Flexible Pipes: Advances in Pipes and Pipelines

By Qiang Bai, Yong Bai and Weidong Ruan

Recent changes in the codes for building pipelines has led to a boom in the production of new materials that can be used in flexible pipes.  With the use of polymers, steel, and other new materials and variations on existing materials, the construction and, therefore, the installation and operation of flexible pipes is changing and being improved upon all over the world.  The authors of this work have written numerous books and papers on these subjects and are some of the most influential authors on flexible pipes in the world, contributing much of the literature on this subject to the industry.  This new volume is a presentation of some of the most cutting-edge technological advances in technical publishing.

This is the most comprehensive and in-depth book on this subject, covering not just the various materials and their aspects that make them different, but every process that goes into their installation, operation, and design.  The thirty-six chapters, divided up into four different parts, have had not just the authors of this text but literally dozens of other engineers who are some of the world’s leading scientists in this area contribute to the work.  This is the future of pipelines, and it is an important breakthrough.  A must-have for the veteran engineer and student alike, this volume is an important new advancement in the energy industry, a strong link in the chain of the world’s energy production.

• Language: English
• Publisher: Wiley
• Release date: Apr 19, 2017
• ISBN: 9781119041276

Qiang Bai

Dr. Qiang Bai obtained a doctorate for Mechanical Engineering at Kyushu University, Japan in 1995. He has more than 20 years of experience in subsea/offshore engineering including research and engineering execution. He has worked at Kyushu University in Japan, UCLA, OPE, JP Kenny, and Technip. His experience includes various aspects of flow assurance and the design and installation of subsea structures, pipelines and riser systems. Dr. Bai is the coauthor of Subsea Pipelines and Risers.

Book preview

Chapter 1

Flexible Pipes and Limit-States Design

1.1 Introduction

The origin of flexible pipes can be traced to pioneering work carried out in the late 1970 s. Initially, flexible pipes were used in relatively benign weather environments such as offshore Brazil, the Mediterranean and the Far East. However, flexible pipe technology has advanced so rapidly that they are now used in various areas in the North Sea [1] and have gained popularity among designers in the Gulf of Mexico. Flexible pipes can be applied in water depths up to 8,000 ft., pressures up to 10,000 psi, high temperatures above 150 °F and can withstand large vessel motions in adverse weather conditions. Figure 1.1 illustrates a typical flexible riser used in deep water and shows the different configurations used for different water depths. This type of dynamic application is typically used in floating production systems with high pressure production risers, export risers, chemical/water/injection lines and gas lift lines.

Figure 1.1 Typical flexible riser configurations [2].

1.2 Applications of Flexible Pipe

This book explores the application of flexible pipes in the oil and gas industry, both onshore and offshore. The flexible pipe’s advantages include its composite structure that combines an internal polymeric sealing layer that transports fluids, helical armoring layers that provide the required strength and a polymeric outer sheath that prevents seawater from interacting with the armor layers. As a result this kind of pipe has a low bending stiffness in comparison to axial tensile stiffness, allowing a much smaller radius of curvature than a homogenous pipe with the same anti-pressure capacity. This particular structure gives the flexible pipe a number of advantages over other types of pipelines and risers such as steel catenary risers, including inherent corrosion resistance, reduced transport and installation costs due to prefabrication and storage on reels and compatibility with compliant structures allowing a permanent connection between a floating support vessel and subsea installations.

Figure 1.2 shows the categorization of flexible pipes based on their configuration and functional requirements. The relationships of different type flexible pipes are illustrated in the figure, and in relation to the applied standards and recommended practices. Some main manufacturers for the different types of flexible pipe are also listed in the figure. Flexible pipes are divided based on the operating pressure and reinforcement material: metal based or composite based. The metal-based flexible pipes are normally designed to withstand high loads such as high internal and external pressures or large axial tension. Flexible composite pipes (FCP) are simplified versions of metal-based flexible pipes due to their simple configuration and lower functional requirements. Both metal-based and composite-based flexible pipes are further divided into two groups based on their configuration: bonded or unbonded. The bonded nonmetallic FCP is also named Thermoplastic Composite Pipes (TCP) or Reinforced Thermoplastic Pipes (RTP). The recommended DNVGL RP F119 [4] outlines the requirements for TCP in offshore applications while API RP 15S [5] focuses on onshore applications. Unbonded FCP includes steel reinforced flexible pipes and unbonded nonmetallic reinforced flexible pipes while bonded FCP includes steel wire bonded flexible pipes and nonmetallic bonded flexible pipes (TCP) such as fiberglass reinforced flexible pipes.

Figure 1.2 Categorization of flexible pipes.

1.2.1 Metal-Based Flexible Pipes

Metal-based flexible pipes are mainly used as flexible flowlines or flexible jumpers in offshore applications due to their resistance to large external and internal pressures and tensile loads. In addition, they are able to withstand large dynamic loads from the operational procedures or the offshore environment. In metal-based bonded pipes, many layers of fabric, elastomer and steel are bonded together into a single structure through a vulcanization process. The bonded pipes are primarily used in short sections such as jumpers in high dynamic applications. Figure 1.3 shows a metal-based bonded flexible pipe cross section. This type of flexible pipe is detailed in Chapter 23 of this book.

