PSI - Issue 45

Mark Mogeke et al. / Procedia Structural Integrity 45 (2023) 36–43 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

40

5

grids from 2011 onwards were forced using Climate Forecast Reanalysis v.2 (CFSv2). Temporal and spatial resolution are 1 hour and 0.2° respectively. On initial comparison of the hindcast data sources, features such as islands and shallow water close to shore were not always adequately captured in sources with larger spatial resolutions. 3. Analysis The process for computing the calculating the stress response spectra is outlined in Fig. 2. Explained in the following subsections, the Finite Element Model was created in MAESTRO, the hydrodynamic model and panel pressures calculated in MAESTRO Wave, the stress and motion RAOs calculated in MAESTRO SFA and coupled to hindcast wave data, then the stress response spectra calculated using STRUC_R.

Hydrodynamic analysis & panel pressures

Finite element model

Stress RAOs calculation

Coupling to wave environment

Stress spectra calculation

RMS stress calculation (for given periods of time)

Fig. 2. Process used to calculate the stress response spectra

3.1. Finite element model and hydrodynamics A Finite Element (FE) model was constructed in MAESTRO 2021.1.0 (DRS Defense Solutions, 2013). The FE model is comprised of 4-node shell and 2-note beam elements. The global model consists of elements with a length of one-frame spacing, within refinement around at the strain gauge locations shown in Fog. 2. MAESTRO Wave was used to calculate the calculate the loads on the ship using an implementation of strip theory that computes both panel pressure and sectional loads (Ma et al., 2014). The calcStressResponse tool permits quick calculation of the root mean square stresses  RMS using provided stress data, operational profiles, and various wave spectra (Thompson et al., 2017). 3.2. Extraction of Wave Environment through coupling of Ship GPS and Wave Hindcast data Open-source libraries in Python were used to create a software tool for coupling data from the ship GPS to wave hindcast data. Both datasets were loaded into memory as arrays. As it is assumed that large gradients in either H s , T p , or χ would not be encountered, the nearest neighbour method is used. 3.3. Computation of Stress Response Spectra and Wave Energy Spectra The stress response amplitude operator (RAO) is used to transform an input wave spectrum into a stress response spectrum. The wave energy spectrum S ζ is defined in Eq. 1 and similarly the energy spectrum of the stress response σ (ω, t) is defined by Eq. 2 , where ω is the wave frequency and ζ α is the wave amplitude. ( ) ⋅ = 1 2 2 ( ) (1) ( ) ⋅ = | ( )| 2 ( ) ⋅ (2) An ocean wave system can be characterised by the shape of the wave energy spectrum. In this study, we applied the Bretschneider spectrum as it is most applicable to open sea areas, where patrol boats often operate in. The Bretschneider wave spectrum is defined in Eq. 3, where ω p is the circular modal peak frequency: