Pipeline Technology Conference 2015
1. Introduction Most power plants require a circulating water system to transfer the waste heat generated by steam cycle into the ambient environment. Recently three kinds of pipe materials have been used widely; these are steel, GRE (Glass Reinforced Epoxy), and HDPE materials. HDPE pipe is selected for the Ras Djinet Combined Cycle Power Plant Project to intake sea water from the intake head located 1 km away shoreline. Ultra large size of 2500 mm diameter of HDPE pipeline was successfully installed by Daewoo Engineering & Construction Co., Ltd. through rigorous analysis with respect to design and installation. A bird's-eye view of the project is shown in Figure 1. This paper covers the design of HDPE pipeline and the procedures of its installation.
Figure 1. A Bird’s-Eye View of the project
2. Material and Design Consideration The seawater intake system of a power plant has the function of delivering cooling water to condenser and auxiliaries. The objective of the design process for pipeline of intake and outfall system is therefore to determine the size of pipeline which ensure flow rate which systems required. The intake system for the project is designed to intake seawater of 108,240 m3 per hour with HDPE pipeline and finally designed to 1
Pipeline Technology Conference 2015 162,360 m3 per hour considering safety factor of 1.5. The design velocity in the each pipeline is limited less than 2.5 m/sec and leads to the size of diameter 2500 mm with 4 pipelines. 2.1 Material Selection The HDPE pipeline adopted for the project has been increasingly used for various marine applications such as water intake, effluent outfalls, river and lake crossings, apart from oil and gas industries. There are various benefits of HDPE such as immunity to galvanic corrosion, light weight and flexibility, which have become excellent solution for water intake system in power plant. Table 1 and Table 2 present the key characteristics of the as designed HDPE pipeline and material properties, and Figure 2 presents its schematic configuration.
Table 1. HDPE Pipeline Key Design Characteristics Description
Intake Pipeline A
Intake Pipeline B
Type
–
SDR30
SDR26
Material
–
HDPE
HDPE
Material Grade
–
PE100
PE100
mm
2500
2500
Service
–
Raw Sea Water
Raw Sea Water
Pressure Regime
–
LP
LP
mm
83.3
96.2
bar(g)
5.5
6.4
Design Temperature
°C
40
40
Operating Temperature
°C
24
24
MPa
10
10
Pipeline Outer Diameter
Wall Thickness Design Pressure
Minimum Required Strength (MRS)
Table 2. High Density Polyethylene Properties for HDPE100 Property
Value
Unit
960
kg/m
Tensile Yield Strength
23
MPa
Elongation at Yield
8
%
Density
2
3
Pipeline Technology Conference 2015 Tensile Break Strength
37
Coefficient of Thermal Expansion
2.4x10
MPa -4
m/m /˚C
Poisson’s Ratio
0.4
-
Minimum Required Strength at 20˚C (MRS)
10.0
MPa
Hydrostatic Design Stress at 23˚C (HDS)
7.7
MPa
Figure 2. Schematic of HDPE pipeline with concrete collar Upper concrete collar Φ2500 HDPE pipeline
Anchor bolt Lower concrete collar
2.2 Design Issues After completion of pipeline sizing, the mechanical design is performed so that the pipeline could be protected from internal and external loads. General key design tasks performed in this project can be summarized as follows: 1. Wall Thickness Design 2. On-Bottom Stability 3. Pipeline Free-Span Analysis 4. Concrete collar structural analysis 5. Sinking Analysis International codes and standards are applied to these design tasks such as DNV, DEP, AWWA, API and CEM (Coastal Engineering Manual). Each of these design activities for the pipeline is discussed in more detail in the following subsections of the paper.
2.2.1 Wall thickness design The wall thickness of pipeline is core resistant factor to endure internal pressure and external loads. While this task involves various technical aspects related to different design cases, primary design loads relevant to the containment of the wall thickness 3
Pipeline Technology Conference 2015 for HDPE pipelines are as follows:
Bursting criteria under internal pressure
Pipe ring deflection under hydrostatic pressure
Compressive stress under burial load and buckling criteria under installation load.
The wall thickness of SDR 30 and 26 pipes is checked to be enough margins to the wall thickness calculated based on the pressure containment because water intake system does not require high internal pressure.
To check the pipe’s reaction to external load, the pipe must be within its safe allowable limit for each of these three reactions – deflection, buckling and ring compression. Ring deflection is a crucial response of flexible pipes to soil load. The modified Iowa Formula is used as specified in the AWWA Manual. For non-pressure applications, a 7.5 percent deflection limit provides a large safety factor against instability. The ring deflection in the pipe caused by the applied external pressure of 2 m soil cover depth is 3.29 %, which is within the limit of recommended deflection 7.5%. Compressive hoop wall stress was calculated to 2.70 MPa and is less than the allowable long-term compressive stress of 3.53 MPa.
