Maintenance pigging of many offshore pipeline assets is, in many cases, becoming increasingly challenging with the development of deep-water applications, the use of exotic materials and more demanding operating conditions. In a number of recent projects, ROSEN has combined its flow assurance and pigging feasibility expertise in order to support operators preparing for and executing several highly challenging offshore pigging applications.
This work has shown significant benefits in confirming pigging feasibility, developing optimized pigging campaigns and preparing robust operational plans. In this article, Ashwin Pinto and Paul Westwood explain how flow assurance inputs help to ensure that project goals are achieved safely, effectively and within budget.
For many operators, pigging is often a non-routine or infrequent operation, making it an unfamiliar task. There can be disagreement on the best way to proceed where pigging requirements are deemed to be non-standard or complex. It is estimated that there are around 3.5 million kilometers of high-pressure pipelines for oil, gas and water worldwide, with about 25,000 kilometers of new pipelines added every year. However, around one third of these pipelines are deemed “unpiggable” using conventional free-swimming pigs. From an integrity management perspective, there is an increased desire to inspect so-called “unpiggable” (or difficult-to-pig) pipelines to be able to demonstrate that they can continue to be operated safely and reliably.
The main challenges for pigging in such pipelines can be broadly classified into “construction challenges” and “operational challenges.” Figure 1 summarizes some of the key technical and operational challenges and shows how these can typically affect pipeline piggability.
PIGGING AND PIPELINE INTEGRITY MANAGEMENT
Pipeline pigging is an integral part of a pipeline asset integrity assurance program. This can include maintenance pigging as part of a corrosion management strategy, helping to minimize excessive liquids or debris hold-up to improve operational capacity or perform an ILI operation.
With any pigging operation, it is important in the first instance to be clear about the ultimate objective before the right pigging equipment is selected (and configured) ahead of operational execution. The complexities and challenges posed by “difficult-to-pig” pipelines are often addressed by pipeline operators through performing a pigging feasibility study. This upfront review should consider the overall pigging project objectives and investigate the range of options available in order to demonstrate and predict the viability and cost of a chosen option. In addition to the obvious risks and practical challenges, such studies can be structured to address other potentially significant external factors as contributors to the full lifecycle costs of a pipeline.
Many of the aspects that need to be considered when preparing and executing complex pigging operations at times present major technical or operational challenges to operators, accompanied by real (or sometimes perceived) risks and significant economic barriers. The challenge is to analyze all factors objectively, drawing on experience where possible, to make an informed decision – and to act on it. Figure 2 shows the process ROSEN adopted to do just that.
Depending on the outcome of the pigging feasibility study, a standard “off-the-shelf” tool may prove to be adequate, but it is often necessary to implement a strategy – and custom-build a tool – that addresses any and all complexities of the pipeline system.
It should also be noted that the term “challenging” is often a function of the project economics, a critical factor that determines the choice between different available options. A robust cost model should account for the global impact of a campaign over the full project lifecycle. Some campaigns can be very difficult to calculate at the feasibility stage. Therefore, they should contain appropriate contingencies or be based on previously performed analogous jobs. Contemplation of ancillary support services such as chemical injection and subsea intervention costs need to be considered, as does a scoring system used to help identify the optimum pigging solution.
FLOW ASSURANCE AND PIGGING
Flow assurance is broadly defined as the ability of a production system to transport fluids from wells through the pipeline in a safe and economical manner. The economics of any production asset depend on the reliable deliverability of the system. Pipelines can plug due to hydrates, wax and asphaltenes severely reducing the production capability, with an eventual risk of blockage. Loss of integrity could occur due to corrosion, erosion or severe slugging. Any deep-water intervention may result in costly downtime, with serious environmental and safety implications.
Maintaining the highest throughput efficiency and pipeline integrity requires a lot of focus on addressing flow assurance challenges such as solids/liquid management, corrosion, slugging, operational pigging, etc. (Figure 3).
