Ensuring the Reliability of Offshore Gas Compression Systems

Summary Selection of offshore gas equipment is based on intended application, efficiency, reliability, and compatibility with platform structural requirements. The compressor system should be free from all potential dynamic problems. This paper discusses potential dynamic problems. This paper discusses techniques for analyzing dynamic vibration and failure problems in reciprocating and centrifugal compressor designs. Introduction To ensure the reliability of offshore gas compression systems, it is necessary to anticipate potential problems in the design stage. Many problems occur problems in the design stage. Many problems occur because equipment selection and design audits on the gas compression units are made for individual units rather than the system as a whole. It is the entire system response that determines the acceptability of the individual units. Selection of compressors, for example, without an adequate understanding of the effect of the piping acoustical characteristics on compressor vibration, pulsation, and performance can lead to many problems.To select proper equipment, first consider the applicable codes such as API Std. 617, and API Std. 618 that deal with centrifugal and reciprocating compressors. Since offshore design requirements are more stringent than normally required by the codes, any design oversights or installation mistakes can be costly, as experience has shown. It is generally preferable to avoid the prototype machines that preferable to avoid the prototype machines that involve significant extrapolation from proven technology in rotor dynamics or machine operating pressures, temperatures, speeds, etc., since the pressures, temperatures, speeds, etc., since the debugging that may be necessary is very costly.The prime concerns of this paper are vibration and fatigue failures of centrifugal and reciprocating machines, the piping, and the platform, and each is discussed, as well as the analysis techniques available to prevent such problems. Pulsation Generation Mechanisms Pulsation Generation Mechanisms Piping vibrations and stresses are major problems Piping vibrations and stresses are major problems encountered in offshore gas compression systems. These vibrations and stresses are generated both by internal flow or pulsation forces in the piping and by mechanical excitation from the machinery. Pulsations usually cause more problems than Pulsations usually cause more problems than mechanical excitation since mechanically induced piping vibrations normally are limited to the running piping vibrations normally are limited to the running speed of the compressor and its lower-order multiples.Pulsation amplitudes in a piping system are dependent not only on the dynamic energy generated by the pulsation sources in the system but also on the acoustical response characteristics of the piping system. Pulsation energy can be generated by a number of mechanisms in a piping system (Table 1), including the following. Reciprocating Compressor Pulsations The intermittent flow of a fluid through compressor cylinder valves generates fluid pulsations that are related to a number of parameters, including operating pressures and temperatures, cylinder horsepower, capacity, cylinder pressure ratios, cylinder clearance volumes, phasing between cylinders, thermodynamic fluid properties, and cylinder and valve design. Pulsations are generated at discrete frequency components corresponding to the multiples of the compressor operating speed. The actual resulting pulsation pressures in the piping system depend on the combination of the pulsation spectrum generated by the compressor and the acoustic resonance effects of the piping. JPT P. 2252