Sacrificial Cathodic Protection (CP) System Inadequacy Due To 2-Phase Operation of 3-Phase Gas / Oil Separator
Authors: Al-Mithin Abdul Wahab , Sabri Hasan , Sardesai Vinayak , Fernando Felix 
Published: March 2015

Sacrificial Anodes based Cathodic Protection System inside a pressure vessel may prove to be grossly inadequate for certain mode of operation if the vessel internals made up of Corrosion Resistant Alloys are not well taken care of during design and / or installation. This paper discusses the causes of accelerated consumption of Aluminium Sacrificial Anodes in a Low Pressure Gas / Oil Separator operated in 2-phase mode instead of 3-phase, in a Gathering Center of Kuwait Oil Company (KOC). The sacrificial anodes which were designed for 10 years of service life were consumed almost completely in less than a year. The factors responsible for accelerated consumption of anodes include inadequate Sacrificial Cathodic Protection (CP) System, operational constraints, ineffectiveness of insulation kit provided between carbon steel vessel and stainless steel internals. It highlights the significance of weir plate in Gas / Oil Separator when the separator is operated in 2-phase or 3-phase. It also stresses the imPortance of using the insulation kit between dissimilar materials. The paper provides the recommendations for the improvement in design of sacrificial CP System taking stainless steel internals into considerations and the need for conducting the continuity test before commissioning. Besides, the need for removing the weir plate in case of 2-phase operation and coating of the stainless steel internals to minimize the Cathode-to-Anode area Ratio has been emphasized upon.

Introduction

The function of a 3-phase Separator is to separate the associated gas, oil and oily water from the Crude Oil flowed from the the oil wells. It works on the principle that the three components have different densities allowing the gas being on top, the oil being in the middle and the water remaining at the bottom. Any solids such as sand would also settle at the bottom of the separator. Gas can be piped out from the top of the vessel after passing through the Demister Mesh / Mist Eliminator. Oil and water are separated by using a weir plate, located at the downstream end of the separator. The weir plate is fixed such that its upper edge is at a level above the oil-water contact level, known as 'Interface Level', allowing oil to spill over onto the other side, while trapping water on the upstream side. The two fluids are then piped out of the separator from their respective sides of the weir plate as shown in Fig. 1. This figure shows the schematic diagram of the general arrangement of the vessel along with the internals. The figure shows the relative locations of the internals inside the separator.

The oil flow and the water flow from their respective outlets are controlled to ensure that the interface is kept at the optimum levels for separation to occur during the 3-phase operation.

In a typical Gathering Center (GC), the crude oil from oil wells is received in the manifolds and then flowed into the downstream equipments for Gas, Oil & Water separation that takes place in 3 stages in High Pressure Separator, Low Pressure Separator & finally in Atmospheric Storage Tank respectively.

A new Low Pressure Separator (30,000BOPD), designed for 3-phase operation was installed and commissioned in July 2008 in one of KOC's Gathering Centers. Although the Separator was designed for 3-phase operation, i.e. designed to separate the Gas, the Oil and the Produced Water, it was being operated in 2-phase due to certain operational constraints. In 2-phase operation it is the separation of the Gas from the Liquid (Oil & Water) and to this the water outlet is closed / spaded and the oil / water mixture is removed from the oil outlet. The Separator vessel was internally coated with Phenolic Epoxy and Sacrificial Aluminum anodes were installed at bottom 5 to 7 'O' clock position. All the internals including Inlet Momentum Absorbers (Porta Test Revolutions Assembly), Coalescer Packs, Perforated Baffles, Weir Plate were made of stainless steel SS 316L material.