Session: 10-01-01 Seabed Properties and Processes

Paper Number: 128176

128176 - Optimizing Subsea Rock Installation for Cost Savings: A Norwegian Shelf Case Study 

Subsea rock installation (SRI) plays a significant role in cost evaluation for offshore engineering projects, with two-fold impacts on both offshore operation time/fee and material expenses. Excessive SRI cost may lead the project to be considered unfeasible at the concept stage, even cannot pass the decision gate to the FEED and detailed design. This study addresses a specific subsea design project involving the installation of a pipe-in-pipe on the Norwegian Continental Shelf, where specific challenges arise due to the soft-clay seabed and the presence of several large iceberg scars.
A well-used approach to subsea rock installation involves a two-dimensional limit equilibrium method (LEM) for assessing the stability of ideal plastic soil/rock. This approach evaluates the side-slope stability of soil/rock efficiently. However, this method may lead to overconservative design, as it simplifies key factors. Three key considerations highlight the need for a more comprehensive approach:
1. Soil behavior is not always ideally plastic. Its strength increases as undergoing consolidation due to the extra load applied by the installed rock. Note that soil consolidation is time-dependent due to the transient drainage of water from soil frame, making it essential to evaluate soil properties at various time points during the installation campaigns.
2. 3-dimensional seabed geometry and rock geometry to be installed are key inputs for simulation. 2-dimensional analysis on the other hand, lacks information about geometry and boundary conditions in the direction normal to the plan of investigation.
3. In comparison to LEM, the finite element method (FEM), widely applied in engineering, enables a more comprehensive assessment. FEM checks the soil stability not only at the potential failure surface but in fact at all elements of the geometry in the simulation. It predicts strain/stress status, deformation, and material failure, providing a more realistic representation of the soil/rock behavior.
Our study aims to optimize rock volume by implementing the techniques outlined above. In this case study, the stabilities are rechecked for the infillings of free span in deep iceberg scars.
Results of analyses are used to evaluate the costs associated with offshore operation time and material for designs in different optimization approaches.
This case study underscores that the rock volume can be optimized by considering more aspects into the design/simulation, resulting in cost savings. The insights gained from this study serve as guidelines for future projects, accelerating the design progress with more confidence.

Presenting Author: Xiyang Xie IKM Ocean Design

Presenting Author Biography: Xiyang Xie received his PhD degree in Rock Mechanics from Norwegian University of Science and Technology in 2019, then worked as a researcher in SINTEF Formation Physics in Norway until 2023. Now he is working for IKM Ocean Design as the Geotechnical Engineer.

Authors:

Xiyang Xie IKM Ocean Design
Otilia Vermeulen IKM Ocean Design
Hao Gao IKM Ocean Design
Moeen Nazari IKM Ocean Design
Per Olav Asklund IKM Ocean Design
Bjørn Lunde IKM Ocean Design
Stian Rasmussen IKM Ocean Design