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Figure 6.13 - 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:

The first and second casing strings are cemented with cement returns to the

ocean floor.

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