Rapid Pressure Cycle Effects On Flexible Pipe
Authors: Hill R.T. , Upchurch John L. , McMahan James  M. 
Published: October 2008

The use of subsea satellite wells tied back to a central manifold unit is a field development concept currently being used by operating companies for staged production of eithercommingled oil or gas. Remote plat form operated control systems that couple the satellite wells and manifold require that safe operating pressure cycle parameters beestablished for all subsea components. Because of start-up and shut-in procedures, extreme pressure variations in the form of rapid pressurization and depressurization must be considered.

This paper describes the test procedures, equipment and results specific to the evaluationof high pressure non-bonded flexible pipe used for subsea production jumpers between satellite wells and manifold system. Recommendation of safe rates of pressurization and depressurization are included.

INTRODUCTION

The Phillips Petroleum Company Garden Banks 70/71 "SeaStar" Development consists of up to four (4) subsea satellite gas wells producing into a 4-slot subsea commingled manifold. The commingled gas production is then delivered viadual subsea pipelines to a host platform processing facility some thirteen (13) miles away in Vermilion Block 386B, offshore Louisiana. An existing subsea sales gas pipeline system then transports the process gas to shore. The gas wells and manifold unit are in 780 feet of water and the platform is in 350 feet of water. A layout of the development is shown in Figure 1.

Multiple lines consisting of dual 6-in flowlines and 4 × 2 in methanol and 1 × 2-in service lines were used to connect the manifold to theplatform. Additionally, an electro-hydraulic control umbilical consisting of twin electric cables and 8 (lxl/2 in, 7x3/8 in) hydraulic lines was installed to provide functional commands to both the manifold and well systems, Tie-in of each well to the manifold using spool piece jumpers. was accomplished.

A 4-in, 10,000 psi design pressure non-bonded flexible pipe was supplied as a 150-ft long production jumper between each satellite well and the manifold unit. To assess what effectsrapid pressure cycling may have on the structure of the flexible pipe, such as collapse ofthe internal carcass, excess pressure elongation or even pull-out of the end fitting, a series ofpressure cycle tests were conducted at H. O. Mohr Research & Engineering, Inc. A 20-ftsection of the 4-in flexible pipe, complete with end fittings and flange connectors identical to the actual pipe supply was used for the evaluation. A drawing of the non-bonded flexible pipe, with each individual layer identified, is shown in Figure 2, with a typical end fitting flanged connector shown in Figure 3.

Description of Test Equipment

The fixture used for the rapid pressure cycle test consisted of a pipe sleeve assembly that was compatible with the 20 ft long flexible pipe, complete with 4 1/16 in, 10,000 psi API 6BXconnectors with blind flanges. One end of the test pipe was bolted to a welded end cap on the sleeve. The purpose of the sleeve was to provide safety containment, add weight to assist in the straightening of the pipe and act as a reference point for extension measurement.