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Figure 2.6 - Semi-submersible platform HENRY GOODRICH
Where rapid mobility is critical, drillships have an edge
over semis. In calm seas, a ship's speed will be from 8 to 14 knots.
Most of the fully loaded self-propelled semis travel at less than 8
knots. For this reason, a drillship usually is preferable when
drilling only one or two wells in a remote area. However,
downtime due to vessel motion must also be considered.
EQUIPMENT
Wellheads and Casing
In many ways, a subsea wellhead performs much like a
land wellhead— it supports the blowout preventer stack, suspends
and supports the casing strings during cementing, and seals off the
annuli between the casing strings while drilling or production
operations are conducted. However, there are some important
differences between floating drilling and land or platform
operations. For instance, the following applies only to floating
drilling:
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