The concept of "defence tines" may be used to illustrate the safety barriers for mobile units.

Defence line 1 is characterized by the structural concept, the stress level, the number and properties of stress concentrations, the material properties, the fabrication standard, supervision during fabrication etc In other words, the inherent standard or quality of the as-built and delivered unit.

Defence line 2 is characterized by the quality of in-service inspection and control, as well as the general up-keep of the structure and its equipment.

Defence line 3 is characterized by the capability of the unit to withstand credible failures of vital stractural elements, whether due to internal causes (eg. fatigue cracks progressing too far) or external causes (eg. damage due to accidental loads).

Defence line 4 is characterized by the capability of the unit to provide survival possibilities to the personnel on board when a grave accident has taken place, this is primarily a matter of giving ample time for rescue of people.

The paper will discuss these defence lines with particular reference to semisubmersibles and, moreover, provide information on items of particular importance for such mobile units.

As it turns out, dramatic changes in current designs are not involved, but rather that of filling some important "holes" in the established practice of design and in-service routines and tasks.

Statistical data for the period 1970–80 will be given for all types of mobile units.

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