Evaluation of Semisubmersible Drilling Vessels
Authors: Stanton P.N. , Kuang J.G. 
Published: December 2007

Evaluation of motion characteristics, structural integrity, and stability of semisubmersible drilling vessels can be done in several ways. Computer programs for predicting semisubmersible motions and for evaluating semisubmersible stability have been developed. Methods for calculating the range of heave values in regular and irregular waves, and for analyzing stresses in irregular seas are presented.

Introduction
This paper describes methods for evaluating and comparing motion characteristics, structural integrity, and stability of semisubmersible drilling vessels. A computer program for predicting semisubmersible motion is presented, along with evidence of its validity. 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 semisubmersible stability is described.

Design Considerations
Any semisubmersible drilling vessel is necessarily a compromise between many design considerations. Not everyone will agree on the best compromise among conflicting requirements, as evidenced by the fact that semisubmersibles 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 that the vessel have minimum motion in waves to provide a good drilling platform. Another consideration is mobility, since platform. Another consideration is mobility, since semisubmersible drilling vessels are used primarily for exploratory drilling. Factors influencing mobility are the vessel's over-all towing resistance and whether the vessel is self-propelled or assisted by propulsion. 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 resupplied. Some rigs have had ease of fabrication and reduced steel weight as major design goals. These 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 a rig. No design is acceptable, however, unless it meets minimum safety requirements. Two of these requirements are adequate structural integrity and adequate vessel stability. (Stability here refers to the vessel's ability to resist capsizing caused by overturning moments such as those from wind forces.)
Evaluation of motions, structural integrity, and stability of semisubmersibles will be discussed. Other design considerations are mentioned because they are interrelated with these; but it is beyond the scope of this paper to discuss them in detail.

Motions
The motion characteristics of a semisubmersible vessel can be evaluated in three ways:observe the motions of a prototype in the field,conduct model tests in a wave tank, andpredict analytically the motion characteristics with a computer.
All three should be and have been used, but the third method has definite advantages in time, cost, and flexibility. An added benefit of using a computer to calculate motions is that the dynamic forces in the structure are also calculated and can be used in the structural evaluation of the vessel.
JPT
P. 422