OTC-20226-PP

Mechanical Design of Subsea and Buried LNG Pipelines Trent Brown, ITP InTerPipe, Inc.; Paul Jukes; and Jason Sun, JP Kenny, Inc.

Copyright 2009, Offshore Technology Conference This paper was prepared for presentation at the 2009 Offshore Technology Conference held in Houston, Texas, USA, 47 May 2009. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract Design of subsea and buried LNG pipelines presents challenges such as low operating temperatures (-160 C), multiple pipe walls, and differential expansion of materials. ITP has developed an LNG pipeline design that has been certified by DNV as Fit for Service for LNG subsea transport. JP Kenny has developed and applied a comprehensive FE model to ITPs triple-walled (PIPIP) LNG pipeline design to successfully demonstrate the robustness of the pipe design. The FE model is used to determine the displacements and stresses in each of the three pipe walls under all the operating conditions and to provide input data for detailed analyses of the bulkheads, risers, riser supports, and tie-ins to external piping. The basis of the model is presented along with a description of the PIPIP. Typical results of the modeling are presented, which demonstrate how the design of the PIPIP limits the stresses, displacements, and end loads. The use of the model results for design of ancilliary and connecting systems is also reviewed. Design Considerations for LNG Loading/Offloading Pipelines LNG loading/offloading pipelines present several design challenges:

Low temperatures (-160 C) Relatively long distances High thermal performance requirements Low risk tolerance

The material selected for LNG pipelines has traditionally been stainless steel 304L. Although suitable for the low temperature, 304L has the disadvantage of a high Coefficient of Thermal Expansion (CTE = 13.6 x 10-6 K-1 between ambient and -160 C). For the stresses in a 304L LNG pipeline to be acceptable, the pipeline must be allowed to contract with temperature. Consequently, either expansion loops or bellows are required. Heat ingress to the pipeline must be limited for operating cost and operational (Boil-Off Gas) reasons. As such, very low overall heat transfer coefficients (U-values less than 0.15 W/m2*K) are required. Low U-values have traditionally been obtained by using either Vacuum Insulated Pipe (VIP) or very thick conventional insulation. All of the product from an LNG export facility must flow through the LNG pipeline. As LNG facilities involve high capital costs, there is a very low risk tolerance for LNG pipelines. If a subsea design is to be preferred over the conventional trestle design, the subsea design must be robust and able to withstand impacts that might occur in near shore regions with ship traffic. To apply an LNG pipeline subsea, the thermal performance must be maintained; the design must be robust and reliable; and bellows and expansion loops must be eliminated. ITP has developed a design that satisfies these criteria using a triple-wall design with different materials for the pipes, and with bulkheads only at each end of the pipeline. The triple-walled design and bulkheads complicate the determination of the stresses, displacements, and end loads. JP Kenny has developed a pipeline analysis model that allows the design to be rigorously evaluated. ITP LNG Pipeline ITP began development of its cryogenic pipe design in 1996 and performed the first full-scale tests in 2001 with LNG. In 2003, ITP was joined by four major oil companies in a joint industry project with the objective of qualifying ITPs LNG pipe

2 OTC OTC-20226-PP

design for subsea transportation of LNG. In 2007, ITP received the Certificate of Fitness for Service from DNV, after completing the rigorous technology qualification process documented in DNV RP-A203. The main characteristics of the design are:

Triple-walled with a sacrificial outer pipe. Inner pipe material, 36% nickel-iron (NiFe), has a very low CTE (less than 2 x 10-6 K-1) High-performance Izoflex insulation. Reduced pressure in the inner annulus. Intermediate and outer pipes both designed for collapse.

Figure 1 shows a cross section of the full-scale, triple-walled pipe.

