Design of Britannia's Subsea Heated Bundle for a 25 Year Service Life
Authors: Brown Lloyd D. , Clapham John , Belmear Clint , Harris Rob , Loudon Allan , Maxwell Stephen , Stott Jim 
Published: October 2008

Abstract
The Britannia project, UK North Sea, brought the first, 15 km subsea, heated bundle into operation in the 3rd quarter of 1998. A hot water heated bundle concept was used as the best technical solution to prevent hydrate and paraffin formation in the 15 km subsea tie back and to assist the co-venturer's environmental impact reduction programs. Corrosion control was a key factor to achieving a 25-year design life for production and safety integrity of the subsea heat exchanger. Corrosion control design required a multi-discipline approach throughout research, design, fabrication, installation, startup, and operations. Successful implementation of this design required close collaboration between the design team, the research team, the fabricators, the operators, and the service companies. The continuity of the multi-discipline knowledge needed to support successful transition into operations was achieved by a core design, research, and service company team. Success should be attributed to the Britannia management team's focus on open, structured teams and support for the team's balance between technology, risk, and cost.

Introduction
The challenges faced in developing the Britannia field for economical production have been reviewed by others.[1-5] A key innovation concept for economic development of the subsea production system was the first use of an actively heated subsea bundle. A similar concept is in use for an onshore sulfur transport pipeline.[6] The Britannia heated bundle system is used to the prevent hydrate and paraffin formation in the 15 Km, subsea production pipeline tie back to the Britannia platform. It accomplishes this by maintaining a minimum temperature of 21 C in the production pipelines. The subsea piping system is in a bundle configuration where a 37" carrier pipe encloses a 14" production, an 8" production test, a 12" heated water, and a 3" methanol supply pipeline. The carrier wall and the 14" production pipeline are separated by less than 40 mm at the 6 o'clock position. The bundle configuration provides the best technical solution for the 140-meter deep subsea tie back. However, unlike typical bundles, 8300 m3 of water media actively heats the bundled piping as the water circulates between the carrier annulus, the 12" hot water pipeline, and the platform at 13,200 m3/d. This circulation of water distributes heat in the subsea system and significantly increases the thermal mass of the system to improve the thermal control required for the subsea tie back. The topside equipment provides heating, expansion tanks, pumps, and system monitoring. For safety monitoring of possible methane in the heating media, the topsides expansion tanks are blanketed with nitrogen having at most 1% oxygen. Figure 1, Figure 2, and Figure 3 show the subsea bundle interior, a subsea bundle layout schematic, and a topside component schematic respectively.

Best Practices
The operating characteristic of the Britannia heated bundle is similar to common industrial closed heating and cooling systems. In its most fundamental concept, it is a long fixed tube heat exchanger with cooling or heating on the shell side. As such, considerable experience exists in literature - of which only a few can be cited here - and technical organizations on the best practices needed to maintain reliable operation and integrity.[7-12] The challenges faced in achieving this onshore identify the risks that result from not following best practices and the remediation required to restore system integrity and reliable operation.