This paper describes methods for evaluating and comparing motion characteristics, structural integrity, and stability of semi submersible drilling vessels. A computer program for predicting semi submersible motions is described and evidence of its validity is presented. The range of heave values calculated for nine different rig designs is shown for both regular and irregular waves. A method for analyzing stresses in irregular seas and a computer program for evaluating semi submersible stability are described.


Any semi submersible drilling vessel design is necessarily a compromise among many design considerations. Not everyone will agree upon the best compromise among often conflicting requirements, as evidenced by the fact that semi submersibles presently under construction or built since 1972 have at least 16 significantly different designs.

What are some of these design considerations? A major operating consideration is for the vessel to have minimum motions in waves to provide a good drilling platform. Another consideration is mobility, since semi submersible drilling vessels are used primarily for exploratory drilling.

Factors influencing mobility are the vessel's overall towing resistance and whether or not the vessel is self-propelled or propulsion assisted. Other design considerations include load carrying capacity, maximum deck load, and storage capacities. These items influence the operating flexibility of the vessel and the frequency with which it must be recompiled. Some rigs have had ease of fabrication and reduced steel weight as major design goals. These latter two goals are part of what must always be a major design consideration, cost, since there is a limit to how much an operator can pay for the rig. No design is acceptable, however, unless it meets minimum safety requirements. Two of these requirements are adequate structural integrity and adequate stability of the vessel (Stability here refers to vessel's ability to resist capsizing due to overturning moments such as those from wind forces).

It is evaluation of motions, structural integrity, and stability of semi submersibles that will be discussed in this paper. Other design considerations were mentioned because they are interrelated with these, but it is beyond the scope of this paper to discuss them in detail.


The motion characteristics of a semi submersible can be evaluated in three ways:

  1. observe the motions of a prototype in the field,

  2. conduct model tests in a wave tank, and

  3. predict the motion characteristics analytically using a computer.

All three should be and have been used, but the latter method has definite advantages in time, cost and flexibility. An added benefit of using a computer to calculate motions is that the dynamic forces on the structure are also calculated and these can be used in the structural evaluation of the vessel.

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