Structural Integrity Management

Structural vibration analysis vs. static analysis

Static, Quasi-Static, and Structural Vibration (Dynamic) Analyses are recommended for offshore production facilities. Since these terms can be confusing, this section explains the differences and who should be involved in the evaluation. Static Analysis focuses on evaluating stress and buckling of members under constant loads. Constant loads can also be described as loads applied at a frequency of 0 Hertz (Hz). This type of analysis can also focus on deflection of skid members as they can affect alignment of equipment.

Quasi-Static Analysis:

evaluates the effects of loads that are periodic, but at a low enough frequency relative to the natural frequencies of the equipment package so that the inertia effects of the structure do not come into play. They tend to have a frequency of less than 3 cycles per second or 3 Hz.

Structural Vibration (Dynamic) Analysis:

predicts the dynamic effects of the machinery itself so that resonance can be avoided. Dynamic loads include imbalance, misalignment, pulsation forces, cross-head guide forces, cylinder gas forces, moments, and other forces (see Figure 5 for an example of dynamic forces in a reciprocating compressor). The machinery related loads occur across different frequency bands and can cause localized structural resonance.

Structural Dynamic Analysis:

focuses on evaluating vibration levels and stress. Limiting vibrations of structural members is important to control vibration of the equipment, vessels, and piping that are attached to it. If a skid member has high vibrations then these components will likely experience high vibrations also.

Structural/ Floor vibration

The term ‘vibrations’ when applied to floors refers to the oscillatory motion experienced by the building and its occupants during the course of normal day-to-day activities. This motion is normally vertical (up and down), but horizontal vibrations are also possible. In either case, the consequences of vibrations range from being a nuisance to the building users to causing damage to the fixtures and fittings or even (in very extreme cases) to the building structure. The severity of the consequences will depend on the source of the motion, its duration and the design and layout of the building. Sensitive process/equipment such as Nano-technologies, microscopes or lasers may be sensitive to a level of vibration which is often below human perception. In such special cases, malfunctioning of the equipment is avoided by limiting the level of vibration to the specific requirement of the equipment. Severe vibration events due to earthquakes and explosions are outside the scope of this article.

Model Testing

Floor vibrations are generally caused by dynamic loads applied either directly to the floor by people or machinery or indirectly by moving floor supports after transmission through the building structure or through the ground. The principal sources of vibration in buildings are:
• Human activity, e.g. walking, dancing, jumping, etc
• Vibrating machinery
• Model analysis
• External forces, e.g. traffic at ground level or underground, or wind buffeting

Source of vibration

The purpose of modal testing is to establish experimentally the modal properties of the structure. There are two types of modal test. Those where:
• The excitation force creating the response is not measured.
• The excitation force creating the response is measured

Model Testing without measuring without excitation force

There are three types of test in common use:
1. Ambient vibration survey (AVS)
2. Heel-drop excitation
3. Rotating mass shaker excitation.
In the ambient vibration survey the floor dynamic excitation is provided by the environment in which it resides. Vibration responses to this kind of excitation are acquired over a grid of test points covering the floor area of interest. This grid needs to be dense enough to describe all of the likely floor mode shapes in sufficient detail. It is also necessary to ensure that the modes of interest are excited by the ambient environment and that the reference transducers are away from the nodal points of the mode being measured.

Static Analysis	Quasi-Static Analysis	Vibration (Dynamic) Analysis
Dead loads, including weight of permanent equipment	Environmental loads including wind, current, wave, earthquake, ice, earth movement, and hydrostatic pressure occurring in any direction	Unbalanced forces created by rotating and reciprocating weights (e.g., crankshafts, piston assemblies)
Thermal loads including forces created by temperature changes and pressure	Construction loads including load out, transportation, and installation	Cylinder gas forces created by the differential pressure between the head end of a cylinder and the crank end
Drive torque of compressors and engines	Fatigue analysis may be done on loads such as those caused by Waves	Vertical forces on the crosshead guides
Lifting or dragging loads when moving the skid with cranes or winches. These loads can include a load factor that considers the impact from sudden stops or motion of the lifting equipment (e.g., offshore lifts). A load factor of 1.15	...	Pulsation-induced shaking forces in the piping system

Rack Testing	Working Pattern	Services	Faults
• The Effect of Floor Vibrations on Racks • The Strength of Material • The Design of Rack • Resonance Effect • Effect of Fatigue for Cyclic Load	• After labeling points, dead load was arranged of approx. 2 ton. • Response of lifter with 2 Ton load was observed at Rack/Floor. • Vibration induced on Rack/Floor while movement of loaded lifter was captured and recorded in Analyzer. • Response in terms of vibration is compared with ISO 4866:2010 Chart which is shown in coming slide.	• Load test • Multi-channel vibration monitoring • Phase analysis • Stress analysis • Model analysis • Fatigue life cycle analysis • Foundation and system dynamic analysis	• Uprights frontal lateral • Braces ( horizontal and diagonal ) • Sheared and twisted columns • Anchoring • Local repairs • Beam reflection • Out of plumb