Subsea Water Pipeline - Report Example

Sub-Sea Water Pipeline Name: Date: Background Subsea or offshore pipeline is a pipeline system laid below or on the seabed. They are used to transport oil, water or gas. The pipelines vary in diameter, generally from 3.0 inches for gas lines to about 80 inches for high capacity oil and water pipelines. Pipe wall thickness typically vary from 0.39 inches to 3.0 inches. This depends on the quantity and type of fluid to be transported, temperature and pressure conditions. In this project task, a design of a subsea pipeline has been done for supply of fresh water to be conveyed from the mainland to an offshore island. The pipeline is to convey water at a flow rate of 50l/sec. over a distance of 300 km from the mainland to the island. The intervening sea is 400 m deep, with a maximum depth of 500 m. The greatest speed of the sea currents flowing across the pipeline is 80 cm/sec. A water reservoir on the mainland is positioned at an elevation of 200 m above the sea level. The average height of the island is 6 m above the sea level. The most important parameters of design considered are pipe size, material selection, and pumping requirements. In addition, the report looks into problems associated with subsea pipeline installations, construction, maintenance, and cost estimation of the fresh water pipeline. Problems Associated with transportation of water over long distances vs. shorter distance Transportation of water to an offshore island over long distances poses significantly different challenges as opposed to transportation of subsea oil and gas. Sub-sea pipeline design engineers are often confronted with a number of problems such as ocean currents and waves, corrosion, reliability of the pipeline, accessibility and maintenance. First, there is the problem of life-cycle corrosion and integrity management. One of the main challenges encountered in the management of corrosion and pipeline integrity in waters as deep as 1500 m is accessibility to the pipelines on the sea bed (Qiang & Yong, 2014). In this environment, it can prove extremely difficult and costly, or even virtually impossible to carry out inspection and repair the pipes. Another challenge is that factors critical for subsea pipelines become more dominated by the requirement to withstand external pressure, especially during the installation process. Local infield lines, e.g. risers, flow lines and subsea umbilicals also present challenges as they are usually small in diameter and resistant to collapse due to hydrostatic pressure. In smaller sizes, local field lines can be economically produced as seamless pipes. Deep water trunk lines pose even a bigger challenge. These lines tent to be of larger diameter and thicker pipe wall to resist hydrostatic pressure as well as bending during the laying process. A further complication of these lines is that they are typically 16 inch to 20 inch in diameter (Koto, 2016). These sizes are on the top end of economical seamless pipe production. The manufacturing process is often slow, and the pipe lengths are short. In addition, the cost of materials is high. Material selection, Construction, and Maintenance The pipes are made from high strength materials, with yield strength of 350 – 500 MPa, weldability being considered as one of the main criteria of selection (Koto, 2016). Carbon steels are the materials generally prefered for manufacturing sub-sea pipelines. The most common used grades are Grade B to Grade X80 with outer diameters ranging from 4.5 – 80 inch. Table 1 shows the tensile strength properties of suitable grades of carbon steel materials for sub-marine pipeline. The pipeline structure is shielded from external corrosion caused by sea environment by coatings such as epoxy supplemented by anodic and cathodic protection (Palmer & King, 2008). Fiberglass or concrete coating can be wrapped around the pipelines to provide extra protection against abrasion. Concrete coating is useful in compensating for negative buoyancy on the pipeline. Table 1: Tensile strength of Carbon steels Higher grades of steel are generally costly (per unit volume) and harder to weld, which is likely to require more operation time. However, using higher grade carbon steels reduces the thickness required, which in turn slightly reduces the cost of pipeline per unit length. Use of higher grade steels produces a lighter pipeline which require lower tension. This factor is very significant, especially in deep waters where the tension required can be a limiting factor. After determining the pipe material, length and diameter required based on hydraulic analysis and design, the pipeline is laid on the safest, shortest and easy to install route to minimize the cost of installation. Factors considered in the selection of the route include depth of water, nature and geotechnics of the seabed, military movement, iceberg movement, fishing and shipping activities, minefields, ship wrecks etc. There are two procedures involved in the construction of subsea pipeline systems. In the first step, the required number of pipe segments are assembled into a full length, and then installation along the