Frequency-Domain Calculations of Moored Vessel Motion Including Low Frequency Effect

Abstract

Frequency-domain analysis can be used to evaluate the motions of the FPSO with its mooring and riser. The main assumption of the frequency-domain analysis is that the coupling is essentially linear. Calculations are performed taking into account first order wave loads on the floating structure. Added mass and radiation damping terms are frequency dependent, and can be easily considered in this formulation. The major non-linearity comes from the drag force both on lines and the floating structure. Linearization of the non-linear drag force acting on the lines is applied. The calculations can be extended to derive the low frequency motion of the floating structure. Second order low frequency quadratic transfer function is computed with a diffraction/radiation method. Given a wave spectrum, the second order force spectrum can then be derived. At the same time frequency-domain analysis is used to derive the low frequency motion and wave frequency motion of the floating system. As an example case, an FPSO is employed. Comparison is performed with time domain simulation to show the robustness of the frequency-domain analysis. Some calculations are also performed with either low frequency terms only or wave frequency terms only in order to check the effect of modeling low and wave frequency terms, separately. In the case study it is found that the low frequency motion is reduced by the wave frequency motion while the wave frequency motion is not affected by the low frequency motion.

Chance Constraint Optimization of a Complex System: Application to the Fatigue Design of a Floating Offshore Wind Turbine Mooring System

“…For the same reasons, T i is also a stationary Gaussian process and its spectral moments are computable from the spectral density of η (h j s ,t j p ,u j ) and the transfer function H T . The mean of the surge and tension processes, denoted respectively µ S and µ T , are computed by the static simulation (see [4] for more details). These properties will be useful for the reformulation of each constraint.…”

“…First, we use the Extreme Value Theory to reformulate the surge and tension constraints. The computations are performed in the frequency domain [4] and the fatigue damage is estimated with the spectral Dirlik empirical formulae [5]. In order to determine the feasibility region defined by the probabilistic constraints, metamodels are then built and sequentially enriched according to a new procedure introduced in this paper.…”

Investigation on Hydrodynamic Characteristics, Wave–Current Interaction and Sensitivity Analysis of Submarine Hoses Attached to a CALM Buoy

“…As a result, hydrodynamic sensitivity analysis of the floating structure's motion behaviour is required. Validated studies avow that hydrodynamic loads are used to assay the strength of various FOS, hull designs, and components utilized in fluid transfer like composite marine risers and offshore hoses [104][105][106][107][108]. However, wave action has an impact on the motion and strength of CALM buoy hose systems.…”

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

“…Thus, having mathematical models for bonded marine hoses on CALM buoys and SPM systems is obviously a requisite for the techno-economic design and operation of these floating structures. Recent innovations in bonded marine hoses include the application of composite materials [8][9][10][11], in composite marine risers [11][12][13][14][15][16][17][18][19], in moored offloading systems with coupled analysis [20][21][22][23][24][25][26][27], and the supporting CALM buoy systems [28][29][30][31][32][33][34][35]. However, these structures, similar to moorings, all depend on existing marine structures such as WECs, breakwater devices, tidal turbines, offshore wind turbines or CALM buoys [36][37][38][39][40][41][42][43][44].…”