Terence Tan Kim Jin, Gary Orgill, Djavid Ahrabian
Esso Production Malaysia Inc.
Iain Smith
Bredero Price (Malaysia) Sdn. Bhd.
Kuala Lumpur
Based on a presentation to Offshore South East Asia 10th Conference & Exhibition, Dec. 6-9, 1994, Singapore.
Esso Production Malaysia Inc.'s evaluation of several insulation-coating systems for the 323 mm (12 in.), 14.0-km Guntong D waxy crude pipeline led to selection of a single-pipe coating that incorporates a fusion-bonded epoxy (FBE) corrosion coating, a syntactic polyurethane insulation coating, and a concrete weight coating. Critical design of the coating focused on shear bond between the concrete weight coating and the syntactic polyurethane, compression set in the syntactic polyurethane under hydrostatic pressure, and cathodic disbondment of the coating at high temperatures.
Temperature difference
Esso Production Malaysia, a production-sharing contractor to Petroliam Nasional Berhad, Malaysia (Petronas), is developing the Guntong field 200 km off the east coast of peninsular Malaysia in the South China Sea.The Guntong D complex (GuD), the latest in that area, consists of a production platform (GuD-P) and a bridge-linked compression platform (GuD-C).
Produced oil from GuD moves 14.0 km via a 323-mm crude export pipeline to Tapis pumping platform (TaP). Sampling of the test wells drilled has characterized GuD crude as waxy.
Crude oil from GuD has a pour point of 35 degrees C., while the sea water temperature averages around 27 degrees C. The predicted sea water temperature range being lower than the pour point makes possible such operational problems as high start-up pressures after a long shutdown and excessive wax deposition in the pipeline and the receiving facilities at TaP.
Consequently, the GuD pipeline was designed to be thermally insulated to maintain the crude temperature at greater than the oil's pour point and thus minimize pigging frequency and addition of wax inhibitors.
Insulation considerations
In the conceptual design, several proven thermal insulation methods were considered for the GuD pipeline to ensure that pipeline outlet fluid temperature remained greater than the 35 degrees C. pour point and that negligible gas breakout would occur at the 110,000 b/d maximum flow rate. Excessive gas breakout at TaP platform would require more facilities to flare the excess gas.The insulation options were pipeline burial; pipe-in-pipe (conventional design and configuration with special joint connectors); and elastomeric insulation coating (syntactic polyurethane elastomer coating).
- Natural and sand back-fill options were considered for GuD pipeline burial. Natural back-fill was not technically acceptable because the sea bed's soft silty clay soil was too weak to prevent upheaval buckling of the hot pipeline (design temperature of 93 C.) for reasonable pipe burial depths. Upheaval buckling would lead to loss of insulating performance and possibly excessive pipeline stresses and would pose a marine hazard. Sand back-fill satisfied the upheaval buckling requirements but failed sufficiently to insulate the pipeline, which was a significant concern at low flow rates. The typical thermal conductivity ( "k" value) range for sands is 2.0 to 2.6 w/m/ degreesK while that for clays is 1.0 to 1.3 w/m/ degreesK.1
- Given the wide use of pipe-in-pipe systems for pipeline insulation on previous projects, two pipe-in-pipe options were evaluated. These included conventional pipe-in-pipe and patented special joint connector (SJC) designs. Both employed a polyurethane foam insulation. The conventional pipe-in-pipe option would involve multicomponent bulkheads and centralizers. Prior experience with this system indicated relatively slow lay rates and fit-up problems associated with welding the split-sleeve field joints. SJC employs a prefabricated pipe-in-pipe joint with SJCs shop welded at both ends. At the SJC, the double-wall pipe converges to a single-wall pipe, making welding on the lay barge substantially easier than for the conventional pipe-in-pipe design. This transition reduces stiffness of the pipeline at the SJC. To limit installation stresses, the wall thickness of the SJC is increased at the butt-weld end. The completed joint can be handled as a single pipe. When compared with other options for the GuD pipeline, these pipe-in-pipe designs represented the highest cost alternative.
- Compared to the pipe-in-pipe design, installation of an elastomeric-coated pipeline is quicker because fewer welding passes are required during pipe laying and damages to welds can be more easily repaired. It was estimated that the pipe lay rate for the elastomeric coated system was significantly faster when compared to the conventional pipe-in-pipe design.
After a thorough screening of all insulation options, syntactic polyurethane (SPU) insulation coating was selected for application on the GuD pipeline. This system appeared to be comparable from a technical standpoint with the pipe-in-pipe systems, satisfied the requirements of the pipeline design criteria, and offered a commercial incentive.
Although polyurethane foam is a slightly better insulator, it was unacceptable compared to SPU because the foam has open cell structure, allowing easy water ingress and, as a consequence, loses its insulating properties. SPU, on the other hand, is a nearly closed cell structure and thus minimizes water ingress.
