Summary of Asgard RFO Pig Development Case Study
The Åsgard Transport (ÅT) 42” Gas Pipeline runs 710km south from the 28” Export Riser Base (ERB) at the Åsgard B platform to the landfall valve station at Kalstø. From here, it crosses three fjords to the Gas Terminal at Kårstø in western Norway.
During pre-commissioning, it was required to waterfill, clean and gauge the line from Kalstø to Åsgard using treated seawater. After the pressure test, it was required to dewater the line using a glycol pig train, again from the Landfall site at Kalstø. This required the use of 42” x 28” dual diameter pigs with a high sealing efficiency.
The following summarises the Pig development programme with reference to:
- 1.1 Details of the pigs developed including the Centreline Multidiameter Suspension System (CMSS-International Patent Application No. PCT/GB00/01159), wheel suspension modules, the dual diameter sealing discs, pig bodies and on-board equipment;
- 1.2 Information on the pre-commissioning tasks performed on the Åsgard line using these pigs:
- 1.3 Important aspects of the development that should be considered in future projects.
2. Details of the 42” x 28” Dual Diameter Pig
Each pig comprised of two CMSS wheel suspension modules, 42” sealing disks with Buckle Inducers, 28” sealing disks and guiders. The pig assembly was completed with the addition of bumper noses. Space was allocated within the pig body for on-board equipment: -
Figure 1 – Åsgard RFO Dual diameter pig with CMSS suspension
2.1 Centreline Multidiameter Suspension System
Each suspension module consists of 8 suspension arms linked to a central piston such that when one wheel arm deflects, the other seven also deflect. A spring-loaded piston forces the suspension arms outward. Pressure balanced dirt seals are mounted on the piston to avoid contamination of the integral piston bearing arrangement.
The basic principle of operation is that the total spring force acting on the piston is greater than the weight of the module. The pig therefore rides on its true geometrical centreline. If the pig were to drop below the centreline, the restoring force of the loading spring would be greater than the weight of the module, thereby placing the pig back on to the centreline. The contribution to total pig friction (normally quantified as differential pressure across the pig) from the wheeled suspension modules is negligible in both line sizes, as it is merely rolling friction from the wheels.
To avoid overloading the wheels and the possible initiation of tyre disbonding, the force/deflection curve imposed by the suspension system upon the wheels, is designed to be as near constant as possible, i.e. on entering the 28" pipeline the load acting on the wheels does not change significantly. This is achieved by careful design of the linkage geometry.
The wheels are selected to meet the duty imposed by distance, velocity and temperature in the pipeline and to avoid tyre disbonding. Previous trouble free experience had already been gained with the chosen wheel/hub/tyre combination in gas pipeline pigging applications.
To distribute the load evenly between the eight wheel assemblies throughout the total pig journey a spiral motion is induced within the vehicle by a small angular offset of the suspension arm. This essentially gives each wheel a respite from supporting the maximum load, due to the weight of the pig, when at the “six o’clock” position. Such a feature is essential if the pipeline is in any way oval.
A fatigue analysis is performed to ensure that the wheels would not fail during the commissioning operation. A special lubricant is used to avoid heat build up in the pressure balanced bearing housing.
Figure 2 - Åsgard Dual diameter commissioning pig during development tests. The pig has just been retrieved from the test loop where it had undergone pull through and sealing disc flip pressure tests. This helped optimize the sealing disc design. Compare them with the final design of sealing discs with those of the pig in Fig 7, which had just completed a 710 km run.
The components used in the wheel module were designed to handle the maximum loads expected in the operation both in steady state and during dynamic events such as exiting from the reducer at speed, for example. A stress analysis was performed on each critical component each being designed to carry half the pig weight plus any additional loading due to imbalance
2.2 The Sealing Discs
The pig uses separate 42” and 28” sealing discs. Figure 3 shows the 42” sealing discs, selected and located to seal in the 42” straight pipe and bends. In the 42” line, these behave just like normal 42” sealing discs. The buckle inducers (six off in this case) cause the seal to buckle in a regular and repeatable manner into the 28” pipeline. The geometry of the sealing disc must be selected carefully such that the seal will buckle into the 28” pipeline but will not buckle in the 42” pipeline.
Figure 3 - Pig in 42” pipeline
The pig is centralised in the 42” pipeline so that the seals are effective over the entire pipeline circumference. Care must be taken to insure that the seals do not buckle or flip forward ( seal flip pressure is critical) in the 42” line.
