System Design for Buried, High Temperature and Pressure Pipelines

Abstract

Abstract Buried pipelines operating at high temperatures and pressures experience extreme compressive loads due to the axial restraint of the soil. The high axial forces combined with imperfections in the seabed may overstress the pipeline or result in upheaval buckling. Typically, expansion loops or dog legs are placed to protect the risers from expansion and to alleviate the axial force. Expansion loops, however, are virtually ineffective when buried due to the lateral restraint of the soil. Alternative methods to reduce expansion may increase the potential of upheaval buckling or overstressing the pipeline. Therefore, a system design must be performed considering the issues of expansion and upheaval buckling together. This paper discusses methods of prevention and control of expansion and upheaval buckling, evaluating the impact on the overall system. A design and construction process is presented that illustrates the steps taken to the final design which utilized buried, enclosed expansion loops to resolve both the expansion and upheaval buckling issues. Introduction A pipeline will expand or contract when temperature and pressure vary from the conditions at the time the pipeline was installed. Therefore, the conditions during construction are the reference temperature and pressure. For the purposes of this paper, the discussion will be limited to expansion; however, Similar Issues may require addressing for contraction. As the pipeline expands, it will follow the path of least resistance, which leads directly to the risers. If the lower riser span is not sufficiently flexible to absorb the expansion within the permissible stress limit, expansion loops or dog legs are typically placed Just before each riser. The expansion loops or dog legs do not function properly when buried and thus overstress at the first bend experiencing the pipeline expansion. There are several methods readily available to reduce the expansion in the pipeline; however, they result in the build-up of axial force in the pipeline leading to upheaval buckling. Upheaval buckling is a mode of failure in trenched and buried pipelines. Pipelines resting on the seabed may also buckle, but they tend to buckle laterally as there is no significant lateral resistance on them. Upheaval buckling results from the axial force generated from the expanding pipeline combined with an uneven trench profile. The trench profile may be uneven due to the trenching process, undulations in the seabed, a rock formation, or an area of denser soil. The trench unevenness is generically referred to as an imperfection. The axial force and imperfection are related. In general terms, the more uneven the trench profile, the lower the axial force required to produce an upheaval buckle. This paper discusses methods to control expansion and upheaval buckling, evaluating their impact on the overall system. From this discussion a design and construction process is presented that provides a system solution. Control of Expansion and Upheaval Buckling Methods to control expansion and upheaval buckling were investigated for the design of a high pressure and temperature, buried sour gas flowline offshore Mobile Bay, AL for a major operator. The methods considered are presented below. Expansion. There are several conventional methods utilized for handling the expansion.