Detailed-estimate designs

When an oil or gas field has just been discovered, the quality of the information available about the well stream may be sparse, and the amount of detail put into the process design should reflect this. However, early models of the process along with broad cost estimates are needed to progress, and both design detail and cost ranges narrow as projects develop through the feasibility study and field development planning phases (see Section 12.0 for a description of project phases). 

The accuracy of a fixed capital estimate tends to be a function of the design effort involved. As the project definition is refined, the estimates evolve from the various preliminary phases, ie, order of magnitude, predesign, factor estimates, etc, into the more detailed estimates used for budget authorization, project control, and contracts. At the same time, the uncertainty in the estimate decreases from 50% to as Htfle as 5%. 

With more and better information regarding the process and layout plans, estimating engineers can prepare detailed estimates which are often quite accurate, usually 10 percent for the best. It is the duty of the process designer to supply the best information in order to contribute to better or improved estimates. 

The terminology will not be repeated here unless the interpretation must be supplemented or modified. The design details will serve as an aid for estimating operating characteristics and not as a final basis for design. Neerken, Lapina, and Fullemann are useful references. 

The accuracy of an estimate depends on the amount of design detail available the accuracy of the cost data available and the time spent on preparing the estimate. In the early stages of a project only an approximate estimate will be required, and justified, by the amount of information by then developed. 

Preliminary (approximate) estimates, accuracy typically 30 per cent, which are used in initial feasibility studies and to make coarse choices between design alternatives. They are based on limited cost data and design detail. 

A document that indicates cleany what is provided in terms of technology, quality, schedule, and cost A basis in enough detail to be used in controlhng the project and its costs to permit proper evaluation of any proposed changes A device to permit subsequent evaluation of the performance compared to the intended performance A document to control the detailed estimate for the final design, construction, and design... 

ASPEN acquired ICARUS in 2000 and developed Process Evaluator based on Questimate that is used for conceptual design, known as front-end loading (FEL). More information on FEE and valueimproving process (VIP) is found later in Sec. 9. Basic and detailed estimates are coupled with a business decision framework in ASPEN-TECH ICARUS 2000. 

Many companies have developed their own factored estimates using computer spreadsheets based upon their in-house experience and cost database information that they have built from company project history. For detailed estimates, the job is outsourced to design-construction companies that have the staff to perform those estimates. 

If the results of the preliminary design show that further work is justified, a detailed-estimate design may be developed. In this type of design, the COSt-and-profit potential of an established process is determined by detailed analyses and calculations. However, exact specifications are not given for the equipment, and drafting-room work is minimized. 

When the detailed-estimate design indicates that the proposed project should be a commercial success, the final step before developing construction plans for the plant is the preparation of a firm process design. Complete specifications are presented for all components of the plant, and accurate costs based on quoted prices are obtained. The firm process design includes blueprints and sufficient information to permit immediate development of the final plans for constructing the plant. 

The preliminary design and the process-development work gives the results necessary for a detailed-estimate design. The following factors should be... 

Factored estimates, especially when used for repeat plants, can be accurate and adequate for appropriation purposes but don t provide the information required for cost and progress tracking. They should be followed as soon as possible with more detailed estimates containing sufficient and suitable information for tracking. On the other hand, the detailed estimates normally prepared by engineering contractors after a certain amount of design work are time consuming and expensive and should be used sparsely. 

While a detailed estimate could not be prepared without the benefit of some detailed engineering, a good semi-detailed estimate can be prepared with the information available at the completion of the Phase 1 design. 

When applied to Phase 1 designs (complete P lD s and arrangement drawings), the quality of the resulting estimates could be as good as the conventional detailed estimates normally prepared by contractors at much greater expense. 

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