MACSI

MACSI (Marine Crane Simulation)

• Hydrodynamic forces
• Motion in irregulars seas
• Precision emplacement
• Guideline operations
• Heave compensator analysis
• Dynamically positioned module handling tools
• Nonlinear analysis

MACSI is a PC-based program for simulation of offshore crane operations, where an object is lifted or lowered through the water to the sea bed. Various configurations and equipment with nonlinear characteristics can be handled by the program.

MACSI computes time series of the crane and object motion, the lift-wire tension and control forces form an active heave compensator or DP forces on the object, all which can be shown graphically.

MACSI allows the user to specify and modify main input data during the execution of the program.

MACSI output gives a direct link between plots, result tabes and the input used to achieve the results - a useful feature for assistance in documentation.

System configurations

MACSI can handle three main configurations:

• Direct winch down from a crane that can be equipped with a passive or active heave compensator. Stabilization by guidelines can be specified.
• Pull down through a sea bed sheave, to the sea bed. The winch can be heave compensated.
• Pull down by a sea bed winch. No heave compensator or guidelines are used. 

Environment

The environmental parameters are:

• Wave direction and spectrum parameters describing either the Pierson-Moskovitz or the JONSWAP spectrum. Alternatively a numerical wave spectrum can be read from an input file.
• Current velocity and direction (no depth or time variation is included).

Vessel and crane top

The crane top motions in irregular seas are computed form frequency response functions of first order vessel motions in waves (six degrees of freedom) for different heading angles.

Alternatively the crane top motion can be specified by:

• Spectrum for motion in one direction
• Amplitude and period of harmonic motion.
• Measured time series.

The crane top position is input to MACSI.

The vertical motion can be modified by a specified transfer function representing an active compensator with known performance.

• Two winch operation modes can be used:
• Constant winch speed, up or down (or zero)
  Transient winch stop

The performance of the passive heave compensator is determined by preset load, stroke, stiffness, damping and friction.

An active heave compensator can be modelled by a PID controller. Noise representing errors in measured position references can be included in the simulation.

Guidelines with constant tension can be modelled.

A DP control system on a module landing tool can be applied. The required thrust forces are computed. Noise in the position reference can be included.

The lifted object is described by:

• Mass, volume and dimensions
• Coefficients for hydrodynamic added mass and linear and quadratic drag for motion in the three directions.

No object rotation is included in the model. Hydrodynamic parameters valid for a fully submerged object, unaffected by boundaries, should be used. (For position dependent coefficients modelling splash-zone and sea bed effects, see the description of MACSI-2).

Static computation

The static position of the load is found by an interpolation towards force equilibrium. Static force components and the resulting lift line tension are computed.

Eigenvalue analysis

Eigenperiods at start conditions for longitudinal and transverse oscillations of the load and lift line are computed by analytic methods.

Dynamic simulations

The static configuration of the system is used as basis for the dynamic simulation, in which the following parameters are computed:

• Motions of the crane top in 3 directions.
• Motions of the lifted object in 3 directions.
• Dynamic lift line tension.
• Control forces form the active heave compensator (if applied).
• Control forces form DP thrusters on the object in the 2 horizontal directions (if applied).

After a simulation the minimum and maximum values of the object position and line tension are displayed. If zero winch velocity has been specified for the simulation, the following are also computed:

• Mean values
• Standard deviations of the parameters and their derivatives.
• Estimated mean (zero-crossing) periods.
• Estimated maximum amplitude values

The risk for slack and snatch loads in the lift line is also evaluated.

MACSI can plot:

• Time series of all output parameters
• XY plots of all output parameters

For more information, please contact: MARINTEK, division Offshore