Troika: Flowline Thermal Performance and Remediation Options
Authors: Beckmann M.M. , Volkert B.C. , McMullen N.D. 
Published: October 2008

The Troika flowline system consists of two 10.75-in. outside diameter 14 mile long pipe-in-pipe insulated oil lines installed by the bottom tow method. The pipe-in-pipe configuration was selected to achieve the required flowline insulating properties. The thermal performance of the flowline system did not meet design expectations. As a result, the risk of hydrate formation at the onset of produced water may negatively affect operability of the flowlines. This paper addresses the evaluation and testing performed to identify the problem and the proposed plan to remediate the Troika flowline insulation system.

Introduction

Troika is a deepwater (2700-ft) oil development located in the Gulf of Mexico approximately 150 miles south of New Orleans, Louisiana. The reserves are being recovered through an 8-slot manifold cluster subsea production system in Green Canyon (GC) 200 tied back to the Bullwinkle platform in 1,350 ft. water depth in GC 65. Commingled flow from the wells is being produced to Bullwinkle through two 14-mile long 10.75-in. diameter pipe-in-pipe insulated flowlines. Well control is maintained with an electrohydraulic control system through separate electrical and hydraulic/chemical umbilicals.

The two 14-mile long flowlines were installed by the bottom tow method in four, 7-mile long segments. Each 7- mile segment was fabricated at a beach makeup site, laterally launched, bottom towed 400 statute miles through a maximum water depth of 3,200 ft., positioned in the field and connected. Connection to the Bullwinkle platform entailed lifting the riser end to the surface and securing it to the jacket leg in a catenary configuration. Insulated steel pipe jumpers were used to join the 7-mile sections at the midline and subsea manifold locations. An overview of the Troika flowline system is shown in Fig. 1. Further details on the Troika system can be found in Refs. 1 and 2.

The flowline system was installed during the summer of 1997 with first oil on November 11, 1997. After startup it was discovered that the arrival temperature of the produced fluids at the host platform was about 50?F lower than expected. An investigation into the cause of the low arrival temperatures was carried out in 1998. The investigation included analysis, field measurements and land tests of an 80-ft. section of the Troika flowline bundle. In addition to evaluating the cause of the poor thermal insulating properties of the flowline system, a remediation plan was developed.

Flowline System Description

The Troika flowline system is depicted in Fig. 1. The upper part of the figure shows the field layout and the lower part shows the various components along the flowline bundle.

The Steel Catenary Risers (SCRs) at the Bullwinkle Platform are a pipe-in-pipe configuration with a 10.75-in. × 1.024-in. production line inside a 18-in. × 0.5-in. outer casing pipe. The 10.75-in. production line is surrounded by a 2-in. thick layer of 2-pcf open cell polyurethane foam. The foam consists of 6-ft long half shells that are secured to the 10.75-in. production pipe using tape and plastic straps.