Study Optimasi Panjang Mooring Line Tipe Spread Mooring pada F(P)SO

Dalam analisa mooring untuk F(P)SO (Floating (Production) Storage and Offloading, banyak hal yang harus dipertimbangkan agar mooring system bisa memenuhi standard kelayakan dan keamanan. Di paper ini akan dibahas mengenai optimasi panjang mooring line untuk mooring system tipe spread mooring pada kapal tanker dengan kapasitas sekitar 10,000 dan 20,000 DWT yang nantinya akan dikonversi menjadi F(P)SO. Jumlah mooring line diberi batasan untuk 8 spread mooring line (tipikal) dengan konfiguarsi mooring line dan pretension mooring line yang sama untuk kedua kapasitas kapal yang berbeda. Variasi hanya pada panjang mooring line untuk melihat secara jelas sejauh mana pengaruh Perubahan panjang mooring line terhadap tension dari tiap mooring line dan offset gerakan dari FPSO saat di tambat dan menerima beban beban lingkungan laut (gelombang, angin, arus) dengan menggunakan full analisa dinamis dengan metode time domain. Dari studi ini bisa diketahui dengan penambahan panjang mooring line maka akan terjadi pengurangan maksimum tension pada mooring line, tapi sebaliknya terjadi penambahan offset gerakan kapal

Kajian Numerik Ketidakstabilan FPSO Tertambat dalam Kondisi Alami Kerusakan pada Kondisi Mooring Line yang Berbeda

Dalam perencanaan suatu kapal yang mendapatkan beban lingkungan seperti angin, gelombang dan arus, adalah sangat penting mengetahui tegangan maksimum yang bekerja pada mooring line. Pada kondisi cuaca yang agak buruk, hal tersebut memungkinkan terjadinya kerusakan/kegagalan pada tali mooring. Kondisi tersebut akan menyebabkan ketidakstabilan terhadap respon gerakan kapal dan tegangan pada mooring line. Suatu pendekatan numerik dilakukan untuk mensimulasikan pengaruh dari mooring line yang mengalami kerusakan serta pengaruh konfigurasi mooring line yang berbeda. Pada akhir pembahasan, suatu analisis dilakukan untuk menentukan kestabilan FPSO dan mooring line dalam kondisi alami kerusakan. Dalam perencanaan suatu kapal yang mendapatkan beban lingkungan seperti angin, gelombang dan arus, adalah sangat penting mengetahui tegangan maksimum yang bekerja pada mooring line. Pada kondisi cuaca yang agak buruk, hal tersebut memungkinkan terjadinya kerusakan/kegagalan pada tali mooring. Kondisi tersebut akan menyebabkan ketidakstabilan terhadap respon gerakan kapal dan tegangan pada mooring line. Suatu pendekatan numerik dilakukan untuk mensimulasikan pengaruh dari mooring line yang mengalami kerusakan serta pengaruh konfigurasi mooring line yang berbeda. Pada akhir pembahasan, suatu analisis dilakukan untuk menentukan kestabilan FPSO dan mooring line dalam kondisi alami kerusakan

Assessment of Wind Turbine Aero-Hydro-Servo-Elastic Modelling on the Effects of Mooring Line Tension via Deep Learning

As offshore wind turbines are moving to deeper water depths, mooring systems are becoming more and more significant for floating offshore wind turbines (FOWTs). Mooring line failures could affect power generations of FOWTs and ultimately incur risk to nearby structures. Among different failure mechanics, an excessive mooring line tension is one of the most essential factors contributing to mooring failure. Even advanced sensing offers an effective way of failure detections, but it is still difficult to comprehend why failures happened. Unlike traditional parametric studies that are computational and time-intensive, this paper applies deep learning to investigate the major driven force on the mooring line tension. A number of environmental conditions are considered, ranging from cut in to cut out wind speeds. Before formatting input data into the deep learning model, a FOWT model of dynamics was simulated under pre-defined environmental conditions. Both taut and slack mooring configurations were considered in the current study. Results showed that the most loaded mooring line tension was mainly determined by the surge motion, regardless of mooring line configurations, while the blade and the tower elasticity were less significant in predicting mooring line tension

