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Damage and defects in the structures of offshore platforms can develop during design, construction, operations, and / or maintenance. Corroded members, cracked joints, dented and bent members are the results. This paper explores the effects of such damage and defects on the lateral loading capacities of a conventional steel 8-leg template-type platform. The computer program ULSLEA (Ultimate Limit State Limit Equilibrium Analyses) is used to evaluate the effects of damage to and defects in the critical load carrying members in such platforms. The effects of repairs on the lateral loading capacities are also studied. The results from the numerical analyses indicate that this type of platform is very robust. This type of platform can tolerate very significant amounts of damage to important load carrying members and not suffer significant decreases in the lateral load carrying capacity of the structure system. The analyses indicate that the robustness in these structures is derived from a combination of redundancy (alternative load paths), "excess" capacity in the important load carrying members, and ductility (ability to deform plastically without loosing strength). Grouting repairs are indicated to be particularly effective in restoring the capacities of damaged and defective braces and joints. INTRODUCTION During their service period, many steel structures sustain damages and deterioration, such as member denting, bending, corrosion, and joint cracking. Repairs to the structure, such as grouting, may be performed to postpone and/or remediate the effects of these damages on the structure's integrity. The degree of damage the structure can withstand before its integrity is lost and its safety is jeopardized varies substantially. Robust or "damage tolerant" structures incorporate redundancy (excess load carrying members), ductility (ability to deform without substantial loss in load carrying capacity), and excess capacity (capacity beyond that required for the "primary loadings").
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems (1.00)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (0.69)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (0.69)
This paper summarizes a simplified method with which strength-level earthquake analyses of jacket-type platforms can be performed. By examining the primary bending, shear, and foundation rotation responses, estimates of platform vibration characteristics can be obtained from which earthquake forces can be estimated by the response spectrum method. This process is referred to as SRSA (Simplified Response Spectrum Analysis). These forces can then be taken together with capacities derived from ULSLEA (Ultimate Limit State Limit Equilibrium Analysis) to develop an evaluation of the demand-capacity behavior of the platform. The ULSLEA-SRSA method is applied to the assessment of two platforms. Results from 3- D frame analyses of the two platforms are used for validation of the simple approach. Agreement between the ULSLEA-SRSA and detailed 3-D analyses is excellent. Several studies related to the simplified assessment of platforms subject to earthquakes are documented in this paper. In the first, a design code approach to earthquake forces based on that contained with the Uniform Building Code is demonstrated and compared to more detailed earthquake force estimates. Next, common simple approximations to pile-head stiffnesses are reviewed, and the impact of foundation flexibility on platform response examined. Last, the impact of local inertia forces on brace axial capacity is studied. INTRODUCTION During the past five years there has been growing interest in the development of simplified structural analysis methods which are inexpensive to apply yet provide sufficiently accurate results to help make timely and economic engineering assessments. A major reason for this development is the re-assessment of aging infrastructure. As many structures (buildings, bridges, offshore platforms, etc.) approach the end of their original service lives, many owner/operators desire to keep these structures in service. As many of the structures in existence today were designed for much less stringent load criteria than current code recommendations, some form of analysis must be performed.