The General HydroStatics SeaKeeping Module "SK" is designed to provide the GHS user with an integrated, general-purpose solution for seakeeping and hydrodynamic analyses. SeaKeeping is the result of significant in-house development work, and while it offers many features that may be found in other motions codes, it is also unique in many respects. Integration within the GHS environment also means calculations may be performed using existing geometry files, loading conditions, and run files. This translates to minimal additional user input, unmatched flexibility in its market segment, and ease of use.
SeaKeeping offers an excellent and economical solution for seakeeping and hydrodynamic analysis that is well suited to many applications. Some of the more common use-cases include:
What Makes SeaKeeping Different?
- Motion Studies: Compute 6-DOF RAOs, statistics, periods and amplitudes, with or without forward speed in long-crested or short-crested seas.
- Cargo, Rigging, and Sea-Fastenings: Forecast accelerations at rigging and fastening points, supporting the calculation of rigging loads, design of cargo fastening systems, and cargo shifting.
- Habitability: Estimate periods, accelerations, and MSI encountered under way in a variety of sea ways to ensure crew comfort and safety.
- Crane Operations: Estimate absolute and relative responses at crane booms and lift points and accelerations at crane bases.
SeaKeeping is a unique and powerful analysis tool. Below are a just a few reasons why SK is different from the competition:
- Forward speed at any heading in any seaway.
- Extensive environment input options: sinusoids, spectra, data, ranges, spreading, and flexible sampling.
- Ability to specify any loading condition to an exceptional level of detail and include all inertial properties.
- Unique and powerful dynamic LIMIT statements simplify operability studies.
- Ability to run many, many cases with little effort and maximum repeatability.
- Robust and flexible damping model: numerical, experimental, or a mix of both.
- Bilge keel and skeg appendage support.
- Full-featured data IO: everything SK computes is accessible.
- Integration with GHS scripting language and a dedicated SK.LIB macro library means you can use all of the data SK creates for powerful post-processing.
- Powerful and unique SEA DATA mode computes motions using external RAO or hydrodynamic data allowing for complex geometry, rapid re-evaluations in different seaways, custom modification to coefficient matrices, and more.
- Full or partial hydrodynamic coupling and the ability to analyze asymmetric conditions.
- Maintained, supported, user driven.
The module offers users the following features:
Don't see a specific feature on the list?
- Integration with GHS command language and run files allows unmatched flexibility
- Minimal syntax makes setting up a seakeeping analysis quick and easy
- Available Acceleration Transform Wizard makes it easy to compute deck-parallel and deck-normal acceleration components (with or without the gravitational acceleration term) for cargo and sea-fastening applications
- Available Long Term Statistics Wizard provides a powerful tool for the input and manipulation of wave scatter data: compute statistics, set up an analysis, and derive insight for transportation, route-planning, and sea-fastening design.
- Use of GHS geometry files (.GF) for seakeeping analysis eliminates the need to create additional models
- Geometry-based appendage support makes no assumptions about the location and/or orientation of appendages
- Ability to model and analyze passive U-style roll tanks
- Minimal input and ease of use compared to other commercial codes
- Recognizes vessel loading condition as specified in GHS, meaning all tank weights, added weights, and lightship distributions are automatically included and there is no need to specify hydrostatic parameters, what is needed is accessed directly
- Full support for monohull vessels, case-specific support for SWATHs and multi-hulls
- 6-DOF motions including displacements, velocities, accelerations, amplitudes and phase angles
- Absolute and relative motions
- Motions may be computed at the vessel center-of-gravity and at specified Critical Points
- Critical Points responses are point-specific, and automatically include any coupling effects (such as pitch-into-heave effects near the bow of a pitching vessel) so there is no need to perform these tedious calculations, the results are direct
- Response amplitude operators (RAOs) for all DOF at CG and/or any Critical Point are automatically computed
- Statistical response data including variance (m0,m2,m4), RMS, average and significant response amplitudes, average periods, and extreme maxima are automatically computed when using wave spectra
- Accelerations may be given in Ft/Sec2, m/Sec2, or gravitational units "g's"
- User specified confidence limits and time intervals allows for custom definition of extreme response amplitudes
