Mooring Line Pretension Research Articles

Experimental investigation of wave severity and mooring pretension on the operability of a moored tanker in a port terminal

This paper investigates the influence of sea severity and mooring line pretension configuration on the operability of a moored vessel at a modified berthing site inside a port. A physical model was constructed to replicate the new layout of the port of Leixões in Portugal, including bathymetry and a future 300 m extension of Leixões’ north breakwater. A tanker ship model was tested with novel custom-made mooring system simulators for two fenders and four mooring lines under various offshore sea states and pretension configurations. The experiments focus on acquiring wave measurements at multiple spots within the port, ship motions, and loads on lines and fenders. The data is analysed in time and frequency domains to examine the relationship between waves, motions, and loads. The results are then compared to standard operational thresholds to estimate downtime and operability for cargo loading operations. The analysis of the results yields several conclusions. It is recommended to use the zero-peak amplitudes in conjunction with the maximum peak-to-peak amplitudes to ensure accurate operability analysis. The application of small-scale physical modelling for a moored tanker in Leixões port is a useful tool not only for investigating the feasibility of port modifications in the existing sheltering structures but also for analysing additional soft countermeasures to strengthen operational conditions at the berth. It also provides site-specific experimental data that may help to develop site-specific safety criteria. The applied mooring simulators help to reduce physical model costs.

Integrated approach to assess resonance between basin eigenmodes and moored ship motions with wavelet transform analysis and proposal of operational thresholds

Waves with periods between 25 s and some minutes can amplify the motions of moored ships, which may result in terminal downtimes and compromise safety. The purpose of this work is twofold: (i) studying infragravity waves and their influence on moored ship motions, including the definition of operational thresholds and (ii) developing a novel and integrated approach to identify and assess resonance situations using wavelet transform analysis. The resonant modes of the harbor basin were identified using a numerical model validated with full-scale data. The motions of five similar LPG vessels moored at two adjacent jetties were analyzed, both in the frequency and frequency–time domains. It was concluded that surge is the most important motion at the berth that has greater operational problems. Moreover, the infragravity motion periods vary with the mooring line pretension and are proportional to the ratio of water depth to vessel draft. Three episodes of mooring line breaking were attributed to large infragravity surge oscillations. In addition, operational thresholds for basin-vessel resonance situations were defined based on the port tide gauge data. A significant wave height of 0.075 and 0.010 m for the 30–65 s period band was established for LPGs and oil tankers, respectively.

大型係留船の動揺特性に与える初期張力と防舷材仕様の影響に関する模型実験

In our previous study, investigations on property of ship motions due to characteristic of mooring system such as mooring line and fender was conducted through a numerical simulation and a field observation of ship motions and mooring loads. From the investigations, it was concluded that large sway motion, namely sub-harmonic motion which was caused by an asymmetrical mooring of mooring line and fender, and large roll motion were induced in beam seas. In this study, we have conducted a physical model test on ship motions and mooring loads for a large vessel moored along dolphin to confirm the effect of pretension of mooring line and fender property.

Experimental evaluation of the tension mooring effect on the response of moored ships

Tension mooring may be applied to improve operational and security conditions of exposed port terminals. This paper analyses the influence of the type of fender facing on the efficacy of tension mooring in terms of reduction of the moored ship motions and improvement of the operational and security conditions at berth. Two mooring line pretension conditions and two types of interfaces between the ship and the fenders were experimentally investigated. The physical model was designed based on the characteristics of a real port terminal with operational problems—the Leixões oil terminal, Portugal. The analysis of the moored ship response shows that high friction fender facings reduce moored ship motions and significantly increase the tension mooring efficiency. The magnitude of pretension forces and the type of ship–fenders interface are important aspects when looking at common environmental conditions, but their contribution to reduce moored ship motions during rough (extreme) conditions is limited. It was also concluded that friction forces developed at that interface provide additional damping to the dynamic system composed of the moored ship and help preventing resonance, in particular when contributions from other sources of damping are minimal, as it is shown for roll.

Rogue wave impact on a tension leg platform: The effect of wave incidence angle and mooring line tension

An understanding of the interaction between extreme ocean conditions and floating offshore platforms is required for their design and safe operation. Rogue wave impacts cannot easily be handled by traditional analytic or computational techniques. Here we use Smoothed Particle Hydrodynamics (SPH) to simulate the fully non-linear dynamics of a large breaking wave on a semi-submersible tension leg platform. We consider the effect of wave impact angle and mooring line pre-tension on the subsequent motion of the platform, and predict the maximum tension in each mooring line. The primary effect of wave impact angle is to determine the peak mooring line tension and line “slackness” during wave impact. A 45° impact results in the maximum tension on the leading line. The maximum heave, surge and pitch vary only slightly with wave angle. As mooring line pre-tension is increased, the duration of heave and pitch excursions is reduced. Peak mooring tension increases only slightly, but the incidence of mooring line “slack” decreases significantly for higher pre-tension. Properly applied, SPH has a wide application in predicting non-linear wave–structure interactions. Genuine opportunity exists for using SPH in the design of structures and mooring systems exposed to extreme ocean events such as rogue wave impact.

부유식 파력발전장치용 계류선의 설계 및 안전성 검토에 관한 연구

A study was performed on the design of a mooring line to maintain the position of a floating WEC (wave energy conversion) system. The procedure to design a mooring line is set up and the safety of the designed mooring system is evaluated using commercial software, Orcaflex. The characteristics curve for one line is analyzed to determine the properties and pretension of a mooring line. While considering the ocean environmental condition and importance of a floating WEC system, a multi-line layout is determined. A 4-point mooring system with 4 lines shows the instability in the yaw motion of the floating WEC system under a designed ocean environmental condition. The redesigned 4-point mooring system with 8 lines is found to be safe on the condition of a harsh ocean environment. The floating WEC system with the redesigned mooring system also shows stable motion in surge and pitch under operating conditions. From a parametric study on the mooring line length, the extreme value of the mooring line tension is found to be very sensitive to the pretension and length of mooring line. The results of this study can contribute to the establishment of a design procedure for mooring lines.