desired route. In oil transportation industries where pipelines are subject to internal pressures of up to 10 MPa and leaks are not acceptable, the pipe segments are joined using full penetration welds (Koto, 2016). There are various methods that can be used to install a subsea pipeline and a suitable method is decide based on the analyses that have to be performed, environmental and physical conditions, cost and availability of equipment, pipe diameter and length, and presence of other pipelines . Installation can be done using a Pull and Tow System, Lay vessel or Reel ship (Qiang & Yong, 2014). Stabilization of the Pipelines Several methods are used to stabilize and protect submarine pipelines and their components. These may be used alone or in combinations. After laying is completed, the pipelines are stabilized and protected using systems such as trenching and burying, rock and gravel dumping, concrete cover and mattresses, ground anchors, saddle blocks, and sandbags and grout bags (see figure 1). (a) (b) (c) (d) Figure 1: (a) Trenching protection, (b) Rockdumping protection, (c) Concrete cover, (d) Matresses Inspection and Maintenance of Subsea Pipelines Generally, subsea pipelines have to be inspected, repaired and maintained after a specified period to ensure that the fluid is transported and delivered safely. Water transportation pipelines are maintained by replacement of anode and cathode protection, remedial burial of pipelines, mattress laying and dredging operations. A pig is useful testing device for hydrostatic pressure in a pipeline, and to check for crimps and dents on the sidewalls, and carry out periodic cleaning as well as other minor repairs. However, monitoring of a pipeline by using a smart pig may increase the construction, operation and maintenance cost due to its scope of design, mechanics of launching as well as receiving subsea pigs from deep waters (Koto, 2016). Electrical resistance probes, non-intrusive acoustic monitors and permanent intrusive coupons have been used in monitoring general erosion and corrosion inside pipelines; although retrieval remains a barrier unless they are located on the topside at user-friendly stations. Other monitoring techniques employed include ultrasonic testing, microbial analyses, in situ spool mapping, guided wave, selective radiography, remotely operated vehicle (ROV), and visual inspections by divers (Palmer & King, 2008). Current methods of inspection employ the use of technology to monitor safety and parameters such as pressure, flow rate and temperature. Estimation of costs Cost estimation of materials used to construct the pipeline was performed using CAPCOST software. The pipe material selected was carbon steel, the type of pump selected is a centrifugal type made of cast iron material. In the estimation of the cost, only major materials were considered, and the labor costs are not included. This is an estimation of pipes and the pump. Pumps (with drives) Pump Type Power (kilowatts) # Spares MOC Discharge Pressure (barg)   Purchased Equipment Cost Bare Module Cost P-101 Centrifugal 30 3 Carbon Steel 10   $ 32,100 $ 128,000                   Pipes Orientation Length/Height (meters) Diameter (meters) MOC Demister MOC Pressure (barg) Purchased Equipment Cost Bare Module Cost V-101 Horizontal 60 0.131 Carbon Steel   10 $ 2000 $ 100,000,000 TOTAL CAPITAL COST 100,128,000 PART B: Sea water is corrosive and the pipe used to convey the water must be corrosive resistant Galvanized steel pipe has a goad corrosive resistance properties. Since the water to be conveyed is fresh- it is non-corrosive the maximum recommended fluid velocity is; Where: - Flow velocity (ft/sec) - Fluid density (Lb/ft3) - 1000Kg/m3= 62.428 lb/ft3 = 12.27ft/sec . Using continuity equation; m2 Determination of whether a pump is required: If the pipeline will be laid at the seabed (400 m depth), the pressure of water from the reservoir located at 200 m above the sea level will be: = 5827 kPa In terms of water head, this pressure equals to 594 m Calculating for head losses in the pipeline; Head loss at entrance: Where: - flow velocity of water through the pipeline (3.74m/s) - Acceleration due to gravity (9.81m/s) Thus, Head loss at pipe exit: = 0.713 m Head loss at bend (4 bends) . HB=1.424m Head loss due to friction. Using the Darcy-Weisbach equation; Where: - Pipe length - Pipe friction factor - Pipe diameter Length 300700 m From the Moody chart (see figure 2), and assuming a laminar flow, carbon steel pipe has a frictional factor of 0.033. Substituting L, v and f in the formula for head loss due to friction; = 53877.58 m Total Head loss = + + + = 0.356 + 0.713 + 1.424 + 53877.58 = 53880 m 53880 >> 594 m; therefore, a pump will be required to compensate for the loss of head. Location of the pump: Available head = 594 m 594 = Substituting the values of and we get the value of; Thus, a pump should be located at 34.6 km from the offshore reservoir. Figure 2: Moody chart Grundfos Pump manufactures produce some of the world’s heavy duty water supply pumps. These pumps can perform well with high pressure, high velocity fluids delivered at high heads. References Read More