- Optional overall or case-specific summary tables include extreme position, velocity, and/or acceleration amplitudes for the CG and/or all Critical Points offering an ideal "one-location" reference table for design values
- Easily compute the maximum response(s) for the CG and/or all Critical Points over a range of headings and/or speeds
- Powerful reporting options manage excess data output, allow for tailored reports
- The ability to analyze vessels with non-zero heel and/or trim means realistic loading conditions can actually be evaluated
- Robust geometry handling alerts the user to potential issues
- Unique trapped-water method offers first-order approach for stern ramps, well decks, and other unusual spaces
- Optional full-coupling, partial-coupling, or full-uncoupling of the equation of motion
- Optionally compute all 36 hydrodynamic added mass and damping coefficients
- Robust linear, rigid-body, frequency domain, boundary element strip theory formulation includes the following theoretical enhancements:
- Computes exact 3D normal vectors using a 3D triangular mesh, does not use 2D or slender-body normal vector approximations, yielding more accurate solutions, and greatly improved solution convergence when increasing the density of sections
- Includes a more complete treatment of speed corrections in forces and coefficients for better forward speed approximations
- Makes no symmetry assumptions
- Boundary element method (2D Rankine panel method) solves the radiation problem directly for each section and can accommodate most ship geometry, incl. anti-symmetric geometric sections (i.e. non-zero heel), fully-submerged sections, and partially submerged sections
- Computes the diffraction potential for all modes using the Haskind relations, yielding robust diffraction forcing solutions with reduced computation time
- Offers a fully-coupled hydrodynamic added mass and damping matrix, meaning all 36 coefficients may be computed
- Offers a generalized mass matrix including all physical mass and mass-moment terms and a fully-defined inertia tensor that interfaces automatically with the current loading condition
- Offers a generalized hydrostatic stiffness matrix including all restoring terms
- Advanced parameterization of hydrodynamic 2D panel mesh
- Robust and modern numerical roll damping model computes additional first- and second-order lift and viscous (skin friction and eddy-formation) damping terms for the hull and any appendages
- Optional input of experimentally derived roll damping coefficients
- Option to input hull damping as a critical damping ratio
- Roll damping input allows mixing of numerical and/or experimental damping terms
- Option to omit all additional damping terms and use only potential wave damping
- Long-crested waves at any wave heading
- Short-crested waves using COS2 or custom spreading functions
- Regular or irregular waves at any wave heading
- Single wave or wave spectra input (via WAVE command extension)
- Available wave spectra include:
- Pierson-Moskowitz (wind speed)
- Pierson-Moskowitz (sig. wave height)
- Bretschneider (General, Narrow-band, and ITTC 1984)
- JONSWAP (wind speed and fetch)
- JONSWAP (sig. wave height and period)
- Ochi-Hubble (6-parameter)
- Ochi-Hubble (most probable, single-parameter via Hs)
- User data file
- Two parameterized wave spectra sampling methods (constant variance or evenly sampled frequency range) for seaway discretization
- Non-zero vessel speed at any wave heading
- Easily run individual or multiple wave headings within a single command
- Easily run one or more forward speeds within a single command
- Create custom dynamic limits using the SEA LIMIT statement syntax
- Automatically evaluate dynamic limits over many cases, generating operability limit plots
- Slamming probabilities, frequencies, and estimated pressures may be computed at a specific location or at frame locations using a frame file
- Deck submergence "Shipping Water" probabilities and frequencies may be computed at Critical Points
- Point emergence (such as propeller emergence) probabilities and frequencies may be computed at Critical Points
- Motion Sickness Incidence (MSI) with variable exposure period may be computed at Critical Points
- Automated polar plotting for multiple headings enables easy generation of polar plots for any response statistic (RMS, Extreme, etc.), and/or derived responses
- Seven (7) optional comma-separated output data files:
- Response Data: includes RAOs and phase angles for all modes and all Critical Points
- Forcing Data: includes total, inertial, and diffraction wave excitation forcing amplitudes per unit wave amplitude and phase angles
- Hydrodynamics Data: includes hydrodynamic and hydrostatic coefficients (i.e. A, B, and C's)
- Wave Data: includes wave sample characteristics such as periods, frequencies, spectral ordinate, sample amplitudes, etc.