2.2.2 On-bottom stability HDPE pipeline resting on the seabed are significantly subjected to the forces in both the horizontal and vertical directions due to wave and current loads. If a pipeline is not stable then it will move under the actions of waves and currents. In the initial stages after finishing installation of HDPE pipeline before backfilling, the pipeline is exposed to the risk of these movements which may cause damage to pipeline. On-bottom stability was performed to keep the pipeline safely no movement based on DNV standards. The pipeline on-bottom stability analysis comprises of both later al and vertical stability of the pipelines during their design life. The lateral stability analysis is carried out to determine the concrete ballast weight (i.e., concrete collars) required to make pipeline stable during installation and operation against the environmental loading caused by waves and currents. The vertical stability analysis is carried out to assess the floatation and/or settlement potential of the pipeline.
The absolute lateral static stability method is a “design wave approach”, i.e. it ensures absolute static stability for a single design (extreme) wave-induced oscillation. Absolute lateral static stability approach is based on force equilibrium ensuring that the hydrodynamic loads are less than the soil resistance under a design extreme oscillatory cycle in the seastate considered for design. 4
Pipeline Technology Conference 2015 In order to keep the pipelines stable vertically, the submerged weight of the pipeline shall be enough large with the safety factor of 1.1 to avoid floatation refer to DNV. (ref). Settlement of the pipeline in the seabed should be then checked not to sink into the seabed, considering with maximum content density, e.g. water-filled. If the specific weight of the pipe is less than that of the soil (including water contents), no further analysis is required to document the safety against sinking. The concrete collar volume is designed to 6.7 m 3 with 6 m span which is equivalent to 135 mm of concrete coating thickness all the way along the pipeline. From onbottom stability analysis, the vertical stability both upward and downward meets the requirements and lateral stability also is above required safety factor as shown Table 3. Table 3. On-bottom Stability Example Results Load Case
Water Depth (m)
Specific Gravity
Installation Operation
15 15
1.251 1.251
Lateral Stability (Vertical) 7.272 14.247
Lateral Stability (Lateral) 2.485 4.528
Collar Volume (m3) 6.7 6.7
2.2.3 Free-span analysis Pipeline spans are caused by a variety of seabed features, the most common of which is an uneven seabed on the selected route. The route for HDPE pipeline was trenched and leveled before installation, but the concrete collars inevitably make spans between them. The allowable span lengths are calculated to maintain the pipelines within the allowable stress limit and to prevent the onset of vortex induced vibrations (VIV). The maximum span length based on static stress considerations are governed by self-weight of the pipe and coatings and the environmental loads. The allowable static span length for a pipeline is calculated by limiting the equivalent stress in the span to allowable bending stress based on the Von Mises equation incorporating the axial stress. The second mode of failure for spans is a fatigue failure due to vortex-induced vibrations (VIV). Vibrations may occur in the pipeline due to vortex shedding because of the flow of water passing a free span. Normally two types of oscillations are encountered:
Oscillations in line with the velocity vector (in-line).
Oscillations perpendicular to the velocity vector (cross flow). 5
Pipeline Technology Conference 2015 For certain flow velocities the vortex shedding frequency may coincide with or be a multiple of the harmonic or sub harmonic excitations. The span length will be selected such that the harmonic frequency or natural frequency of the span will be less than the vortex shedding frequency either for in-line or cross flow oscillations. The pipelines are designed such that no oscillation is allowed and to that effect, the maximum allowable span for the VIV criteria shall be less than that for the onset of in-line oscillation requirements. As in-line oscillations are not allowed, a fatigue analysis will not be required. The static and dynamic free spanning analysis is performed for installation condition because backfilling will be followed to installation. For conservative approach, fully restraint condition is considered. The summary of free span analysis for installation case is presented in Table 4. Table 4. Free Span Analysis Results Description
Unit
Max. Allowable Static Span Length Max. Allowable Dynamic Inline flow VIV Span Length Cross flow VIV Governing Span Length Recommended Allowable Span
m m m m m
Design Condition Installation Hydrotest 10.42 10.42 33.72 34.74 34.66 35.68 10.42 10.42 6 6
2.2.5 Concrete collar Structure Analysis Previously explained, the concrete collar is attached on the pipeline with 6 m span by combining upper and lower parts of precast concrete collar. The standards of CBA 93 and BAEL 91 are applied for design concrete structures. The type of cement was determined to CPA with C3A to be contained less than 10 % considering full immersion conditions for seawater. The concrete is designed to 35 MPa of the compressive concrete strength and 2.55 ton/m 3 of dry density. To prohibit that the pipe is rotating when floating on the water surface, the bottom part of the collar should be heavier than the upper part. Accordingly, the concrete collar volume comprises 2.4 m 3 in the upper part and 4.0 m3 in the lower part. The blocks are attached on the pipes with 4 bolts. Bolt forces are controlled by means of measuring the compression of the rubber compensators on each bolt. In order to secure that the block will not slide on the pipe during the submersion of the pipes a sliding test was performed. A minimum bolt force at the assembly of collars on the pipes is expected to be around 8 ton. But, based on 0.5 friction coefficient between rubber pad and HDPE pipe, considering more than 4 times safety factor for 6
