Design and Analysis of High Temperature, Thermally Insulated, Pipe-in-Pipe Risers

Abstract

Abstract Bottom tow pipeline bundle installations are gaining acceptance in deepwater developments. This is particularly the case where paraffins and hydrates pose a problem, since insulation and heating lines may easily be added inside the buoyancy casing, with less cost impact than in vessel based methods. However, barriers to more extensive use of the bottom towed bundle technique are perceived problems of potential leakage into the outer casing and perceived problems related to the pressurization of the casing. Approaches to mitigation of such problems, such as Bulk heading and foam filling are proposed and evaluated herein. Introduction Deepwater developments currently underway are increasingly requiring either insulation or heating to prevent hydrate formation or waxing in flowlines traversing cold ambient waters. Flowlines may be insulated either with directly applied insulation, pipe-in-pipe insulation, or insulation wraps within cased bundles. Heating may be achieved utilizing heating water in separate lines or annular jackets, or electrically, utilizing skin effect current transfer or other means. Possible installation methods, already in use or imminent, include reeling, J-lay, S-lay or towing of bundled pipelines. In a towed installation, the pipelines and casing are welded into strings on a beach. To date, all Gulf of Mexico bottom towed bundles have been fabricated at Matagorda Peninsula, just down the coast from the mouth of the Colorado River. Thus site accommodates bundles up to 10 miles long. The bundle is then deflected laterally off of the beach and towed to the site. Pipelines may be towed on the sea dace, at mid- depth, or on bottom. All the Gulf of Mexico bundled installations have been by bottom tow.@ The bundled approach to installation of multiple pipelines, is to gather the several pipelines into a single package, and to provide a casing around the entire package which brings the total bundle submerged weight into a desired range. For a bottom towed bundle this submerged weight must be great enough to provide stability during the launch and tow process but low enough to permit towing by available vessels. The casing diameter required for correct submerged weight is normally sufficiently large such that an insulating wrap layer may be added to the flowlines without change in the design of the bundle, and the low density of insulating materials makes only a modest change in the submerged weight. In shallow water depths the casing may resist hydrostatic collapse through its inherent stiffness, however, as water depths increase the increase in wall thickness required to prevent buckling makes it impossible to achieve a desired submerged weight.