The assessment of mooring line damping for offshore structures

With increasing water depth of oil and gas exploration, greater importance has been attached to the damping force from mooring systems. The effect is significantly important to slow drift motion of the floating structure, and it is also coupled with its motions. Coupled analysis is thus preferred to be applied to estimate the floating structure motions and to calculate the mooring system response, especially for offshore structures in deep-water.;In this study, the aim is to achieve a better understanding of mooring line induced damping estimation. Drag forces normal to the mooring line due to the motion of the mooring line through the water, are the main source of hydrodynamic damping of the mooring line. A method of energy dissipation based on the mooring line dynamic response obtained by Orcaflex is developed. The validation is established through two types of mooring lines in shallow water and deep water.;The present approach shows a good agreement with the published results, but with two exceptions. One is for the mooring line oscillated by a very slow LF motion in shallow water, when the hydrodynamic damping is very small. Another one is for the wire mooring line oscillated by WF motion in deep water, the result shows significant discrepancy with that from the quasi-static method.;Then a non-dimensional analysis is completed, due to the strong complexity of the mooring line induced damping. The effects of the factors can be divided into three groups: first, the effects from pretension and scope are related to the geometry changes of the mooring line; second, the oscillation, current velocity and drag coefficient make contributions to the drag forces of mooring line directly; and the last, the effects of stiffness and seabed friction which, it was found, can be neglected.;In order to experimentally investigate the chain behaviour moving in water, a series model tests for the damping characteristics of a single chain line is implemented through oscillation tests of various parameters. The drag coefficient ( C D ) variations with different Reynold ( Rn ) and KC numbers are investigated. The drag coefficients in this study range from 1.5 to 4.0, which is case-dependent, because both Reynold ( Rn ) and KC number affect them.;With the increase of KC number, the drag coefficient shows a decrease with exceptions occurring in low KC cases. In addition, it is shown that the chain segments near the fairlead and touch down area are most sensitive to the drag coefficient, which is consistent with the velocity distribution along the mooring line.;Finally, the validation is established by comparing the results of experimental tests and numerical simulations. Based on the assessment of drag coefficient from the scaled experimental investigation, numerical simulations of estimated drag coefficient are carried out within Orcaflex. A good agreement is achieved between the numerical calculations and experimental measurements, which illustrates that the present method can be applied for mooring line damping estimation. Meanwhile, the effects of the amplitude and frequency of the oscillation are studied.With increasing water depth of oil and gas exploration, greater importance has been attached to the damping force from mooring systems. The effect is significantly important to slow drift motion of the floating structure, and it is also coupled with its motions. Coupled analysis is thus preferred to be applied to estimate the floating structure motions and to calculate the mooring system response, especially for offshore structures in deep-water.;In this study, the aim is to achieve a better understanding of mooring line induced damping estimation. Drag forces normal to the mooring line due to the motion of the mooring line through the water, are the main source of hydrodynamic damping of the mooring line. A method of energy dissipation based on the mooring line dynamic response obtained by Orcaflex is developed. The validation is established through two types of mooring lines in shallow water and deep water.;The present approach shows a good agreement with the published results, but with two exceptions. One is for the mooring line oscillated by a very slow LF motion in shallow water, when the hydrodynamic damping is very small. Another one is for the wire mooring line oscillated by WF motion in deep water, the result shows significant discrepancy with that from the quasi-static method.;Then a non-dimensional analysis is completed, due to the strong complexity of the mooring line induced damping. The effects of the factors can be divided into three groups: first, the effects from pretension and scope are related to the geometry changes of the mooring line; second, the oscillation, current velocity and drag coefficient make contributions to the drag forces of mooring line directly; and the last, the effects of stiffness and seabed friction which, it was found, can be neglected.;In order to experimentally investigate the chain behaviour moving in water, a series model tests for the damping characteristics of a single chain line is implemented through oscillation tests of various parameters. The drag coefficient ( C D ) variations with different Reynold ( Rn ) and KC numbers are investigated. The drag coefficients in this study range from 1.5 to 4.0, which is case-dependent, because both Reynold ( Rn ) and KC number affect them.;With the increase of KC number, the drag coefficient shows a decrease with exceptions occurring in low KC cases. In addition, it is shown that the chain segments near the fairlead and touch down area are most sensitive to the drag coefficient, which is consistent with the velocity distribution along the mooring line.;Finally, the validation is established by comparing the results of experimental tests and numerical simulations. Based on the assessment of drag coefficient from the scaled experimental investigation, numerical simulations of estimated drag coefficient are carried out within Orcaflex. A good agreement is achieved between the numerical calculations and experimental measurements, which illustrates that the present method can be applied for mooring line damping estimation. Meanwhile, the effects of the amplitude and frequency of the oscillation are studied

