MIMOSA

MIMOSA analyses mooring systems and moored vessels. It uses efficient frequency-domain techniques to calculate wave frequency and low frequency vessel motions and mooring tensions.
For more information please contact: Division of Offshore Structures

MIMOSA is up-to-date with respect to all calculations required by the Norwegian Maritime Directorate (NMD) and the American Petroleum Institute (API) for approval of positioning systems, e.g. calculation of slow drift, line dynamics and transient motion.

MIMOSA offers a variety of options and is well suited for tasks like early design studies, long term simulation, and parameter studies. The program may also perform location approval analysis as required by authorities and insurance companies for vessels like semi-submersible drilling and accommodation platforms, turret moored ships, and deepwater floaters for production.

MIMOSA employs efficient frequency domain techniques to compute vessel motion and dynamic mooring tension. Time domain transient analysis may also be performed.

The fast calculation methods make it particularly attractive as a basis for reliability analysis of mooring systems in which a large number of environmental conditions has to be evaluated.

MIMOSA covers:

• Static and dynamic environmental forces due to waves, wind and current
• Wave induced motions
• Slow drift motions
• Static and dynamic mooring system analysis
• Composite mooring lines
• Transient motions after line breakage
• Non-Gaussian statistics
• Dynamic positioning (DP) with thrusters
• Stability analysis of turret-moored ships
• Long term simulation

User Inferface

MIMOSA has a menu driven, interactive input. The user selects options from a hierarchy of menus and responds to program requests for additional data, e.g. whether and how (tables, plots) to present and store results.

With a macro command facility the user can easily repeat often used sequences of commands. The commands are stored in a macro file and thereby easily executed whenever desired. In addition to saving time this will reduce the risk of giving erroneous input. A macro can be instructed to pause at selected points during execution to allow the user to enter data from the keyboard. The macro facility is therefore well suited for parameter studies.

This command storage facility can also be used to ensure documentation and repeatability of all calculations. This can otherwise be a problem with programs for interactive design and verification analysis.

Input Data

The input data required by MIMOSA are:

The vessel description including mass, force coefficients for wind, current drag and wave drift excitation, motion transfer functions, and damping coefficients.

MIMOSA is interfaced with the SESAM programs WADAM and WAVESHIP to ease the input of frequency dependent transfer functions and wave drift coefficients.

The mooring system description including the number, direction, pretension and fairlead position of the mooring lines. The possibilities for describing the mooring lines are numerous: Each cable may be composed of several segments with different lengths, diameters, weights, elasticities and drag coefficients. Buoys and clump weights may also be included. The slope of the seabed may be specified for each line individually. Flexible risers may be modelled as mooring cables.

• Dynamic positioning controller gains and thruster parameters such as type, location, capacity and direction of force.
• Waves and wind modelled as irregular (stochastic) processes defined by power spectra. A number of spectrum types are available (e.g. Pierson-Moskowitz for waves and Harris for wind). The current is specified by constant magnitude and direction.

Interactive modification of the mooring and thruster system makes it easy to perform parameter studies. Modified data sets may be saved on file for later use.

Results from MIMOSA

The results computed by MIMOSA are:

• The environmental forces due to wind, current and waves.
• The equilibrium position in which the mooring and thruster forces balance the static components of the environmental forces.
• Represented by standard deviation, mean oscillation period, significant value and expected maxima, results are available for:
  - Motion of any point in the vessel for:
  - slow drift (or low frequency) motion in the horizontal plane.
• Mooring tension:
  static mooring tension for the equilibrium position or for any position and heading, dynamic mooring tension for the combined wave frequency and slow drift motion in the chosen weather condition.
  Maximum slow drift motion and maximum dynamic mooring tension are based on nonGaussian statistics.
• Static and dynamic forces from thrusters under dynamic positioning control.
• Optimum distribution of tension based on either minimizing the maximum tension in the mooring system, or least squares minimization including thrusters.
• Run length of winches required to move the vessel to a new position or to obtain optimum tension distribution.
• Stability of the vessel in single-point mooring or turret mooring checked by eigenvalue analysis.
• Transient motion after a line breakage or thruster failure in terms of time traces of motion and tension, motion trajectory, maximum excursion of any given point on the vessel, and maximum mooring tension. This also includes free drift (DP black-out).
• Tension and displacement characteristics, line profile, clearance to other lines and objects in the vicinity. Various relevant data for any mooring line and user-specified conditions like upper end tension or distance to anchor.
• Long term simulation results based on using a macro command facility for running a set of environmental conditions and producing corresponding sets of results, covering e.g. 5-20 years of operation.

Background

MIMOSA is based on the following design requirements: interactive use, user-friendliness, and prompt response to user input. This posed a great challenge in the program development as hydrodynamic and structural analysis generally require massive computer resources. Ingenuity has therefore been put into simplification of methods while retaining the required accuracy.

Since the first version of MIMOSA was developed by MARINTEK in the mid seventies the program has continuously been enhanced. The program has for several years been among MARINTEKs most important in-house tools, and a number of results from MARINTEKs research activities - from theoretical work and model basin tests to full-scale field tests - have been implemented in MIMOSA.

MIMOSA is currently being used by several oil and engineering companies as well as rig and ship owners.

Benefits for the User

• The frequency domain analysis enables operating directly on statistical quantities such as standard deviations and power spectra rather than calculating these parameters based on lengthy time domain simulations.
• Fast and efficient analysis methods provides for conventional design and verification work as well as for optimisation studies and reliability analyses.
• Reliable service by well reputed organizations
• Extensive use in research and development ensures continuous improvement of analysis methods.
• A proven solution

MIMOSA is owned, developed and maintained by MARINTEK, - marketing and sale is handled by DNV Software (http://www.dnv.com/software)

MIMOSA Product pamphlet in PDF format