Geotechnical analysis for offshore developments

Ternan Energy routinely performs geotechnical analysis for offshore developments within the Oil & Gas and Renewable Energy sectors.

Ternan perform foundation analysis of the following foundation types: pin-pile, monopile, gravity base and suction caissons. Various other types of geotechnical analysis including pipe-soil analysis, cable burial risk assessment, trencher performance predictions and assessment of rock placement structure stability on the seabed are also carried out when required.

Our client (Zennor Petroleum) was developing an oil and gas project involving two new oil wells tied back to a platform by a new 20km pipeline. As part of the scope Ternan performed an independent review of all pipeline related geotechnics performed by the installation contractor. This determined the lateral deflections of their well conductors when exposed to a fishing load of 650kN acting on the wellhead 4.8m above seabed.

Phase 1: Lateral Conductor (Pile) Analysis

Following review by Ternan of the site investigation data, the ground conditions at the conductor location were found to be soft clay with an approximately linearly increasing shear strength profile. The lateral behaviour of the conductor was modelled by employing non-linear soil p-y curves which represent the relationship between lateral soil resistance (p) and lateral displacement (y) and applied as uncoupled springs down the length of the pile. This was all performed within the commercial software package LPile. Figure 1 below presents some of the inputs and results of the LPile analysis.

 

Case Study 9 LPile Example

Figure 1 - Outputs from a Lateral Pile Analysis

 

Phase 2: Downwards Resistance of Pipeline in Rock Dumped Trench

The downwards resistance of the trenched and rock dumped pipeline was verified by Ternan Energy using commercial software PLAXIS 2D. Figure 2 below presents the geometry of the ploughed trench and pipeline covered by rock dump. The shading shows the magnitude of displacements in the soil. As the pipeline is displaced downwards the hot colours denote the greatest displacements. The graph insert shows the calculated downwards resistance - pipe displacement curve from the PLAXIS 2D analysis. 

 

Case Study 9 PLAXIS 2D Example

Figure 2 – Soil Displacements and Calculated Downwards Resistance of Pipeline in Rock Dumped Trench

The findings of our lateral conductor analysis were provided to the client in a report which was both on time and within budget. The PLAXIS 2D analysis Ternan performed provided a diligent and well received review of the contractor’s analysis.

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