Combined Environmental Loads Research Articles

Effects of one mooring line failure on the behaviour of a work ship

The dynamic performance of a work ship with a mooring line failure was studied. A fully coupled dynamic analysis program (Ansys Aqwa) was applied to simulate ship motions and mooring line dynamics. The first- and second-order wave excitation forces were calculated using the three-dimensional diffraction/radiation theory, and the behaviours of the mooring lines were analysed using a non-linear finite-element method. Under the combined environmental loads of waves and wind, one mooring line was intentionally disconnected at a certain moment. Then, the dynamic response after the mooring line breaks, the energy distribution characteristics in the motion response spectrum and the mean tension variation of the remaining mooring lines were assessed. The results showed that sudden failure of the mooring line significantly affected the performance of the ship. Therefore, to ensure the structural integrity of the ship system and the safety of personnel on deck, the influence of a mooring line failure on a ship should be investigated in advance.

Effects of installation on combined bearing capacity of a spudcan foundation in spatially variable clay

Mobile jack-up drilling rigs with spudcan foundations are extensively used in offshore industry due to their flexibility, mobility and cost-effectiveness. A sufficient bearing capacity of the spudcan foundation is needed to resist the combined environmental loading associated with storms, winds, waves and currents. In most previous studies, the combined bearing capacity was defined for the wished-in-place spudcan foundation, ignoring the disturbance to the seabed soils cause by spudcan installation. It is shown in this paper how the installation affects the failure mechanism and bearing capacity of spudcan foundation in spatially variable soil. Random large deformation finite element method is first used to simulate the installation process of a spudcan foundation in order to obtain the disturbed soil strength profiles. Then the profiles are mapped into random small-strain finite element model for calculation of bearing capacity of the spudcan foundation. Results show that a large amount of the softer soils in the surface of seabed flow back to top of spudcan foundation, which causes the bearing capacity to reduce significantly. Moreover, a Monte-Carlo simulation is carried out to define the probabilistic failure envelopes of the spudcan foundation, considering the disturbance of installation to the spatially variable soils.

Dynamic response of the mooring system in the floating photovoltaic power station

The floating photovoltaic power (FPV) station becomes popular to decrease carbon emission. However, limited research has been done on the dynamic response of the mooring lines of the FPV array. Based on a typical 2.14MW FPV array, this study investigates the displacement of the array and the mooring tension of the mooring lines. The numerical model of the FPV array is built through three-dimensional potential theory with 124 mooring lines. Firstly, the effect of the environment on the response is investigated under wave-only, current-only and wind-only conditions. Then, the tension and motion in the combined environmental loads are analyzed. It is found that the wind load has the greatest influence on the motion and mooring tension on the FPV power station, the effect of wave and current on the response is very limited.

Failure behavior of woven fiberglass composites under combined compressive and environmental loading

The purpose of this study is to quantitatively characterize the compressive and damage behavior of a woven fiberglass composite under combined environmental loading. Cuboidal samples of a commercially available woven fiberglass epoxy resin composite, garolite G10, are examined under uniaxial compressive loading perpendicular to the plies at quasi-static (10−3 s−1) and dynamic (103 s−1) strain rates using a standard load frame and Kolsky (split-Hopkinson) bar. In order to simulate environmental conditions, a subset of samples were soaked in either distilled or ASTM standard seawater prior to loading. Two time periods of environmental conditioning were investigated: short term at two weeks and long term at four months. Results demonstrate that, on average, the dynamic compressive strength of the fiberglass increased 35% from the quasi-static. Moreover, environmentally treated samples generally experienced a decrease strain to failure, and composites exposed to water for only short periods exhibited signs of the absorbed water sustaining additional load under quasi-static rates. Ultra-high-speed photography combined with digital image correlation, a full-field surface kinematic measurement technique, is used to map 2D strains on the sample during loading. In all cases, a clear shear failure mechanism from local instabilities appears, and a Mohr–Coulomb failure criterion is used to extract a mesoscale cohesive shear stress and coefficient of internal friction.

Global design and analysis of deep sea FRP composite risers under combined environmental loads

The use of composite materials in offshore engineering for deep sea oil production riser systems has drawn considerable interest due to the potential weight savings and improvement in durability that can be achieved. The design of composite risers consists of two stages: (1) local design based on critical local load cases (LCs) to obtain the geometric configuration of the riser which will be analysed in the global design stage, and (2) global analysis of the full length composite riser under global loads including top tension force, platform motion, hydrostatic pressure, gravity, buoyancy, wave and current loads to determine and assess critical locations. This study describes the methodology, LCs, analysis procedure and results of the global design of the composite riser based on the geometries of the tubular optimised in the local design stage. The results show that a careful local design of the tubular using inclined reinforcements in addition to axial and hoop reinforcements can offer substantial weight savings and at the same time ensure that the structure is capable of withstanding the global loads applied on it.