Pipeline Technology Conference 2015 uncertainty during bolt tightening & sinking pipe and etc., a maximum bolt force at the assembly of blocks on the pipes is designed to be around 33.5 ton (335kN). As a result M33 size of bolt is used. The overall structure modeling and analysis are performed using Midas Civil, finite element analysis (FEA) software. From the concrete collar structural analysis, it could be learned that the most critical engineering parts are not concrete structure integrity such as bending moment, shear stress, and crack is not critical, but the bolts combining each parts of the collar and rubber part making the gap between the pipe and concrete. These parts are shown in the figure 3. Figure 3. Precast Concrete Collar
2.2.6 Installation Analysis The HDPE pipeline installation method for this project adopted the float-and-sink method, so-called, Rentis installation. It has been used widely for near shore underwater pipeline installation. In traditional Rentis method, the required pipe string length is fabricated onshore and fitted with buoyancy devices at a given spacing, then is launched and finally towed to the desired offshore location. After positioning and aligning of the pipe string, the buoyancy devices are stripped in a control manner so that the pipeline settles to seabed due to its own weight in a controlled manner. The main difference between HDPE pipeline and traditional Rentis installation is that HDPE don’t have to consider buo yancy devices because HDPE is buoyant itself. Critical issues are therefore to attach concrete collar to submerge pipeline and pump water into the pipeline to change it from positive buoyancy to negative. Accordingly pipeline settles on seabed due to its own weight including concrete collar in a controlled manner. The commercial software Orcaflex was used to perform the analysis of Rentis 7
Pipeline Technology Conference 2015 installation. Orcaflex developed by Orcina have been used widely for static and dynamic analysis of a wide range of offshore systems including all types of marine riser, floating facilities, moorings, installation and towed systems. Orcaflex is a fully 3D non-linear time domain finite element program capable of dealing with arbitrarily large deflection of the flexibles from the initial configuration. The sinking analysis model consists of HDPE pipeline, concrete collar, buoyancy tank and winch as shown Figure 4. As Orcaflex is limited to simulate change of pipe content from empty to flooded condition, Pipeline was modeled as flooded status with seawater in the beginning but attached additional buoyancy tanks to compensate the initial flooded water weight inside the pipe. Analysis was then performed by removing the attached buoyancy tanks, which could simulate sinking of HDPE pipeline in the offshore site. The weather condition is assumed as “Calm-day working”. As a result of analyses minimum bending radius along pipeline length is larger than 50m of the pipeline as shown in Figure 5. According to AWWA Manual (Ref), the minimum short-term bending radius shall be larger than 20 times pipe OD i.e., 50m for this project in a short-term period. Maximum von-Mises stress was calculated to 21.2 MPa which is within the tensile yield strength of 23 MPa refer to Figure 6. These results are based on the tension from pulling boat with the magnitude of 70 ton.
Figure 4. Simulation Progress with Time (a) Initial floating out
(b) Sinking gradually by remove buoyancy tank
(c) Sinking completion
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Pipeline Technology Conference 2015
Figure 5. Maximum/Mean/Minimum Curvature along Offshore Intake Pipeline Length
Figure 6. Maximum/Mean/Minimum von Mises Stress along Offshore Intake Pipeline Length
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Pipeline Technology Conference 2015
3. Construction and Installation 3.1 Manufacturing of 2500mm OD of HDPE & Accessories The manufacture and production of long module 2500m OD HDPE pipe and accessories required for the project were carried out by PipelifeNorge The pipes are produced in Norway where fabrication yard is located and then delivered to Algeria. 3.2 Towing of pipe from factory, receiving and storage The pipelines are towed to the port near the site in Algeria under responsibility of Pipelife. The towing plan was prepared to ensure that pipes to be towed are free f rom damages and not exposed to extreme hazards during transport. To increase visibility of the cargo, a standard yellow color is painted every 5 meters on the HDPE pi pe and blinking light at the end of the tow is attached as shown in Figure 7. Tug boat usually travels with the speed under 7~8 knots and a 1 knot per minute acceleration would be permitted for increasing the speed. Figure 7. Towing of HDPE Pipeline (PIPELIFE)
When long module HDPE pipes approach to the port, the harbor authority grants permission to proceed. The harbor tug holds the pipes and the assisting vessels position and secure them.
