Session: 04-04-05 Free Spans and Routing

Paper Number: 126850

126850 - Escarpment Crossing Design and Analyses of Large Diameter Offshore Pipelines 

Subsea escarpment crossing is always the most challenging part in offshore pipeline projects.  The escarpment crossing design is complicated and time consuming. This is because the pipeline will experience an excessive plastic strain at the overbend and fatigue damage in the long span due to a steep seabed, especially for the large diameter pipeline. Therefore, general understanding of large diameter pipeline in-place behaviors over escarpment is very important, which is the key factor to make projects successful in terms of quality, cost, and schedule.

 

Due to great variabilities and/or uncertainties, the escarpment pipeline crossing design typically uses conservative assumptions and only allow a limited bending moment and span length. Consequently, mitigation measures such as costly seabed excavation, risky span supports on a sloping seabed, and/or highly restricted installation methods are often required for escarpment crossings.  Instead of expensive and complex mitigations, this paper presents an alternative pipeline crossing design allowing plastic strain at the escarpment top without jeopardizing the pipeline integrity and utilizing more accurate but less conservative design methodologies.

 

Finite element (FE) models of 24-in gas export pipelines in an African field, are utilized to investigate in-place behaviors of the pipeline from a water depth of 25m rapidly down to 250m. The pipeline strength check is assessed with the displacement-controlled condition (DCC) at the escarpment top where the pipeline longitudinal strain is determined by the seabed profile and the load-controlled condition (LCC) is used to assess the rest of pipeline, in accordance with the DNV offshore pipeline design standard (DNV-ST-F101). The numerical model simulates the S-lay, pre-commissioning, and operating processes with sensitivity studies on key input parameters. The mitigation of the longitudinal/bending strain and DCC utilization is then investigated, such as optimizations of the pipeline route, wall thickness, lay tension, and crossing protection.

 

In this paper, several analysis methodologies and parameters are discussed to remove unnecessary conservatism for a cost-effective pipeline design. This includes using elbow elements instead of pipe elements for the overbed areas to account for pipeline cross-section deformation, pipeline crossing protecting modules (e.g., modified Uraduct modules) to distribute the highly localized contact force more evenly, and a reasonably conservative condition load effect factor given less design uncertainties in the escarpment crossing design when the worst soil stiffness and lay corridor are considered.

Presenting Author: Orjan Kvist Genesis

Presenting Author Biography: Orjan Kvist is a Project Manager for the Houston office of Genesis, a business unit of Technip Energies. He has worked for 27 years in the oil and gas industry in Houston, Texas. He received his Bachelor of Science degree in Marine Technology from Bergen College of Engineering, a Bachelor of Science degree in Maritime Systems Engineering from Texas A&M University in Galveston and a MS in Civil Engineering from University of Houston. Orjan’s experience includes offshore pipeline design, offshore survey support, offshore installation support and project management.

Authors:

Xinhai Qi Genesis
Orjan Kvist Genesis
Jian Chen Golden Shield Technology
Gang Duan Golden Shield Technology
Lan Ren Golden Shield Technology
Howard Wang ExxonMobil
Jundong Lan ExxonMobil