Double Catenary Offloading Lines for Deepwater Fields
Authors: Chaudhury Gautam 
Published: October 2008

The use of non-weathervaning floating production units is found feasible for fields in the deep waters of West Africa. The lack of pipeline infrastructure in these remote areas requires export of produced fluid from an offloading system. The transfer of product from the production vessel to offloading buoy is carried out by large diameter suspended flexible flow-lines. The flow-lines are configured in a double wave shape, "W" concept, by introducing distributed buoyancy along the middle section of the lines.

The paper presents an alternative concept, a naturally suspended double catenary ("U" configuration) rigid steel flow line. A preliminary, but comprehensive, study was conducted, which considered strength and fatigue performance, technical merits and demerits, economic gains and losses, and safety and reliability concerns between the two concepts. The results of the study confirmed the feasibility and potential savings of the proposed "U" concept.

Introduction

In recent years, there have been several field developments in the deep waters of West Africa, and there are prospects for more. The mild environment makes it possible to use spread moored Floating Production and Storage Units (FPSU). Wind and current in these fields are mostly uncorrelated and nondirectional, which leads to more possible downtime and increased risk of collision between production unit and cargo tanker if traditional tandem offloading is employed. This has resulted in the use of offloading buoy located at a safe distance from the production unit. Flow lines are connected between the FPSU and the offloading buoy to transport produced fluid. The integrity of these flow lines is very important for safe and reliable operation of the offloading system.

At present, flexible flow lines are suspended in a double wave ("W") configuration with the use of syntactic foam buoyancy elements over some carefully optimized length in the middle of the line (Figure 1.0). Generally, 20% to 25% of the length of the flow line is fitted with buoyancy elements. The primary reasons for the use of wave configuration, as reported (1), are to reduce hang-off loads at both ends and to accommodate large amplitudes of low frequency motions associated with platforms in deep waters. Flexible flow lines are very expensive and require zero local contents. The amount of local contents used is an important selection parameter when considering a design concept for West African field developments.

The elimination of flexible pipe will reduce cost, introduce local contents, and add potential for increased flow rate through use of large diameter steel pipe. Proposals (1,2,3) were made to replace flexible pipe with steel pipe, in a double wave "W" configuration with the use of foam buoyancy. Recently, the first 16 inches diameter rigid steel pipe offloading flow lines have been installed by Stolt Offshore, employing "W" configuration, for Girassol field in West Africa. The concept was selected for reason of single piece surface tow, which, nevertheless, demonstrates that rigid steel pipe works. Now, it is time to reap the further benefits rigid steel pipe has to offer.