NEMOH: Open-source boundary element solver for computation of first- and second-order hydrodynamic loads in the frequency domain

Abstract

NEMOH is developed to solve potential flow problems using the boundary element method for computing wave loads on offshore structures. The solver solves the first-order problem in the frequency domain. An extended module enables computation of the second-order loads on a structure in bi-chromatic bi-directional waves. NEMOH is released as open-source, developed, and distributed under the terms of GNU General Public License (GPLv3). Along with the source code, a user manual is available which facilitates the compilation and execution of the source files. In this new release, NEMOH v3.0 has new features; irregular frequencies removal method, new linear system solvers for enhancing computational efficiency, and a new module for computing the full quadratic transfer function (QTF) of drift forces. Verification cases are shown for a truncated cylinder, a truncated cylinder with a thin heave plate, a spar buoy platform (SOFTWIND), and a semi-submersible platform (OC4). The results are compared with reference data from commercial software, WAMIT, DIODORE, and HYDROSTAR, and the comparison showed that NEMOH v3.0 is accurate and efficient. Program summaryProgram Title: NEMOHCPC Library link to program files:https://doi.org/10.17632/k9v4hjb5xj.1Developer's repository link:https://gitlab.com/lheea/NemohCode Ocean capsule:https://codeocean.com/capsule/9256007Licensing provisions: GPLv3Programming language: FortranExternal libraries and programs: BLAS [1], LAPACK [2]Nature of problem: NEMOH solves the potential flow problems of diffraction and radiation in wave-structure interactions. Typical use is the estimation of the dynamic response of floating structures, e.g. floating offshore wind turbine platforms, or wave-energy converters.Solution method: The linear potential flow problems, diffraction and radiation, are expressed in the boundary integral of the source distribution formulation. The equations are solved in the frequency domain and implemented using the boundary element method with quadrilateral panels. The source distribution on the body panels is obtained by solving the linear system. The hydrodynamic coefficients are computed with the known source distribution. The second-order potential flow problem is not solved directly; the Green formulation is used with an assisting function which is the instantaneous radiation potential, that allows the computation of second-order force for given first-order hydrodynamic quantities.Additional comments including restrictions and unusual features:•The previous version, NEMOH v2.0 [3], was available only for computing first-order hydrodynamic coefficients without the irregular frequency removal method. NEMOH v3.0 has new features, the extended boundary integral method is implemented for removing the irregular frequencies, new linear system solvers are added for enhancing computational efficiency, and a new module is added for computing second-order wave loads. NEMOH v3.0 has better accuracy than v2.0 due to finer interpolation points of the free-surface Green function and a better integration method for constructing the influence coefficients.•For the free-surface piercing bodies problem, it is known that non-unique solutions exist in the boundary integral equation at certain frequencies. In NEMOH, the extended boundary integral equation is applied to remove the irregular frequencies by providing the lid panels on the body waterplane. The second-order loads require the computation of the free-surface forcing terms in the integral forms. These free-surface integrals are essential for the sum-frequency wave loads but may be negligible in the difference-frequency loads. For now, it is suggested not to apply the lid panels when the free-surface integrals are computed.