THPS Degradation in the Long-Term Preservation of Sub-sea Flow-lines and Risers

The Thunder Horse project’s sub-sea flow-lines were installed in 6,000’ water depths; the installation of the risers followed a year later.  These lines were initially filled with chemically treated seawater in anticipation of hydro-testing and start-up.  The initial chemical treatments were in accordance with BP guidelines and EPA GoM specific regulations.  The initial chemical treatments consisted of a THPS biocide (~270 ppm), oxygen scavenger (~100 ppm), corrosion inhibitor (~100 ppm), and dye (~30 ppm).

The initial chemically-treated fluids remained in place for up to two years during which time installation operations continued.  Due to the extended length of fluid residence time, investigations were undertaken to determine the continued effectiveness of the chemical treatments in maintaining the mechanical integrity of the risers and flow-lines.  The primary interest of this investigation was to determine the degradation rate of the active biocide product, THPS, under in-situ conditions.  With an understanding of the biocide degradation rate, the Project Team could effectively and efficiently maintain the appropriate concentrations of biocide in the fill fluids for the control of microbial influenced corrosion.

This paper summarizes the work done by the Thunder Horse Sub-sea Commissioning Team to investigate THPS degradation of chemically treated seawater in sub-sea risers and flow-lines.  The investigation included the following.

1.      Literature review of associated THPS degradation data,

2.      Laboratory study to investigate the degradation of THPS in the sub-sea risers and flow-lines, and

3.      A sampling campaign during the execution of the Thunder Horse project’s installation and commissioning scopes of work to quantify the actual THPS initial and final concentrations.