Added flexibility for strip-theory hydrostatic load calculation

[luwang00]

This PR is ready to be merged.

Feature or improvement description

This PR adds a new input switch HstMod under Strip Theory Options to HydroDyn. HstMod controls the computation of distributed hydrostatic loads on strip-theory members. If HstMod = 0, the hydrostatic pressure is always integrated on the instantaneous wetted surface up to the SWL, even if wave stretching is enabled. If HstMod = 1 and one of the wave stretching model is selected with WaveStMod > 0, the hydrostatic pressure will be integrated up to the instantaneous wave free surface, considering both wave elevation and wave slope. If wave stretching is not used, HstMod will always be set to zero internally, disregarding the user input. Irrespective of HstMod, the exact displaced position of the structure is always used when computing hydrostatic loads to obtain the correct hydrostatic restoring effect.

Generally, when strip theory is used for both hydrodynamic and hydrostatic loads, wave stretching with HstMod = 1 can be used. However, for hydroelastic simulations, a mixed approach is sometimes preferred. With this approach, the hydrodynamic loads are computed using multiple potential-flow bodies, while the hydrostatic loads are computed using the nonlinear strip-theory approach (with PotMod = false but all hydrodynamic coefficients zeroed out). This approach provides the correct mean loads in the structure from hydrostatic effects that cannot be easily obtained with displaced volumes and hydrostatic stiffness matrices. In this case, inconsistencies can arise between the hydrodynamic loads based on perturbation theory and the fully nonlinear hydrostatic loads up to the instantaneous free surface when simulating large incident waves (see Issue #3216). To address this problem, HstMod = 0 can be used, allowing wave stretching to be used when computing the distributed drag force only.

Related issue, if one exists
Issue #3216

Impacted areas of the software
HydroDyn, openfast_io

Test results, if applicable
No change to existing test results. All HydroDyn input files, openfast_io, and user documentation have been updated with the new HstMod input.

• r-test branch merging required