Onbottom stability
This article is an introduction, and a brief how-to guide for performing a simple On-bottom stability analysis.
Jacket resting on the mud mat in its pre-piled state.
Notice: This article is currently being written. The final article would not have this notice.
Introduction
In its pre-piled condition, an offshore jacket structure resting on the sea bed1 is vulnerable to sliding and overturning. On-bottom stability analysis is performed to assess, and optimize jacket’s mudmat2 size.
Following factors influence the size of the mudmat:
- Weight of the structure.
- Hydrodynamic loading on the structure.
- Undrained shear strength of soil at mudline.
Analysis
On-bottom stability analysis of a substructure is performed in two following stages.
- Determining hydrodynamic loading.
- Iterative stability analysis—per environmental attack direction.
Hydrodynamic loading during On-bottom
For pre-service operations, such as On-bottom, and pile-driving, milder weather windows are typically considered. The hydrodynamic action criteria during such operations, therefore, is usually milder than the 1-year (Operation), or the 100-year (Storm) condition.
For example, the following is used typically in tropical regions.
| Wave height (m) | 3 |
|---|---|
| Wave period (s) | 6 |
| Current (m/s) | As per Metocean data. |
Computer Model
A fairly detailed finite element model shall be used to determine hydrodynamic action on the substructure.
In addition, drag coefficients (Cd) shall either be calculated, or suitably considered (in terms of percentile) on members that carry unmodelled items such as anodes.3 Failing to account for forces on unmodelled items could result in underestimating overall hydrodynamic loads on the structure, and may severly limit its capacity to resist overturning moments and base shear.
Boundary condition: It is considered adequate to fix the substructure (jacket) model at its work-point elevation, which is usually higher than the sea-deck elevation.
A number of wave directions shall be considered.
| 3-legged | At least 12-directions—4 orthogonal, and 8 diagonal. |
|---|---|
| 4-legged and above | At least 8-directions—4 orthogonal, and 4 diagonal. |
The angles of direction shall be chosen such that all significant wave attacks resulting in maximum overturning and base shear are covered.
Iterative stability analysis
Iterative stability analysis is performed to determine the resistance of an unpiled substructure for a given attack direction, and address it by sizing the mudmat adequately, where necessary.
Iterative stability analysis could easily be performed either by hand-calculations, or by use of spreadsheets.