1.1.Parts of an offshore structure
- Superstructure
- Substructure
- Foundation
1.2. Classification
Jackets
Towers
Compliant Towersr
Gravity base structures
1.3. Codes and standards
2. Loads and Load Cases
Assumption for rough hydrodynamic load estimation
- We consider only the horizontal components of hydrodynamic loads
- the time variation of the load and the dynamic response of the structure is not taken into consideration
- We consider only the time moment that generates the maximum horizontal load and/or the maximum overturning moment
- We assume 100-year wave plus (100 year) current.
5. The phase differences due to horizontal spatial separation is neglected - The spatially distributed horizontal load on individual members is replaced by a number of horizontal point loads at the nodes of the space frame
Fh=ΣFhi Moh=ΣhiFhi - The structure is represented by a collection of vertical members or by just one vertical member (equivalent stick model) FdD
3. Morison Equation
Morison Equation
Example 1
Example 2
4. Structural Analysis
Main Structural concerns:
Strength
Stability
Deformations
Vibration and Fatigue
General Analysis Approach
1. Determine the support reactions (satisfy the global equilibrium)
2. Consider the structure as a continuous beam and determine the distribution of shear forces and bending moments over the height of the structure
3. Determine the internal forces of the actual space frame and of the foundation piles.
Structural Design
1.1. Rules of Thumb
1.2. Fatigue
Damage ˶̴ (stress range)^3
Yield stress does not affect the fatigue resistance of welded connection
2% higher stress range will result to a reduction of 1.2^3 of the fatigue lifetime ( three since m=3)
3 years/1.2^3=1.73 years
Nnew=c/(1.2Δσ)^3
7. Inspection & Maintenance
8. Decommissioning