A Neural Network Approach to Estimate Buoy Mooring Line Sensor Deflection

Instrumented moorings are often used to measure characteristics, such as temperature and current, over the water column. However, the moorings deflect from the effects of currents and waves, which could lead to innacurate measurements. In this work, a computationally efficient method to compensate for mooring sensor position errors is developed. The two-step process first uses a hydrodynamic model of the buoy and mooring line system to create estimated mooring line deflections in a steady current. A neural network model is trained to approximate the hydrodynamic model’s mooring line displacement given the spatial location of the buoy and current profile measurements. The method is illustrated using the Mackinac Straits West buoy that is part of the Upper Great Lakes Observing System (UGLOS). Its mooring line is instrumented with 10 thermistors, attached to the mooring line at varying intervals. Since the approach naturally provides interpolation, it allows researchers, with access to publicly available UGLOS data, to request temperatures at any depth. While the vertical deflection compensation method is illustrated here is for a particular mooring system, the process involved is applicable to a wide class of instrumented mooring systems. It was found that access to the current data of a mooring line increases the accuracy of the Neural Network, but knowing the position of the buoy in relation to the anchor can still give adequate results

An Improved Stiffness Model for Polyester Mooring Lines

The stiffness model of highly extensible polyester mooring lines is studied. Mooring lines are considered within coupled dynamic model of a moored fl oating object. In more detail, deepwater mooring with taut polyester mooring lines is observed. In this case mooring line is modelled as an extensible cable without bending and torsional stiffness. Movements are assumed to be three-dimensional, so it is necessary to examine large displacement model. In longitudinal strain calculation the material of the mooring line is considered as nonlinear. A large elongation value is examined within the stiffness model. Inertial forces of the mooring line are also considered. Hydrodynamic loads due to surrounding fl uid are taken into account with the Morison equation. Due to nonlinear properties of mooring lines calculations have to be done in time domain. On these assumptions, derivation of a mooring line fi nite element is presented for static and dynamic analysis. A fl oating object is modelled as a rigid body with six degrees of freedom and with small displacements assumption. Hydrodynamic coeffi cients are calculated in a specifi ed frequency domain; therefore, mapping from the frequency to the time domain is necessary. Comparison between the improved model developed in this paper and current equivalent model is done. A simple mooring line that can be analytically described was the base for comparison. The improved model achieved better agreement with the analytical result

NUMERICAL AND EXPERIMENTAL STUDIES ON THE SLOW DRIFT MOTIONS AND THE MOORING LINE RESPONSES OF TRUSS SPAR PLATFORMS

An efficient methodology has been developed for the dynamic analysis of offshore floating structures. In this methodology, special attention was given to the second order difference frequency forces and responses. According to this numerical scheme, a MATLAB program named TRSPAR was developed to predict the dynamic responses of truss spar platform in time domain. In this program, the truss spar platform was modeled as a rigid body with three degrees of freedom. Hydrodynamics of the structure, which include the linear and second order wave forces, mean drift forces, added mass, radiation damping, wave drift damping and system stiffness were included in the program. Current and wind forces were also considered showing their effects on the slow drift responses. The wave forces, including inertia and drag forces, were calculated using Morison equation assuming the wave field as undisturbed. An efficient time domain integration scheme was adopted based on Newmark Beta method. Comprehensive experimental studies were conducted and the numerical predictions were systematically compared with model test results. These comparisons consisted of structure’s dynamic responses in different environmental conditions and two structural situations. The first situation was the structure with intact mooring lines and the other one was the structure under mooring line failure. The responses of the platform with mooring line system damage were investigated with the emphasis on finding the critical effects of line failure on the resonant responses. The effects of the second order difference frequency wave forces on the truss spar motion characteristics were examined numerically. Published numerical results were used to verify the developed numerical model in predicting the truss spar dynamic responses when subjected to combined wave, current and wind forces. The effects of strengthening mooring line system on the motion characteristics of the structure were examined numerically. For the assessment of the fluid to mooring nonlinear interactions, a deterministic approach based on lumped mass method with equations of dynamic equilibrium and continuity was adopted. Finally, parametric studies on deepwater mooring line analysis have been conducted for investigating the contributions of the various design parameters on mooring line tension. The experimental results verified the validity of the developed numerical scheme for prediction of the wave frequency and low frequency motions of the truss spar platform with its intact mooring and in the case of mooring line damage condition. RMSD values for the numerical and the experimental results show that the simulated wave frequency responses (WFR) trend was relatively agreed well with the experiments compared to the low frequency responses (LFR). For the intact mooring line condition, RMSD values for the WFR ranged from 109.9 to 182.4 while for LFR were ranged from 499.6 to 550.2. The same has been noticed in the mooring line damage condition in which RMSD values ranged from 107.4 to 323.6 and 209.1 to 1074 for WFR and LFR respectively. With regard to the peak responses, good accuracy has been achieved between the predictions and the measurements. The percentage errors for the peak responses in the intact mooring and the mooring line damage conditions were ranged from 9.5% to 17.3%

An enhanced stiffness model for elastic lines and its application to the analysis of a moored floating offshore wind turbine