Also, foam can, in certain cases, become acidic with water absorption resulting in the possibility of increased corrosion risk. An added disadvantage of foam is its relatively weak mechanical strength compared to SPU resulting in possible damage during installation and operation.
Syntactic polyurethane
The selected GuD insulation coating is a three-layer system ( Figs. 1 (56839 bytes) and 2 (61948 bytes)) consisting of FBE, SPU, and concrete coatings.The first layer is the FBE corrosion coating which can withstand contact temperatures up to 100 degrees C. when used with the SPU top coat. This base FBE layer bonds between the SPU and pipe surface, provides corrosion protection, and resists cathodic disbondment.
The second layer is the SPU insulation rated to withstand temperatures up to 100 degrees C. SPU is a heat-resistant polyurethane matrix that incorporates microspheres that provide thermal insulation.
SPU insulation is produced from a filled polyether polyol blend which is reacted with isocyanate to produce a tough, low-density polyurethane elastomer. The final product can have a typical "k" value as low as 0.11 w/m/ degrees K.
The basic strength and mechanical performance of the syntactic (artificial foam) polyurethane comes from the high temperature-resistant polyurethane, while the low thermal conductivity derives from the microspheres dispersed throughout the polyurethane matrix.
Concrete coating is applied over the SPU layer for seabed stability. This also acts as extra protection for the insulating material against mechanical damage. To ensure high shear bond strength at the SPU-to-concrete interface, grooves or castellations are cut into the SPU material before the concrete is applied.
Fig. 3 (61948 bytes) shows the configuration of the field joint on the concrete-coated GuD insulated pipeline. The heat-shrink sleeves, capable of withstanding temperatures up to 120 degrees C., act as the primary corrosion coating layer for bare pipe at the field joint.
The SPU half shells are strapped over the heat-shrink sleeves for insulation. A polyurethane filler, compatible with the heat-shrink sleeves, SPU half shells, and FBE coating, creates a water-proof chemical bond at the interfaces with SPU and FBE.
Anodes are installed above the SPU coating to avoid high contact temperatures of the steel pipe; temperatures greater than 45 degrees C. are assumed to decrease significantly the anode current capacity output.
Thickness of the anode is such that the outside surface is flush with the concrete coating, thus maintaining a continuous surface profile of the pipe. This is to avoid problems at the stinger rollers during pipe laying.
For electrical continuity between anode and steel pipe, copper cables are cad welded to a doubler plate and bracket configuration (Fig. 4 (127969 bytes)).
The anode bracket is installed ahead of the SPU coating. Once the SPU has set up, a small area of the coating is removed to expose the bracket. This procedure minimizes the potential for water ingress to the FBE at the anode bracket.
Laying Performance
Because of extensive previous testing for other projects and installation of SPU-coated pipelines (reeled and "S" laid), simulated lay trials for the GuD SPU configuration were unnecessary. The following reviews the capabilities of the coating for either reel barge or conventional "S" lay.But reeling was never seriously considered for GuD because suitable installation equipment is not readily available in Southeast Asia. Further, the effect of the concrete is not considered (with the exception of the shear loading calculations) because the project does not represent worst-case conditions.
During reeling of a pipeline, the coating is exposed to a compressive (internal) and tensile (external) strain which could be cycled up to a maximum strain of 3%. The material elongation at break is a minimum of 60%. Therefore, reeling is viewed as technically feasible, although further work may be necessary to confirm the integrity of the field joints during reeling.
Because conventional "S" lay is the most likely method of installation, more detailed investigation of concerns was pursued. Specifically, two forces must be considered:
- The squeeze load applied through the coating to the steel to prevent the pipe slipping
- The load applied by the stinger rollers to control the angle of lay.
The squeeze load applied is a function of depth, pipe size, coating friction coefficient, and overall weight. To date, the worst case seen has been a heavy wall pipe in 290 m depth.2 At these conditions, the applied forces were line tension = 873 kN; stinger force = 45 kN (per roller box).
Since this scenario is much more severe than GuD, additional testing for GuD was not carried out. Using simple "average" loads defined as force overall contact area, the shear and compressive loads applied to the coating can be calculated: S = T; where: S = Squeeze load; = Coating coefficient of friction (compliant and surface effects); and T = Line tension. The applied squeeze load gives rise to two effects: it causes a compression within the coating and coupled with the line tension generates a shear load. The stinger rollers support the coating and pipe, allowing a controlled entry into the water as such a compressive load is applied. Hence the resulting bearing stresses (tension and stinger) are approximately:
(Formulas unable to be viewed)
The shear stress is calculated from the contact area and squeeze load applied and will be maximum around the tensioner pads. The critical areas tend to be at material interfaces. Hence:
(formula unable to be viewed)
From practical testing of coated pipe at the conditions expected for the deepwater lay (as given earlier), the following stresses are obtained: bearing stresses from tension pads = 3.1 MPa; bearing stresses from stinger rollers = 1.8 MPa; induced shear stress = 0.6 - 0.3 MPa (100-275 mm pipe).