The number of buckle inducers and the hardness of the sealing discs are selected in order to minimise the stresses in the disc and reduce compression set and wear as much as possible while maintaining a high sealing efficiency.
An analysis is performed in order to select the sealing disc geometry, hardness, number of buckle inducers, size and location. Care must be taken to bolt up the pig correctly in order to avoid sealing discs pulling out of the boltholes. Figure 4 shows the view from the back of the pig with the sealing disc neatly buckled into the 28” line:-
Figure 4 – 42” seals buckled and folded into the 28” pipeline
The Buckle Inducers force the 42” seals to fold up in a repeatable manner into the 28” pipeline. This allows reduced stresses in the disc and low pig differential pressure in the 28” pipeline. This photograph is taken looking at the rear of the pig in the 28” temporary receiver.
The use of disc type seals, running at, or very near, to the geometrical centre line of the pipeline, results in high seal efficiency. As the seal wears, a new sealing surface is developed (enhanced by the induced spiraling motion) which maintains high contact pressure. In this way, the seal efficiency is maintained over the length of the line.
The 28” sealing discs are again standard disc type seals selected and located to operate in the 28” straight pipe and bends. 28” guider discs are also used in front of these seals. These guiders are used mainly to support the 42” seals, to avoid forward flipping of the sealing elements.
2.3 The Pig Body
The pig body is designed to accommodate the seals and suspension modules, to allow correct assembly of the pig and to provide space allocation for on-board equipment. The initial design stage is used to ensure that there is sufficient space on-board the pig for equipment such as transponders, isotopes, and cleaning magnets and that the pig will negotiate pipe features such as bends while maintaining a positive seal.
Front and rear bumper noses are essential in dual diameter pigging as there may be a risk of one pig in the train becoming jammed into the rear of the pig in front in the smaller diameter. This can cause serious damage to both pigs as the seals could lock onto the pipewall.
It is necessary to avoid:
- pressure cavities inside the pig and(1) pressure cavities inside the pig and
- leakage through the pig.This is addressed in the design of the pig body and the suspension system. Methods for eliminating “through bolt” leakage past the discs are implemented during assembly. To help ensure that the seals will not pull out from the flanges, bolt torque loading and flange design are important. Metal ferrules are provided to avoid the over tightening of the sealing discs. The flange diameter must be selected carefully to match the sealing disc geometry (to avoid buckling in the 42” sealing disc but to allow buckling in the 28” sealing disc).
Gauging was provided in the Åsgard pig by using a linear displacement transducer to measure the deflection of the suspension arms. This was recorded using a data logger in the pig. Thorough project management was required to allow the suspension modules, sealing system and body to work together and meet the pig functional requirements. Full records of the pig design, build and operation were maintained.
3. The Waterfill and Dewatering Runs
There were two pigging operations during RFO for this pipeline: -
- Waterfill, cleaning and gauging
Figure 5 shows the pig train used for waterfill: -
Figure 5 – Waterfill, Cleaning and Gauging Train
Six pigs were launched with treated seawater between each. The motion of the pigs was simulated using a pig motion model to allow the operation to be monitored. The pig position was recorded using the transponders. The inlet pressure was checked daily. This was then compared with the expected pressure and location from the simulation and so the operation was monitored continuously.
The subsequent pressure test showed that there was very little air remaining in the liquid. There was progressively less ferrous debris on the magnets after each pig arrived at the receiver, and coupled with the high level of Quality Control during construction, the line was considered clean. The gauging tool showed that there were no defects in the line.
Figure 6 – Dewatering Train
Again, six pigs were launched with glycol slugs between the first four pigs, and dry air between the last pigs. This was to pick up the remaining glycol and water in the line from pipe components such as tees. The operation was monitored using a simulation of the pig train compared with the pig location and inlet pressure as before. This shows that the operation was progressing satisfactorily. No change in the relative position of the pigs was reported, probably due to the efficiency of the sealing discs.
At the end of the run, the three glycol slugs were sampled and the percentage water content measured to indicate the efficiency of the operation. 0.4% water in glycol was recorded in the last liquid slug, compared with 3 to 4% from previous dewaterings. This is the most efficient Glycol Dewatering recorded by Statoil to date.
4. Important aspects to note
The following important points were noted during the development: -
- 4.1 The Åsgard Pig Development project was begun by defining a list of Pig Functional Requirements that had to be met. An example of such requirements was that the pig differential pressure in the 28” line must be less than 2 bar. These requirements were specific to each project and operation. These were agreed with the project team and used to monitor the pig development. The meeting of these functional requirements were verified by means of calculation, test or using CAD.
