A great number of offshore marine structures such as jacket platforms, monopiles, FPSOs and subsea pipelines are reaching the original design life and several alternatives for retrofitting are available to extend their service life. These can include impressed current or sacrificial anodes.
Among other parameters, the design should consider the current state of the structures using data from recent field survey and the possible interactions of the retrofit anodes with the existing structures. In complex scenarios, the main challenge to obtain a cost-effective solution is to minimise the number of retrofit anodes whilst mitigating possible interactions with other structures to achieve protection of the asset while avoiding overprotection. Computational modelling can assist in optimising not only the location and number of anodes, but also avoiding over-protection and considering additional aspects like interactions and changes in the polarisation status of the structures for example.
This paper illustrates the optimisation of a retrofit design with the aim of extending the service life of a system of jacket structures and submerged pipelines using ICCP remote anode, contemplating strategies to overcome the screening effect caused by big number of conductors present in the jacket structures.
The complex of jacket structures and flowlines subject to retrofitting consists of 5 jacket structures and 8 subsea flowlines. The arrangement of assets and dimensions used in this work is designed to mimic the challenges posed by real complex scenarios. The complex has three central structures bridge linked (J1, J2 and J3) and two distant structures (J4 and J5) connected to J1 through underwater flowlines. All assets are electrically connected.
The assumptions are that the structures were originally designed with a CP sacrificial system, but as the complex reaches its end of life, recent surveys indicate that anodes in the different structures are almost depleted. Therefore, a retrofit system consisting of remote impressed current sled anodes is to be designed to provide protection during the life extension period.
The challenges in this work are to provide protection to structures J2 and J5, which have 25 conductors each avoiding over-protecting the nearby flowlines.
With the objective of optimizing the design, pursing a cost-effective solution where minimal number of sleds are used, several arrangements were modelled using BEASY †1 software. In this way, a mapping between space and solution can be established and the optimal arrangement be found.
