Active Heating For Flow Assurance Control in Deepwater Flowlines
Authors: Esaklul Khlefa A. , Fung Gee , Harrison Gary , Perego Ron 
Published: October 2008

This paper describes the flow assurance challenges faced during the development of a multi-well field located approximately 27 KM (17 miles) from the host facility and how those challenges have been solved using pipe-in-pipe active heating. The selection criterion demonstrates that active heating has advantages over chemical options. Active heating systems are a practical and cost effective development option for control of hydrate and wax deposition during operation and planned or emergency shutdowns. Active heating provides increased operational flexibility through control of the pipeline temperature above the hydrate formation and wax deposition temperatures, allowing for prolonged shutdowns, conditioning of the line for resuming operation, or melting of hydrate or wax plugs formed during extended shutdowns.

These flowlines were constructed by Allseas' Solitaire pipelay ship and the S-Lay method and commissioned successfully using treated fresh water for hydrotesting to eliminate contamination, particularly for the annulus section.

A Glycol - water solution was selected as the heat transfer medium for its heat transfer efficiency, low corrosivity and inherent microbiological control (biocide) properties. The performance of the first active heating system in the Gulf of Mexico has exceeded the design criterion and provided better heating efficiency than anticipated.

Introduction

The King field is located approximately 96 KM (60 miles) East of Venice, Louisiana and is made up of four blocks: Mississippi Canyon MC 84, 85, 128 and 129. Water depth in the field ranges from 5000 to 5400 ft. The King field development consists of two Phases. Phase I is comprised of two production wells with first oil in April 2002 and expected peak production of approximately 29 MBOPD, 31 MMSCFD, and as much as 5 MBWPD. Phase II (King West) will consist of one additional well and is expected to begin production in July 2003.

The development consists of a subsea production system tied back to the Marlin TLP. Marlin is located in Viosca Knoll Block 915 at a water depth of 990 m (3250 feet).

The subsea wells are tied back to the Marlin TLP using two flowlines. Each flowline connects to Marlin with a pipe-inpipe steel catenary Riser. Flow assurance was one of the key factors in the development of the field with hydrate and wax deposition as the primary design parameters.

Several development options were considered including insulation, chemical mitigation and active heating.

Active heating was selected as the preferable option for its cost effectiveness, avoidance of pigging for wax control, and ability to maintain the flowlines above the hydrate formation and wax deposition temperatures during shut downs and prior to start-ups after prolonged shut downs.

The heated flowline system was chosen primarily to provide a means of preventing and/or remediating hydrate formation in the subsea production system during both normal operations and shutdown conditions. In addition, the heated pipe design mitigates wax deposition in the subsea flowlines by maintaining system temperatures above not only the hydrate formation temperature but also the wax appearance temperature (WAT) - effectively eliminating the need to run scraper pigs for wax buildup control.