Abstract
Traditional methods used in buoyancy control for offshore pipelines involve application of concrete weight coatings by either high velocity impingement or by wrapping techniques. The density of concrete that can be applied by such methods is limited. In addition, damage to the underlying corrosion coating can occur with high velocity impingement application if due care is not taken. The technology presented in this paper describes a method of application that allows very high density concrete to be applied with minimal impact on the corrosion protection coating. High-density concrete application allows reduction in the overall thickness of the concrete coating and can improve on bottom pipeline stability and reduce the impact of wave action on the pipeline. In addition, the paper will discuss thermoplastic coating materials that can be used for pipeline buoyancy control.
Introduction
In designing a subsea pipeline, buoyancy and seabed stability are paramount concerns. Large diameter pipelines displace a significant volume of water leading to buoyancy issues. The large surface area of a pipeline also impacts the interaction with seabed currents and can lead to stability issues. Overcoming these issues can be addressed by increasing pipeline steel wall thickness, but the cost of this option is often prohibitive. Therefore, the pipeline design is usually based on providing sufficient steel for pressure containment along with enough steel for corrosion allowance. Concrete weight coating is then used to provide negative buoyancy and control the stability of the pipeline on the seabed. In some instances, the concrete coating is also used to provide mechanical protection of the pipe against impact damage.
The method of concrete application is usually either by high velocity impingement or by wrapping. In both cases, the concrete is reinforced with either a steel cage or by use of wire mesh. A third option for concrete weight coating involves a vibratory slip forming technique. This application method enables significantly higher density concrete to be applied to pipes and does not damage the underlying anti corrosion coating. This has the benefit of reducing the thickness of the concrete required to achieve a specific negative buoyancy level and also results in a lower interaction of the pipeline with seabed currents due to the reduction in overall surface area of the coated pipe.
As an alternative to the use of concrete weight coating, certain polymers can be modified to produce materials with densities of 2200 kgm-3. In this study, a commercially available polypropylene material has been evaluated as part of a combined weight coating and insulation system.
ConcreteBasics
Concrete is a mixture of cement, water and coarse and fine aggregates. The cement and water produce a paste that wets and coats the surface of the aggregate materials. When appropriately mixed together, these materials produce a product that is initially in a plastic state that can be worked into a pipe coating. A chemical reaction takes place as the paste hardens and binds the aggregates together. Once the material has cured, the finished product is a hard, dense lid with high compressive strength. The key to producing a successful concrete product is the result of the careful proportioning and mixing of the raw materials. The concrete mix design must take into consideration the environment to which it will be exposed. As concrete weight coated pipes are exposed to seawater, the cement chosen is normally either a type II or type V Portland Cement based on ASTM specification C150 (1), due to the moderate and high sulfate resistance respectively. This resistance is related to the level of a component in Portland cement known as tricalcium aluminate (C3A). The presence of large amounts of sulfate in seawater would suggest the possibility of sulfate attack being a concern due to deleterious chemical reactions producing insoluble products that cause cracking of the concrete. However it has been shown that such reactions do not take place and that the reaction products produced are soluble in the presence of chloride ions and are therefore leached out and cannot cause deleterious expansions (2). It has been reported that the use of sulfate resistant cement in concrete exposed to marine environments is not warranted (3). Therefore, more important than the resistance to sulfate attack is low permeability of the concrete for offshore pipeline use. This can be achieved by using a low water to cement ratio along with adequate consolidation and curing of the concrete.
| |
| Page 1 of 5 << First < Prev | Next > Last >> |
The proportion of each component in the mix design has an impact on the finished product properties, primarily the concrete strength and density. For an offshore pipeline coating, compressive strength is very important. The strength of the concrete coating increases with increased cement content and decreases with increased water content. For this reason, the ratio of water to cement is controlled to produce a finished product with the required compressive properties. Figures 1, 2 and 3 depict these relationships. Density is function of the type of aggregate used and for high density finished product good quality iron ore is required.
Curing of concrete is also extremely important. During the curing process, water evaporation needs to be controlled to ensure that the required compressive strength is attained and to prevent spalling and cracking of the concrete. The longer the concrete is kept moist, the stronger and more durable it will become. Most of the strength achieved from the hydration of the cement is achieved within the first month of the concrete's life, but the hydration continues slowly for many years so that the concrete gets stronger as it gets older. Maintaining the required moisture levels during the curing process is achieved either by wrapping the coating with a thin film of polyethylene after concrete application, or through spraying the finished concrete coated pipe with water during the curing process.
Concrete Weight Coating Application
The first step in the application of concrete to pipelines is to ensure that an appropriate concrete mix is produced. Raw materials are proportioned accurately into a batch mixing plant to achieve the required product density. Typically, the concrete is then applied by either a compression wrap process or a high velocity impingement process. In most cases, steel reinforcement either in the form of welded wire mesh or a specifically manufactured welded steel cage is used to provide additional structural strength to the finished product. The wrap process uses welded wire mesh reinforcement whereas the impingement process generally uses welded steel cage reinforcement.
