Steel quantity and cost comparison of modular construction options for sea-transported pipe racks

Abstract

This thesis is aimed at finding the most cost effective way of executing Modular Execution Strategy (MES) for building pipe racks of a project that an engineering company Fluor B.V. is currently executing in Kuwait. A pipe rack is a steel structure which is constructed to efficiently place and support multiple levels of pipelines for industrial plants such as refinery plants, chemical plants or power plants.

The Modular Execution Strategy aims at relocating parts of fabrication and assembly activities of a pipe rack construction to potentially low cost locations at which the conditions for fabrication and assembly activities are more favorable. The pre-assembled pipe racks will be transported to the onshore installation site by a vessel, which results in sea-transport design requirements (due to vessel motions) in addition to the in-place design.

Three options of different configuration for MES were considered. The first option is to transport only upper parts of the pipe racks without their bottom columns and assemble the bottom columns at the installation site. The second option is to transport the complete pipe racks including bottom columns which are stiffened by temporary bracings. The last option is to transport complete pipe racks with strengthened columns having a larger profile dimensions.

In order to consider various sizes of pipe racks, 27-representative configurations of pipe racks of the project were selected. These pipe racks were designed to withstand in-place loadings and sea-transport loadings with a quasi-static analysis method. The in-place loadings are weight of pipe lines and wind force. The sea-transport loadings are forces due to motions of a vessel and critical sea-transport loadings come from roll + heave and pitch + heave. Quantities of steel for each option were found after completion of the design. Subsequently, the quantities were translated into steel work cost which includes procurement, fabrication, assembly and installation costs of steel work.

As a conclusion, it was found that considering the quantities and costs of steel work for the project, option 1 (transport the pipe racks without columns) is the most cost effective solution. If pinned supports are used at the vessel deck, which are more favorable for the company, it was calculated that option 1 requires, on average, 15% and 30% less cost than option 2 and option 3 respectively. For clamped supported conditions, option 1 still requires 15% less cost than both option 2 and option 3.

Furthermore, it was demonstrated by performing a resonance check and a dynamic analysis for a tall two-dimensional frame, that a quasi-static analysis method could be used to assess the sea-transport loadings. It was found that there is very low possibility of resonance and only low dynamic amplification.

In this thesis, the focus has been on differences in the structural configurations. Other aspects, some of which may be difficult to express in cost terms such as logistical difficulty, safety/risk, and project schedule, were not taken into account. Therefore, in order to verify the attractiveness of each option in more detail, it is suggested to also make a complete assessment of those mentioned aspects.