Page 61 - Hydrocarbon Exploration and Production Second Edition

48 Rig Types and Rig Selection

be determined. Wellhead locations, well design and trajectory are aimed at
minimising the combined costs of well construction and seabed/surface facilities,
whilst maximising production.
The accuracy of the parameters used in the well planning process will depend on
the knowledge of the field or the region. Particularly during exploration drilling
and the early stages of field development considerable uncertainty in subsurface data
will prevail. It is important that the uncertainties are clearly spelled out and
preferably quantified. Potential risks and problems expected or already encountered
in offset wells (earlier wells drilled in the area) should be incorporated into the design
of the planned well. This is often achieved by using a decision tree approach in the
well planning phase. The optimum well design balances risk, uncertainty and cost
with overall project value.
The basis for the well design is captured in a comprehensive document. This is
then ‘translated’ into a drilling programme.
In summary, the well engineer will be able to design and cost the well in detail
using the information obtained from the petroleum engineers, geoscientists and
production engineers. In particular, he will plan the setting depth and ratings for the
various casing strings, cementing programme, mud weights and mud types required
during drilling, and select an appropriate rig and related hardware, for example drill
bits.
The following sections will explain, in more detail, the terms introduced so far.

4.2. Rig Types and Rig Selection

The type of rig which will be selected depends upon a number of parameters,
in particular:

cost and availability
water depth of location (offshore)
mobility/transportability (onshore)
depth of target zone and expected formation pressures
prevailing weather/metocean conditions in the area of operation
experience of the drilling crew (in particular the safety record!).

The following types of rig can be contracted for offshore drilling (Figure 4.1).
Swamp barges operate in very shallow water (less than 20 ft). They can be towed
onto location and are then ballasted so that they ‘sit on bottom’. The drilling unit is
mounted onto the barge. This type of unit is used in the swamp areas of, for
example Nigeria, Venezuela and US Gulf Coast.
Drilling jackets are small steel platform structures which are used in areas of shallow
and calm water. A number of wells may be drilled from one jacket. If a jacket is too
small to accommodate a drilling operation, a jack-up rig (see Figure 4.2)is usually
cantilevered over the jacket and the operation carried out from there. Once a viable
development has been proven, it is extremely cost-effective to build and operate
jackets in a shallow sea environment. In particular, they allow a flexible and step-wise