Convection Heat Losses through Installation Gaps between Pipe and Insulation and between Insulation Half Shells
Authors: Wang Xuegeng , Chin Y. Doreen , Perera Ravi M. , Prescott C. Neal , Raborn Gene O. 
Published: October 2008

Abstract
After field production data indicated that a recently installed insulated deepwater pipe-in-pipe (PIP) flowline with pressurized nitrogen gas in the annulus did not perform as designed, tests were conducted to investigate the thermal performance of the PIP. Pre-cast, half C-shell, rigid, open-cell polyurethane foam insulation was used in the PIP with the annulus filled with pressurized nitrogen gas. The thermal test results showed that extensive heat losses were due to the gaps in the insulation system, which exist because of the method of insulation application. This paper presents the numerical simulation results of the PIP heat transfer and the effects of gap sizes. Comparisons with test data are also presented.

Introduction
Long distance subsea tiebacks offer cost saving alternatives to using surface facilities for deepwater field development. The produced hydrocarbons are transported through flowlines to a shallow water facility for processing. As the flowline length increases, product delivery temperature can drop below the hydrate or wax formation temperature. Insulating the flowline is one effective way of reducing the risk of flowline blockage by preserving the heat and maintaining the delivery product temperature above the hydrate or wax formation temperature. An individual flowline (PIP) or a group of flowlines (bundles) may be wrapped with insulating materials and placed inside a casing pipe. Depending on the installation methods, the casing pipe of an insulated PIP or bundles may be designed to contain air at standard pressure or nitrogen gas at elevated pressure to resist external hydrostatic pressure.
A recent Gulf of Mexico field development project involved two 14 mile insulated PIP flowlines. Each 14-mile line was installed in two 7-mile sections and connected with a midline jumper. The 10-inch flowline was insulated with 3-inch polyurethane foam and encased in a 24-inch casing. The annulus of the PIP was pressurized with nitrogen gas to 1,475 psig to resist external hydrostatic pressure up to 3,300 ft of water depth. To prevent insulation material collapse under nitrogen pressure, rigid open-cell polyurethane foam was cast into C-sections and strapped around the flowline. After the flowlines were commissioned, it was observed that the delivery temperature at the host platform was lower than design estimate. Although delivery temperature depends on many factors, i.e., wellhead temperature, ambient temperature, heat loss through jumpers and sled piping, etc., flowline insulation performance was believed to be a key factor.
A mock up 100-foot section of the insulated PIP was tested to evaluate its thermal performance. The annulus of the PIP was pressurized with nitrogen gas to various pressures. The test section was immersed in the water and the flowline was heated through hot water circulation. After the temperature in the flowline was stabilized, the water circulation was shutoff and the flowline was allowed to cool down for approximately 40 hours. The temperatures in the flowline and ambient were recorded during the cool down. Data reductions were then performed to determine the PIP overall heat transfer coefficients (OHTC) at each nitrogen pressure.