New Technology In Insulateo Offshore Pipelines - Design And Installation

ABSTRACT A new insulation system for offshore pipelines has been designed and installed as part of a new field located in the Arabian Gulf. This system of insulation will be used to prevent the formation of hydrates in dual flowlines transporting high temperature crude oil between six wells and a central production platform. The insulated pipeline system, designed and installed as described in this paper, is applicable to future design of insulated offshore pipelines. The system is designed to operate at a constant temperature of 200-deg F with excursions to 225-deg F at a water depth of 50-ft. Over a period of two years, 4-in., 6-in., and 8-in. pipelines were designed through analysis and testing. The testing included field joint feasibility and reliability, flexure of insulated pipe joints at midspan and field joints, and full scale laybarge tension loading of insulated pipe joints. From this design program an insulation system consisting of polyurethane foam, polyethylene jacket, heat shrinkable polymer waterstops and field joint materials, and weight coating was developed. The insulated pipelines were installed by the laybarge method in the Fall of 1978. In this paper the development and installation of the insulated pipeline system are described. General results and descriptions of the field joint, flexure and tension testing programs are given. A description of the method of installation, and its analysis and the actual field installation are also included. INTRODUCTION As the development of the field was being studied, a system of satellite wells connected to a central production facility was determined to be optimum. The possibility of hydrates forming in the crude oil in the flowlines before the oil was processed loomed as a major question to the development plan. Providing two insulated flowlines to each well, one for production flow and the other for startup circulation, to maintain the high crude oil temperatures was the answer to the process scheme for the field. The ability to safely install an insulated pipeline system in a 50-ft water depth became a key to field development. Table 1 lists the pipeline requirements that are the basis of the insulated pipeline design. The pipe diameters were determined by flow requirements and the wall thickness by a shut-in pressure of 3,000 psi and corrosion allowances. The crude oil to be transported contains a high concentration of hydrogen sulfide which dictated the metallurgy, material grade and corrosion allowance for the pipe. Insulation requirements determined by the process scheme are 3-in. of polyurethane foam with a maximum thermal conductivity of 0.20 (BTU-in)/(ft2-hr-deg F). Maximum design operating temperature for the pipeline system was established as 200-deg F with excursions to 225-deg F. Also included in Table 1 are the on-bottom stability concrete coating requirements which were determined from 100-year storm conditions acting on the pipelines, coated with 3-in. of polyurethane foam, resting on the sea floor. As can be seen from the table, the concrete requirements were large to compensate for the buoyant effect of the insulation.