FCG

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Pre-Commissioning Training #3

FLOODING, CLEANING & GAUGING

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Introduction to the FCG of a pipeline

One of the essential Pre-Commissioning operations to keep the pipeline functioning at its best is the Flooding, Cleaning and Gauging (FCG). This involves a combination of flooding, cleaning, and gauging, which is often carried out using multiple pigs in a single pass. The pig train is carefully designed to ensure that every aspect of the operation is completed efficiently.

Multipurpose pig used during a FCG

Flooding

The first operation done through FCG is flooding. This process is critical for cleaning, gauging, and hydrotesting, and we do it usually at a speedy 0.5 m/s - 1 m/s pig speed. But before we start flooding, we need to make sure that the fluid we're using won't harm the pipeline or its coating. If we don't know the quality of our water source, we'll take a sample and analyze it. And if we plan to keep water in the pipeline for an extended period, we'll use chemical treatments to prevent bacterial growth and internal corrosion.

Water intake for water winning lines to be used for FCG. Here, you already know you'll need strong filters and maybe divers to prevent all this trash from going into the lines!

Divers musn't go near a working water intake. Everything must be stopped before they perform their job.

Pre-flooding

What you can also do before starting pigging operations is to pre-flood the line. This is something usually done when you flood from shallow water / topside towards deep water. Indeed, it may be necessary in order to add back-pressure to slow down pigs with air in front of them and water in the back. Indeed, high speed may damage pigs, reduce their sealing ability, create air pockets, etc. Alternatively to pre-flooding, if for instance it is difficult to handle the water volume of pre-flooding, you can do air packing to control the pig speed with higher air pressure. Air packing can however increase the risk of air pockets with high pressure air bypassing pigs. In any case, flooding from the lowest point is the preferred option to control the pigs and have as little residual air as possible.

Different flooding methods

Simultaneous flooding, cleaning, and gauging are common practices for offshore pipelines. Pipeline flooding can be performed using different methods depending on offshore or onshore conditions: - Free Flooding / Forced Flooding: Seawater fills the pipeline freely before caps are installed. Pigs may be used, but the main idea is simply to let water enter and fill the line. This can be followed by forced flooding using pumps to complete the filling remove residual air. - Unassisted Flooding / Assisted Flooding: The pipeline is prepared for pigging with a pig train ready to launch. Water enters through an orifice plate connected to a subsea pumping unit with filtration and chemical injection. Caps are already in place. - Unassisted flooding occurs when most of the line (>90%) is already flooded and relies solely on differential pressure until front and back pressures equalize.- Assisted flooding uses boost pumping to finish the pigging and can either follow unassisted flooding or start at the beginning of the FCG operation. Topside flooding typically uses only this phase.

Different flooding methods

For unassisted flooding, an orifice plate can be used to control the flowrate, with its design based on water depth and the required pig speed. When performing free flooding, caution is required: filters should be used to minimize the ingress of organic particles or sand, and the water intake should be positioned away from the seabed. Additionally, it is important to assess the impact of exposing the pipeline to untreated water if no chemicals are added during free flooding.

Orifice plate (the holes allow the seawater to enter the filtration & chemical treatment system)

Hot stab (tool used to connect and inject fluid into a subsea structure / pipeline)

Flooding requirement

For instance, a requirement could be: "Water to be used for flooding should have a minimum quality corresponding to filtration removing suspended particles larger than 50μm, and should have an average content of suspended matters not exceeding 20g/m3." You can still write a Deviation Request during the project to modify filtration level for precom activities:

Flooding chemicals

Chemicals are a big part of any flooding operation, and it's crucial to get the concentrations just right. The client specifies the amount of chemicals we'll use, based on how long they want the pipeline to be protected. And because there's always a small chance we'll find damage during precom we usually protect the pipeline for at least 12 months.We also generally use a dye to help us identify any leaks tests. This dye is critical because it allows us to inspect the pipeline with a ROV and detect any issue. So if you see some fluorescent dye swirling around, you'll know exactly what's going on (and that’s not good…).

Flooding chemicals

Various chemicals may be used during FCG. It typically includes oxygen scavenger, biocide, corrosion inhibitor as well as dye. Most of the time, a cocktail of these will allow to only use a single chemical to preserve the pipeline. Here are some examples: - Roemex RX-5255 : a 4 in 1 liquid cocktail comprising corrosion inhibitor, biocide, oxygen scavenger and leak detection dye. - Roemex RX-5208 : a combined oxygen scavenger and biocide designed to allow the oxygen scavenger to be released into the seawater before the biocide. This allows the seawater to be deoxygenated prior to release of the biocide. This way there is no adverse interaction / inhibition between chemicals. - Nalco Champion Hydrosure O-3670R : a combined oxygen scavenger and corrosion inhibitor.

