The concept of "defence lines" 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. 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 the capability of the unit to withstand credible failures of vital structural elements, whether due to internal causes (e.g. fatigue cracks progressing too far) or external causes (e.g. 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. 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. Introduction The history of mobile offshore units is about 25–30 years. The first units were used in the shallow waters of the Gulf of Mexico, and the design principles were largely based upon civil engineering experience and tradition. Until the end of the 60's, the design of the first generation mobile units was dominated by experience gained in the Gulf of Mexico. The second generation of mobile units was taken in use in the beginning of the 70's and considered explicitly for the first time hostile areas like the North Sea. Experience from the marine technological field was much better included than before. An example in this regard is the Aker H-3 design. The third generation of mobile units came about towards the end of the 70's and the beginning of the 80's, primarily as a consequence of needs to operate in deeper waters and with considerably increased pay load (3-4000 t). Examples in this regard are deep-water jack-ups 90 meters of water and semisubmersibles of the Paceserter and Ocean Ranger type. The most recent Norwegian requirement have brought about an extension of the third generation type of semisubmersibles. These units satisfy special requirements regarding among others intact and damaged stability, operational mooring and accidental loads. Examples in this regard are some Pacesetters, "Bingo", Aker H-3.2 and Gotaverken GVA 4000. About 500 mobile units are today in operation. Within two years this number will have been increased to 700. The fleet of today is dominated by jack-ups (60%), while the second largest group is semisubmersibles (25%). The fleet of jack-ups will grow somewhat stronger than semisubmersibles in the next few years. This paper focuses first of all on amended safety principles for the design of mobile units, as considered by VERITAS. The "Alexander L. Kielland" accident in the North Sea in March, 1980, and experience on a broad basis from hostile areas, has emphasized the need to reconsider safety standards and strategies to achieve a better standard. This reconsideration has brought about necessary improvements in our view, both by closing previous "open" areas and by focusing more explicitly on accidental, but still credible situations. Concept of Defence Lines For illustrative purposes, it has been found to employ the concept of "defence lines" in connection with safety considerations of mobile offshore units. For convenience four lines of defence will be used. The implication of these lines of defence will be discussed in detail below. First, however, some general remarks will be offered on the topic of safety. Safety represents certain margins against certain events or situations to occur. Basically one tries to prevent undesirable events leading to Damage to or loss of property Pollution of or damage to environment Damage to or loss of life. Obviously item (i) also represents directly situations when availability of an installation is diminished. This may occur from the fact that drilling, production to a structure, for example, necessities shutting down production, fully or partly. Item (ii) may also lead to hault in production. Moreover, it is normal that items (i) and (ii) have direct economic consequences, while loss of life is not so easy or acceptable to measure in economic terms. Basic links in the safety chain are – Standards (rules/regulations) – Design – Fabrication – Operation.