- 4.2 Due to tight space constraints within the pig, it was important to allocate space for all on-board equipment, and determine the requirements for the sealing discs and suspension units as early as possible. This needed to be coordinated carefully.
- 4.3 Simulations of pig motion, to monitor the movement of the pig trains were employed for both the waterfill and dewatering operations. Coupled with knowledge of the location of the pig against time and inlet pumping pressure, it is possible to check the pig train progress in real time.
- 4.4 Selection of the sealing disc geometry, material properties and Buckle Inducer design was a vital part in getting the pig design to work in both test and in the field. Incorrect geometry, for example, can lead to potential buckling of the seals in the 42” line. A design methodology for the seals was developed to allow correct seal selection.
- 4.5 It is essential to select the correct wheels (hubs and tyres), bearings and lubricants for the suspension modules. The experience of FTL Seals Technology, in the design and manufacture of the CMSS suspension module was important in order to ensure that the best wheels and bearings were specified for the job;
- 4.6 Finally, the launcher and receiver should be designed to suit the pig if possible (not the other way around). The trap design needed input from the pig designers.
Figure 7 – One of the Åsgard RFO pigs after the pipeline dewatering run.
5. Norne-Heidrun Pre-commissioning pig development
Fig 8 - Complete 10” x 16” dual diameter pig assemblies.
The 10” x 16” Norne Heidrun pipeline presents a considerable challenge in terms of pig development for both pre-commissioning and operations. Provision for a pig for this duty involves:-
- A pig design which can operate effectively and safely in both a 10” line and a 16” line;
- A sealing system design which can overcome compression set in the 10” pipe in order to recover and provide sealing in the 16” line;
- A pig design that can negotiate tight bends, a Y-piece and other inline features.
Statoil has developed the pre-commissioning concept which employs five 10” x 16” Dual Diameter pigs, Figure 8, for pipeline dewatering. These pigs employ the CMSS wheel suspension system to guarantee centralisation in the large pipeline. Buckle Inducers are used for efficiently folding the 16” seals into the 10” line. Correct selection of seal geometry and properties allows the seals to buckle when required and recover sufficiently from compression set.
The Norne Heidrun 10” x 16” pipeline initiates at Norne FPSO. From here a 900m 10” flexible riser runs to the seabed, depth 300m. On the seabed, the pipeline expands to 16” after the riser termination hub. 18.5m downstream from this point, there is an asymmetrical 16” equal Y-piece for possible future tie-in or subsea launch of an inspection pig. From here, the line runs for 124km, which includes a number of 5D bends, where it ties into the Åsgard Transport Gas Export line along with the 16” pipeline from Heidrun.
In order to negotiate the 16” wye piece and the 16” 5D bends, it was necessary to provide two pig modules, articulated by a universal connecting bar, see Figure 8. In previous projects that employed such a system, it was found that this bar is subject to high compressive and tensile loads. Therefore it is necessary to design the connector to suit these conditions. The loadings were determined by simulation of the pig motion in the line.
The dewatering pig train consists of four such pigs run with a glycol batch between each and a trailing final pig run in dry air or nitrogen. As the final pig exits the 10” flexible and enters the 16” line, a potential problem arises. Due to the sudden drop in friction, the pig will accelerate suddenly to a relatively high velocity. Such acceleration can cause the pig to compress the gas in front of it, decelerate and finally reverse. Therefore, the final pig could potentially reverse into the 16” Y-piece thus damaging either the wye or the pig. This scenario must be avoided.
To investigate this problem, the dewatering operation was modelled using Piglab, a pig motion model from Pipeline Research Limited and the pig train designed to avoid this problem. Figure 9 shows the velocity profile of this pig as it exits the reducer showing the reversal of the pig near the wye piece. This demonstrates the successful use of dynamic pig simulation in decision-making and problem solving for pigging operations.
Fig 9. Possible reversal of the rear dewatering pigs across a Wye can be avoided by careful design of the pig train. The simulation shows pig velocity against time.
6. Future Developments
Currently we are actively involved in several new design proposals being undertaken for dual diameter and constant bore commissioning and cleaning pig systems. Working in conjunction with industry leaders we are able to offer oil and gas operators, world wide, a total engineered and operational package.
Figures 2, 3, 4, 7, 8 and 9 with kind permission of Statoil.