Although the "modular approach" to plant construction has been utilized for offshore construction and for specialized "Vendor modules" in land based process plants it is only since the inflationary seventies that this technique has become an increasing1y attractive approach for many land based process plants. Remote plant locations such as the Arctic or offshore, skilled labour availability or Government policy will dictate the construction mode to be employed. The process plants we are concerned with do offer a choice and the benefits derived from modularization may not be obvious.
Offsite prefabrication and preassembly are concepts developed that benefit from a shop environment and are familiar to the Energy Industry. Every Construction project employs some combination of offsite construction techniques, whether consisting of prefabricated pipe components, control valve stations or prefabricated structural steel. Modular construction has evolved from the early concepts of prefabrication. In the following Sections we will examine the various parameters of modularization in more detail.
In order to determine if the modular approach is going to be cost effective, one should develop productivity factors for the plant site and the module fabrication shop. The cost breakdown in Fig.1 typical of several gas processing plants built by Delta Projects Inc. The values assigned to factors which affect productivity are averages for similar gas processing plants. The incremental productivity factor between field labour and fabrication shop labour can thus be applied against an estimated percentage of labour transferred to the module fabrication facility. Historically Delta's percentage for manhours transferred to the module shop from the field average from 25% to 30%.
Major Equipment costs vary little with either the Construction method or site location and can be considered a constant. Engineering and Expediting cost will increase with an increase in modularization. Since this cost, a 10% increase in the "Home Office" component is tolerable. Freight, which normally constitutes 1 ? % of the project may double in cost, but as with the Home Office labour component it is still a relatively small component of total job costs. Foundation costs will decrease s1ightly with a corresponding move toward modularization.
Field labour and plant site construction methods are the most sensitive components of plant cost It should therefore be closely scrutinized.
Labour productivity is a function of a number of variables, most of which can be analyzed and their impact predicted in advance.
To derive a factor for each variable being considered, it is necessary to divide each into sub-elements. The productivity impact for each sub-element is evaluted to estab1ish an overall factor for the variable. Once this is completed, the product of all variables will determine the productivity factors for the project.
Crew Interface Surveys indicate that on larger projects, between three and four hours per week per person are lost because of crew interfaces. These delays occur between interfacing groups of different crafts.