Because the material is compliant and elastic, it has no fixed compressive strength; its pseudo compressive strength (10% strain) is around 4 MPa and its shear strength exceeds 4 MPa. Therefore, the general properties of the material appear to be more than adequate to meet the GuD laying requirements.
Service performance
On the seabed, the coating acts to prevent heat loss from the product and to protect the pipe from corrosion.The coating's performance under the combined effects of time, temperature, and pressure is therefore required in order to predict "U" value (overall heat-transfer coefficient), creep rate, and cathodic disbondment for structural and performance integrity throughout its design life.
Most elastomeric insulation materials, when subject to an increasing hydrostatic load, show an initial elastic response followed by a crushing of the material. The point where crushing occurs depends upon both the magnitude and duration of the applied load with materials being able to withstand high loads for short periods and low loads for long periods.
For all materials used subsea, the applied loads during field installation are well within the compressive strength (for example, PVC foam compressive strength 3.8 MPa, applied load in the North Sea is typically 1.5-2.5 Mpa)3 4 and therefore short-term loading poses no problem. When a coating is subjected to a force imbalance (uniaxial or biaxial loading) over a long-term, it can and will undergo creep with a reduction in the coating thickness.
Most elastomeric materials show small creep when in service because of the inherent free volume present in the material on a macromolecular scale. But this effect can be neglected because it is very small.
Materials which have a cellular or syntactic nature can give rise to large creep and therefore can significantly change in thickness over time. These materials require further design.
For the SPU, typical creep rates in the order of 0.05-0.1% loss in thickness per decade hour are expected. ( "Decade hour" refers to a log change in time; that is, from 1 hr to 10 hr, from 10 hr to 100 hr, from 100 hr to 1,000 hr, etc. Creep rates tend to be linear when plotted on a log scale.)
For a 25-year design life, the creep rate is therefore around 2.5-5% depending upon depth and temperature.
Table 1 (17436 bytes) demonstrates the predicted and actual performance of the GuD coating in terms of coating compression, "k" value, "U" value, and creep rates.
The actual data in Table 1 (17436 bytes) were taken from a test carried out at Heriot Watt University, Edinburgh, where a pipe section was coated and subjected to conditions expected subsea. Changes in coating thickness and heat flow through the coating were recorded. Fig. 5 (82358 bytes) illustrates the results of the simulated service test for compression and creep rate. The predictive values were derived from a computer model which breaks down the coating into a series of concentric rings. Each ring was assigned a temperature based upon the thickness of the ring and its "k" value.
The model then follows an iterative process in which it calculates the degree of compression expected for each ring, the resulting change in density and therefore "k" value. The model also takes into account the effect of water pickup and the resulting increase in the material's "k" value. The results of the tests are given in Figs. 6 (30249 bytes), 7, 8 (24142 bytes) and 9 (21514 bytes).
Cathodic disbondment tests were also carried out at Bredero Price (Malaysia) Sdn Bhd in Kuantan, Malaysia. The various configurations, conditions, and final results are tabulated in Table 2 (18244 bytes).
Shear performance
As noted, the GuD pipeline has a concrete outer coating to provide on-bottom stability. In order to lay the pipe and to aid its operation subsea, there must be a defined shear resistance between the concrete and the SPU. It was this area in which a detailed study was performed and supplemented by testing.The study was used to determine the theoretical loads being applied both in laying and in operations (due to thermal expansion of the pipeline). In parallel, practical testing on coated pipe was performed to investigate the level of shear bonding that could be obtained. From the theoretical study, it was determined that to lay the pipe without slippage between the SPU coating and concrete, a shear bond strength of 0.04 MPa was needed. Also, a maximum shear loading of 0.15 MPa would be set up between the SPU and concrete due to the thermal expansion seen in service.
In order to evaluate the shear strength between SPU and concrete, a trial was set up to evaluate the bond of cured concrete on cured SPU. The results gave a bond strength which was too low to record and well below that required.
Detailed consideration of the method of production led to a decision to cut grooves into the SPU coating sufficient to allow penetration of concrete (Fig. 7). Trials were performed with 1, 2, and 3 grooves per meter to assess shear strength and shear characteristics.
Fig. 8 (24142 bytes) shows the results of those trials. When the test area is taken into account, Fig. 9 (21514 bytes) shows that the shear performance of the test is uniform.
The results of Figs. 8 (24142 bytes) and 9 (21514 bytes) were taken from coated pipe containing 2 or 3 grooves per meter and were used to establish the shear characteristics needed for the GuD pipeline. The final configuration, with due allowance for safety, was 25 or 26 grooves of 30 mm wide x 8 mm deep equally spaced on each pipe joint.