Wrap Application
Wrap application of concrete is also often termed compression coating. During the wrap application process, the corrosion coated pipe travels through the plant on a conveyor in a spiral motion past the concrete application belt. In general, a conveyor belt system feeds pre-mixed concrete from the batch mixing plant to the application belt. At the same time, wire mesh and an outer plastic wrap are fed to the application belt and the concrete wire mesh and outer plastic wrap are simultaneously wrapped around the pipe. Modifications to the process have enabled raw materials to be mixed immediately prior to the application head. Moisture control is key to achieving the correct mix properties. Cut backs are prepared for each coated pipe and the pipe weights checked prior to moving the pipe to an area where it is left to cure. The outer plastic wrap keeps moisture in the concrete so that the concrete doesn't cure too rapidly enabling the required strength to be developed.
Impingement Application
During application by the impingement method, the concrete mix is thrown onto the pipe surface by a set of rollers. The throwing unit transfers the concrete at high velocity so that it is compacted onto the pipe surface. End rings are used to perform concrete cutbacks. The concrete is reinforced with a specially manufactured cage that is installed prior to concrete application. Care is taken during application so that damage of the corrosion coating does not occur. The high velocity imparts significant energy to the impinged particles. This force is transferred initially to the corrosion coating so that this has to have the ability to withstand this impact. Once the concrete is applied the pipes are again weighed and end rings removed to form the cut backs.
Vibrodens Application Process
This application method has been developed to take advantage of the beneficial effects of vibration to produce a dense impermeable concrete. As discussed above, density and impermeability are keys to the performance of the concrete coating. The concrete mix is applied to the vertical pipe by a purpose built slip-form process that allows the pipe to be handled immediately after the slip-form has been raised along the pipe.
The coating tower is equipped with a tilting mandrel, elevator platform and a top mandrel. The tilting mandrel turns the anti corrosion coated pipe including the reinforcement cage to the vertical position. Concrete from a batching plant is transferred by conveyor to the elevator platform where a rotator distributes the concrete evenly around the pipe in a cylindrical mould. Electromagnetic vibrators compact the concrete as the elevator platform is raised along the pipe length.
Once the coating is applied, the pipe is rotated back to a horizontal position and held in a specially designed buggy. The weight of the pipe moving to the horizontal position is used to raise the next pipe into the vertical position. Advantages of this application method include:
• Compaction through vibration allows production of high density, durable concrete
• Compaction through vibration reduces voids and enhances the mechanical strength of the concrete
• Compaction through vibration produces a low permeability concrete that is resistant to seawater degradation
• The application process produces uniform concrete thickness.
Figure 4 provides an example of coating being applied by this method.
The coating shown in Figure 4 has been applied to 48 inch outside diameter pipe. The concrete density in this instance is 3600 kgm-3
Figure 4. Coating applied by the Vibrodens Process. On the left the coating is being removed from the coating tower and on the right, an example of finished coating on 48 inch outside diameter pipe.
The ability to increase the concrete density using the Vibrodens application method allows a reduction in the overall concrete thickness to achieve a given specific gravity as presented in Table I.
|
|
Case 1
|
Case 2
|
|
Pipe Diameter (mm)
|
1219.2
|
1219.2
|
|
Wall Thickness (mm)
|
26.8
|
26.8
|
|
Concrete Thickness (mm)
|
110
|
80.91
|
|
Concrete Density (kg/m3)
|
3040
|
3700
|
|
Concrete OD (mm)
|
1449
|
1391
|
|
Negative Buoyancy (kg/m)
|
529
|
488
|
|
Specify Gravity
|
1.31
|
1.3
|
Buoyancy and Thermal Insulation Coatings
One method of providing thermal insulation for offshore pipelines involves application of a blown polypropylene material commonly termed PP Foam. Often PP Foam coated pipelines require coating with concrete in order to provide negative buoyancy for the insulated pipeline. An alternative to applying concrete is to use a modified polypropylene product with a high density. Commercially available polypropylene compounded with barium sulfate filler can be produced with a density of 2200 kgm-3. Where moderate buoyancy control is required, this product can be used to replace concrete weight coating. In addition, the thermal properties of this material are superior to those of concrete so that additional thermal insulation can be provided by using this material. Figure 5 shows an example of a pipeline that is coated with a multi-layer coating system designed to provide negative buoyancy as well as thermal insulation.
Figure 5. An example of insulation and buoyancy control using a multi-layer polypropylene coating system.
The system depicted in Figure 5 is produced as a multi-layer extruded product. The design basis separates the corrosion protection requirement from the insulation and buoyancy requirements. This enables much better control over the application process when compared to traditional extrusion methodologies because the heating and cooling processes required during coating application are managed more effectively. Three-layer polypropylene is firstly applied in a conventional application process to provide a corrosion protection barrier. High-density polypropylene and polypropylene foam layers are also applied by a side extrusion process to provide the required buoyancy control and thermal insulation. A final layer of solid polypropylene provides mechanical protection of the system.
Summary
The Vibrodens application process for concrete weight coating of large diameter pipes has several advantages over conventional weight coating methodologies. These include the ability to achieve higher density, reducing overall diameter of the coated pipe and providing low permeability weight coating. An alternative to concrete weight coating for insulated flowlines involves the use of barium sulfate filled polypropylene to provide negative buoyancy.
References
1. ASTM CI50, American Society for Testing Materials.
2. A.M Neville, "Properties of Concrete," Longman Group Limited, UK, 4th Edition, 1995.
3. V. M. Malhotra, "Durability of Concrete" Uhlig's Corrosion Handbook, Second Edition, Edited 'by R. Winston Revie. John Wiley & Sons, Inc, 2000.
|