The performance of a polyester mooring line is non-linear and its elongation plays a significant role in the dynamic response of an offshore moored structure. However, unlike chain, the tension-elongation relationship and the overall behavior of elastic polyester ropes are complex. In this paper, by applying an enhanced stiffness model of the mooring line, the traditional elastic rod theory has been extended to allow for large elongations. One beneficial feature of the present method is that the tangent stiffness matrix is symmetric; in non-linear formulations the tangent stiffness matrix is often non-symmetric. The static problem was solved by Newton-Raphson iteration, whereas a direct integration method was used for the dynamic problem. The computed mooring line tension was validated against the proprietary OrcaFlex software. Results of mooring line top tension predicated by different elongations are compared and discussed. The present method was then used for a simulation of an offshore floating wind turbine moored with taut lines. From a comparison between linear and non-linear formulations, it is seen that a linear spring model under-estimates the mean position when the turbine is operating, but over-estimates the amplitude of the platform response at low frequencies when the turbine has shut down

Effect of Mooring Line Layout on the Loads of Ship-shaped Offshore Installations

An offshore mooring system stations a ship-shaped offshore installation in place while withstanding incoming loads from the marine environment with short-term and long-term uncertainties. This study aims to develop a novel framework for analysing the loads on floating systems, namely mooring line tension, mooring line fatigue damage, and hull bending moment, as a function of the mooring layout design variables and environmental random variables. The nonlinear influence of those variables is assessed by means of advanced techniques using response charts, response divergence charts, and Sobol's total-effect sensitivity indexes. The developed procedure includes a probabilistic selection of mooring scenarios, station-keeping numerical analyses, and metamodel selection to define input loads. An example of a hypothetical floating production storage and offloading (FPSO) unit with taut legs in the Gulf of Mexico illustrates the procedure. The details of the computations are documented, and the findings show that the mooring line top-tension has a high total-effect index for the wave-induced bending moment and the total mooring line tension, whereas the fatigue damage is mostly affected by the chain diameter. The results of this research offer useful insights to designers and propose the use of a surrogate model to be used in the reliability-based design of mooring systems

Desain Mooring Line Pada Struktur Pantai Floating Breakwater Menggunakan Catenary Mooring Line

Breakwater merupakan struktur perlindungan pantai untuk memecah gelombang. Akhir-akhir ini para peneliti telah mengembangkan struktur breakwater menjadi pemecah gelombang terapung (floating breakwater). Untuk menahan floating breakwater tersebut maka di perlukan sistem penambat (moring line). Pada penelitian ini difokuskan untuk mendesain mooring line pada struktur single floating breakwater menggunakan catenary mooring line yang dimana terjadi variasi kedalaman laut, arus, pasang surut, sudut arah datang gelombang, dan tinggi gelombang. Floating breakwater memiliki dimensi 10 m x 3 m x 1,3 m dan draft 0,8 m dan freeboard 0,5 m pengerjaan ini dibantu dengan menggunakan software MOSES dan Orcaflex. Material yang digunakan untuk mendesain mooring line ini menggunakan chain (rantai). Jumlah mooring line yang diperlukan sebanyak 4 buah mooring line. Hasil dari simulasi menggunakan software tersebut berupa Response Amplitude Operators (RAO) saat kondisi terapung bebas pada moda gerak surge, sway, heave, roll, pitch, dan yaw. Selain itu hasil simulasi juga menghasilkan tension maximum pada mooring line dan offset maximum pada floating breakwater yang lebih besar dan lebih dominan terjadi pada pasang 11,2 m dengan arah datang gelombang 900 dan tinggi gelombang 1,5 m. Setelah itu dapat ditentukan spesifikasi mooring line yang digunakan pada floating breakwater tersebut. ========================================================================================================================Breakwater is a coastal protection structure to break the waves. Lately researchers have developed breakwater structures into floating breakwaters. To hold the floating breakwater is in need of a mooring system. This research is focused on designing mooring line in single floating breakwater structure using catenary mooring line where variation of ocean depth, current, tidal, wave direction angle, and wave height. Floating breakwater has dimensions of 10 m x 3 m x 1.3 m and a draft of 0.8 m and 0.5 m freeboard this work is assisted by using MOSES and Orcaflex software. The material used to design this mooring line using chain. The number of mooring line required is 4 mooring line. The result of the simulation using the software is Response Amplitude Operators (RAO) when free floating conditions in the mode of motion surge, sway, heave, roll, pitch, and yaw. In addition, the simulation result also produces maximum tension on mooring line and offset maximum on larger floating breakwater and more dominant occurs at pairs 11.2 m with the direction of wave 900 and 1.5 m wave height. After that can be determined mooring line specifications used on the floating breakwater. Keywords: Floating Breakwater, Catenary Mooring Line, Response Amplitude Operators (RAO), Tension mooring line and Offset floating breakwater, mooring line Specificatio