Comprehensive Integrity Solution for a Subsea Loading Line
High-quality data collection with a free-swimming, bi-directional MFL-equipped solution
ROSEN was contracted by a South American company to provide a solution to inspect two offshore loading pipelines, one 24” pipeline with a length of 16.7 km and the other 36” pipeline of 10.1 km. These unique systems connect onshore storage facilities to two subsea pipeline end manifold’s (PLEM’s), which are connected to a buoy by a flexible hose for the critical task of loading tankers. Due to its position in the pipeline system, this asset was deemed critical since there was no alternative means to import or export the product.
Asset description:
- Location: Offshore
- Product: Crude Oil
- Pipeline diameter: 24” & 36”
- Survey Length: 16.7 & 10.1 km
- Outer Diameter: 588 mm / 914.4 mm
- Wall Thickness: 11.91 mm / 15.87 mm
- Min. ID: 564 / 882
- Bend Radius: 5D
- Operating Conditions: 10 bar & 1.0 m/s
The challenge
The loading line in this case, by design, could not be inspected with a conventional inspection approach due to the following challenges:
- Single access
- No traps - subsea line ends at PLEM
- Unknown cleanliness
- Single flow direction
Additionally, the customer’s comprehensive integrity system does not allow for reduced quality in their inspection datasets. As such, it was requested that ROSEN provide a high-resolution solution with optimal defect detection and identification for both internal and external corrosion features. The data analysis scope also required a post-inspection Corrosion Growth Analysis to be provided by ROSEN’s Integrity Services. The customer also requested subsea georeferencing of their loading line system.
Our solution
Although conventional inspection methods, in combination with subsea traps or tethered UT solutions, could be sufficient for an in-line inspection, it would not obtain the high-quality data requested by the customer. Therefore, ROSEN’s Solution Experts worked closely with the customer to tailor the optimal solution package.
It was determined that a free-swimming, bi-directional, MFL-equipped solution, with a reversed cleaning approach, was the most suitable in order to obtain the high-quality results. The benefits of this approach are:
- The reversed cleaning approach allowed for the collection of debris to take place onshore, thus minimizing expensive subsea operations.
- The MFL technology provides a robust design on a proven concept known to be least sensitive to debris.
In order to provide a suitable solution for georeferencing the pipelines, an in-depth market assessment on available technologies and potential local vendors was performed by ROSEN. The approach that was identified to be the most suitable for this application was a combination of technologies:
- Highly accurate DGPS system for position verification above sea level
• The system consists of a DGPS base station established on a known location within the terminal (known geodetic coordinates).
• The other component consists of a mobile DGPS station, also called a rover, that is placed on the vessel. - The USBL tracking system consisting of multiple components
• The USBL tracking system mounted on the vessel from which the measurements are conducted, and which can determine the 3D angle to transponders.
• The transponders that respond to an ultrasonic signal from the USBL tracking system.
In addition, at all pre-determined subsea reference points the salinity, conductivity, temperature and pressure depth profile of the water was obtained to establish the speed of sound value.
The overall accuracy is influenced by the quality and diligence of the involved personnel during execution. Each individual step requires detailed preparation procedures, onsite calibration, proper execution, and supervision.
Your benefit
Despite the challenging offshore conditions, a very demanding scope, and the fact that this was the first utilization of the georeferencing technology, ROSEN was able to complete all onsite activities within the requested time frame. Furthermore, the seven in-line inspection surveys were completed without any accidents or incidents and to the full satisfaction of the customer. The operator obtained valuable information on the integrity status of the asset, including the identification and localization of subsea defects and accurate strain analysis as a result of pipeline movement. This has allowed the customer to continue safe operation of the loading line system.