Pipe-in-Pipe Global Buckling and Trawl Design on Uneven Seabed

摘要

The use of Pipe-in-Pipe (PIP) systems is emerging for Norwegianoffshore projects in the North Sea, the last three years. The designmethods of these projects, as applied for pipeline expansion and trawlgear interaction, appear to differ. The installed and planned reeled PIPsystems typically have an OD smaller than 15” for the carrier pipe, andmany of these are installed in areas where trawl gear with weights of upto 6-7 tons is used by the fishing industry. This could result in burial orrock covering in order to protect the PIP system from excessive loads.This paper describes the design process of a typical 9”ID/15”OD Pipein-Pipe (PIP) system, installed on an uneven seabed and exposed toglobal buckling and heavy/large trawl gear. The free span assessment isfurthermore a vital part of the design, especially for the operationalphase. The expansion design is based on recommended practice DNVRP-F110. This paper describes the typical design challenges of a longPIP system (installed length more than 10 km), which is installed on anuneven seabed. The Marintek Software SIMLA is used for analyzingthe design, determining the behavior of full PIP system in a singlemodel. The mechanical behavior of the PIP system is analyzed in arobust manner by FE modeling, using a number of techniques.Long pipelines are typically divided into a number of overlappingsections for analysis. More design confidence is gained analyzing thecomplete pipeline route in a single model, including the varying pipesoilproperties, rock intervention work and taking into account thecomplex PIP model, rather than modeling the PIP system section bysection. The observations of this paper should provide a suitablereference for other projects designing HT/HP PIP systems.