Mooring with Multicomponent Cable Systems

1980 ◽  
Vol 102 (2) ◽  
pp. 62-69 ◽  
Author(s):  
K. A. Ansari
Keyword(s):  
Dynamic Response ◽  
System Design ◽  
Mooring Line ◽  
Analysis Tool ◽  
Mooring System ◽  
Practical Application ◽  
Analysis Techniques ◽  
Anchoring System ◽  
Cable Systems ◽  
Stiffness Characteristics

Relative to the present-day need for offshore operations involving the use of moored vessels, mooring system design has become quite complex. Since a proper choice of mooring line components in the form of anchors, clump weights, chains and cables is vital for vessel station-keeping and mooring system survival, the adequacy of the mooring system under consideration must be checked out by suitable analysis techniques. This paper gives a general discussion of the various mooring line components available for use, presents an analysis tool to determine the stiffness characteristics of a multicomponent cable including the effect of line stretch, and demonstrates how these could be included in the dynamic analysis of an offshore construction vessel moored by a multileg anchoring system. The maximum limiting tension of the mooring line considered, which is a combination of anchors, clump weights, chains and cables is determined from the several breaking strengths and anchor capacities associated with the various cable configurations that can occur. Finally, in order to illustrate a practical application, the dynamic response of a moored production barge subjected to external environmental forces is examined.

Sensitivity of Mooring Line Pretension in the Design Cycle for Floating Caissons

2004 ◽  
Author(s):  
Adinarayana Mukkamala ◽  
Partha Chakrabarti ◽  
Subrata K. Chakrabarti
Keyword(s):  
Reinforced Concrete ◽  
System Design ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Design Cycle ◽  
Overall Performance ◽  
The Difference ◽  
Tacoma Narrows Bridge ◽  
System Layout

The new parallel Tacoma Narrows Bridge being constructed by Tacoma Narrows Constructors will be mounted on two towers and these towers in turn will be supported by reinforced concrete caissons referred to as East Caisson (Tacoma side) and West Caisson (Gig Harbor side). Each Caisson is towed to the location and several stages of construction will take place at the actual site. During construction, the floating caissons will be moored in place to hold it against the flood and ebb currents in the Narrows. During the mooring system design, a desired pretension is established for the lines at each draft. However, due to practical limitations in the field some variations to this design pretension value may be expected. It is important to study the effect of this variation on the overall performance of the mooring system. In this paper, the sensitivity of the mooring line pretension on the overall performance of the mooring system for the above caisson is presented. During this study, all the variables that affect the mooring system design such as mooring system layout, mooring line makeup, anchor positions, fairlead departure angles, and fairlead locations are kept constant. The only variable changed is the pretension of the mooring lines. Two approaches for defining the variations in the pretension have been studied in this paper. In the first approach, the pretension is changed in a systematic way (predicted approach). In the second method the pretension is changed randomly. The latter is considered more likely to occur in the field for this type of complex mooring system. Both sets of results are presented for some selected drafts attained by the caisson during its construction. The difference in the results from the two methods is discussed.

scholarly journals Dynamic Response for a Submerged Floating Offshore Wind Turbine with Different Mooring Configurations

2019 ◽  
Vol 7 (4) ◽  
pp. 115 ◽  
Author(s):  
Yane Li ◽  
Conghuan Le ◽  
Hongyan Ding ◽  
Puyang Zhang ◽  
Jian Zhang
Keyword(s):  
Dynamic Response ◽  
Wind Turbine ◽  
Dynamic Responses ◽  
Offshore Wind ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Intermediate Water ◽  
Mooring System ◽  
Mooring Lines ◽  
Floating Offshore Wind Turbine

The paper discusses the effects of mooring configurations on the dynamic response of a submerged floating offshore wind turbine (SFOWT) for intermediate water depths. A coupled dynamic model of a wind turbine-tower-floating platform-mooring system is established, and the dynamic response of the platform, tensions in mooring lines, and bending moment at the tower base and blade root under four different mooring configurations are checked. A well-stabilized configuration (i.e., four vertical lines and 12 diagonal lines with an inclination angle of 30°) is selected to study the coupled dynamic responses of SFOWT with broken mooring lines, and in order to keep the safety of SFOWT under extreme sea-states, the pretension of the vertical mooring line has to increase from 1800–2780 kN. Results show that the optimized mooring system can provide larger restoring force, and the SFOWT has a smaller movement response under extreme sea-states; when the mooring lines in the upwind wave direction are broken, an increased motion response of the platform will be caused. However, there is no slack in the remaining mooring lines, and the SFOWT still has enough stability.

