Point Mooring System Research Articles

Centrifuge model tests on pile groups under oblique pullout load for single point mooring system through transparent sand

Single point mooring systems anchored by pile groups have been widely applied to support floating offshore platforms. Centrifuge model tests on the pile group performance and soil displacement field were conducted using transparent sand. The pile group consisted of six piles connected by steel wires and divided into two groups. The two groups were symmetric in the x-direction, and the three piles in each group were symmetric in the y-direction. The influences of pile group arrangement and loading direction were investigated. With the increasing angle between adjacent piles in the same group (β), the pile group resistance for a given displacement increased under the x-direction load but decreased under the y-direction load. The difference in the pile load distribution increased with β, and the pile group resistance reduced significantly once one pile failed at β = 60°. The soil displacement field indicated that the overlap effect of the influence zone increased with the decreasing β, leading to a reduction in the pile group resistance at β = 30°. Therefore, the pile group arrangement with β equal to 45° was optimal in this study. Additionally, the pile group resistance improved as the loading direction changed from the x-direction to the y-direction.

Influence of Grease Contamination on Acoustic Emission Monitoring of Low-speed Roller Bearings

Large-scale low-speed rolling element bearings form crucial connections in offshore installations such as heavy-lifting cranes, single point mooring systems, and wind turbines. Due to the stochastic nature of wind and waves, the applied loads on these bearings are hardly predictable. Furthermore, the remote nature of the offshore environment requires a high standard of operational safety and reliability, reinforcing the need for advanced inspection and monitoring methods. Acoustic emission (AE) is a continuous monitoring technique for condition assessment of low-speed bearings, as it may detect the stress waves associated with developing degradation within the rolling elements. This paper presents an investigation on the influence of grease contamination on acoustic emission (AE) monitoring of low-speed bearings. It discusses several experiments that have been conducted in various regimes of particle contaminated lubrication, and proposes a strategy for condition monitoring. The presented experiments have been conducted with differing bearing geometries in multiple testing environments. The contamination particles have been both naturally developed and artificially introduced. The results total of 9 experimental cases are discussed and compared. All experiments follow the same data-acquisition principles, utilising arrays of three AE transducers sensitive in a range between 40-580 kHz. Data is recorded while operating the bearing at low speed under load. Abrasive wear of the rolling elements and crushing of the particles are concluded to be the main sources of AE due to particle contamination. Processing consists of waveform cross-correlation clustering and feature analysis. The results suggest that hard abrasive particles make significant contribution to the AE signals, which is associated with abrasive wear of the rollers and raceways. Comparisons with the contribution of softer and finer particles are presented as well.

Launch and recovery of a work class ROV through wave zone in small offshore service vessel

The deployment of remotely operated underwater vehicle (ROV) from a small offshore service vessel (OSV) based on single point mooring system (SPMS) method is recently adopted in offshore renewable energy sector. However, the tension spike in wire, also known as snap load, often occurs when the ROV passes through the wave zone in launching and lifting operation of deployment. In this study, a practical numerical model for predicting wire tension during launch and recovery of ROV is developed and validated by wave flume test of a 1:10 scaled model. The numerical simulations reveal that the ROV deployment at vessel stern along with an appropriate reduction of horizontal distance from the hull are reliable safety strategies for reducing wire tension. By adopting the new deployment strategy, the annual operational capacity can be expanded by approximately 6% when the safe operational limit of ROV under a significant wave height of 1.25 m. Based on the comprehensive numerical simulation, the newly developed safe operating envelope provides a further guidance for onboard ROV operation in the O&M of offshore wind farms.

