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142 3 Drilling into Geothermal Reservoirs
The calculations of pressure capacities and load capacities are done normally
according to the formulae given in API Bulletin 5C3. Here, API gives formulae
for ‘‘burst’’ (= internal yield pressure) and for four different cases of the D/t
relation (= nominal outside diameter of pipe divided by the nominal wall thickness)
for collapse calculations. As mentioned above, the respective yield strength is a
function of temperature. Typical yield strength reductions of different material
grades are shown in (Figure 3.18).
Because – particularly on long casing strings – the external and internal pressures
and loads change with depth very often, a combination of different wall thicknesses
and/or grades can be selected for a single string in order to find an optimum
solution of total string weights and cost. However, for all cases to be investigated the
calculated values of all expected load situations must be less than the corresponding
values of casing material selected.

3.6
Drilling a Well

After having finalized the planning process of the well the decision has to be made
as to what type of drilling contract is planned because – depending on the contract
type – the further steps may be different, particularly concerning the material
supply and the contracting of subcontractors for special services. There will be also
differences in the further project organization needed to proceed successfully.

3.6.1
Contract Types and Influence on Project Organization

Several contract types are common for the execution of deep drilling projects; four
typical contract types are described below.

3.6.1.1 Turnkey Contract
With a turnkey contract a lump sum is paid for the complete well, including all
materials, services, and energy which are necessary for drilling and completing the
well. This contract type may not be the cheapest solution because a certain risk
margin has to be granted to the contractor, but it is the easiest contract type for
the operator. All responsibilities (except the ‘‘geological risk’’) are relegated to the
contractor. On the other hand, the operator cannot order the contractor to do the
work in a manner the operator would like to have it done.
The operator is also responsible for the proper organization of all subcontractor
work.
The turnkey contract is mainly used when the operator is not experienced in
drilling wells.
It has to be pointed out that a turnkey contract for only one or two wells is a
significant risk for the contractor; the risk margin is therefore high and the target
to reduce cost might be difficult to achieve.