Experimental and Numerical Study on Large Truncation of Deepwater Mooring Line

2009 ◽  
Author(s):  
Yihua Su ◽  
Jianmin Yang ◽  
Longfei Xiao ◽  
Gang Chen
Keyword(s):  
Dynamic Response ◽  
Water Depth ◽  
Numerical Study ◽  
Unit Length ◽  
Low Frequency ◽  
Model Tests ◽  
Mooring Line ◽  
Mooring System ◽  
Mooring Lines ◽  
Wave Basin

Modeling the deepwater mooring system in present available basin using standard Froude scaling at an acceptable scale presents new challenges. A prospective method is to truncate the full-depth mooring lines and find an equivalent truncated mooring system that can reproduce both static and dynamic response of the full-depth mooring system, but large truncation arise if the water depth where the deepwater platform located is very deep or the available water depth of the basin is shallow. A Cell-Truss Spar operated in 1500m water depth is calibrated in a wave basin with 4m water depth. Large truncation arises even though a small model scale 1:100 is chosen. A series of truncated mooring lines are designed and investigated through numerical simulations, single line model tests and coupled wave basin model tests. It is found that dynamic response of the truncated mooring line can be enlarged by using larger diameter and mass per unit length in air. Although the truncated mooring line with clump presents a “taut” shape, its dynamic characteristics is dominated by the geometry stiffness and it underestimates dynamic response of the full-depth mooring line, even induces high-frequency dynamic response. There are still two obstacles in realizing dynamic similarity for the largely truncated mooring system: lower mean value of the top tension of upstream mooring lines, and smaller low-frequency mooring-induced damping.

Design Considerations for Polyester Construction Stretch Removal and its Impact on Mooring System Design

2021 ◽  
Author(s):  
Hisham Moideen ◽  
Stephane Le-Guennec ◽  
Alaa M. Mansour ◽  
Cheng Peng ◽  
Mark Huntley
Keyword(s):  
System Design ◽  
Cost Effective ◽  
Mooring Line ◽  
Mooring System ◽  
Fiber Elongation ◽  
Test Program ◽  
Design Considerations ◽  
Key Driver ◽  
New Challenges ◽  
Floating Systems

Abstract Polyester based mooring systems offer unique challenges from design, analysis and installation perspective. Non-linear elongation behavior of the fiber is key to these challenges. A good understanding of the fiber elongation characteristics is critical to the design of a robust and efficient mooring system. Mooring systems are one of the primary contributors to the CAPEX of the Floating Production Units (FPU) and the drive to develop cost effective systems has led to improvements/changes to mooring systems. Off-vessel tensioning has been in use for several years, but the option has recently received more in-depth consideration. Off-vessel tensioning is becoming the standard for floating systems with the elimination of on-vessel tensioning systems. The elimination of on-vessel tensioning system brings upon new challenges in terms of mooring line installation and tensioning. A key driver for these systems is the polyester rope pre-load criteria primarily used to remove construction stretch and jump the creep curve that the system may experience. The challenges to each project are unique, however, understanding and assessing the key design impacts will be beneficial to the industry. This paper focuses on the design of a polyester based mooring system with focus on polyester fiber elongation characteristics and its impact on the design and installation. A polyester test program is established to understand the rope elongation behavior and impact of various pre-load levels. Installation assessment is also performed to understand the installation vessel requirements to achieve various pre-loads. Based on the studies, design updates are made to the mooring system and a novel two tension regime mooring system is proposed. Design impact of the two pre-tension system on various design criterions are evaluated and presented in this paper. The paper also makes recommendations on target pre-load and elongation to be considered in systems that do not plan to re-tension their system post installation. Authors appreciate that recommendations and observations reported may not be applicable for all types of floaters and mooring system.

scholarly journals Dynamic Analysis of Embedded Chains in Mooring Line for Fish Cage System

