of semisubmersibles is stability. They are more frequently out of the range of motion-creating wave periods than are ship vessels. Therefore, their range of acceptable operating conditions is greater.
The special designs used for environment operating efficiency have some disadvantages:
• Semis have stricter deck-loading limits than drillships. It is not uncommon that semis are limited to 50% of the deck loads that could be carried
Fig. 16-14 Rig layout schematic (Courtesy Western Oceanic Inc.)
by drillships. The overall well plan such as pipe and mud requirements should be checked, • Since a semi is a specially designed vessel, it is generally much more expensive to operate daily due to higher initial required investments.
The advantages and disadvantages of drillships and semisubmersibles must be carefully evaluated prior to actual rig selection.
Rig mobilization costs must be considered when selecting marine rigs. These costs can be significant and greater than the differences in daily operating rates between comparable rigs. In many cases, the number of wells involved in the contract will influence the selection decision due to high initial mobilization costs.
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