In this environment, it can prove extremely difficult and costly, or even virtually impossible to carry out inspection and repair the pipes. Another challenge is that factors critical for subsea pipelines become more dominated by the requirement to withstand external pressure, especially during the installation process. Local infield lines, e.g. risers, flow lines and subsea umbilicals also present challenges as they are usually small in diameter and resistant to collapse due to hydrostatic pressure.

In smaller sizes, local field lines can be economically produced as seamless pipes. Deep water trunk lines pose even a bigger challenge. These lines tent to be of larger diameter and thicker pipe wall to resist hydrostatic pressure as well as bending during the laying process. A further complication of these lines is that they are typically 16 inch to 20 inch in diameter (Koto, 2016). These sizes are on the top end of economical seamless pipe production. The manufacturing process is often slow, and the pipe lengths are short.

In addition, the cost of materials is high. Material selection, Construction, and Maintenance The pipes are made from high strength materials, with yield strength of 350 – 500 MPa, weldability being considered as one of the main criteria of selection (Koto, 2016). Carbon steels are the materials generally prefered for manufacturing sub-sea pipelines. The most common used grades are Grade B to Grade X80 with outer diameters ranging from 4.5 – 80 inch. Table 1 shows the tensile strength properties of suitable grades of carbon steel materials for sub-marine pipeline.

The pipeline structure is shielded from external corrosion caused by sea environment by coatings such as epoxy supplemented by anodic and cathodic protection (Palmer & King, 2008). Fiberglass or concrete coating can be wrapped around the pipelines to provide extra protection against abrasion. Concrete coating is useful in compensating for negative buoyancy on the pipeline. Table 1: Tensile strength of Carbon steels Higher grades of steel are generally costly (per unit volume) and harder to weld, which is likely to require more operation time.

However, using higher grade carbon steels reduces the thickness required, which in turn slightly reduces the cost of pipeline per unit length. Use of higher grade steels produces a lighter pipeline which require lower tension. This factor is very significant, especially in deep waters where the tension required can be a limiting factor. After determining the pipe material, length and diameter required based on hydraulic analysis and design, the pipeline is laid on the safest, shortest and easy to install route to minimize the cost of installation.

Factors considered in the selection of the route include depth of water, nature and geotechnics of the seabed, military movement, iceberg movement, fishing and shipping activities, minefields, ship wrecks etc. There are two procedures involved in the construction of subsea pipeline systems. In the first step, the required number of pipe segments are assembled into a full length, and then installation along the desired route. In oil transportation industries where pipelines are subject to internal pressures of up to 10 MPa and leaks are not acceptable, the pipe segments are joined using full penetration welds (Koto, 2016).

There are various methods that can be used to install a subsea pipeline and a suitable method is decide based on the analyses that have to be performed, environmental and physical conditions, cost and availability of equipment, pipe diameter and length, and presence of other pipelines . Installation can be done using a Pull and Tow System, Lay vessel or Reel ship (Qiang & Yong, 2014). Stabilization of the Pipelines Several methods are used to stabilize and protect submarine pipelines and their components.

These may be used alone or in combinations. After laying is completed, the pipelines are stabilized and protected using systems such as trenching and burying, rock and gravel dumping, concrete cover and mattresses, ground anchors, saddle blocks, and sandbags and grout bags (see figure 1).

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