Centrifuge model tests on an anchor pile under cyclic oblique pullout load using transparent sand

Cyclic load on anchor piles of single point mooring systems is an important issue, and the cyclic behavior of anchor piles has a connection with the surrounding soil displacement. Based on transparent sand and particle image velocimetry (PIV) technique, centrifuge model tests on the pile subjected to oblique pullout load were conducted. The cyclic pile performance and the soil displacement field under various cyclic load characteristics were studied. The mechanism for the cyclic response of anchor piles was investigated. It can be observed by transparent sand that the failure mode of the pile is a combination of vertical and lateral translation without rotation. The post-cyclic pullout bearing capacities are decreased with the increasing of the cyclic amplitude and number of cycles, but the pile vertical displacement and the soil displacement are increased at the same post-cyclic reload. During the cyclic load period, the pile produces cumulative displacement due to soil particle movement, especially vertical displacement that can reduce embedment depth and bearing capacity. However, the shakedown behaviors of the cumulative displacement and post-cyclic bearing capacity of the pile occur, because the increasing rate of the soil displacement is decreased with loading cycles.

Reversed tension characteristics in subsea mooring connectors for an FPSO SPM in shallow water

The reversed tension on subsea mooring connectors is studied to prove its appearance. The mooring line is chosen from the real single point mooring system of Floating Production Storage and Offloading Unit (FPSO) wording in the South China Sea. The forced motion excitations are applied on the top end of the mooring line with the data monitored by the GPS device on the FPSO. The simulations are carried out with the lumped mass model and the nonlinear finite element model to verify the accuracy of the whole process. In the results of the forced excitation case, the reversed tensions are found three times. The occurrence of the revered tensions is consistent with the minus peak of the vertical displacement in time history, and the vertical displacement is recognized as the key factor for the reason of reversed tension. The reversed tension will apply a compression force on the rigid subsea mooring connect after a generous vertical displacement change. It may lead to the failure of the connector, and then the mooring line. Therefore, the minimum tension design is suggested to be added to the rules of classification society to avoid the reversed tension and slack condition of the mooring line.

Experimental investigation on the coupling response of a multi-floating side-by-side system between a single-point mooring carrier and an articulated floating platform

As an important equipment, side-by-side mooring system and its safety are crucial during offloading operation. Experimental investigation on coupling response of an articulated floating platform and cargo carrier side-by-side single point mooring system is presented. In the case of regular wave and irregular sea, the properties of relative motions in a multi-floating side-by-side single point mooring system is analysised. Comparisons of performance levels between connection systems in different sea conditions reveal effects of wave load are drawn based on the analysis results, which can be used to guide further studies of the multi-floating side-by-side single point mooring system offloading operations.

Study on dynamic tension estimation for the underwater soft yoke mooring system with LSTM-AM neural network

It is significant to estimate the mooring tension for the safety, operation and maintenance of single point mooring system of floating production, storage and offloading unit (FPSO). In this study, a neural network model named long-short term memory combined with attention mechanism (LSTM-AM) is adopted to estimate the mooring legs dynamic tension of an underwater soft yoke mooring system in time domain. The mooring legs tension and FPSO motions are set as the training features in the LSTM-AM neural network, together with the corresponding first-order and second-order central moments. The training data collection is implemented with the numerical hydrodynamic coupled analysis of FPSO and an underwater soft yoke mooring system. Different variables are studied to determine the optimal structure of the LSTM-AM neural network model, including the time window, the layer numbers, the neural units and the optimizer. Through cases studied, it is proved that the LSTM-AM neural network model is suitable to estimate the mooring legs tension of FPSO and the underwater soft yoke mooring system in different sea states.

A novel risk analysis approach for FPSO single point mooring system using Bayesian Network and interval type-2 fuzzy sets

Floating Production, Storage and Offtake (FPSO) is one of the key components of offshore oil development and the single point mooring system is the key technology affecting the safety of the FPSO. Therefore, risk analysis is useful in preventing the failure of the FPSO single point mooring system. However, traditional techniques fall short in terms of quantitative assessment, dynamic control, and handling uncertainty. This study provides a risk analysis approach that can be used to assess the FPSO single point mooring system safety risk. The Bayesian Network and interval type-2 fuzzy sets are used to overcome the difficulties in gathering and evaluating the data required for the calculation of the FPSO single point mooring system safety hazards. The proposed approach allows for the identification of the primary underlying causes of these occurrences. The effectiveness of the proposed methods for risk analysis of FPSO single point mooring systems is proved using a case study. According to the findings, it is believed that this study could be useful in providing a thorough and all-encompassing evaluation of the safety risk in multi-process systems and in suggesting a strategic strategy for reducing the FPSO single point mooring system safety hazards.