Outer carbon steel pipe

Outer annulus

Intermediate pipe

Inner annulus at reduced pressure

Izoflex insulation

36% nickel inner pipe

Outer carbon steel pipe

Outer annulus

Intermediate pipe

Inner annulus at reduced pressure

Izoflex insulation

36% nickel inner pipe

Figure 1: ITP LNG PIPIP

Due to the low CTE of 36% NiFe, the pipeline does not require expansion loops or bellows, independent of the length of the pipeline. The intermediate and outer pipes are carbon steel and are both designed for collapse in the maximum water depth. The purpose of the outer pipe is to provide protection from external damage. The outer pipe is sacrificial it can be damaged or even breached without compromising the integrity of the pipeline or the thermal performance. Thus, the PIPIP provides excellent protection against external impacts (fishing equipment, anchors, and other dropped objects), with the outer pipe acting as a crumple zone and absorbing impact energy. The inner annulus is typically continuous along the entire pipeline length. The inner annulus is maintained at a reduced pressure. This partial vacuum has two advantages. First, the thermal performance of the Izoflex insulation is increased by a factor of three. Second, the partial vacuum acts as a straightforward, robust, and very sensitive leak detection system. The pressure of the inner annulus is monitored continuously, and, due to the low pressure, any leak of LNG into the annulus is quickly detected. The reduced pressure is easy to achieve with standard vacuum equipment and can be maintained indefinitely no special handling of the pipe is required as the pressures are not low enough for degassing to be an issue. The outer annulus is filled with dry nitrogen at a pressure above hydrostatic head. The pressure of this annulus is also continuously monitored. This allows for a simple, straightforward, leak detection system. The leak detection capabilities of the system have been reviewed by Offredi, et al., 2008. Bulkheads are located at either end of the pipeline (and potentially at bends along the pipeline route). One bulkhead connects the inner and intermediate pipes. The second bulkhead connects the intermediate and outer pipes. These bulkheads connect the inner, intermediate, and outer pipes to form two sealed annuli. When the intermediate pipe is stainless steel, the triple-walled pipe design provides double containment for the LNG and a second barrier (the third pipe) for protection of the pipelines from external damage. All joints between the bulkheads are simple butt welds. This is possible because of the Izoflex insulation material application procedure and the mechanical strength and thermal properties of the Izoflex. In the pipe-in-pipe fabrication process, the

OTC OTC-20226-PP 3

intermediate pipe can be slid over the Izoflex insulation and the intermediate pipe can be welded directly over the insulation without damage to the insulation because the insulation can tolerate the high temperatures of the welding process. Butt welds are the most robust weld for a pipeline and the most readily inspected. Welds on all three pipes are 100% inspected. JPK Pipeline Model A global, 3-D FE model has been developed to assess the LNG pipeline expansion/contraction and the stresses and load response under extreme ambient and operational loading conditions. The model was developed using ABAQUS version 6.7.1. The FE global model meshes were initially created using the ABAQUS pre-processor (ABAQUS/CAE), which is a user-friendly graphical interface that can easily generate the entire system including the bend and riser curvature. Load case runs are conducted using input decks and restart techniques for simulation efficiency. The modules can perform parametric studies by simply changing the input parameters of the input script code. Upon completion of a single analysis, the following results are available:

Submerged weight; Axial and lateral displacements for all three pipes; Effective and true axial forces for all three pipes; Bending moments for all three pipes as a function of position; Longitudinal, hoop, and Von Mises stresses for all three pipes as a function of position; Elastic/plastic strain, bending curvature. End loads

Finite Elements Two-node 3D pipe elements are used to model all three pipes. All of the nodes and elements of the three pipes are

assumed concentric initially. The intermediate pipe and inner pipe then naturally settled to their correct positions. Thus, any bending moments associated with the eccentricities of the pipes are accounted for in the model. The interactions between the three pipes are modeled by tube-to-tube contact elements with gaps between and friction effects were counted for. Single node connector elements are used to model the restraints of the onshore tie-in, bottom anchor mechanism, and riser clamps. Boundary Conditions

The interaction between the seabed and the pipeline is defined as a hard contact, and the effect of friction on the pipeline is accounted for via equivalent frictional coefficients in both the axial and lateral directions. The seabed is assumed to be flat and rigid. The landfall is also modeled by soil/pipe interaction with a high level of friction with the soil. The onshore tie-in mechanism is modeled as a low-strength spring (connector element). The riser clamps are modeled as a low-strength springs (connector element). The bottom anchor mechanism is modeled by a high strength spring (connector element) connected to the offshore riser platform. Other Information

Inline and end bulkheads are modeled by joining the corresponding pipe nodes in all translational and rotational degrees of freedom. To mimic the spacers between the intermediate and outer pipes, every 6 m the outer pipe nodes are kinematically coupled to the corresponding intermediate pipe nodes. This allows axial movement between the outer and intermediate pipes. Friction between the two pipes iss considered. A gap is specified to account for the separation between spacers and the outer pipe inner surface. The Izoflex insulation in the inner annulus is modeled in a similar way, with every inner and intermediate pipe node coupled. A general view of the model is given in Figure 2.

