Subsea Integrity Management is defined as the management of a subsea system or asset to ensure that it delivers the design requirements while not adversely affecting life or the health of the environment throughout the required life of the field¹.
Achieving the ideal dryness for a subsea natural gas pipeline is a crucial step not only for commissioning of a pipeline, but also for is subsequent integrity management. Attaining the correct dryness level can help inhibit microbiologically influenced corrosion (MIC), hydrates and other issues.
Drying of a pipeline is usually completed in stages that involve one or more of the following techniques: pigging, methanol/glycol swabbing, air drying, vacuum drying and nitrogen packing.
Selection of methods for drying is often driven by economics and time restraints, without sufficient consideration given to operability and corrosion issues.
Leak testing, commonly confused with hydrostatic testing, is a means of verifying the quality of facility construction. Hydrostatic testing uses liquid media under pressure to test the structural integrity of weld joints and piping spools, while leak testing uses gas or service media, at or close to the maximum working pressure of the system, to serve as a final confirmation that the system is “leak tight” and ready for service. Leak testing simulates “live” conditions without actually using hydrocarbons, allowing flange connections to be checked for “tightness”.
In this GATEKEEPER series, we introduced the Ultimate Gain Plot (Figure 1) and the variables dead time (DT), time constant (TC), and controller gain (Kp). These four aspects along with a basic understanding of the control loop to be tuned, are all that is required to develop preliminary tuning parameters. The examples in this GATEKEEPER demonstrate simple tuning rules.
This is part two of the GATEKEEPER series on control systems tuning. To effectively tune a control loop, there needs to be an understanding about the dynamics of the system. The intention of this GATEKEEPER is not to provide a detailed review, but to provide an 80/20 solution.
Effectively tuned control loops provide for more efficient and safer operation. After process startup, there are various techniques available for tuning controllers ranging from trial and error methods to mathematically sophisticated programs. Few options are available for tuning loops prior to startup. Many control systems are started with the manufacturer’s default tuning parameters. This series of GATEKEEPERS will provide methods for using readily available process design data for determining effective tuning parameters before startup.
A great deal of work goes into making operating procedures accurate, but a procedure that is accurately written may be implemented incorrectly.
Studies suggest that humans conducting simple, mundane tasks make an error roughly 1% of the time. Error rates for complex tasks are much higher. Some procedures are more error-prone than others. It is incumbent upon us to write procedures that are not only accurate, but that are likely to be implemented without error.
The airline industry has dramatically decreased the incidence of human error, in part by focusing on development of effective procedures and on instilling a culture in which the procedures are actually used. We can do the same in the oil industry.
It is frequently necessary to displace the contents of a pipeline or umbilical tube (fluid B) with another fluid (fluid A). If we don’t use a pig to separate the liquids, there will be mixing at the interface (axial mixing). The mixing zone requires us to overflush the line to effectively remove fluid B from the line. Below, we address a method of calculating the length of the mixing zone in order to determine the effective overflush requirement for a given pipeline or umbilical tube.
It comes up very often in projects—”Why can’t the construction team do the commissioning”. On smaller projects this may be acceptable with an experienced team and involvement with operations, but for major capital projects it pays to have a dedicated seasoned commissioning team to execute pre-commissioning and commissioning work.
Hydrotesting of pipelines and equipment, including tanks and vessels, is a key part of ensuring that they are fit for purpose depending on factors such as contact time, chemicals used, oxygen and bacteria. This may result in general corrosion, crevice corrosion, pitting corrosion, differential aeration corrosion or microbially induced corrosion (MIC). MIC will take place due to the introduction of bacteria during the hydrotesting and/or parking of equipment. Corrosion caused by any one or combination of these mechanisms may reduce pipeline and equipment service life and in extreme cases make it unfit for purpose.
Deepwater oil and gas facilities are designed, constructed and commissioned by multiple teams with multiple objectives. Consequently, design disconnects and lack of foresight become apparent during the commissioning and startup phase.
Recent commissioning and startup experiences on an FPSO have provided many valuable lessons regarding such issues. This GATEKEEPER explores how a foreknowledge of design flaws and common discussion would have prevented significant problems seen during the final commissioning and processes startup.
Either by nature, or by training, engineers are conservative. That is generally a good thing, but we sometimes go too far. For example, chokes and control valves are often oversized even for normal operation, and are sometimes far too large to provide adequate control of low flow rates at initial startup. Startup planning should include an assessment of the operability of chokes and control valves.
Five different conditions exist for flow through restrictions:
- Liquid flow
- Non-critical gas flow
- Critical gas flow
- Non-critical two-phase flow
- Critical two-phase flow