Motion response analysis of FPSO with single point mooring system based on fully nonlinear method

The mooring system of FPSO has characteristics of large curvature ratio and high deflection, and the existing hydrodynamic load calculation methods ignore some nonlinear wave factors. In this paper, the mooring system analysis was achieved by using the slender rod theory and finite element method. Based on the boundary element method, the particular auxiliary function and mixed Lagrangian-Euler method were employed to solve the simultaneous hydrodynamic force at each time step. Finally, a hull and mooring coupled motion forecast method was established by integrating the forward two methods and a self-programming code was developed. The model is verified by comparing the numerical results of a FPSO under RAO conditions with experiment data.

Centrifuge model tests on anchor pile of single point mooring system under oblique pullout load using transparent sand

Single point mooring system anchored by piles has been widely used to support floating offshore platforms. However, the failure mechanism and soil displacement of the anchor pile subjected to oblique pullout loads are still unclear. Based on transparent sand and PIV technique, a visual centrifuge test system for anchor piles of single point moorings was established. A series of centrifuge model tests were performed on the pile under oblique pullout loads, and the measured soil displacement field results were compared with the finite element simulation, indicating that the centrifuge modeling using transparent sand are suitable for studying the anchor pile behaviors and soil deformation. A modified semi-empirical method was also proposed, which can predict the bearing capacity of piles under difference loading angle and loading position in sand. It shows that with the increasing loading angle and pad eye depth, the pile horizontal translation and soil horizontal deformation become more obvious, and the pullout bearing capacity increases. The proportion of the soil horizontal displacement for θ = 75° is nearly 4.1 times of that for θ = 37°, and the horizontal displacement proportion for z = 0.75L increases approximately 25% of that for z = 0.67L.

Evaluation of Dynamic Tensions of Single Point Mooring System under Random Waves with Artificial Neural Network

Real-time monitoring of the mooring safety of floating structures is of great significance to their production operations. A deep learning model is proposed here, based on the long short-term memory (LSTM) artificial neural network. Firstly, the numerical simulation is carried out with the single-point mooring system of a Floating Production Storage and Offloading (FPSO) as the training data of LSTM. Then the proposed LSTM is performed. Finally, taking the motion of FPSO which is not encountered by LSTM neural network model as input, we predict the mooring line tension with this model. Here, one FPSO in the South China Sea is taken as a research case, hydrodynamic and mooring models are established, and the network structure and hyper-parameters of the LSTM model are determined. The prediction results of the LSTM under different combinations of wind, wave, and current are compared with the calculation results of AQWA software. The model constructed here can well predict the mooring line tension of different combinations of wind, wave and current.

An Overview on Bonded Marine Hoses for Sustainable Fluid Transfer and (Un)Loading Operations via Floating Offshore Structures (FOS)

Due to the demand for oil production in varying water depth regions, the advantage of flexible buoyant conduits has led to an increase in bonded marine hoses for fluid transfer and (un)loading operations. The fluid transfer system for bonded marine hoses is dependent on floating offshore structures (FOS). This paper presents an overview of different systems for sustainable fluid transfer and (un)loading operations via FOS, such as Single Point Mooring (SPM) systems. SPMs are component aspects of the techno-economic design and FOS operation. This review aims to present sustainable fluid transfer technologies while addressing the subject of bonded marine hoses based on application, configuration, test models, hose selection criteria, hose-mooring configurations and operational views. This paper also includes an overview of the hose dynamics, with the loading and unloading (or discharging) techniques for sustainable fluid transfer via marine bonded hoses, based on operational challenges encountered. To dynamically present the hose performance in this review, an overview of the test methods’ guidance as specified in available industry standards was conducted. The pros and cons of marine hose application were also presented. Finally, this study presents different marine hose types and novel design configurations applied in implementing hose-mooring systems. Some concluding remarks with recommended solutions on the technology were presented in this review.

