Abstract
During sub-sea pipeline commissioning operations using a surface support vessel, a substantial amount of non-productive time is endured during the hold period of pipeline hydrotesting. During this time, which may last up to several days, instruments on the support vessel monitor the pipeline pressure and temperature using long hoses and cables. The vessel is unable to move away and perform more productive operations elsewhere. In addition, adverse weather conditions may force the vessel to disconnect and move off station causing data loss and the requirement for a retest. With vessel day-rates in the tens, or even hundreds, of thousands of dollars per day, it is apparent that freeing the vessel during the hydrotest substantially reduces overall project costs and saves time.
This paper describes the development of an electronic sub-sea data acquisition and transmission system which achieves these goals by transmitting data to the surface using through-water two-way acoustic communications. Even if surface data collection becomes impossible due to the vessel moving out of range, or a deterioration in the acoustic environment, successful test verification is still possible as the data is permanently recorded in a non-volatile memory card sub-sea.
The paper also describes a number of challenges to successful acoustic data transmission in the offshore oil & gas marine environment. These include the naturally varying properties of the water column (water density, salinity, temperature and seasonal variations) as well as the relatively large horizontal displacement with respect to the shallow water depth (commonly found in many producing offshore fields) which lead to a heterogeneous acoustic velocity profile and multi-path effects. In addition, significant vessel-produced noise requires the use of advanced signal processing techniques to provide successful communications. It is believed that this technique could be applied to other sub-sea applications requiring remote monitoring & effective data acquisition.
