Pipeline Hydrotesting, Dewatering, and Commissioning
Authors: Mappus B.E. , Torstrick A.G. 
Published: October 2008

All newly constructed pipelines must go through a series of activities to ensure the pipelines meet regulatory laws and the requirements of the pipeline's owner and operator. The requirements depend largely on the type of service the pipeline is to be used for and the materials used during construction. The Independence Trail Gas Export Pipeline has several unique challenges not only due to the intended service, but also the pipe diameters with various wall thicknesses and the configuration of the system.

It is necessary to break the flooding operations into two stages due to the significant lapse in time from when the pipe is installed on the seafloor to when the pipeline is completed as a system. The first stage is the initial flooding of a portion of the pipeline to aid with burial and to tie-in the shallow water platform. The second stage occurs months later, after the installation of the Independence Hub platform and lifting the export 20-inch SCR to flood the entire system. Following the flooding operations, the pipeline must be hydrotested to a minimum test pressure of 4,550 psig for eight continuous hours.

The dewatering and purging operations require the use of a vessel to stage the large spreads of equipment. The Hub platform has very limited open deck space, not allowing adequate space for sufficient air compression or nitrogen equipment. The equipment spreads consist of more than 16,000 scfm of available compressed air and 5,000 scfm of generated nitrogen. It is one of the largest commissioning spreads taken offshore to date.

To commission the Hub platform, gas is taken from the outgoing system on WD 68. The flow of the gas is diverted into a custom-built flow control skid and custom-built dryer prior to entering the pipeline.

The various activities required in the pre-commissioning and commissioning operations of the pipeline are explained in full detail within this paper. These are the same activities required before introducing product into any newly constructed pipeline but with a different method of execution designed specifically for this project's scope of work.

Introduction

The Independence Trail Gas Export Pipeline must go through the pre-commissioning activities before being put into service. These activities include: flooding, hydrotesting, dewatering and purging. Each activity has its own challenges, unique to each project's scope of work.

The pipeline is currently the largest, longest, and deepest in the Gulf of Mexico with diameters of 20-inch and 24-inch for a stretch of 134 miles from water depths of 8,000 fsw to 115 fsw. The flooding operations require high volume pumps to push a pig through the pipeline at the desired pig speed. The dewatering and purging operations requires as much air and nitrogen equipment that can be brought together to push a pig through the pipeline to remove the seawater.

Each activity requires extensive planning and coordination for well executed operations and a successful conclusion. Each operation requires the coordination of several contractors' personnel and equipment in the field at one time, working towards a common goal of a safe and efficient completion of work.