Design Dead Load in Buried Pipe

Existing soil above buried pipe is categorized as external dead load. Unlike truck load which is categorized as live load, soil load is given evenly along buried pipe. Soil load (Figure 1) can be calculated using the following formula:

P_d = g H γ_s OD

Where:

P_d         = soil load (N/m)

H         = depth of buried pipe in the ground (m)

γ_S         = density of soil (kg/m³)

OD      = outer diameter of buried pipe (m)

So the total amount of external load is transferred to buried pipes due to soil load (dead load) and truck load (live load) is:

P = P₁ + P_d

Where:

P          = total load on buried pipe

P_l         = load truck

P_d         = soil load

As an interpretation of the model loading pipes in the soil with the combination loads, in this case is a truck, it can be seen in Figure 2 below.

Figure 1: Design Dead Load in Buried Pipe
(source: book-Structural Mechanics of Buried Pipes by Reynold KW, Loren RA) 

Figure 2: Modeling Load on Buried Pipe
(source: book-Structural Mechanics of Buried Pipes by Reynold KW, Loren RA) 

Design Live Load in Buried Pipe

The external loads in the buried pipe stress analysis that must be considered in calculation are live load and dead load. Live load will always changes according to position or distance, while dead load is not depend on these factors but it is determined by design value of dead load itself.  The examples of design live loads in buried pipe are truck load, car load, train load and the others.

As for example: truck is located above buried pipe or truck across ground area where pipe is buried in it. Truck is modeled as live load so it will add external load to buried pipe. This truck load is vertical load for buried pipe. It is assumed that the truck load is single load (W) above ground as shown in Figure 1 below.

Figure 1: Truck as Live Load on Buried Pipe
(source: book-Structural Mechanics of Buried Pipes by Reynold KW, Loren RA) 

The amount of truck load that is received by buried pipes at point A (right above pipe) is:

P₁ = N W / H²

Where:

P_l         = truck load (N/m²),

W        = single truck load (N)

H         = height of soil above pipe (m)

R         = horizontal distance from the center of pipe to single load (m)

N         = coefficient Boussinesq = (3 (H/R)⁵) / (2π)

Coefficient value of N can be seen from Figure 2. To know the value of N can use the relationship of R (distance W of center pipe) with H (depth pipe in soil). Then the relationship between R / H with N can be obtained as shown in the chart below.

Figure 2: Relationship of R/H vs N
(source: book-Structural Mechanics of Buried Pipes by Reynold KW, Loren RA) 

Multiphase Flow

A fluid can flow along pipes which have different shape; fluid flow direction can be divided into four categories, namely: vertical flow, horizontal flow, inclined flow, and directional flow. The direction of inclined and directional flow make angle of inclination between 0 to 90 degrees to horizontal axis. Figure 1 below shows the contribution of each category of flow direction on a production system.

Figure 1: Direction of Flow in a Production System

Vertical and directional flow direction is typically used in pipelines beneath surface, the pipe connecting fluid from reservoir to surface (wellhead) of offshore platform. Once the fluid reaches surface, directions of flow is frequently involved are horizontal and inclined flow direction. The use of flow direction type is closely related in terms of determining the value of changes in pressure along pipe flow.

Multiphase flow in pipes can be defined as the concurrent movement of free gas and liquid in the pipe that can occur in various flow patterns. Gas and liquid can flow as homogeneous mixture, the liquid is at the front with the gas push behind liquid, liquid and gas may flow in parallel, or in various combinations of flow patterns that may occur.

Generally, the pattern of multiphase flow is divided into three types as follow:

1. Segregated multiphase flow

In the segregated multiphase flow, the flow of gas phase is separated from liquid phase; it means that gas phase can flow above liquid phase or between the flow of liquid phase as shown in Figure 2.

Figure 2: Segregated Multiphase Flow

2. Intermittent multiphase flow

Flow pattern that can be included in the form of intermittent multiphase flow is the flow of liquid phase which hit the gas phase (Figure 3 bottom side) or the flow of liquid phase inhibit the flow of gas phase (Figure 3 top side).

Figure 3: Intermittent Multiphase Flow

3. Distributed multiphase flow

The pattern of distributed multiphase flow, gas and liquid phase are dispersed uniformly inside pipe flow, as shown in Figure 4.

Figure 4: Distributed Multiphase Flow

Flow in Pipe Concept

Flow in pipe can consist of single phase flow (gas, oil, or water only) or more which can be called multiphase (gas and liquid). In general, fluid in pipe can flow from an inlet point to outlet point if the pressure at outlet point is smaller than the pressure at inlet point, Pout < Pin (ΔP>0). If the pressure at inlet point is equal to the pressure at outlet point, Pout = Pin (ΔP=0), then fluid flow along pipe will not happen.

Figure 1: Flow in Pipe Concept

Figure 1 is a figure that show pipeline with inlet point consist of three pipe segments, segment 1 with pressure P1 and flow rate Q1 , segment 2 with pressure P2 and flow rate Q2, and segment 3 with pressure P3 and flow rate  Q3. The fluid of three pipe segments will be streamed to a gathering pipe with outlet pressure as Pout and fluid flow rate as Qout. Flow rate Qout shall be the sum of Q1, Q2, and Q3, because the pipeline in Figure 1 is assumed there is not inhibitor there.

If Qout is not summation of three flow rate (Q1, Q2, and Q3), it indicate that there is leak in pipe flow. As was explained earlier that the fluid flows from high pressure to low pressure, so fluid from the third inlet segments will flow into outlet segment if the value of P1, P2, and P3 respectively is greater than Pout.

A case that could possibly occur on the fluid in pipe is the pressure in the inlet segment is smaller than Pout, so the fluid will experience back-flow. Backflow can occur in variety of possibilities, namely the fluid from inlet segment which has higher pressure will flow into inlet segment which has lower pressure. When this condition occurs, then the flow rate that reaches outlet point will not optimal. Similarly, for fluid flow in the production pipeline, if backflow occur, the fluid flow rate through separator will not optimal.

Pipeline System in Offshore Platform

A pipeline consists of several segments of pipe that has very complex and diverse forms. Figure 1 shows a scheme of pipeline surface that connects the fluid of some wellheads which are located at each platform to separator. The pipeline consists of several platforms, each of which has several wellheads.

Figure 1: Scheme of Pipeline Production System between Offshore Platform

Fluid flow that occurs comes from well which then when it reaches the surface of fluid will go to the wellhead. Fluid from each wellhead will be streamed into a pipe flow, starting point of the meeting is called gathering point fluid, fluid then flows into the header platform. Fluid derived from multiple platforms header will be channeled back into separator. This section serves as separator between phases of fluid.

More details of the pipeline production scheme that lies beneath surface to the surface is shown in Figure 2. In the Figure 2 shows that the fluid source is derived from 3 pieces of reservoirs. Fluid jetting processes that occur on the surface similar to those seen in Figure 1, it is from wellhead to separator. In an oil field, production processes often experience a variety of obstacles, one of them is bottleneck. This bottleneck can lead to the production of wells are not optimal.

Figure 2: Scheme of Pipeline Production in 1 Offshore Platform