Mathematical Modelling of Bonded Marine Hoses for Single Point Mooring (SPM) Systems, with Catenary Anchor Leg Mooring (CALM) Buoy Application—A Review

The application of mathematical analysis has been an essential tool applied on Catenary Anchor Leg Mooring (CALM) buoys, Wave Energy Converters (WEC), point absorber buoys, and various single point mooring (SPM) systems. This enables having mathematical models for bonded marine hoses on SPM systems with application with CALM buoys, which are obviously a requisite for the techno-economic design and operation of these floating structures. Hose models (HM) and mooring models (MM) are utilized on a variety of applications such as SPARs, Semisubmersibles, WECs and CALM buoys. CALM buoys are an application of SPM systems. The goal of this review is to address the subject of marine hoses from mathematical modeling and operational views. To correctly reproduce the behavior of bonded marine hoses, including nonlinear dynamics, and to study their performance, accurate mathematical models are required. The paper gives an overview of the statics and dynamics of offshore/marine hoses. The reviews on marine hose behavior are conducted based on theoretical, numerical, and experimental investigations. The review also covers challenges encountered in hose installation, connection, and hang-off operations. State-of-the-art, developments and recent innovations in mooring applications for SURP (subsea umbilicals, risers, and pipelines) are presented. Finally, this study details the relevant materials that are utilized in hoses and mooring implementations. Some conclusions and recommendations are presented based on this review.

A study on the design and performance of ModuleRaft wave energy converter

This paper presents a novel wave energy converter (WEC), referred to as the ModuleRaft WEC. The WEC consists of a floating modular flap and four rafts hinged at the main floating structure. ModuleRaft WEC is unique due to its ability to convert both wave potential energy and wave kinetic energy by utilizing the pitch motion of rafts and floating modular flap. The motion characteristic, performance and optimization of the ModuleRaft wave energy converter are investigated under regular wave conditions using ANSYS-AQWA. The effect of wave frequency, power take-off (PTO) damping coefficient and floating/fixed main structure on capture factor is analyzed. Results indicate that using a single point mooring (SPM) system, the ModuleRaft WEC can operate optimally by utilizing all directions wave energy. In addition, comparing WEC with and without rafts, it was found that the capture factor of the modular flap with rafts is much better than the conventional floating modular flap-type WEC. Moreover, the three most influential parameters; wave frequency, PTO damping coefficient and fixed main structure, have a significant effect on the capture factor of the ModuleRaft WEC.

Design and Application of Buoy Single Point Mooring System with Electro-Optical-Mechanical (EOM) Cable

This paper presents the design of a hybrid system named the Mooring Buoys Observation System for Benthic with Electro Optical Mechanical Cable (MBOSBC) for long-term mooring buoy observation in deep oceans. MBOSBC is comprised of three main modules: a surface buoy, a benthic node, and an Electro-Optical-Mechanical (EOM) cable. The Surface buoy provides energy for the entire system, the Benthic node is used to observe scientific phenomena on the seabed, and the Electro-Optical-Mechanical (EOM) cable connects the sea surface buoy and the benthic observation node, serving as the information and power transmission link. This paper provides design criteria for a single point mooring buoy using an EOM cable. The environmental load of the buoy under varying wind, wave, and current conditions is analyzed and the mooring hydrodynamic force of EOM is calculated. After calculating the load of the EOM cable under extreme marine conditions, the hydrodynamic force needed for the system is analyzed. After physically testing the strength of the designed EOM cable, a near-shore test of MBOSBC was carried out, in order to verify that the system has the expected function.

Availability assessment of a monobuoy-anchored ship to ship offloading system

Cargo transshipment operations between large vessels are increasingly recurrent in the oil and gas industry scenario. Due to the operational limitations of Brazilian terminals and ports and recent discoveries of large oil and gas offshore fields in the pre-salt area, new types of configurations are being studied to propose alternatives for operators. The present work proposes a methodology for the analysis of the availability of a ship to ship offloading operation using a monobuoy single point mooring system. The study initially contemplates a quasi-static analysis considering a 10-year history of measurements of environmental conditions such as wind, current, wave, and swell, which to obtain the system heading, loads on the monobuoy, and static tensions in the mooring system between ships. In sequence, the dynamic tensions are calculated as a function of the first-order relative movements caused by the wave and swell incidence. Also, it is proposed to use a tugboat to align the system with the waves’ incidence angle in order to reduce motions and increase availability, which is confirmed by the obtained results. Furthermore, the system’s performance is assessed according to the variation of loading conditions, the relative positioning of the vessels, and different operating scenarios.