Figure 2: Overview of the Pipeline Model

Spacer, 6m

Inner Pipe 36% Ni Steel

Outer Pipe CS Steel Intermediate Pipe CS Steel

Concrete Coating

Izoflex Insulation

Bulkhead Bulkhead

Boundary Boundary Total Length

4 OTC OTC-20226-PP

Sample Case A pipeline design has been conducted for a generic, subsea, LNG export pipeline of typical dimensions. The pipeline is 7000 m from the onshore tie-in to the offshore platform. This length is selected because it is sufficient to address all of the analysis requirements, as the middle portion of the pipeline will be locked axially due to the soil and other boundary restraints. It is assumed that all of seabed pipeline will be buried (either deliberately or naturally) before operation of the LNG pipeline begins. The model incorporates the inner pipe internal pressure and the outer pipe external pressure, but does not consider the inner and outer annulus pressures. No installation residual tension is considered in the model. The water depth is assumed to be 15 m, which is typical for LNG facilities and the elevation changes along the route are negligible, other than the steady slope. In detail, the FE model includes the landfall, the pipe on the seabed, and the bend/riser as shown in Figure 3. The restraints at the pipeline onshore end, offshore end, and riser are also modeled. Sensitivities were evaluated for the coefficient of friction between the outer pipe and soil and for the stiffness of the connection to the offshore structure. The dimensions of the system are given in Table 1. In the base case the following parameters were used:

Coefficient of friction between the pipeline and the soil: 0.94 Coefficient of friction between pipes 0.1 Internal Pressure: 30 barg (design pressure) Onshore tie-in stiffness 100 kN/m Connection to offshore structure stiffness 105 kN/m

In addition to the base case parameters above, results are presented for sensitivities in the soil friction coefficient (2.92) and the stiffness of the connection to the offshore structure (106 kN/m). The soil friction is subject to a large uncertainty and the stiffness of the offshore connection is a design parameter.

Figure 3: Layout of the Generic Pipeline Model

OTC OTC-20226-PP 5

Table 1: PIPIP Dimensions

Pipe or Layer Material Outside

Diameter(mm)

Thickness(mm)

Inner Pipe 36% Ni-FE 762.0 8.5 Insulation Izoflex 40.0

Intermediate Pipe Carbon Steel 914.4 14.3 Outer Pipe Carbon Steel 1016.0 17.5

Weight Coating Concrete 50.8

Expansion/Contraction Since an LNG pipeline operates about 190 C below ambient air temperature, thermal contraction is a significant issue.

For instance, if a stainless steel pipeline were completely unconstrained (i.e., free to move) it would contract about 8.4 m on each end (16.8 m total), resulting in severe problems for both the onshore and offshore connections. Alternatively, if a stainless steel pipeline is completely restrained (not allowed to move) it would experience longitudinal stresses well beyond the yield stress. For example, for a stainless steel pipe a linear calculation gives stresses more than twice the yield stress. For stainless steel LNG pipelines, the expansion/contraction issue requires either expansion joints or bellows.

To minimize the expansion/contraction issues and eliminate the need for expansion joints or bellows, the inner pipe of

ITPs PIPIP is made of 36% NiFe with a CTE less than 2 x 10-6 K-1. Additionally, the high-performance insulation ensures that the intermediate and outer pipes are close to ambient temperature, so that these two pipes resist the contraction of the inner pipe. The result, as discussed below, is that the axial displacements and end loads are very small.

The displacements of each of the three pipe walls of the PIPIP, as calculated by JPKs FE model, are shown in Figure 4.

The positive direction is from the onshore tie-in to the offshore tie-in. For a pipeline that is 7000 m long, the total displacement at the onshore end is only 43 mm (or 1.7). At the offshore end, the displacement is just -21 mm (0.8). The signs of the displacements indicate that both ends of the pipeline are contracting toward the middle of the pipeline. There is less contraction offshore than onshore, because the connection to the offshore structure is specified as being stiffer than the onshore tie-in. For the middle 4-5 km of the pipeline, friction is sufficient to hold the pipeline in place and prevent axial displacement.

At the two ends of the pipeline, all three pipes are constrained to have the same displacement, because they are tied

together via the bulkheads. Away from the ends, however, the individual walls are free to move relative to each other, subject to frictional forces. This can be seen more clearly in Figure 5, where the relative displacements of the pipes are shown. The movement of the inner pipe relative to the intermediate pipe is of particular interest as the pipeline design avoids the use centralizers. Over the middle 4-5 km, the inner pipe does not move relative to the intermediate pipe due to friction between the two pipes. Near the ends, the inner pipe moves by, at most, 3 mm (0.11) relative to the intermediate pipe.

Figure 6 shows a comparison of the displacements of the inner pipe for the base and sensitivity cases. Increasing the

friction obviously reduces the overall movement. Variations in the friction factor with the range of uncertainty change the displacement by roughly 30% relative to the base case; however, this still only results displacements between 30 and 43 mm (1.2 to 1.7). Increasing the stiffness of the end connection has more pronounced effect. The maximum stiffness for the connection to the offshore structure gives a displacement of only 3 mm. Anchoring the pipelines at the ends (effectively high-stiffness connections) does not present any problems regarding stresses, as discussed in the following section.