- Variance Data: includes response variance (m0, m2, m4) for all modes and all Critical Points
- Statistics Data: includes response statistics such as periods, RMS, average, significant, and extreme amplitudes
- Tank Data: includes tank moments for any roll tanks
- Dedicated SK.LIB seakeeping macro library provides useful macros and automated data file parsing to make data easily accessible as variables, and allows for powerful post-processing
- SEA DATA mode provides flexbility and efficiency allowing SeaKeeping to run on pre-existing RAO or hydrodynamic data, allowing for rapid re-evaluation in different seaways or modification of coefficient matrices
- Full-featured and comprehensive report file includes all RAO and phase angle tables and plots, response statistics, continuous and sampled spectra tables and plots, summary tables, derived response tables, and polar plots
Contact Customer Support
to inquire about current and prospective feature updates. SeaKeeping is always being updated and improved.
Seakeeping calculations create a lot of data, which is both a necessity and a curse when you are trying to review results. Seakeeping output is designed to make it as easy as possible to reference specific RAOs or response results for certain Critical Points or heading/speed combinations (a.k.a. Cases) without omitting important supporting information. SeaKeeping reports are organized according to the following structure, with options to include or omit various sections:
- INPUT SUMMARY
Summary of general input parameters, including method type, meshing parameters. coupling parameters, and a table of seakeeping geometry part\component names and types.
- CRITICAL POINTS
Table of user specified Critical Points, overall vessel CG, and any optional derived response points (SHW, MSI).
Case number, case-specific wave heading and ship speed
- Wave Components
Summary of overall wave/spectra type, wave samples, sampling method parameters, and numerically derived variance and significant wave height. Also includes tabulated wave period, frequency, LWL/WvLen, encounter frequency, ordinate, and amplitude for each wave component.
- RAOs and Phase Angles (Position, Velocity, and/or Acceleration)
Tabulated RAOs and phase angles for all modes for each Critical Point. Also includes a plot for each mode, with results for all Critical Points plotted together.
- Response Statistics (Position, Velocity, and/or Acceleration)
Tabulated response statistics for all modes for each Critical Point. Includes variance (m0, m2, m4), average periods (ZUC/Maxima), RMS, average, significant, average maxima, and optional confidence/interval extreme amplitudes.
- Derived Responses (Optional)
- Deck Submergence/Shipping Water "SHW"
- Point Emergence "EMG"
- Motion Sickness Incidence "MSI"
- Case Summary Table (Optional)
Table of user requested absolute response amplitudes for all modes for each Critical Point. Displays the response for the preceding case.
- POLAR PLOTS (Optional)
Polar plots for each user requested response for each Critical Point for all modes.
- LIMIT PLOTS (Optional)
Polar limit plots for each user defined dynamic limit over all cases.
- OVERALL SUMMARY TABLE (Optional)
Table of overall user requested absolute response amplitudes and corresponding case numbers for all modes for each Critical Point. Displays the maximum over all cases.
To help parse especially large runs, SeaKeeping offers optional case-specific or overall summary tables which display user requested position, velocity, and/or acceleration amplitude components in the x (Surge), y (Sway), z (Heave) translational directions, and the X (Roll), Y (Pitch), Z (Yaw) angular directions. These tables are an ideal "one-location" reference point for design values or initial review, and are always located at the end of each reported case. Automated response and limit polar plotting is also available to further aid in the review process.
In addition to comprehensive reports, SeaKeeping offers seven (7) optional comma-separated-variable (CSV) data files that include any of the information found in the report (and more) in a convenient form for input into spreadsheets, third-party programs, or further post-processing by run files.
Get Access to a Trial Version
If you are interested in testing the SeaKeeping Module, contact sales
to inquire about a temporary license. We will work with you to set up a trial period that meets your evaluation needs.
A number of core validation studies, including coefficients, forcing, RAOs, relative motions, and accelerations, may be found here.