Figure 3 – ROSEN Flow Assurance Service
From a pigging perspective, flow analysis is essential to estimate the amount of solid deposits such as sand, wax and liquid accumulation at various production conditions. Transient pigging analyses are performed to assess the sufficiency of pigging pressure, suitability of pig design and configuration. A worst-case sensitivity could predict a pig getting stuck due to excess deposit build-up in front of the pig. This allows for the determination of the pigging frequency required to avoid excessive inventory build-up, which is a function of the rate of solid/liquid deposition and downstream facility processing constraints.
In certain cases, bypass pigging is advantageous in order to maximize production flowrates during pigging operations. Bypass ports allow pigs to “bypass” production fluid through the pig body via bypass ports (see Figure 4). Pig bypass reduces the pressure drop across the pig and therefore reduces its velocity when compared to the bulk fluid velocity. This means that cleaning pigging can be effectively carried out at increased production flowrates and with decreased load on the downstream liquid handling capacity.
Figure 4 – Bypass Pig – Working Principle
The calculation of an optimum bypass opening requires careful consideration of the fluid properties, pigging flowrates, system backpressures, liquid hold-up, slug handling volume, drainage rate, tool design and pig-pipe wall frictional forces. Flow modelling is used to model the bypass pig behavior and effectively size the bypass port opening.
ADVANTAGES OF COMBINING FLOW ASSURANCE AND PIGGING FEASIBILITY
Every pipeline asset has a unique process operational envelope (Figure 5) bound by the limits set by the production targets, costs, design constraints and allowable process limits. Similarly, a pigging operational envelope exists for a conventional cleaning pig and, more importantly, for in-line inspection pigging.
For an “unpiggable” system, flow analysis plays an important role in determining the flexibility of the process and pigging operational envelopes. In most cases, depending on the pipeline asset, the process operational envelope may be modified to suit the pigging envelope, or the pigging envelope may be modified to fit into the process operational envelope. In some cases, neither the pigging nor the operating envelope can be modified, which may classify the operation as “unpiggable.” However, a thorough flow assurance analysis combined with a detailed pigging feasibility study may make it possible to tailor the pig type, design, configuration and pigging strategy, along with the process conditions, to ensure piggability of the system.
CLEANING PIGGING STRATEGIES
In challenging applications, a particular cleaning pigging methodology ultimately chosen for a specific pipeline is a function of several factors, including:
- Process operating conditions (flows, pressures, temperature)
- Risk of the pig blocking the line
- Time available for cleaning operations
- The degree of cleanliness to be achieved
- Operational constraints
A progressive pigging strategy may be required, depending on the internal conditions, operational limits, pipeline geometry and types of pigs considered. A progressive pigging campaign involves running a series of pigs that progressively increase in aggressiveness in order to remove deposits gradually during each run.
Prior to performing pigging, however, the flow conditions in the pipeline should be ascertained by performing flow tests to minimize the associated risks:
- Eliminate the presence of restrictions due to excessive deposit build-up
- Confirm the communication of fluids end-to-end in the pigged section
- Identify if the system is configured correctly
Typically, the aggressiveness of the subsequent pig run will be determined by the results of the previous run. In cases where significant deposits are suspected present in the line, a scout pig, such as a medium-density foam pig, may be used. This pig is designed to pass through restrictions and potentially disintegrate where the passage is too narrow due to a restriction possibly caused by stubborn heavy deposits. Once passage is confirmed, this may be followed by solid body pig(s), increasing in aggressiveness with each run. If the pig returns with a lower amount of deposits, a more aggressive pig can be considered for the next run. An aggressive pig may use a combination of cups/discs with brushes, studs, magnets, bypass, etc.
Flow assurance analysis is essential in the development of pigging programs aimed at understanding internal pipeline conditions, quantifying the associated risks and eventually optimizing cleaning pigging campaigns.
Flow assurance and pigging feasibility studies allow facilitate a detailed assessment and the quantification of the risks of performing pigging in challenging pipelines.
Combined, such studies can provide pipeline operators with cost-effective solutions to minimize and manage threats in complex operating scenarios on challenging assets, providing assurance of pigging and operational readiness to support safe operations.