Seismic Design of Large Scale Integrated Subsea Facilities
- Majid Hesar (Subsea 7) | Tzi Piau Cheong (Subsea 7) | Qingjing Meng (Subsea 7) | Leonardo Gitahy (Subsea 7) | Carlos Charnaux (Subsea 7)
- Document ID
- International Society of Offshore and Polar Engineers
- The 28th International Ocean and Polar Engineering Conference, 10-15 June, Sapporo, Japan
- Publication Date
- Document Type
- Conference Paper
- 2018. International Society of Offshore and Polar Engineers
- Seismic Design, Time Domain Analysis, Hysteresis, Earthquake Design, Soil-Structure Interaction, Implicit Dynamics
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Subsea facility clusters typically consist of large stiff structures, interconnected with spools or jumpers. The seismic response of these slender elements, particularly their connection points are heavily dependent of the system response and the clusters need to be modelled in an integrated fashion. In recent earthquake design practice Subsea 7 uses a novel approach to ensure that the so called “system effects” are captured. Soil-structure interfaces are modelled with special kinematically hardening elements distributed around the foundations. An implicit dynamic approach in Abaqus is used to pass each model through a minimum of four earthquakes at both ELE and ALE level. All parts of the system are code checked against the relevant design codes using Python scripts.
Subsea assets located in earthquake prone regions of the world can be subjected to severe excitation. According to the ISO 19901-2 standard, (2004) these assets need to be designed in a two-tier manner against both ELE and ALE level earthquakes. The return period of these earthquake levels can be typically 100-300 years and greater than 1000 years, respectively. These facilities may typically consist of rigid pipelines terminating in sliding PLET structures on skirted or hybrid mudmat foundations, connected via rigid spools or jumpers to manifolds on suction pile foundations or X-mas Trees on well conductors cantilevering a few meters above seabed. Several Flexible flowlines may be hanging off these structures via goosenecks.
Experience shows that the seismic response of such clusters is complex. Although larger individual structures can resist the earthquake excitation, the so called “system effects” can be intractable and require the use of integrated models. The response of other more flexible elements inter-connecting the larges structures can be surprisingly significant even in moderate magnitude earthquakes. Examples of the latter are rigid spools and jumpers that span large distances between PLEM, PLET and X-Tree structures without touching the seabed.
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