• Perform a stress analysis,
• Consider floating equilibrium,
• Account for ballast in compartments,
• Allow things to be flooded,
• Consider the force of the sea upon a manifold of different types of bodies,
• Allow for bodies to undergo large motions, and
• Consider the forces which connect bodies together.

Modeling structural elements is quite straight forward. A MOSES structural model is much like a SACS, STRUCAD, or STRUDL model. The elements are associated with a set of "intrinsic" load attributes. In other words, the basic properties of the elements are used to compute the weight of each element, the wind force, buoyancy and hydrodynamic forces. The only other thing to say here is that any hydrodynamic force computed for beams and plates will be computed using Morrisons' equation.

When one considers something like a barge, however, he needs another concept to consider the force of the water. Plate elements can also attract pressure, but they are assumed to act independently. The hull of a ship is a skin where the force is due to the elements acting together, or in concert. One models this skin as a collection of things we call panels. A panel is like a plate in that it must be a plane, but there the similarity ends. Panels attract hydrostatic and hydrodynamic pressure. In other words, the portion of the skin below the water will have a pressure on it due to the water. The integral of this pressure over the skin will be the force that the water exerts on the vessel. Statically, this is a relative simple process. Dynamically, however, it is quite a bit more complex. There are two ways that MOSES can compute hydrodynamic forces on a mesh: with a two dimensional approximation (Strip Theory), or with a three dimensional diffraction theory. While the details of what is required for a strip theory model differ a bit from the general theory, the basic result is the same. Normally, one can use strip theory for "ship like" vessels, but three dimensional diffraction should be used for semisubmersibles, spar structures, or things for which length and breadth are approximately equal.

You must ask MOSES to compute the hydrodynamic forces on a mesh. Before asking, put the vessel into a condition. The results you receive are strictly applicable only for this condition. What MOSES produces is a set of hydrodynamic pressures which give rise to added mass, radiation damping, and an excitation force. Additionally, MOSES uses the first order pressures to compute second order hydrodynamic forces. These second order forces are not important for many simulations, but for situations such as a moored vessel, they can be quite important.

Ballast in tanks is also accounted for with panels. In MOSES, a compartment is defined exactly as the exterior. When you instruct MOSES to put water into a tank, it produces pressure on the panels which define it. In general, interior water adds weight and inertia to the system. In addition to "putting ballast" into tanks, one can also flood them. Here, they fill up to the current water line, and increase the inertia of the system. In addition to tanks defined with a mesh of panels, one can also define "tube tanks" which are simply the inside of tubes. This allows one to simulate controlled flooding during a jacket uprighting process.

Unfortunately, we still need more ways to model other than that discussed above. These other things are lumped together into a category called load group attributes. Here, you can model concentrated areas, plane areas, concentrated buoyancy, tubes, weights with radii of gyration, things which allow wind and current forces to be defined as functions of the heading, and wind and current forces via the OCIMF formulae. For any of these things, hydrodynamic forces are computed with Morrisons' equation. Finally, viscous damping in roll is computed for a diffraction according to the method of Tanaka.

In summary, the following types of forces are produced by different element types:

• Radiation Damping - diffraction mesh
• Added Mass - diffraction mesh, any Morrison element
• Viscous Damping - Morrison elements, and "Tanaka" damping on diffraction meshes
• Buoyancy - any structural element, a diffraction mesh, and most load group attributes
• Wind Force - any structural element, and most load group attributes
• Weight - any structural element, and most load group attributes
• Second Order Wave Forces - a diffraction mesh

Finally, in the above where we speak of Viscous damping, we also get excitation due to wave velocity and current since the forces depend on the relative water/vessel velocity. These are really second order wave forces, but most people do not talk about them.

Perhaps the most important thing to remember is that MOSES does what you say to do. The variation in force along the model is computed according to the model you defined. MOSES itself makes no assumptions.