2018 ◽  
Vol 25 (4) ◽  
pp. 83-97
Author(s):  
Hui-Min Hou ◽  
Guo-Hai Dong ◽  
Tiao-Jian Xu ◽  
Yun-Peng Zhao ◽  
Chun-Wei Bi
Keyword(s):  
Numerical Model ◽  
Dynamic Response ◽  
Single Point ◽  
Mooring Line ◽  
Mooring System ◽  
Lumped Mass ◽  
Model Case ◽  
Cage System ◽  
Net Cage ◽  
Offshore Aquaculture

Abstract Investigation of the embedded chains in soil starts to play an important role in understanding the structural performance of mooring system, when the embedded anchors will be employed to sustain large loads with the gradually growth of installation depth of offshore aquaculture farm. The aim of this study is to investigate the dynamic response of mooring line considering the influence of embedded chains in clay soil for net cage system. Lumped-mass method is used to establish the numerical model for evaluating the performance of mooring line with embedded chains. To validate the numerical model, comparisons of numerical results with the analytical formulas and the experimental data are conducted. A good agreement of the profile and the tension response is obtained. Then, the effect of embedded chains on the static and dynamic response of mooring line is evaluated, and the dynamic behavior of mooring system considering embedded chains for net cage system is investigated. The results indicate that the soil resistance on embedded chains should be included to predict the mooring line development and the load on the embedded anchors in the numerical simulations. An appropriate safety factor should be included if employing the simplified model Case C at the initial design phase. And the effect of embedded chains on the holding capacity of embedded anchors in single-point mooring system for single net cage cannot be negligible during the design and operation phases. Consequently, it is profound to take into account the interaction of embedded chains and soil for accurately predicting the reliability of mooring system for fish cage.

scholarly journals Transient Responses Evaluation of FPSO with Different Failure Scenarios of Mooring Lines

2021 ◽  
Vol 9 (2) ◽  
pp. 103
Author(s):  
Dongsheng Qiao ◽  
Binbin Li ◽  
Jun Yan ◽  
Yu Qin ◽  
Haizhi Liang ◽  
...  
Keyword(s):  
Service Condition ◽  
Mooring Line ◽  
Mooring System ◽  
Transient Effects ◽  
Floating Platform ◽  
Transient Responses ◽  
Mooring Lines ◽  
Performance Deterioration ◽  
Steady State Responses ◽  
Influence Process

During the long-term service condition, the mooring line of the deep-water floating platform may fail due to various reasons, such as overloading caused by an accidental condition or performance deterioration. Therefore, the safety performance under the transient responses process should be evaluated in advance, during the design phase. A series of time-domain numerical simulations for evaluating the performance changes of a Floating Production Storage and Offloading (FPSO) with different broken modes of mooring lines was carried out. The broken conditions include the single mooring line or two mooring lines failure under ipsilateral, opposite, and adjacent sides. The resulting transient and following steady-state responses of the vessel and the mooring line tensions were analyzed, and the corresponding influence mechanism was investigated. The accidental failure of a single or two mooring lines changes the watch circle of the vessel and the tension redistribution of the remaining mooring lines. The results indicated that the failure of mooring lines mainly influences the responses of sway, surge, and yaw, and the change rule is closely related to the stiffness and symmetry of the mooring system. The simulation results could give a profound understanding of the transient-effects influence process of mooring line failure, and the suggestions are given to account for the transient effects in the design of the mooring system.

scholarly journals Design and Automated Optimization of an Internal Turret Mooring System in the Frequency and Time Domain

2021 ◽  
Vol 9 (6) ◽  
pp. 581
Author(s):  
Hongrae Park ◽  
Sungjun Jung
Keyword(s):  
System Design ◽  
Time Domain ◽  
Initial Conditions ◽  
Minimum Weight ◽  
Optimization Algorithms ◽  
Local Optimization ◽  
Mooring System ◽  
Floating Offshore Wind Turbines ◽  
Global And Local Optimization ◽  
Global And Local

A cost-effective mooring system design has been emphasized for traditional offshore industry applications and in the design of floating offshore wind turbines. The industry consensus regarding mooring system design is mainly inhibited by previous project experience. The design of the mooring system also requires a significant number of design cycles. To take aim at these challenges, this paper studies the application of an optimization algorithm to the Floating Production Storage and Offloading (FPSO) mooring system design with an internal turret system at deep-water locations. The goal is to minimize mooring system costs by satisfying constraints, and an objective function is defined as the minimum weight of the mooring system. Anchor loads, a floating body offset and mooring line tensions are defined as constraints. In the process of optimization, the mooring system is analyzed in terms of the frequency domain and time domain, and global and local optimization algorithms are also deployed towards reaching the optimum solution. Three cases are studied with the same initial conditions. The global and local optimization algorithms successfully find a feasible mooring system by reducing the mooring system cost by up to 52%.

