The TLWP Cathodic Protection System
Authors: Evans S. 
Published: October 2008

The sub sea corrosion protection system of the tension leg well platform which incorporates coatings, cathodic protection, and high initial current densities, is detailed. The results of surveys undertaken after installation of the TLWP reveal potentials at least 150 millivolts more protective than the minimum potential required for protection. The system life will exceed the design life of 20 years. The TLWP protection system weight is 434,000 pounds less than that associated with conventional cathodic protection design, with 286,000 fewer pounds on the floating portion of the TLWP.

INTRODUCTION

In late 1989, Conoco installed the tension leg well platform (TLWP) in 1760 feet of water in the Jolliet Field in Green Canyon Block 184 (Figure 1).1 The TLWP is comprised of 4 main elements:2 drilling template, foundation template, tension legs (or tendons), and the TLWP itself (Figure 2). The objectives of the sub sea protection system, which combined coatings and cathodic protection (CP), were: to provide a safe, low maintenance protection system with a life of 20 years, to minimize the anode weight on the floating components (hull, tendons, risers), and to eliminate risks that could result in premature retrofits in deep water.

BACKGROUND

The CP design philosophy involves minimizing anode weight by coating all of the components of the structure, and by ensuring a high initial current density on the structure such that a low maintenance current density is required. The combination of coatings and cathodic protection has resulted in significant weight and cost savings with two Conoco fixed platforms recently installed in 617 feet of water at Green Canyon Block 52.3 The synergistic combination of coatings and CP is reflected in several technically and economically successful applications in the Gulf of Mexico, Abu Dhabi, the southern North Sea, and the northern North Sea4,5,6,7.

In addition to anode weight reduction, it is important to strategically place the anodes such that much of the anode weight required to protect the tendons and risers is concentrated on the templates, located on the sea floor. Our experience in the Gulf of Mexico and the Arabian Gulf is that CP is markedly enhanced on well coated pipes, with an individual anode having a throwing power to at least 3,000 feet. Exxon has reported similar observations for coated guy lines on its Lena structure.8 Numerical modeling, with the computer program SEACORR, 9 has also shown CP to be effective over large distances for coated cathodes.

The TLWP CP design is based on achieving an initially high structure current density (e.g., 30 Ma/ft2) that will result in a structure potential at least as negative as -0.950 V with respect to the silver- silver chloride reference electrode (Ag/AgCl). Meeting this initial polarization criteria enables the safe use of a maintenance current density of 3.5 Ma/ft2 - The use of high initial current densities along with lower maintenance current densities has been well documented for platforms in the Gulf of Mexico, the Arabian Gulf and the North Sea. 10,11,12 The use of lower maintenance current densities results in lowering the anode weight requirement. Initial current densities of this order have been found to be effective in deep waters in both the Gulf of Mexico and the North Sea. 3,13