Introduction to Offshore Structures - Offshore Structures: Design, Construction and Maintenance - page 4

At depths of 300 m (1000 ft), floating drilling units require special mooring
arrangements or a dynamic positioning system. A floating semisubmersible
drilling rig is capable of operating in water as deep as 900

- 1200 m (3000 -

4000 ft).
Exploratory drilling work follows the discovery well. This generally
requires three to six wells drilled at selected points of a reservoir. These activ-
ities and production testing of the wells where oil and gas are encountered give
reasonably detailed information about the size, depth, extent and topography of
a reservoir, such as the fault lines, impermeable layers, etc., and its recoverable
reserves, viscosity (API grade), liquid properties (e.g., the oil/water ratio), and
impurities, such as sulfur or another critical component.
Reservoir information enables geologists and geophysicists to estimate the
location and number of wells that will be required to produce a field and the
volumes of oil, gas and water production. This information is used to determine
the type of production equipment, facilities and the transport system needed to
produce the field.
Obviously, the accuracy of reservoir data has a major effect on the selection
of a field-development concept. In marginal or complex reservoirs, reliable
reservoir data and the flexibility of the production system in accommodating
changes from the reservoir appraisal are very desirable.

1.3.1 Field-Development Cost
Field development for a new project or for extending existing facilities is a multi-
step process. The first step is gathering input parameters, such as the reservoir
and environmental data; the selection and design of major system components,
such as the production drilling and the wells, facilities and offtake system; and
the decision criteria, such as the economics. The next step is evaluating the
different field-development options that satisfy the input requirements and
establishing their relative merits with respect to the decision criteria. In this
design process loop, not only alternatives for field-development systems, but
also alternatives for each system, need to be taken into account.
At the next stage, a preliminary design for the selected system is started. In
this phase, the selection activity is focused on the system components and detail
elements. During this phase, design iterations are generated until all the members
of all engineering and operation disciplines are satisfied from a technical point of
view. All the system components and construction activities must be well defined.
Once the design is complete, few changes to the system and its components can be
made without suffering delays and cost overruns.
The operation phase includes maintenance, production, repair and reassess-
ment and transportation activities. Viable field-development options are identi-
fied and developed and selection of the most suitable option occurs in parallel
with the acquire, explore and appraise cycle. All project activities that precede
the start of the basic design phase are called the FEED (front-end engineering