Cathodic Protection For Deepwater Pipelines
Authors: Bomba J.G. , McCasland F.D. , Agosta P.J. 
Published: October 2008

Deep water presents additional problems when designing a cathodic protection (CP) system for a pipeline. Various factors resultingj70m deep warer alone influence how a cathodic protection system operates. As Thompson and Fischer stated in OTC 68881, "The effect of deep water on cathodic protection is simply a cumulative result of the seawater characteristics (dissolved oxygen, temperature, salinity, pH, sea currents, pressure, and fouling) that vary with depth and effect cathodic polarization and calcareous deposit formation. " With these factors in mind, the cathodic protection system for the deep water Mississippi Canyon 441 pipeline system was designed. This paper summarizes that design process.

INTRODUCTION

Enserch Exploration Inc., (EPOC) along with partners Petrofina Delaware Inc. (Fins) and AGIP Petroleum, is currently developing the deep water gas reserves in Mississippi Canyon Block 441, approximately 50 miles south of Grand Louisiana. The discovery wells are located in to 1520 feet of water.

A complicating factor to the development of the project is the Louisiana Offshore Oil Port (LOOP) safety fairway which covers the eastern 75 percent of block MC441, effectively ruling out surface production facilities over the well locations.

This constraint led Enserch and partners to the development of special subsea well completions designed to produce through flow line bundles to a fixed platform location in shallow water inside the fairway. The platform, known as the shallow water facility (SWF) is located in Ewing Bank Block 482.

The subsea facilities, particularly the unique "stackable templates, is described in an Oil & Gas Journal article, entitled "Stacked Subsea Templates Accelerate Deepwater Development".2 This article describes the drilling program, development strategy for the field, and briefly describes the flowline bundles and the method chosen for their installation.

This paper presents some of the factors affecting the design of the cathodic protection design for the Mississippi Canyon 441 pipeline bundles and risers system. Thepipeline iselectfically isolatd from the shallow water facility (SWF) platform structure and the templates have been pre-fitted with sufficient anodes to protect the well casings and trees, effectively insuring that these structures will not interfere with the CP on the pipeline bundles. Background information on the pipeline and the selection criteria used in determining proper anode materials, anode spacing, external coating, and electrical isolation of the flowline bundles is included herein. Much of the design work was based on theoretical calculations, as little hard data exists regarding deep water corrosion, due to the difficulty in simulating the subset conditions in a laboratory. An in-situ ROV (remote operating vehicle) survey is planned to examine the polarization status at completion of tie-in activities. Potentials are expected to be close to the open circuit values.

BACKGROUND DESIGN INFORMATION

Initial consideration was given to the use of an impressed current system to protect the bundle assemblies because of the relatively short line lengths (the longest 5.8 miles) and the inherent flexibility of an impressed current system.