A methodology for the rigorous assessment of pipeline freespans is described, together with a description of its application to the Beryl Field network of pipelines.
The methodology is in two parts, each with two stages, and comprise preliminary stress and vibration frequency checks followed by detailed strain and fatigue life checks where appropriate. Comprehensive software, automatically linked to the inspection database, has been written to allow efficient use of the methodology.
The use of a new ROV based freespan rectification technique is also described.
Both the assessment and rectification techniques were successfully used in Mobil's Beryl field and the SAGE pipeline in 1992.
The Beryl 'A' platform was installed in block 9/13 in 1973. Subsequent developments in the block include the Beryl B platform, the Ness, BWISS, Linnhe and NESS II multi-well subsea developments and 4 single subsea wells. A total of 25 pipelines are laid in the block; the majority are 6" diameter flowlines from the subsea wells to the platforms, but include 1 No. 16" and 1 No. 20" hydrocarbon transfer lines between the production platforms and 2 short 361, oil export lines to loading buoys. All the facilities noted above are operated by Mobil North Sea Limited (MNSL) on behalf of the block 9/13 co-venturers, Amerada Hess Ltd, Enterprise Oil plc, BG North Sea Holdings Ltd and OMV (UK) Ltd. In addition, the recently installed SAGE Gas export line runs 323 km from Beryl A to St. Fergus, and is operated by MNSL on behalf of the Beryl and Brae groups.
Annual inspection of all the lines is undertaken in order to comply with regulatory and MNSL requirements and to ensure continued fitness for purpose. As with most pipelines, the annual surveys identify numerous freespans which have to be assessed and, if necessary, rectified.
MNSL has now implemented a number of improvements to its methods for collecting and recording pipeline inspection data, and for automatically assessing the significance of freespans in relation to the service, pressure, temperature and orientation of pipelines. The methods of data storage and freespan assessment are based on user friendly PC-based programs for use both offshore and onshore.
The basis for recording and displaying the inspection results is a database system (COABIS). Survey data can be entered into the system in real time on the inspection vessel as the inspection is performed. Further checks can be undertaken on the data in the office including graphical comparison of all features from one year to another.