Subsea Chemical Storage and Injection Station - Single Line Batch Re-Supply of Chemicals - Operation Strategies and System Design

Abstract

The offshore industry is facing many challenges when it comes to maintaining operational efficiency. One of the biggest problems is related to flow assurance problems. To avoid such issues, different chemicals are transported through individual lines in an umbilical from topside into a x-mass tree or a manifold. Total has for some years worked on a new concept where they want to replace the traditional umbilical with big tanks that are stored on the seabed, which are refilled every sixth month. However, if it is possible to simplify this, then the capital cost would be reduced even more.

The aim of this thesis was to look at the operational strategy, meaning how we can transport these chemicals from one place top-side to a place subsea. Furthermore, find a solution for the design of the system, both regarding chemicals being into separate tanks, and tanks that contain a mixture of two or more of the chemicals, making a so-called chemicals combination.

The approach used to answer the objectives of this thesis involves collecting relevant scientific reports and theory by search in scientific databases as well as books and other relevant sources online. Furthermore, calculations with focus on flow behaviour have been conducted. Illustrations have been used to a large extent to illustrate flow behaviours in pipelines of different diameters, and when the liquid spacer are of different length. In addition flow diagrams have been sketched to illustrate the transportation and the distribution of the chemicals.

Due to the uncertainties on how the chemical will behave when they are sent through the same line, they will be separated by a liquid spacer. Uzu et al. (2000) say that the liquid spacer can vary from 100 meters to 500 meters depending on the size of the pipeline and the length it is going to travel. However, since there is little specific knowledge on how long it should be between different chemicals, the liquid spacer has been seen as a parameter.

Based on the injection rates provided by Total for one field case, it was founded that biocide was injected at a much higher rate than the other production chemicals. It was, therefore, suggested that biocide should be injected every other time.

By sending the chemicals as combinations rather as individuals a tank module based on the chemicals being sent through a 0,5 pipeline was reduced from 16 tanks down to 10 tanks.