Coolerless Design: Achieving the High-Temperature Pipeline Goal
Authors: Richards Shannon L. , LeBlanc Steven J.H. , Surke Michael B. 
Published: October 2008

High-temperature wells on remote wellhead platforms typically require high facilities costs due in part to large wellhead coolers and associated utilities. Coolers are required due to temperature-based design limitations on buried, subsea pipelines. In May 1999, commissioning of a high temperature (300°F design) subsea flowline was completed. This high-temperature design allows for the elimination of the wellstream cooler and associated facilities. Thermal expansion, cathodic protection consumption, and coatings degradation are among the major design concerns that were addressed early in the project. Sand free expansion boxes allow the flowline to expand longitudinally on either end of the line, freeing the buried flowline from the natural constraints of sand. The cathodic protection design utilized an attenuation model, which resulted in the installation of two remote anode sleds, one on either end of the flowline. The flowline is coated with a high-temperature fusion bonded epoxy. Both cathodic protection and coatings are being monitored in situ to confirm design assumptions. Start up was monitored by divers, underwater cameras, and survey equipment.

Introduction

A typical one-well development for a high-temperature well in shallow water includes a three or four-pile platform, wellhead coolers, and utilities. These facilities are required to protect the flowline from the elevated corrosion and mechanical difficulties that it will experience at full wellstream temperatures. In the subject area, these temperatures range from 265°F to 315°F. The challenge of this design was to minimize the wellhead facility by eliminating the cooler and its associated utilities, while mitigating these technical concerns.

The mechanical design concerns for high-temperature, buried flowlines are thermal expansion and upheaval buckling. Thermal expansion will occur when the flowline operating temperature is higher than its as-installed temperature. In buried environments, natural flowline expansion is constrained by the axial and lateral loads of the soil on the flowline. Where there are imperfections in the flowline profile, the line will tend to experience lateral or upheaval movement.

Corrosion concerns at elevated temperatures include high cathodic protection consumption and coatings degradation. An anode's current density increases and its capacity decreases with increasing temperatures. Both of these conditions work against a standard system of bracelet anodes installed directly onto the flowline. The challenge in selecting a suitable coating is to find a system that stands up to high temperatures and resists cathodic disbondment.

This paper outlines the design that was selected to control these mechanical and corrosion concerns, ultimately eliminating the need for a wellhead cooler. Start-up and operational history of the flowline are also discussed.

Mechanical Design

The mechanical design considerations for this flowline include thermal expansion and upheaval buckling. The design for each is discussed below.

Thermal Expansion.

The thermal expansion analysis was performed with the assistance of commercially available software. First, a differential temperature profile was developed based on soils and expected flow conditions. The flowline is designed for an inlet temperature of 300°F, with an installation reference temperature of 65°F.