3.4 Installation of concrete weights The concrete weight production was carried out in parallel with the delivery of HDPE pipe by pre-cast method. The upper collar and the lower collar were fixed to the HDPE pipe by using a lift frame. The lifting frame will then be loaded with a set of the 10
Pipeline Technology Conference 2015 collar weight, the upper and the lower section, and then it is carefully lowered to the marked area on the HDPE where the collar weights will be put in place. The upper collar will then be slowly lowered to connect the two sections by hot dip galvanized bolts and nuts. This procedure is repeated until installation of all collar weights is completed. Figure 8 shows the installation of concrete collar performed in the site. . Figure 8. Setting of Collar using Lift frame
3.5 Sinking process of weighted HDPE pipe After installation of concrete collar with other required accessories to the long module pipe, it was towed from the port to the site. The most important thing is to know the local weather condition because sea condition must be calm to sink the HDPE pipeline. The pipeline to be sunk is positioned in the designated route by the use of tug boats, barges and small boats. The inmost end is connected to the flange, and there must be an entry pipe so that seawater can be allowed to enter during sinking. The outmost end is fitted with a hose connected to the compressor to serve controlling air pressure inside the pipe if required. To be ready to sink, the location of the pipe route should be pre-determined and marked by buoys using GPS equipment. The first end to be sunk is the inmost part and it is anchored to concrete anchors by the use of a wire to the end flange of the pipe. This is to ensure inmost end of the pipe string settles to a suitable distance from the end of the onshore distance. The flooding of the pipe start by opening one of the valves for inflow of water, and as 11
Pipeline Technology Conference 2015 the water flows into the pipe, the inshore end become sink down to rest on the seabed. Initial stage of the sinking shows an inverse J -figure and continuous inflow of water inside of the pipe leads gradually to the shape of S-bend configuration. The configuration of the pipeline is controlled to maintain curvature above 60 OD using the valve and pulling force. It is very important that pull on the offshore is maintained to prevent the sudden significant increase of curvature to the pipe. Maintaining the pull force applied at the offshore end will control the buckling failure to the pipe. When the air in the pipe are fully evacuated, and the offshore end of the pipe is fully submerged, the pulling force applied is gradually reduced until the offshore end of pipe reaches the seabed, and the installation of the pipe could be said to be successful. The configuration during sinking of the HDPE pipeline and process are shown in Figure 9 and Figure 10. Figure 9. S-shape Configuration of HDPE Pipeline
Figure 10. Initial Sinking
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Pipeline Technology Conference 2015 3.6 Spool piecing and connection works To connect the two pipe strings which are inshore and offshore section respectively, the pipe spools are required. The length is measured before as per the actual distance between the ends of the pipe to be spool pieced. There must be a clearance for the spool piece in between flanges but not wide, the gap shall be minimized in order to prevent excessive longitudinal strain to the pipe in the flanging operation when bolts are tighten. Sinking of spool piece to join submerged pipe carried by the use of barge with crane. The bolts were completely tightened by pneumatic type under water.
4. Conclusion There are many advantages to use HDPE pipeline for seawater intake and outfall in power plant projects. Continuous manufacture of long module has the pipeline more integrity by reducing the welding joint in the site and contributes saving time requiring installation. The engineers should make a careful assessment in design and installation for HDPE to be installed with no harm and keep safe during its operation. In terms of design large diameter HDPE pipeline might adopt traditional offshore pipeline engineering procedures. But on-bottom stability analysis is to be performed most critically when assessing the pipeline movement underwater. For rigorous approach the projected area of submerged concrete collar should be considered in calculating the force caused by currents and waves. These forces are not much small to be neglected. The connection between lower and upper part of concrete collar is also be stringently checked because of possibility to breaking and loosening of bolts in the harsh condition. Daewoo Engineering & Construction Ltd., have been conducting research about optimizing concrete collar and miscellaneous parts. In addition to design, the installation of HDPE pipeline should be analyzed and investigated previously before start up the sinking so that it could be controlled delicately. The results of simulation could provide information such as what tension would be required to hold the pipeline to keep the S-shape and to avoid buckling. These meticulous engineering analyses and controlling performed in the site lead the pipeline to be finally said “installed successfully”.
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