Figure 1.3 Metal-based bonded flexible pipe [3].

On the other hand, metal-based unbonded flexible pipes are manufactured for static and dynamic applications in lengths of several hundred meters. Unless otherwise stated, unbonded flexible pipes refer to metal-based unbonded flexible pipes in this book. Figure 1.4 shows a typical cross section of an unbonded flexible pipe and clearly identifies nine layers with different function. The space between the internal polymer sheath and the external polymer sheath is known as the pipe annulus. This type of flexible pipe is detailed in Chapters 5 to 13.

Figure 1.4 Metal-based unbonded flexible pipe [6].

Figure 1.5 shows the distribution of the internal diameter of unbonded flexible pipes for different water depths in various offshore applications. The deepest water depth in which a flexible riser is installed is about 6,234 ft. (1,900 m) with a flexible pipe internal diameter (ID) of about 7.5 in. as shown below. Although flexible risers with an internal diameter of more than 16 in. have been installed offshore, these are in water depths that do not exceed 1,312 ft. (400 m).

Figure 1.5 Water depth vs. internal diameter for unbonded flexible pipes [7].

Figure 1.6 plots the design pressure vs. internal diameter of unbonded flexible pipes in operation. The data was gathered from industry applications worldwide by the SureFlex Joint Industry Project (JIP) deliverables completed at the end of 2010. The database shows that 76% of all flexible pipes have a design pressure below 345 bar (5000 psi), 90% below 10 in. and 70% designed for temperatures less than 80 °C. The pressure by internal diameter (P*ID) is an important characteristic for unbonded flexible pipes and four constant P*ID are plotted based on the data collected from the database. The line with the largest P*ID value in operation is at 80,000 psi-inch and is for a 12-inch flexible pipe. The majority of unbonded flexible pipes in use have a P*ID value below 50,000 psi-inch. Figures 1.5 and 1.6 provide a good indicator of the current capacities of unbonded flexible pipes in operation.

Figure 1.6 Overview of unbonded flexible pipes in use – pressure/diameter [17, 18].

1.2.2 Composite-Based Flexible Pipes

Composite-based flexible pipes are also called Flexible Composite Pipes (FCP) and are a continuous spoolable line pipe product composed of a reinforced thermoplastic liner covered by a protective layer. Flexible composite pipes are used for water as well as hydrocarbon transportation. FCP is a proven technology in onshore applications while qualification efforts have recently made the pipe suitable for offshore applications in shallow water. A range of FCP products based on different fabrication methods and materials resulting in different performances have been used in the oil and gas industry. The reinforcement material (such as aramid, steel cords/stripes, fiberglass, etc.) varies significantly, resulting in great difference in performance. The reinforcement can be unbonded in strings or tapes, secured with adhesives or fully bonded into an integrated matrix, resulting in different applications and prices. Table 1.1 presents an overview of the application range for different types of FCP. A technical assessment qualification program for unbonded FCP is required for shallow water and offshore dynamic condition applications.

Table 1.1 Overview of flexible composite pipe applications.

FCP is a flexible reinforced thermoplastic pipe and includes three main characteristics:

Flexible: FCP allows large deflections without a significant increase in bending stresses and may be manufactured in continuous lengths and stored or transported on reels.

Thermoplastic: the inner liner of the FCP containing the bore fluids is made of thermoplastic. A high density polyethylene (HDPE) is typically used while other plastics such as PA and PVDF are increasingly applied.

Reinforced: the reinforcement layer is composed of thermoplastic composites, thermoset composites, fiberglass, steel wires, steel cords or steel strips and is often a combination of multiple materials.

Figure 1.7 shows a typical bonded FCP (TCP or RTP) construction including a polymeric liner or barrier, a structural layer and an outer polymeric cover. The function of the inner liner is to offer leak-proof capacity, corrosion resistance and contain the transported fluid. The function of the structural layer is to provide the mechanical strength to withstand the loads applied during service and installation. The structural layer typically consists of an even number of balanced helical windings of continuous aramid or other types of fiber reinforcement. The reinforcement is helically wound yarns or fiber reinforced preformed tapes and is encapsulated by a thermoplastic resin. The outer cover is added on the top of the structural layer to protect the structure during installation and operation. This kind of bonded flexible composite pipes is detailed in Chapters 24 to 35.

Figure 1.7 Typical construction of bonded FCP (courtesy of OPR Inc.).

Figure 1.8 shows a typical construction of a four-layer unbonded steel reinforced flexible pipe and includes (1) an innermost layer that is an extruded thermoplastic tube (PE) that seals the conveyed fluid; (2) two (or four) contra-wound layers of carbon steel strip reinforcements spirally wrapped on top of the liner roughly at a 55° lay angle. The steel strips provide strength that resists the internal pressure and tensile loads on the pipe and are plain carbon steel strips that combine economy with high strength and good toughness. The tensile layers are not exposed to the bore fluid and exist in a considerably milder environment in the annulus between the inner and outer extruded layers; (3) an outermost PE layer that is a thick extruded external shield protects the underlying layers of the pipe from the external environment. This kind unbonded steel reinforced FCP is detailed in Chapters 14 to 22.

