Challenges of long-distance tiebacks
Another outcome from rough terrain is the occurrence of freespan when the pipe is resting on the seabed. A low bottom tension also is required to minimize the length of each freespan. Acergy Falcon can adjust the angle of the lay ramp from shallow to vertical during laying (21º to 90º). This enabled an adjustment of the lay parameters such as bottom tension during laying to align with the requirements for both the shallow and deepwater part. When the vessel performed pipeline down the Storrega Slide (30º slope from 250 to 650 m [820 to 2,133 ft] water depth), the pipe was laid with bottom tensions below 1.5 metric tons (1.7 tons), with a high stylization of the moment capacity in the sag bend. This was to reduce the length of some long freespans and, hence, the seabed intervention cost. This left only small margins for vessel positioning and dynamic impact from weather. Good control over vessel station keeping and dynamic movement was required.
Both static and dynamic analysis was performed for various water depths, seabed terrain, and environmental conditions. Knowledge of the station keeping of the vessel plus its dynamic response in various weather conditions was required for a realistic analysis. This has been obtained for the Acergy Falcon from research on hull models and experience from many years of pipelay in various conditions with various pipe sizes. Current speeds at almost 1 m/s at the surface and 0.5 m/s at the bottom when the water depth was more than 850 m (2,789 ft) created several tons of drag force on the pipe. This has an impact on the configuration of the pipe catenary, especially since the low bottom tension leaves the catenary in a very “slack” shape.
Online monitoring
To meet the routing requirements and not over-stress and damage the pipe, the vessel needed to respond quickly to sudden strong current. Therefore, online monitoring of the catenary was used. Online monitoring consisted of an acoustic Doppler current profiler continuously measuring current direction and speed through the whole water column. Together with the actual vessel position, touch down position, water depth, and departure angle of the pipe, the current profile was an input to a continuous static calculation. The actual static catenary with the current taken into account was monitored always. Tables with vessel position and ramp angle for most possible current scenarios were developed prior to going offshore. These tables and the online monitoring system gave the crew information about which actions to take once the current increased.
During the installation of the Ormen Lange MEG lines, strong current coming in transversely to the lay direction required the vessel to offset as much as 90 m (295 ft) from the lay route and rotate 37° to the laying direction.
The actual residual bottom tension also was captured from the online monitoring. The tension was higher with a current in the lay direction. The residual bottom tension along the whole route from shore to the field was captured for the MEG lines. This was essential for the post-lay trenching.
Previous Page
|
Next Page
Page 2 of 5
Volume 68 Issue 9
September 2008
Add RSS Feed
