# SIMDYN

## SIMDYN - Time Domain Hydrodynamic Simulation Tool

In the course of my research towards my PhD dissertation, I have developed a time domain simulation program (in FORTRAN) to simulate the nonlinear motions of a vessel at sea. Although the original idea was to create a tool to simulate parametric roll motions of ships, the code has come way far ahead and has taken shape into a generalized time domain hydrodynamic tool. It is a time domain simulation program which can handle the large amplitude of motions of vessel by using an Euler angle formulation and also computes the nonlinear force vector to calculate the nonlinear motions of a vessel in an irregular seas. In particular it includes both nonlinear Froude Krylov and nonlinear hydrostatic forces and moments and also includes a viscous roll damping model for ship shaped structures to help simulate nonlinear phenomena such as parametric roll.

While including the nonlinear Froude Krylov and nonlinear hydrostatics allows modelling strong nonlinear motions, the program is also capable of handling the traditional consistent second order analysis where the second order drift forces and moments are included to evaluate the performance of the mooring system. A mooring model based on the catenary equation is also included to allow for the analysis of moored offshore platforms.

### Salient Features of SIMDYN

1. Large amplitudes of rotation considered using Euler angle formulation

2. Solves exact nonlinear equations of motion

3. Nonlinear Froude Krylov forces and moments

4. Nonlinear hydrostatic forces and moments

5. Viscous roll damping model to predict linear and quadratic roll damping

6. Allows analysis in both zero and forward speed

7. Pressure history output for mapping into structural analysis programs

Relevant Publications

1. Somayajula, A., & Falzarano, J. (2015). Large-amplitude time-domain simulation tool for marine and offshore motion prediction. Marine Systems & Ocean Technology, 10(1), 1–17.

## Nonlinear Pressure Mapping

In the latest version of the time domain simulation tool SIMDYN capability has been provided to output the pressure time histories which can then be mapped to a structural analysis grid to perform strength analysis. The video below shows the nonlinear pressure time history on KCS container ship in oblique waves. Note that the inclusion of nonlinear Froude Krylov and hydrostatic pressure helps estimate pressure history up to relative waterline instead of the mean waterline.

## Parametric Excitation

Parametric response is an instability which results in large responses even when the excitation is close to zero. This nonlinear phenomenon is primarily caused by the time variation of a system parameter such as stiffness (in waves) or inertia.

### Parametric Roll Motion of APL China Container Ship

Typically, for a long time parametric roll motion was believed to be a problem for fishing vessels in following waves. However, in the recent years it has come to light that even fast container ships (which have a slender form to achieve higher operational speeds) are highly susceptible to parametric roll in head seas. This instability manifests when the incident regular wave has an encounter frequency which is twice the roll natural frequency. The video below demonstrates this instability of C11 (modified APL China hull form) simulated using SIMDYN (Numerical simulation tool described in detail below).

Relevant Publications

### Parametric Excitation of Single Column Floater

In spar type of platforms, when the damping is low, a coupled instability in the heave and pitch modes is observed when the difference frequency between heave natural period and the incident wave period matches with the pitch natural frequency. This effect has been extensively studied for the classical spar. However, even some of the other designs such as the Single Column Floater are found to be susceptible to this instability. The video below demonstrates the instability of Single Column Floater simulated using SIMDYN.