6 OTC OTC-20226-PP

Pipeline Axial Displacement Design Pressure and Temperature and Base Boundary Conditions

-30

-20

-10

0

10

20

30

40

50

0 1000 2000 3000 4000 5000 6000 7000

Distance from Onshore Tie-in (m)

Dis

plac

emen

t (m

m)

Inner PipeIntermediate PipeOuter Pipe

Figure 4: Base Case Axial Displacements

Relative Movement Between Pipes Design Pressure and Temperature and Base Boundary Conditions

-5

0

5

10

15

0 1000 2000 3000 4000 5000 6000 7000

Distance from Onshore Tie-in (m)

Rel

ativ

e A

xial

Mov

emen

t (m

m) Intermediate Pipe vs. Outer Pipe

Inner Pipe vs. Intermediate Pipe

Figure 5: Base Case Relative Displacements

OTC OTC-20226-PP 7

Inner Pipe Axial Displacement

-30

-20

-10

0

10

20

30

40

50

0 1000 2000 3000 4000 5000 6000 7000

Distance from Onshore Tie-in (m)

Dis

plac

emen

t (m

m)

Base Case

Maximum Soil Friction

Maximum Offshore Connection Stiffness

Figure 6: Comparison of Inner Pipe Displacements for Different Friction and Anchor Stiffnesses

Stresses

For the stress analysis, the pipeline can be divided into three sections: the onshore tie-in, the offshore end, and the center of the pipeline where friction prevents movement of the pipeline. The Von Mises stresses for the three cases studied are shown in Table 2. For each case, the pressure is the maximum possible pressure in the system (full surge pressure), so the hoop stress is the dominant stress for the inner pipe. The maximum Von Mises stress in the inner pipe occurs at the offshore end of the pipeline and is 61% of the 36% NiFe materials SMYS at ambient temperature. At LNG temperature the SMYS of 36% NiFe increases by 82% relative to ambient temperature, and the maximum Von Mises stress is only 34% of the SMYS. Because the design is based on ambient-temperature strengths, the design at LNG temperature is very conservative.

The choice of material for the inner pipe has an important effect. The longitudinal stress due to the change in temperature

is only about 50 MPa in the fully restrained center section of the pipeline even without bellows or expansion loops. The controlling design criterion for the inner pipe is burst at the full surge pressure, which occurs only in an Emergency Shutdown (ESD). Under the normal operating conditions, the maximum Von Mises stress is typically 40% of SMYS assuming ambient temperature properties.

The stresses in the intermediate and outer pipes are small, with a maximum of only 65 MPa. As these pipes are carbon

steel, this represents only 16% of SMYS.

8 OTC OTC-20226-PP

Table 2: Combined Stresses

Case Pipe Von Mises Equivalent Stress (MPa)

Onshore Tie-in Riser End

Center of Pipeline (Anchored Section)

Base Case Inner 126 132 117

Intermediate 35 57 43 Outer 40 29 34

Maximum Friction

Inner 127 133 117 Intermediate 37 58 43

Outer 43 30 34 Max Offshore

Connection Stiffness

Inner 126 134 117 Intermediate 35 65 43

Outer 40 35 34 End Loads

Important design criteria are the loads that the pipeline applies to the offshore structure. In this case, the pipeline is to be constructed onshore and pulled offshore into its final position. The pulling sled is designed such that most of the contraction load from the pipeline is transferred to the offshore structure through the pulling sled. Table 3 shows the loads that the pipeline applies to the offshore structure due to the pressure and temperature forces. The model assumes that the the offshore structure is completely rigid.

The offshore structure can be designed to accommodate even the most severe of the loads. In practice, the model is used to

generate a set of results spanning the practical range of connection stiffnesses. Since all of the results are acceptable to the pipeline, these results allow the installation and structural engineers to optimize the design of the installation sled and offshore structure.

Table 3: Offshore Loads and Displacements

Case

Pipeline Axial

Displacement(mm)

Offshore Connection

Load (kN)

Maximum Riser Clamp

Stiffness (kN)

Base -22 2180 2.0 Maximum Friction -14 1420 1.3

Max Offshore Connection Stiffness -4 3810 0.6 Application of Pipeline Model Results

In addition to the basic pipeline design, the model results are used as inputs for the design of other pipeline components and connections. These are discussed briefly in the sections below.

