AQWA Hydrdynamic Diffraction+AQWA NAUT

[NVN NAMMA]

Dear team,

I’d like to get some clarity on some of the aspects I faced while modelling in AQWA. My questions are related to (1) moon pool treatment in AQWA NAUT, (2)multiple structures with diffracting and Morison elements in the motion equation, and (3) Linear cable treatment in the Cummins formulation

• In AQWA-NAUT time-domain simulations, when linear elastic cable moorings are used with a floating diffracting body containing a moonpool, are the mooring restoring effects incorporated directly into the global stiffness matrix K, or are they applied separately as external force terms updated at each time step?
• For linear cable mooring elements in AQWA, are hydrodynamic drag and inertia forces on the mooring lines neglected by default, or can they be included within the standard Workbench setup?
• In a coupled model where the main floater is represented as a diffraction body with a moonpool and the attached pipe is suspended, represented as a free-flooded pipe using Morison elements (fully submerged), how does AQWA internally assemble the Morison element inertia/added-mass contribution into the global equation of motion?
• For a floating diffracting body with an attached Morison-modelled free-flooded pip, does the added-mass matrix A∞ contain only diffraction-body hydrodynamic coefficients, while Morison added mass is handled separately during time integration?
• In AQWA-NAUT time-domain simulations, how is a moonpool treated when the Internal Lid (ILID) option is not specified? If the moonpool is modelled with panelled internal walls and open top/bottom boundaries, does the standard diffraction/radiation solution adequately capture internal water-column resonance, added mass, and damping effects, or is ILID necessary for reliable moonpool dynamics?

[NVN NAMMA]

Please

[NVN NAMMA]

 

Hi, Thank you for your time, Kindly look into these..I was using 2022 R1, Full Institute License

 

[Mike Pettit]

Hello,

I'll start by assuming that you are not using the moonpool feature that was introduced into Aqwa at Release 2023 R2, as this functionality is not yet available in Aqwa-Naut.

To answer your questions:

• In Aqwa-Naut the mooring forces are applied separately as external forces at the attachment points, and are updated at each time step.
• It is not possible to include drag or inertia loads on linear cables. Only nonlinear catenary cables, with cable dynamics, can account for these terms.
• Where a structure includes Morison elements (or consists only of Morison elements), the Aqwa time domain solver will calculate the hydrodynamic forces on each element, accounting for its instantaneous position relative to the incident wave surface. The inertia and added mass contribute to the total hydrodynamic load via the Morison equation. Please refer to the Aqwa Thoery Manual for more information: 6.1. Morison Equation
• Referring to the Aqwa-Line LIS output file, the frequency-dependent added mass matrices include the constant contributions from Morison elements, but the matrices at low (0.001 rad/s) and high (100.0 rad/s) frequencies do not. The Morison added mass is handled separately during the time integration.
• Aqwa-Naut has no special treatment for moonpools, they are just considered as surfaces like the rest of the structure. So, it is recommended to use internal lid (ILID) elements inside the structure, but also external lid (VLID) elements over the free surface of the moonpool, to dampen any standing waves that form within. Without VLID elements the distributed free surface may become unrealistic, which will affect the structure motions.

I hope this helps.

Mike