A collection of common questions from SK users may be found here.
Publications Featuring SK
Kyle E. Marlantes, Peter Kim. Addressing the New IMO Guidelines for Second Generation Intact Stability. MarineLink: November, 2020.
Abstract: The common perception of intact stability has remained largely unchanged over the last few decades, where a vesselís stability is evaluated using classical and static means: limits on righting arms, residual areas, and determining maximal VCG (or minimal GM) composite curves. These methods are familiar to most naval architects and are taught at a fundamental level in most naval architecture engineering programs. But repeated incidents of dynamic failure in recent decades brings question to the adequacy of classical static stability criteria to provide a complete understanding of, and adequate safety margin for, a vesselís stability. Obviously, strictly static methods are not wholly sufficient in assessing the dynamic stability of a vessel in waves. Stability criterion are designed to protect people and property, and with safety and liability a primary concern, an improved regulatory framework to assess dynamic stability is required.
Marlantes, K. E. (2019, October 30). Asymmetric Conditions and Ship Motions: Investigating the Ubiquitous Symmetry Assumption. The Society of Naval Architects and Marine Engineers.
Abstract: This paper explores the effect of asymmetric conditions, specifically non-zero heel, on the formulation of the ship motion problem using a fully-coupled, linear seakeeping code developed by the author. A theoretical formulation is provided under the auspices of an eliminated symmetry assumption, and numerical predictions of the radiation and excitation forces are given to explore unique aspects of the problem. Symmetric and asymmetric numerical heave, roll, and pitch RAOs for a generic naval frigate are compared to third-party model test data. It is found that asymmetry can have marked effects on the ship motions problem, most notably in the roll excitation moment, physical mass matrix, and the hydrodynamic cross-coupling of the six modes of motion. The location of a so-called ďcenter of motionĒ is found to be important in the formulation, suggesting that the origin cannot be arbitrarily placed at the center of mass. Some discussion addressing the practical nature of asymmetry in seakeeping computations is provided, attempting to relate the theoretical and numerical findings back to the practical application.
Kyle E. Marlantes, Brandon M. Taravella, A fully-coupled quadratic strip theory/finite element method for predicting global ship structure response in head seas,
Ocean Engineering, Volume 187, 2019, 106189, ISSN 0029-8018.
Abstract: This paper outlines the theoretical development and some validation of a quadratic strip theory method coupled to a global finite element model to predict the global structural response of the Korea Research Institute of Ships and Ocean Engineering (KRISO) hull geometry due to regular, head seas waves in the time-domain. The method attempts to capture some body-nonlinear effects of the dynamic problem due to time-varying underwater hull geometry by drawing a relationship between the coefficients, A33, B33, and C33 and the local draft, Ts. In addition, the hull girder is considered flexible and structural damping may be included. A segmented model test in head seas was also performed, and the linear and nonlinear numerical results are compared to the experimental data. It is found that the theory shows reasonably good agreement with the model test data, and that nonlinear effects account for a significant increase in predicted bending moment.
GHS-SK: An Integrated, General-Purpose Approach to Seakeeping and Hydrodynamic Analysis
Abstract: The General HydroStatics SeaKeeping software module, referred to in short as "SK", is Creative Systemsí entrant into the world of hydrodynamics. Introduced in January of 2018, the module aims to provide users with an integrated, general purpose approach to seakeeping and hydrodynamic analysis. This paper is a moderately technical introduction for the interested individual or prospective SK user. After some introduction, the overall focus will be on the four (4) main areas that make SK unique in its class: Automation, Customization, Technical Capability, and Development & User Support. While some theoretical content is presented here, those specifically interested in a deeper mathematical basis of the module are invited to contact Creative Systems Inc. for a copy of the SK Userís Manual.
Kyle E. Marlantes. Tech Talk: GHS Adds 'Seakeeping'. MarineLink: February, 2018.
Abstract: General HydroStatics is no longer just about hydrostatics, and will soon offer capabilities in the world of hydrodynamics with the introduction of a long awaited addition to the GHS product family: an optional seakeeping module.