Experimental investigation of a shuttle tanker in irregular waves with hawser single point mooring system

ABSTRACT Hawser single point moored (SPM) shuttle tankers are widely used in various offshore oil development system. In this paper, the experiment of 1:40 shuttle tanker model with hawser SPM system in irregular waves is carried out. The effects of loading status, wave parameters, hawser line length and stiffness on the motion response of the shuttle tanker are investigated, including the amplitude, frequency and trajectory of the shuttle tanker. Meanwhile, the influences of hawser length and stiffness on the hawser line tension have also been analysed. Then based on the experimental results, some suggestions for the design of hawser SPM system of the shuttle tanker are given.

Hydrodynamic performance of a semi-submersible offshore fish farm with a single point mooring system in pure waves and current

A physical model of a semi-submersible offshore fish farm with a scale of 1:30 has been tested in a wave-current tank. The mooring force of the fish farm in waves was measured and compared for different single-point mooring arrangements, and henceforth, a safe and reliable mooring form is chosen as the mooring system for the fish farm. On this basis, the tested results for the mooring force, heave, pitch and roll of the fish farm in waves and current are presented, in which the effect of drought was discussed. The performance of blocking water flow for the fish farm in pure current was also evaluated. It has been found that increasing the anchor chain length and adding a sinker suspending from the chain could reduce the mooring force significantly despite the fluctuations in the mooring force. During the experiments, the tested value for the heave and pitch as well as the roll has been small, which indicates that the fish farm has a good stability. The mooring force has become larger, while the heave became smaller as the drought increased. Furthermore, the current velocity inside the fish farm is significantly lower than that of its outside. According to the field test results obtained in the fish farm, the farm exhibited good adaptability and it formed a suitable environment for growing cultured fish.

Design and research on optimization of single point mooring system

In this paper, we establish a statics model to analyze the force balance of the buoys, four steel pipes and steel buckets, and we derive the relationship between the inclination angle of steel pipes and steel buckets and the draught depth and wind speed, which is solved by moment in mechanics. Next, by applying the catenary curve’s equation to per unit of anchor chain, we can obtain the vertical height of anchor chain, thus the total depth of sea water can be obtained. Finally, the variable step search algorithm is used to search the draught depth, and we can obtain each parameter of the optimization result.

Multibody dynamical modeling of the FPSO soft yoke mooring system and prototype validation

The Soft Yoke Mooring System (SYMS) is a single point mooring system for shallow water. It is composed of a mooring framework, mooring legs, yoke, and single point, and is located at the Floating Production Storage and Offloading (FPSO) through 13 hinge joints, such as universal joints and thrust bearings. Mooring restoring force, motions and postures of mooring components, and mechanical behaviors of hinge joints are major criteria for the structural design of the SYMS. Aiming at the difficulties of the multibody dynamics in traditional design of the SYMS, a multi-body dynamic mathematical modeling with seven independent degrees of freedom (DOFs) which is applicable to prototype field engineering was developed in this study. The proposed mathematical modeling of the SYMS multibody dynamic system has several advantages: 1. Internal tribological behaviors in hinge joints are considered within the presented multibody dynamics model to illustrate the good dynamic effects of the SYMS. 2. The multibody dynamic model can be applied in field service. Correctness and feasibility of the proposed multibody dynamic simulation method for describing motions and postures of hinges and single-body were validated by the prototype monitoring data. 3. The horizontal restoring force of the SYMS was calculated according to field measurement data. The motion state of each single body and internal stress distributions at each hinge joint in the SYMS are given. 4. The multibody dynamics calculation program can be directly used for the real-time monitoring of mechanical behaviors of the SYMS under the service state. The simulated results can provide real-time guarantee for safety alarming of the system. The vulnerability of the mooring system in service was evaluated based on long-term monitoring data analysis.