Introduction to Offshore Structures - Offshore Structures: Design, Construction and Maintenance - page 5

design) phase. FEED is the most important phase of a field-development
timeline. An ideal field-development schedule should allow sufficient lead-time
for performance of all FEED work before basic design starts.
In the FEED phase, after the facilities required on the topside of the deck
have been defined, the geometrical shape of the platform is defined. A prelimin-
ary structure analysis is run through structure analysis software to identify the
section that matches the loads and the geometrical shape with appropriate deck
dimensions that serve the required facilities on the deck. The structure system of
the platform subsea structure, which is called a jacket, and a construction
method based on the water depth are selected in this phase. In management
of a new project, this is called the select phase.
Experience shows that the FEED phase will identify viable options;
develop, evaluate and select concepts; and provide a conceptual design. The
FEED phase usually consumes only about 2 3% of the total installed cost

(TIC) of field development and has the highest impact on cost, schedule, quality
and success. It is not uncommon to observe major cost overruns when a full
FEED phase is not performed. Reanalysis of projects that did not have a satis-
factory FEED phase because of political factors indicates that a 50% TIC reduc-
tion could have been achieved if a satisfactory FEED phase had been
performed.
Figure 1.1 shows the variation in the accuracy of the TIC estimates in dif-
ferent phases of a project. In general, understanding of the economics and other
features of a field-development system improve as we move along the field-
development timeline. At the start of the FEED phase, a number of options
are available and identifying the right field-development concept will pro-
foundly influence a project

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s success. During the conceptual design phase of
FEED, each system component, such as well systems, platform(s), topside facil-
ities, transportation and their subcomponents, such as hull, the mooring system,
tethers, living quarters, processes, utility systems, pipelines, storage and risers,
are defined and a cost and schedule estimate is prepared. Selection and defini-
tion of the system components and subcomponents also have a significant
impact on costs and/or the schedule. At this phase, the accuracy of the TIC esti-
mates is approximately

25%.
Experience shows that a standard project with routine components and
almost no innovations will experience very low operability problems. A recent
survey by Karsan (2000) of offshore projects indicated that 90% of projects
with substantial innovations had major operability problems; therefore, care
must be taken not to introduce low value when adding new ideas and
components into a field-development system.
The preliminary or basic design phase includes a definition of the process
through process flow diagrams (PFDs) and preparation of the field and equip-
ment layouts: piping and instrumentation diagrams (P

± 15% to ±

IDs), general platform
drawings, materials and equipment lists, data sheets, specifications and a
final engineering, procurement and construction (EPC) cost and schedule

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