The design water depth of recent jack-up drilling units have remarkably increased and it enables the compact semi-submersible drilling units to compete with these large jack-up rigs economically.

Taking consideration of this trend, we developed a compact semi-submersible drilling unit, named MD-I 02, which has the box type configuration to be different from he conventional semi-sub rigs; and we carried out the computer analysis and the model tank test to confirm the motion characteristics in waves.

As a result, it was clarified that the experimental motion characteristics have a good agreement with the calculated ones and the downtime of the design is almost the same as that for the other large semi-sub rigs in the comparatively moderate seas such as off South East Asia. It was also clarified that it is not possible to apply the added mass coefficient of heaving in infinite fluid to that for such special lower hulls and it is necessary to pay particular attention to it.


The design water depth of the recent jack-up type drilling units have remarkably increased and the jack-up rig for 400 ft water depth has been constructed.

The tendency of increase of operating water makes the building cost grow in geometrical progression. In other words, the water depth in such area is too deep for the jack-up rig to operate economically.

So, the operation of the competitive semi-submersible drilling unit with jack-up rig is able to be taken into consideration. However, it is too shallow for the large semi-sub rig to operate economically. It is therefore necessary to develop the compactly designed semi-sub rig which is not achieved by the reduced scale version of large semi-sub rig due to stability and performance.

We have developed the compactly designed semi-submersible drilling unit, to meet the marketability above-mentioned, which has the box type columns and lower hulls.

To confirm the motion characteristics in waves of this compact semi-submersible unit which has a different configuration from the conventional semi-sub, the model tank tests were carried out in the regular and irregular waves.

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