Bulkhead Design

At the ends of the pipeline and around some bends, such as the base of the riser, bulkheads connect the three pipe walls. The primary purposes of the bulkhead are:

Connect all the pipes together so that the pipeline can be installed as a single unit; Seal the annulis for pressure reduction and leak detection; Transfer loads from the inner pipe to the intermediate outer pipes; Prevent damage to the insulation due to movement of the inner pipe relative to the intermediate pipe;

Two different types of bulkheads are utilized. First, at the ends of the pipeline, end bulkheads terminate the PIPIP and

allow the pipeline to be welded directly to standard, single stainless steel pipe. Second, at bends, inline bulkheads connect the pipe walls together, but allow the PIPIP to continue on both sides of the bulkhead.

The bulkhead designs are not covered by DNV codes, so they are designed to pressure vessel codes. A fine-scale, 3D FE

model of the bulkhead location is constructed. Internal and external temperatures and pressures are applied to the FE model as well as the longitudinal forces and bending moments as determined by the pipeline model. Table 3 shows the loads to be applied as determined by the pipeline model for the base case. The FE model calculates the temperature distributions and the

OTC OTC-20226-PP 9

stresses throughout the bulkheads. The stresses at key locations throughout the bulkhead are then checked against the acceptance criteria in the pressure vessel code. The calculated stresses are acceptable at all locations for load cases.

Table 3: Offshore Loads and Displacements for the Base Case

Load Condition Item

Onshore End Bulkhead

Riser End Bulkhead

Base of Riser Inline Bulkhead

Inner Pipe

Mid Pipe

Outer Pipe

Inner Pipe

Mid Pipe

Outer Pipe

Inner Pipe

Mid Pipe

Outer Pipe

Base Case

Effective Axial Force (kN) 354 793 -1143 821 165 -250 479 1285 645

True Wall Force (kN) 1656 793 -1222 1384 165 -329 1781 1285 566

Bending Moment (kN-m) 139 136 -267 0.04 0.2 0.3 166 278 364

Riser and Tie-in Designs

The pipeline model is also used for the design of risers; design of the onshore and offshore tie-ins of the PIPIP to conventional, single-walled, stainless steel pipe; and fatigue analyses. For the riser, a separate, detailed FE model is constructed, including the inline bulkhead at the base of the riser, the bends at the top and bottom of the riser, the main riser section, and the end bulkhead. Internal and external temperatures and pressures, wave and current loadings, and the longitudinal forces and bending moments as determined by the pipeline model are applied. Althought the riser is short, there are difficulties in the design:

Multiple pipe walls; Highly restrained pipes (bulkheads at each end of the riser); Different materials throughout (bulkheads vs. pipe); Large temperature differences;

Again, the choice of material for the inner pipe is important. With 36% NiFe the riser is acceptable. However, other

materials such as stainless steel present a serious problem because the stresses imposed by thermal contraction are unacceptably large. This is because the inner pipe in the riser is coupled to the other pipes by bulkheads at the top and bottom, preventing the inner pipe from contracting.

For the design of the tie-ins to the onshore and offshore piping, local FE models are created. The loads from the pipeline

model are input into the local tie-in models to determine the local axial forces, shear stresses, and the bending moments. The loads arere then used to calculate the stresses in the flange to compare against the allowable stresses in pressure vessel design codes.

For fatigue analyses, the simulations with the pipeline model are made with the successive steps in the simulations being

the operating conditions for relevant operating cycles such as LNG recycle LNG loading LNG recycle and LNG loading ESD (with surge) shutdown. Temperature and pressure loads are changed according to the actual operating conditions at each step. The stresses obtained from the pipeline model across pipe welds at successive steps are taken to define the stresses ranges used in the fatige anaysis. Conclusions ITPs design for LNG subsea and buried pipelines is a triple wall pipeline utilizing a 36% NiFe material for the inner pipe, which meets all of the design requirements for subsea LNG pipelines. JP Kenny has developed and implemented a global FE pipeline model which includes the three pipe walls, the insulation the offshore and onshore tie-ins, and friction between all materials. The model has been used to demonstrate that the PIPIP design results in minimal contraction and/or longitudinal stresses even for long distance pipelines. Further, the results of the model are usefule for design pipeline end connections and other components of the pipeline such as bulkheads.

Nomenclature

CTE Coefficient of Thermal Expansion FE Finite Element ITP ITP InTerPipe, Inc. JPK JP Kenny, Inc. LNG Liquified Natural Gas

10 OTC OTC-20226-PP

PIP Pipe-In-Pipe PIPIP Pipe-In-Pipe-In-Pipe SMYS Specified Minimum Yield Stress VIP Vacuum Insulated Pipe

References DNV OS-F101 DNV RP-A203 Leak detection paper. PD 5500