Note: Chemical use for Pre-Commissioning will be more detailed in the chemicals training module.

Cleaning purpose

It's good practice to remove any removable debris to prevent product contamination. A clean pipeline performs better, requiring less energy to drain the product, especially in long pipelines where debris significantly affects flow. Without debris, corrosion inhibitors adhere better to the pipe surface, enhancing their performance. During drying, once a certain level of dryness is achieved, a cleaner pipe traps fewer particles (and thus retains less water), making the drying process more efficient. Finally, this practice ensures that equipment is not damaged by debris. Please note that operations are supposed to clean the quads of pipeline in the firing line before installation. If it's not done correctly, the pigs will suffer during FCG...

Carcass of a flexible pipe can easily trap lots of water / debris in its cavities

Cleaning concept

The pipeline cleaning concept should consider among other things:

• Protection of pipeline components and facilities (e.g. valves) from damage by cleaning fluids and pigs

• Testing devices

• Removal of substances that may contaminate the product to be transported

• Particles and residue from testing and mill scale

• Organisms and residue resulting from test fluids

• Chemical residue and gels

• Removal of metallic particles that may affect future inspection activities

Cleaning process

Typically, clients must approve a single-pass cleaning operation according to a specified procedure. This approach minimizes the need for further cleaning requests and reduces the risk of delays. Since methods for verifying pipeline cleanliness can differ, effective risk mitigation is crucial. For pipelines with internal coatings, it's important to carefully select the materials used in mechanical cleaning pigs to avoid damaging the coating. For example, wire brush pigs are unsuitable for pipelines coated with materials like Inconel, as they can cause damage. Instead, nylon brushes may be preferred.

Nylon brushes

Cleaning inspection

After the pig train run is completed and the PLR is recovered to the vessel, each pig is removed and any debris in front of the pigs is collected and weighed. The amount and composition of the debris recovered is assessed. An usual cleanliness criteria is less than 0.5 kg/km of dry debris collected in front of the pig train. This is a client requirement that may change.

Pigs after a cleaning run

Gauging

Pipeline gauging is a critical aspect of pigging operations that cannot be overlooked. It ensures that the pipeline is free from any internal geometrical flaw that could hinder its efficient functioning. The gauge pig, equipped with an aluminum gauging plate, must be carefully designed for each system. For the gauging of a multi-diameter pipe, a caliper pig will be necessary. A smart gauge plate might be used to timecode the moment the plate gets damaged. Flexible gauge plates are now also being used.

Gauge plate

Smart gauge plate

Multiimpact and multidiameter gauge plate

Gauging requirement

For instance, a typical client specification could be: “Gauge pigs shall be identical to cleaning pigs and the pigs shall include a gauge plate mounted in front of the last set of sealing discs with two sets of separate guiding and sealing discs. Gauge plates shall be machined aluminum 6 – 10mm thick and removable. The plate should have 8 radial slotted incisions at 45 degree intervals to allow the plate to buckle in the presence of a bore reduction, ovality or foreign debris. The leading edge shall be chamfered 45° to half the plate thickness. Each gauge plate shall be permanently marked with a unique identifier before insertion into the pipeline. Gauging pigs shall be fitted with a pig detector and its position in the line monitored. Gauge plates shall be 95% of the minimum nominal ID of the pipeline." While the client specification above outlines the standard gauging process, several other options are available, depending on the pipeline's complexity. These options range from electronic equipment to data recording capabilities, providing clients with a wide range of choices.

Gauging acceptance

To be accepted, a gauge plate must present no deflection. Minor abrasions must be analyzed by the operator for acceptance. The gauging can be deemed successful as long as one of the plates is in top shape. If none of the plates is good enough to conclude the gauging, there will be investigations that will take into account the amount of debris discharged, pipeline installation history, pig position during eventual pressure spikes etc.

Accepted gauge plates

Discharge of debris

Topside FCG spread

Now that you're aware of what the FCG of a pipeline is, let's take a look at the spread used to perform it! The required equipment and resources vary depending on factors such as the required flowrate, pipeline diameter, length, water depth, etc. However, sub-contractor services companies are usually equipped to perform flooding operations from either topside or remote subsea locations.

