SIMO is a time domain simulation program for multibody systems allows non-linear effects to be included in the wave-frequency range. Flexible modelling of stationkeeping forces and connecting force mechanisms (anchor lines, ropes, thrusters) is included.

The results from the program are presented as time traces, statistics and spectral analysis of all forces and motions of all bodies in the analysed system.

SIMO is a modular and interactive computer program with batch processing options.

Figure 1. Simulation of SNORRE TLP.

Figure 2. Photo from SNORRE TLP installation (Photo: Saga Petroleum)

Figure 3. Alternative TLP installation.
Typical applications

TLP Installation
Extensive studies on the SNORRE installation and the HEIDRUN installation have been performed. Such installations are typically modelled by the TLP hull positioned by a number of anchored and/or dynamically positioned semi-submersibles and tugs as shown in Figures 1, 2 and 3.

Offshore Crane Operations
Since the equations of motion are solved in time domain, this allows for coupling between the surface vessel(s) and the load module to be accounted for. Examples are installation of the Concrete Foundation Templates (CFT) of SNORRE TLP (Figure 4), installation of the Sleipner A riser jacket, and numerous subsea template installations.

Floating production systems
Several studies have been undertaken on turret moored ships and semi-submersible platforms as the Troll Olje platform. A shuttle tanker connected to a production storage tanker may also be modelled.

Dynamic Positioning Systems
Simulations are useful for predicting motions of flotels with gangway connection to fixed or floating structures.

Environment
Waves can be modelled by regular waves and by various model spectra for irregular waves.

Average wind velocity is constant, but gust in both propagation- and lateral directions can be specified. Wind forces are defined through directional dependent quadratic wind force coefficients.

Current speed and direction are assumed to be constant with time. A current profile can be included by specifying velocity and direction at a set of water levels. Current forces can be defined through directional dependent force coefficients.

Hydrodynamic Modelling - Large Bodies

Figure 4. Installation of SNORRE CFT's.

Hydrodynamic loads of bodies may be described by frequency-dependent coefficients.

1st order wave exciting forces are described by linear transfer functions between force amplitude and wave amplitude. Time series are obtained by multiplying interpolated transfer functions with the wave spectrum.

According to linear potential theory, a large-volume body in a fluid with free surface can be described by frequency-dependent added mass and damping, which are transferred to the time domain.

Hydrodynamic interaction between bodies can be included.

Diagonal elements of the 2nd order wave drift force coefficients or full 2nd order transfer functions may be given as functions of wave frequency and direction. Time series of slowly varying drift forces are calculated by a method according to Newman.

Hydrodynamic Modelling - Small Bodies

For small bodies viscous hydrodynamic forces can be calculated according to Morison's formula. Position dependent added mass, linear and quadratic drag coefficients and buoyancy can be used when the body is near the surface or bottom. Slamming forces are also included.

Damping

Damping forces are often the most uncertain parameter when modelling marine operations, and flexible selection of models are available.

A body can be modelled with linear and/or quadratic drag coefficients as a function of direction relative to the body.
Linear and/or quadratic damping can be specified in any degree of freedom.
A simplified wave drift damping model is available.
A Strip model using Morison's equation is available.

Positioning Systems

Anchor lines or hawsers may be specified by a general tension elongation relationship. Hysteresis may be included. It is also possible to describe the anchor line by any number of segments, buoys, and clump-weights. The line characteristics will then by calculated.

Thrusters may produce constant forces or may be controlled by a dynamic positioning module (based on Kalman filtering). This module resembles systems being used in practice onboard offshore vessels.

Mechanical Coupling

Couplings between bodies may be described by a general tension-elongation relationship. Hysteresis may be included. It is also possible to model crane wire and slings. The crane wire length may be varied during the simulation, updating the axial stiffness accordingly. Varous contact elements can be modelled such as fenders, bumbers, guide posts and docking cones.

Future Extensions

The program is designed with flexibility in selection of force models, and new models will be added as the further development takes place. Examples of future extensions are: