Offshore Platform Materials Integrity - Remediation Measures
Authors: Mahajanam Sudhakar P.V. , Joosten Michael W. , Craigie Bob 
Published: July 2010

Offshore oil and gas production environments provide severe challenges in terms of materials selection and day-to-day operations. Several issues encountered during commissioning of a North Sea platform and the remedial actions are presented. Iron contamination from debris generated during topsides construction and chlorides from the marine atmosphere provided the conditions necessary for ferric chloride pitting corrosion of uncoated 316 stainless steel (SS), duplex and super duplex SS, and 6%Mo SS pipework and vessels. Several offshore cleaning/coating methods were evaluated and a new procedure for cleaning was identified that achieved the desired goal without removal of metal or affecting the corrosion resistant alloy's passive film integrity. Corrosion protection of carbon steel bolting by encapsulation was employed to stop deterioration of an improperly applied anodic coating. Insulated SS instrument tubing was susceptible to crevice corrosion and chloride stress corrosion cracking (Cl-SCC) under wet insulation at the temperatures generated by the heat tracing. Solutions were suggested to minimize such forms of corrosion. Coatings exceeding the manufacturer's recommended thickness had been applied to several high temperature vessels and pipework to prevent SCC as well as ferric chloride pitting. The potential for coating disbondment at elevated temperatures due to high dry film thickness was evaluated through a testing program and onsite inspections.

Introduction

A platform topsides facility, shown in Figure 1, installed on a North Sea jacket, deteriorated substantially during the time between installation and the start of hookup/commissioning due mainly to the substandard coatings and workmanship. This degradation was exasperated by construction contamination and exposure to the marine environment. To safely, cost-effectively and in a timely manner remediate this situation, a rectification team was established, which included materials and corrosion specialists. Implementation of pragmatic solutions to the various issues encountered was required to ensure an on-time start-up. The required work scope entailed:Bolting remediation with thermoplastic encapsulationIron and chloride contamination removalAluminum foil for heat tracing of corrosion resistant alloy (CRA) instrument tubingHigh dry film thickness (DFT) coating qualification.

Bolting remediation with thermoplastic encapsulation

Corrosion of bolting has been long recognized as a problem for offshore platforms.(O. Andersen et al., 1996, K.P. Fischer, 2002, K.P. Fischer, 2003, I. A. Muzghi, 2004, K. Stevens, 2000, K. Stevens et al., 2003, B.M. Willis, 2000) Various remediation techniques have been used in the past including, fluoropolymer coatings, electroplating/galvanizing, and use of CRA bolting materials. At this facility, most bolts were manufactured from low alloy carbon steel to provide a good combination of high strength and reasonable cost. Zinc/nickel electroplating followed by a passivation treatment was specified to provide corrosion protection. During deck integration, it was evident that a significant quantity of bolting materials on the platform suffered corrosion due to failure and premature consumption of the electroplated " protective?? coating. As most piping systems were already installed and hydrotested, it was important to avoid replacement of bolting materials unless safety or short-to-medium term operability were threatened. The corrosion product was superficial, giving an unsightly appearance to the bolts, adjacent flanges and piping components as shown in Figure 2. No pitting corrosion was observed.