CONTINUED OPERATION OF LARGE OFFSHORE PLATFORMS REQUIRES INNOVATIVE NEW CP SYSTEMS TO BE RETROFITTED
by Alex Delwiche and Jim Britton
Many large structures, and other offshore producing areas in the North Sea, are needing to stay operational beyond their original design life. The reasons for this are typically driven by improved geological survey and recovery technologies, that keep the reservoirs viable, and the advances in subsea production systems which, through subsea tiebacks, keep the oil and gas flowing.
This article discusses the difficulties in undertaking cathodic protection upgrades to offshore oil and gas structures, and discusses examples of where the authors’ company have undertaken upgrades or retrofits to a number of structures in UK waters.
Most of these structures were commonly equipped with sacrificial anodes in the fabrication yard, a relatively straightforward process, as the anodes were usually attached when the jacket framing was at ground level before being “rolled up into the vertical” position. In the seventies, some operators promoted the use of coatings on these facilities to reduce the number of anodes required, especially where zinc anodes were favoured over AL-Zn-In or Al-Zn-Hg. Other operators reviewed the CAPEX financial figures and opted to just increase the number of anodes, which was normally the most cost effective option. This resulted in the case of a large bare steel structure with the attachment of around 2476 anodes, with a combined weight of 666,000 kg (1). Many of these structures were designed to provide around a 20 to 25 year life.
When designing large sacrificial anode systems, two things are very important. The first being to provide sufficient anodes to deliver the required current density (mA/m2) to the submerged structure, to ensure that optimum levels of cathodic protection are established, and that it is evenly distributed to all areas of the submerged structure. This current density is normally referred to the initial, or polarization current density, and following the present design codes, is recommended to be 150 to 180 mA/m2 in the Southern and Northern North Sea areas respectively (2). The second important item is to ensure that there is sufficient mass of sacrificial material to provide the required current for the design life, given that aluminium anodes typically waste at a rate of 3.5 Kg/Amp/ Year. However, to calculate the mass of anodes required, the current density can be reduced to a lower level of 90 mA/m2 which is the number quoted in the design standards as the mean or maintenance current density. The large difference between these numbers is significant when considering the question of continued extended operation.
If a cathodic protection system that has established protective deposits on the steel surfaces, and is still able to “maintain” these deposits, at about half of the theoretical design current requirement, then it is strategically advantageous to extend the system life while it is operating at this maintenance level, rather than going back to the initial, or polarization level again. It is for this reason that a pro-active intervention is recommended, whereby cost savings can be significant.