A further detail of the grooves was their depth: they had to be sufficiently deep to give a key and to allow for any coating compression as a result of hydrostatic load and temperature in service.
It was determined that the loss in thickness would not exceed 2 mm over the life of the pipeline and therefore the 8 mm deep grooves would not allow the concrete to slip at a later date.
SPU performance
The GuD coating was carried out by Bredero Price (Malaysia) Sdn Bhd in Kuantan, Malaysia, and represented the first application of its kind in Southeast Asia.Bredero Price mobilized the specialized SPU molding equipment from its plant in Leith, Scotland, and delivered the specialized SPU materials used in the molding process. Whereas the equipment is designed to run warm (and therefore the local temperature was a benefit to the process), the material required a controlled temperature both throughout 6 weeks of shipping and on-site storage.
The isocyanate could not be allowed to cool to less than 12 degrees C. at which point it would start crystallizing. The polyol, on the other hand, could not be allowed to heat past 45 degrees C. after which point its shelf life shortens significantly.
As a result, the material manufacturer, responsible for shipping, chose special insulated containers. While these were in the U.K., they could be heated (the material being manufactured in the British winter). And while aboard ship, they would retain sufficient heat until the ships moved into warmer climates.
To monitor the temperature, sensors were located inside the containers to record the temperature throughout the voyage.
To check for any degradation of the material during transportation to or storage in Malaysia or from environmental conditions, the material was pumped into test molds such as those used by the manufacturer and subjected to a range of tests both by the manufacturer and by external test houses. The results of these tests, with due allowance for scatter, demonstrated that the material had not degraded (Table 3 (23777 bytes)).
Because of differences in test methods (within the same standard) and with the equipment, there is some variance in the results between tear and "k" value. The increase in "k" value reported by the external test house is proportional to the water uptake. The reported figures appear high but result from the equipment used.
Application
Following is the general coating application process:In the application of the SPU, it was important that the humidity around the pipe and the pipe and mold temperature be controlled within set parameters. If the uncured or curing SPU is exposed to high humidity, it will pick up moisture which in turn causes foaming.
While the generation of a foamed material improves the thermal conductivity, it reduces the material's inherent strength and results in a coating which cannot be laid or used subsea. To overcome this humidity problem and to stop condensation, both the mold and pipe were heated. When the pipe was located inside the closed mold, the local environment around the pipe was sufficient to overcome these problems.
With the local humidity in Kuantan, Malaysia, significantly greater than in Scotland, humidity could cause a problem.
The only time the SPU is significantly exposed to air is during the filling of the mold. Using temperatures greater than ambient and heating the mold prevented high humidity from being a problem.
Care had to be taken to ensure that the pipe and mold temperature did not rise high enough to reduce the gel time of the SPU and prevent easy processing. It was determined that the optimum temperature range for the pipe and mold was 40-60 degrees C. with the mold being at a marginally higher temperature than the pipe.
Completion of detailed design, simulated service tests, and coating application for the 12-in. Guntong D syntactic polyurethane insulated pipeline led to the following conclusions:
- Although previous experience with SPU coating was primarily in the cooler European climate, no difficulties were experienced in SPU coating in the tropical Malaysian climate. Pipe coating was successfully completed on schedule by the Malaysian coating contractor in accordance with Esso Production Malaysia specifications.
- Introducing a series of circumferential grooves into the SPU coating permitted the required shear bond between the concrete and SPU coating.
- The compression and creep rate of SPU meet the requirements of hydrostatic loading for the design life of the pipeline.
- Cathodic disbondment of the FBE and SPU system at elevated temperatures (98-102 degrees C.) was within the design requirements.
Acknowledgments
The authors wish to thank Petroliam Nasional Berhad and Esso Production Malaysia Inc. for their permission to publish this article and personnel with Bredero Price (Malaysia) Sdn Bhd for their contributions.References
1.Intec, Joint Industry Study -- "Design and Installation of Insulated Submarine Pipelines," Job No. H-053.1, June 1986.2.Haldane, D., Statoil Satellites Project Report, Heriot Watt University, Scotland, May 1992.
3.Diab Technical data sheet SPI 220, Diab Box 201 S-312 01 Laholm Sweden.
4.Collins, M.H., and Lyle, A.R., "Recent Developments in Thermal Insulation for Subsea Flowlines," 8th International Conference 1990 -- Internal and External Protection of Pipes.
The Authors
Ahrabian holds a BS from the University of Nottingham and MS and PhD in mechanical engineering from the University of Manchester Institute of Science & Technology. He is a member of the Royal Institute of Naval Architects and since 1986 has been a registered chartered engineer with the Engineering Council in the U.K.
