Innovative Method for Ship Navigation Safety Risk Response in Landslide-Induced Wave

When the bank of a reservoir slope slides along a weak structural plane at a high speed, “landslide slamming” will occur in the nearby water. The formation of landslide-induced waves is a serious threat to the safety of wharfs, shore marks, buildings in the water, and vessels navigating in reservoir areas. To ensure the safety of navigating ships, this study proposes a landslide-induced wave water ship navigation safety risk response technology. The propagation characteristics of landslide-induced waves are analysed based on a physical model experiment, and the characteristics of a ship's motion response and mooring cable tensions are studied under conditions of bow and stern mooring and multipoint mooring. The influences of the landslide-induced wave direction and ship navigation position on the ship rolling motion characteristics are discussed. The results of this study can further improve the navigation safety of ships in landslide-induced wave waters.

Study on Mooring Design and Calculation Method of Ocean Farm Based on Time-Domain Potential Flow Theory

In order to calculate the mooring force of a new semi-submerged Ocean Farm quickly and accurately, based on the unsteady time-domain potential flow theory and combined the catenary model, the control equation of mooring cable is established, and the mooring force of the platform under the wave spectrum is calculated. First of all, based on the actual situation of the ocean environment and platform, the mooring design of the platform is carried out, and the failure analysis and sensitivity analysis of the single anchor chain by the time domain coupling method are adopted: including different water depth, cycle, pretension size, anchor chain layout direction and wind speed, etc. The analysis results confirm the reliability of anchoring method. Based on this, the mooring point location of the platform is determined, the force of each anchor chain in the anchoring process is calculated, and the mooring force and the number of mooring cables are obtained for each cable that satisfies the specification, the results of this paper can provide theoretical calculation methods for mooring setting and mooring force calculation of similar offshore platforms.

Uncertainty Quantification in Mooring Cable Dynamics Using Polynomial Chaos Expansions

Mooring systems exhibit high failure rates. This is especially problematic for offshore renewable energy systems, like wave and floating wind, where the mooring system can be an active component and the redundancy in the design must be kept low. Here we investigate how uncertainty in input parameters propagates through the mooring system and affects the design and dynamic response of mooring and floaters. The method used is a nonintrusive surrogate based uncertainty quantification (UQ) approach based on generalized Polynomial Chaos (gPC). We investigate the importance of the added mass, tangential drag, and normal drag coefficient of a catenary mooring cable on the peak tension in the cable. It is found that the normal drag coefficient has the greatest influence. However, the uncertainty in the coefficients plays a minor role for snap loads. Using the same methodology we analyze how deviations in anchor placement impact the dynamics of a floating axi-symmetric point-absorber. It is shown that heave and pitch are largely unaffected but surge and cable tension can be significantly altered. Our results are important towards streamlining the analysis and design of floating structures. Improving the analysis to take into account uncertainties is especially relevant for offshore renewable energy systems where the mooring system is a considerable portion of the investment.

Stochastic dynamic response analysis of submerged multi-body structure considering the uncertainty of connection gap

The aim of this study is to determine the stochastic dynamic characteristics of the system due to the uncertainty of the gap contact between the mooring cable and the structural body of the submerged multi-body structure. The sag effect caused by the light, flexible, and low damping characteristics of the mooring cable and its dynamic elongation under the action of the flow field and the impact of the gap connection is considered. The sag effect is corrected using the equivalent elastic modulus method; the stochastic generalized non-deterministic dynamic model of the system considering the dynamic elongation and clearance contact of the mooring cable is established; and the Newmark-β method and the interval algorithm are used to solve the problem. The calculation results show that the mean and peak values of the kinematic parameters of the structure are larger than the ideal articulated state; the dynamic tension is changed randomly; and the deflection angle of the gap-contact state is greater than that of the articulated state, which may lead to the instability of the system. Therefore, when studying the concrete structure in water, we should consider the contact state of the joint and analyze its stochastic dynamic characteristics.

Effects of Co-Located Floating Wind-Wave Systems on Fatigue Damage of Floating Offshore Wind Turbine Mooring Cables

Industrial realization of both floating offshore wind and wave energy technologies requires reductions in the current high levelized cost of energy. Reducing mooring fatigue loads could decrease levelized cost of energy, as mooring is expected to be a major cost of these systems. Previous work on improving mooring reliability and costs has focused on material and design. In this exploratory study, we quantify how placing WECs in front of a FOWT could reduce fatigue damage incurred by FOWT mooring cables in long-crested wave conditions. We use SWAN to quantify the WEC-induced sea state modifications and obtain wave spectra at the FOWT location. The spectra are then input into WEC-Sim and MooDy to model the mooring cable behavior. Fatigue analysis with Rainflow Counting is used to quantify the fatigue loading on the mooring cables. Results from this study show a 8% reduction in fatigue damage to mooring cables over the lifetime of the structure. These results indicate that co-location could have a beneficial effect on FOWT mooring cable fatigue. In future work, these results will be leveraged to perform optimal O&M planning and reliability-based design optimization of floating offshore wind turbines.

Autonomous Ocean Turbulence Measurements From a Moored Upwardly Rising Profiler Based on a Buoyancy-Driven Mechanism

An autonomous Moored Reciprocating Vertical Profiler (MRVP) has been developed and tested for measuring ocean turbulence. The MRVP is designed to combine the advantages of long-term moored measurements at specified depths with those of short-term ship-supported continuous profiling performed at high vertical resolution. The profiler is programmed to repeat vertical motions autonomously along the mooring cable based on a buoyancy-driven mechanism. A sea trial has been conducted in the South China Sea to evaluate the performance of the profiler. The shear probe data are unreliable when the flow past sensors is not sufficiently greater than an estimate of turbulent velocity. For 65% of the dataset, turbulence measurements are of high quality and the magnitude of dissipation rates is up to O(10−10) W kg−1. To minimize the contamination induced by instrument vibration and improve the estimation of turbulent kinetic energy terms, an advanced cross-spectrum algorithm is implemented to the measured shear data. The corrected spectra agreed well with the empirical Nasmyth spectrum, and dissipation rates had averagely decreased a factor of 2 and 8 times lower than the raw spectra. The autonomous MRVP is proven to be a stable platform, and the novel upward measurement provides a new perspective for measuring long-term time series of turbulence mixing.