Less=Moor: A Time-Efficient Computational Tool to Assess the Behavior of Moored Ships in Waves

2017 ◽  
Author(s):  
Yijun Wang ◽  
Alex van Deyzen ◽  
Benno Beimers
Keyword(s):  
Dynamic Response ◽  
Research Effort ◽  
Equations Of Motion ◽  
Line Tension ◽  
Mooring Line ◽  
Induced Response ◽  
Wave Forces ◽  
Computational Tool ◽  
The Time Domain ◽  
Nonlinear Equations Of Motion

In the field of port design there is a need for a reliable but time-efficient method to assess the behavior of moored ships in order to determine if further detailed analysis of the behavior is required. The response of moored ships induced by gusting wind and/or waves is dynamic. Excessive motion response may cause interruption of the (un)loading operation. High line tension may cause lines to snap, introducing dangerous situations. A (detailed) Dynamic Mooring Analysis (DMA), however, is often a time-consuming and expensive exercise, especially when responses in many different environmental conditions need to be assessed. Royal HaskoningDHV has developed a time-efficient computational tool in-house to assess the wave (sea or swell) induced dynamic response of ships moored to exposed berths. The mooring line characteristics are linearized and the equations of motion are solved in the frequency domain with both the 1st and 2nd wave forces taken into account. This tool has been termed Less=Moor. The accuracy and reliability of the computational tool has been illustrated by comparing motions and mooring line forces to results obtained with software that solves the nonlinear equations of motion in the time domain (aNySIM). The calculated response of a Floating Storage and Regasification Unit (FSRU) moored to dolphins located offshore has been presented. The results show a good comparison. The computational tool can therefore be used to indicate whether the wave induced response of ships moored at exposed berths proves to be critical. The next step is to make this tool suitable to assess the dynamic response of moored ships with large wind areas, e.g. container ships, cruise vessels, RoRo or car carriers, to gusting wind. In addition, assessment of ship responses in a complicated wave field (e.g. with reflected infra-gravity waves) also requires more research effort.

North Sea Mooring Systems: How Reliable Are They?

2014 ◽  
Author(s):  
Will Brindley ◽  
Andrew P. Comley
Keyword(s):  
Continental Shelf ◽  
North Sea ◽  
Recent Decade ◽  
Mooring Line ◽  
Mooring System ◽  
Failure Rates ◽  
Floating Structure ◽  
Strength Capacity ◽  
Failure Statistics ◽  
Overall Reliability

In recent years a number of high profile mooring failures have emphasised the high risk nature of this element of a floating structure. Semi-submersible Mobile Offshore Drilling Units (MODUs) operating in the harsh North Sea environment have experienced approximately 3 mooring failures every 2 years, based on an average population of 34 units. In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UK Continental Shelf (UKCS) [17]. Although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the question: what is the reliability of the mooring system? This paper aims to answer this question by evaluating failure statistics over the most recent decade of available data. Mooring failure rates are compared between the Norwegian Continental Shelf (NCS), the UKCS, and with industry code targets to understand how overall reliability is related to the strength capacity of a mooring system. The failure statistics suggest that a typical MODU operating in the UKCS would experience a mooring line failure in heavy weather approximately every 20 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. Despite the increased strength requirements for the NCS, failure rates do not appear to be lower than the UKCS. This suggests that reliability does not correlate well with mooring system strength. As a result, designing to meet the more rigorous HSE requirements, which would require extensive upgrades to existing units, may not significantly increase mooring system reliability. This conclusion needs to be supported with further investigation of failure statistics in both the UKCS and NCS. In general, work remains to find practical ways to further understand past failures and so improve overall reliability.

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