Topside flooding equipment is typically modular and tailored to the project's maximum requirements. This includes a break tank, fuel tank, flooding pumps, filter skid, chemical injection skid, hose reel, test/control cabin, pressure/flow metering system, hoses...

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Subsea FCG spread

For deep waters where handling and deploying longer hoses is difficult, a subsea-based or remote flooding and testing system can be used. It can be connected to a PLET for instance and offers an alternative to topside spread. The Subsea Pumping Unit will be deployed on the seabed near the structure and hydraulically powered by a ROV when needed.

Subsea pump connected to a PLET

Subsea pump with flooding arm (the arm is used to connect to subsea structures)

Be aware that a subsea spread is rated for a max water depth as flow will be limited as depth increases.

Note: A subsea pump can be connected to a structure either with an arm or a hose.

FCG pig train design

During the design of the pig train that will be used for FCG, the following shall be considered among other things:

• The number of pigs needed for the operation will often depend of the length of the line. Different clients will have different requirements for the number of pigs required to perform FCG.

• For short lines, a single multi-purpose pig may be used.

• However, some clients may require at least three pigs in the pig train to avoid spurious damage. This is done by having two gauge plates on separate pigs and a cleaning pig in the front to avoid damaging gauge plates.

• During Pre-Commissioning, pigs with bypass are almost never used, if at all...

Multipurpose FCG pigs

FCG pig train design

• Regarding the wire brushes, they are usually not compatible with Corrosion-Resistance Alloys, flexibles and internally coated lines.

• Foam pigs are also not recommanded for FCG as they are less effective at cleaning / gauging than metal bodied pigs.

• Only "Bi-Di" pigs ( i.e. Bi-Directional, pigs which can be run in reverse) will be used for the operation.

• The fluid pumped in front of the first pig and in between each pig is proportional to the length of the line.

FCG standards

You will rely during the engineering of the FCG not only on the company's applicable standards but also on international standards. You can keep those in mind for general FCG requirements:

DNV-RP-F115

DNV-ST-F101

ASME B31.4

API RB 17B

ISO 13628

Pre-Commissioning of submarine pipelines

Design and operation of subsea production systems

Pipeline transportation systems for liquid hydrocarbons and other liquids

Submarine pipeline systems

Recommended Practice for Flexible Pipe

For your information, "RP" means "Recommended Practice", it is not mandatory to follow those guidelines.

General checks

Refer to Pre-Commissioning department checklists to see general checks to be done before, during and after FCG:

CHECKLIST: FCG

Some international standards such as DNV-GL also provide checklists for FCG operation.

FCG report

Throughout the entirety of the FCG, you will, with the client and the subcontractor, sign off documents that will appear in the final report such as:

• the Pig inspection report pre-FCG
• Deployment form if needed (for a Subsea Pumping Unit for instance)
• Sites queries if needed
• Operational commencement form
• Logged readings
• Pig inspection report post-FCG
• Gauge plate acceptance certificate
• etc.

The final report will also include instrument calibration certificates.

Examples of checks in Pig inspection report post-FCG

FCG interfaces

During FCG, precom engineers are communicating with numerous internal / external interfaces. Here are a few of the many times a precom engineer will have to communicate with other entities (press the + button for more infos on internal interfaces!):

Now that you have a clearer view of what pigging during precommissioning involves, let's practice with some exercises!

• ROV for the study of the accessibility of ROVs to PLRs for example.

• INMET for the deployment and the recovery of the precom auxiliary equipment, especially PLRs that need to be installed either topside or subsea. INMET may be contacted for the valve management plan (all valves must be open when the pigs pass) and the deck layout of the pigging spread. Pigs may also be preloaded in units prior to their installation. Finally, precom department must look at the level line of the layout planning from INMET to verify if there is any declivity which could influence pigging direction.

• Flexible for pig design in hoses

• Structure design for the piggability of the proposed design, the PLR design.

• Equipment for piggability given the valve design, ensure flow rate when pigging.

• Welding for the presence of welding tools, paper plugs etc. in the pigged pipeline

• HSE: Chemical discharge authorization or plan

• ROV: Connection to precom equipment, accessibility to subsea units, timing for ROV change of configuration

• Corrosion: Chemicals needed

• Installation methods: Sequence of activities between installation and precom for FCG, deployment of precom ancillary equiment (PLR, hoses...), Caps installation/removal procedure and sequence, valve management

• Structures design: Piggability, PLR design, testing pressures

• Equipment: Valves design, connectors, flow assurance calculation for pigging

• Survey: Free span prior flooding, frequency management system