ShipX

SINTEF offers numerical calculations of ship motions as an alternative to, or in addition to model tests. The Veres (Vessel Response) programme offers the ability to calculate ship motions and global loads, including the calculation of short-term statistics, long term statistics and operability.  The programme is integrated in the hydrodynamic workbench ShipX.

The study of wave induced vessel responses is essential in the design of new ships. To optimize the operability of the vessel in a seaway, it is important to minimize the motions of the ship. If the loads are decreased, the steel weight can be reduced.

Further, hydrodynamic loads and motions are important from the standpoint of safety of the ship and its crew. Veres is a tool which can be used in early design, in defining and evaluating model tests and in obtaining supplementary results. The programme calculates:

• Motion transfer functions in six degrees of freedom
• Relative motion transfer functions
• Motion transfer functions at specified points
• Unsteady global loads
• Short term statistics of the above mentioned
• Long term statistics of the above mentioned
• Operability limiting boundaries
• Percentage operability

In this context, motions include displacements, velocities and accelerations.

Calculations can be performed for monohulls and multi-hulls at low as well as high speed. At low and moderate speeds, the traditional strip theory, developed by Salvesen, Tuck & Faltinsen is applied. At higher speeds, the high-speed formulation developed by Faltinsen & Zhao is applied.  

Roll damping 

The programme can include viscous roll damping from hull friction and bilge keels, as well as the effects of roll stabilizing tanks and active roll stabilizing fins. 

Motion control 

The programme can include the effects from passive free-surface roll stabilizing tanks, as well as active and passive U-tube tanks, rudder control and active and passive fins such as roll stabilizing fins and T-foils. The programme can also include the effects from air cushions on Surface Effect Ships. 

Short term statistics 

Short term statistics of the data from the calculations includes

• Standard deviations
• Significant values
• Expected maximum in a seastate of a given duration (e.g. 3 hours)
• Average of the 1/nth largest values
• Response zero-upcrossing period
• Plotting of response spectra

The calculations are based on selected standard wave spectra as well as measured wave spectra and can be performed for long- and short-crested seas. 

Long term statistics 

Long term statistics of the data from the calculations can be calculated based on a specified scatter diagram.

The long term statistics can be calculated for each wave heading separately, or with a specified probability of each wave heading. A speed curve specifying the vessel speed as function of significant wave height can also be specified. 

Operability  

The calculation of operability is available in three modes:

1. Operability limiting boundaries presented as limiting significant wave heights as a function of the wave period
2. Operability diagram
3. Percentage operability

The operability can be calculated based on the following criteria:

• Motions in six degrees of freedom
• Relative vertical motions
• Probability of slamming
• Probability of green water on deck
• Probability of air exposure
• Vertical accelerations according to ISO 2631 (motion sickness)
• Motion-Induced Interruptions (MIIs)
• Motion Sickness Incidence (MSI)

Time-domain calculations

The ability to perform time-domain calculations is also available in Veres. In the time-domain calculations, the linear hydrodynamic coefficients of the ship hull can be combined with non-linear wave excitation forces and restoring forces, as well as non-linear effects from motion control systems.

VeRes 3D 

Contrary to the strip theory formulation of the standard VERES, VERES3D is a linear, three-dimensional, frequency domain radiation/diffraction code. VERES3D is developed to analyse the hydrodynamic interaction of waves with large volume structures for zero and forward speed. Large-volume structures are defined as structures where diffraction is essential. Both finite and infinite water depth configurations can be computed.

VERES3D is based on potential flow theory applying a panel method in the solution procedure. The fluid is assumed ideal, and the flow time harmonic. The free surface condition is linearised, so only terms proportional to the wave amplitude are retained. The second-order mean wave drift forces are included in the analysis as they may be obtained from the linear solution.

The radiation and diffraction velocity potentials on the mean wetted surface of the body are determined from the solution of an integral equation obtained from Green’s theorem. Without current, a Green function satisfying the linear free surface condition is used, and only the mean wetted surface of the body is discretised into panels. A numerical formulation using the Green function satisfying the far-field free surface condition in the outer domain is applied.

This formulation is often referred to as the direct method since the potential is obtained directly from the solution of the integral equation. Surface elevation, pressure and fluid velocities in the surrounding fluid without wave-current interaction are obtained by applying an equivalent source distribution on the mean wetted surface of the body. This source distribution is calculated by using the computed potential distribution on the body.

VERES3D uses a low-order panel method where flat quadrilateral panels define the surface of the body, and the solution for the velocity potential (and thus also source strengths) are assumed constant over each panel.

The programme may be applied to single bodies in waves with forward speed. The bodies may be located on the free surface, and be fully submerged.

Viscous forces may be applied in the motion response calculation. On ship structures, viscous roll damping may be specified. The viscous forces contain quadratic terms, which are linearised by equivalent linearisation. The motion responses are then obtained by iteration.

The following quantities can be evaluated in VERES3D:

• Hydrostatic coefficients
• Added mass and damping coefficients for all modes of motion
• Wave excitation forces and moments from direct pressure integration from the diffraction potential or from control surface integration (with or without line integral).
• Motion amplitudes and phases for a freely floating body. Additional mass, damping and restoring forces may be specified to account for forces restraining the body from moving in some modes. These additional matrices may also be used to tune the natural periods against model tests or full-scale results. Viscous forces may also be specified.
• Horizontal mean drift forces and mean yaw moment

The horizontal drift forces and moments are obtained by either a direct pressure integration method or a control surface integration based on the conservation of momentum. In the control surface integration method, a vertical control surface must be positioned around the body. Far-field method can also be applied when computing zero-speed.