Page 126 - Drilling Technology in Nontechnical Language

Chapter 5 – RIG SELECTION AND RIG EQUIPMENT 117

The limit of water depth for a semisubmersible rig is dictated by the
amount of riser pipe that can be carried or run or by the capability of the
anchoring system if the rig is anchored.

A semisubmersible rig (also called a semisub rig) might cost between
$300,000 and $400,000 per day, depending on the age, equipment, and
area of operation.

Drillship

A drillship has a ship-shaped hull. Approximately centrally located
is a derrick, under which is a large hole through the hull. This is called
the moonpool.
Drillships vary in size, but the biggest can carry everything needed
to drill fairly deep holes in fairly deep water without resupply. They can
be moved quickly between locations. Drillships are often positioned
dynamically over the well rather than being anchored in place.

Drillships can be expensive to hire! Daily expense can range
from $230,000 for an old one to more than $400,000 for a modern
DP drillship.

Drilling tender

A drilling tender has a ship- or barge-shaped hull containing the
accommodation and all equipment except the derrick, BOP, and ancillary
equipment. The tender is moored against a platform, and the derrick is
installed on the platform deck. Cables and hoses are run from the tender to

the rig to provide power, drilling fluid, compressed air, communications,

and control systems, etc. (fig. 5–7).
A ramp can be suspended from the platform, using cables from posts
on the platform to the end of the ramp. To transfer personnel between the
platform and tender, it is necessary to climb a set of steps on the front of
the tender, step (or jump) onto the ramp, and then walk up the ramp stairs
to the platform. The ramp is called a widow maker with typical offshore
humor and some truth. Happily, widow makers are rare nowadays, and
better designs allow safe transit between the stationary platform and the

floating tender.

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