chain mooring line is proved to be fairly economic, but this may provoke some corresponding problems. Te aim of this paper is to compare the fatigue damage of two mooring system types, taking into account corrosion effects. Using a semi-submersible platform as the research object, two types of mooring systems of the similar static restoring stiffness were employed. Te mooring lines had the chain-wire-chain and chain-polyester-chain structure, respectively. Firstly, the numerical simulation model between the semi-submersible platform and its mooring system was built. Te time series of mooring line tension generated by each short-term sea state of South China Sea S4 area were calculated. Secondly, the rain flow counting method was employed to obtain the fatigue load spectrum. Thirdly, the Miner linear cumulative law model was used to compare the fatigue damage of the two mooring system types in long-term sea state. Finally, the corrosion effects from zero to twenty years were considered, and the comparison between the fatigue damage of the two mooring system types was recalculated.

Comparative Analysis on Fatigue Damage of Deepwater Hybrid Mooring Line

2010 ◽  
Author(s):  
Dongsheng Qiao ◽  
Changxin Zheng ◽  
Binbin Li ◽  
Jinping Ou ◽  
Gangjun Zhai
Keyword(s):  
Finite Element ◽  
Fatigue Damage ◽  
Nonlinear Finite Element ◽  
Mooring Line ◽  
Inertia Force ◽  
Short Term ◽  
Sea State ◽  
Spar Platform ◽  
Mooring Lines ◽  
The Wire

In the deepwater exploitation of oil and gas, the replacement of polyester rope for the wire in chain-wire-chain mooring line is proved to be fairly economic. Previous studies are mainly conducted to hydrodynamic analysis and dynamic response calculation, but there are few studies on the fatigue damage analysis of hybrid mooring line. Take the mooring system of a Spar platform as the research object, and the multi-component mooring lines are usually made up of chain-wire-chain. The representative mooring line has been considered while other conditions are kept unchanged, and top end tension-static offset characteristic curve of the two mooring lines are consistent as much as possible, meanwhile the polyester rope is adopted as the substitute for the wire, then the comparative calculation on fatigue damage of deepwater hybrid mooring line is carried on. The analysis of Spar platform response is carried out in which the wave force is calculated under diffraction theory. After the establishment of two-dimensional nonlinear finite element dynamic model of the hybrid mooring line, the calculation on stiffness of the hybrid mooring line is accomplished through the iterative method based on the empirical formula proposed by Del Vecchio in 1992. The mooring line and seabed interaction is based on the hypothesis of rigid seabed. The fluid drag force and inertia force on the mooring line are calculated according to the Morrison formula. The dynamic analysis is executed through time-domain nonlinear finite element method accounting for the response of the Spar platform which is calculated as above. The stress time histories of the hybrid mooring line in each short-term sea state of South China Sea area are calculated, and then the rain flow counting method is employed to obtain the fatigue load spectrum in each short-term sea state. The Miner linear cumulative law model is used to compare the fatigue damage of the hybrid mooring lines in long-term sea state which consists of tens of short-term sea states. The results show that the using of polyester rope as the substitute for the wire can significantly improve the fatigue resistance.

Sensitivity of a Floating Bridge Global Responses to Different Wave Drift Force Models

2021 ◽  
Author(s):  
Carlos Eduardo Silva de Souza ◽  
Nuno Fonseca ◽  
Marit Irene Kvittem
Keyword(s):  
Mooring Line ◽  
Sea State ◽  
Mooring Lines ◽  
Wave Drift ◽  
Floating Bridge ◽  
Promising Solution ◽  
Bridge Girder ◽  
Wave Drift Forces ◽  
Wave Incidence ◽  
Drift Forces

Abstract Floating bridges are a promising solution for replacing ferries in the crossing of Norwegian fjords. Their design involves the adoption of accurate, but at the same time efficient models for the loads the structure is subjected to. Wave drift forces at the bridge’s pontoon may excite the bridge’s lower horizontal modes, with consequences to the loads on the bridge and mooring lines. Newman’s approximation is normally adopted to calculate the wave drift forces in such applications. A common simplification is to assume that the pontoons are fixed in the calculation of wave drift coefficients, while it is known that wave frequency motions may significantly influence drift loads. This paper evaluates the consequences of this simplification, in comparison to coefficients obtained considering the pontoons’ motions. First, the effect of the bridge deflection, due to mean drift, on the pontoon’s motions, is evaluated. It is found that this effect is negligible. Then, the RAOs are used in the calculation of wave drift coefficients, showing very different results than those obtained with fixed pontoons. Time-domain simulations are then performed with wave drift coefficients calculated with both approaches, with focus on the bridge girder moments and mooring line tensions. It is shown that using wave drift coefficients obtained with fixed pontoon is a non-conservative simplification, depending on sea state and wave incidence direction.

Ultradeepwater Monobuoys

2003 ◽  
Author(s):  
Mauro C. Oliveira
Keyword(s):  
Numerical Simulation ◽  
Time Domain ◽  
Calibration Procedure ◽  
Coupled Analysis ◽  
Global Behavior ◽  
Test Results ◽  
Mooring Lines ◽  
Analysis And Design ◽  
Deep Waters ◽  
Hull Forms

This work deals with the analysis and design of monobuoys for deep waters. The monobuoy performance evaluation is carried out using a time domain computer program due to the non linearities present in this problem. This program is used to simulate the behavior of the monobuoy under the action of waves, wind and current. A coupled analysis between the floater and the mooring lines, considering its inertia, is also employed. Initially a validation study is conducted comparing the numerical simulations with model test results for a 400 meters water depth CALM buoy. The test comprises an operational condition with a tanker connected to the buoy under the action of wave, current and wind loads. From these results a calibration procedure of the numerical simulation is proposed and different hull forms are assessed in order to verify its global behavior. The main objective is to check if there are improvements in comparison with more conventional shapes relatively to the riser forces in the connection point.

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