Figure 1.8 Typical construction of unbonded FCP (courtesy of OPR Inc.).

When comparing unbonded flexible pipes (shown in Figure 1.4) with unbonded steel reinforced FCP (shown in Figure 1.8), the unbonded flexible pipe normally has two more layers than the unbonded steel reinforced FCP: the interlocked metal carcass layer and the metal armor layer. The interlocked stainless steel carcass layer prevents the collapse of the pressure sheath and provides erosion protection from the conveyed medium and is an optional component. The metal armor layer used in the unbonded flexible pipe provides strength to withstand the hoop stresses due to internal and external pressure loads. These two layers are replaced by the typical 55 degree reinforcement layer in unbonded FCP that resists the pressure and axial tensile loads and is made of thermoplastic composites, thermoset composites, fiberglass, steel wires, steel cords or steel strips.

Figure 1.9 illustrates a simplified overview of the application range of flexible pipes. (Please note that the application range is for illustrative purposes only.) Unbonded flexible pipes can be used in a wider temperature and pressure range with larger diameters. However, they are much more expensive than FCP.

Figure 1.9 Application envelope of flexible pipes.

1.2.3 Design Codes and Specifications

Several recognized industry norms for flexible pipes have been developed by the American Petroleum Institute (API). All aspects of flexible pipe design and technology, from functional definitions to installation, are addressed in the following standards and recommended practices of the API.

API 17B, Recommended practice for flexible pipes, equivalent to ISO 13628-11 [8, 9];

API 17J, for non-bonded flexible pipes, equivalent to ISO 13628-2 [10];

API 17K, for bonded flexible pipes, equivalent to ISO 13628-10 [11];

API 17L1 and L2 for ancillary components of flexible pipe systems [12, 13];

API Technical Report 17TR1, Evaluation standard for internal pressure sheath polymers for high temperature flexible pipes was developed to identify key parameters for high temperature liner materials such as PVDF in order to avoid future field failures due to insufficient understanding of the material characteristics [14];

API Technical Report 17TR2, The aging of PA11 in flexible pipes was developed to provide an understanding of hydrolysis aging of the most frequently used flexible pipe liner material. It also proposed an acceptance standard for aged material [15];

API RP 15S, Qualification of spoolable reinforced plastic line pipes [5].

In the late 1990 s the API documents (API 17J, 17K and 17B) were reformatted and issued as ISO documents (ISO 13628-2, -10 and -11) to widen the availability of international standards for flexible pipes. However, the ISO documents are not supported by either standing committees or quality programs.

API RP 15S covers: (1) product construction; (2) raw material selection; (3) qualification requirements, and (4) quality control requirements and prescribes an extensive program of product testing including regression testing, cyclic testing, joint testing, gas testing, bent testing, and axial load testing for onshore FCPs.

Supplementary standards for the definition of hydrodynamic loads, sour service qualification, integrity management, etc., are required when designing a flexible pipe system, typically from sources such as:

DNV

NORSOK

ASME

NACE

None of the traditional design standards for pipeline systems such as API RP1111, DNV-OS-F101, ISO 13623 and BSI PD8010-1/2 have specific guidance for flexible composite pipe systems. Regulatory codes such as ASME B31-4/8 specifically exclude the requirements for reinforced thermoplastic pipe systems and references API RP 15S, API Spec 17J (ISO 13628-2) or API Spec 17K (ISO 13628-10). ASME B31-8 specifies the pressure limitation for reinforced thermosetting pipelines but acknowledges in the interpretation document that this does not apply to reinforced thermoplastic pipes.

For unbonded flexible composite pipe systems, API Spec 17J (or alternatively DNV-RP-F202 [16]) is often applied in the industry. However, API Spec 17J excludes the use of nonmetallic reinforcements. Sometimes, the newer API RP 17B (ISO 13628-11) in addition to API Spec 17J and 17K are referenced, but they are only applicable to subsea and marine applications.

API RP 15S is widely applied for onshore FCP and includes guidelines for determining materials properties, pressure ratings, safety factors, service factors, and minimum performance requirements. It also includes guidelines for manufacturing, quality control tests, and typical installation methods. API RP 15S uses proven ASTM testing methods to establish performance (e.g., ASTM D1598 and D2992). API RP 15S only applies for onshore applications.

Some manufacturers recognize the limitations of currents standards for composite pipe systems and take the approach described by DNV-RP-A203 in combination with DNV-OS-C501 [17]. However, loads and/or specific tests still may be taken from various applicable ISO or API standards in the qualification procedure. DNV GL organized a Joined Industry Project to develop a new recommended practice for the offshore use of bonded FCP, named DNV GL RP-F119 Thermoplastic Composite Pipes [4].

1.3 Comparison between Flexible Pipes and Rigid Pipes

The use of flexible pipelines in offshore oil and gas applications has increased in recent years. They are considered to be an efficient solution in terms of technical as well as economic performance due to their easy and fast laying procedure, durability and recoverability. Many risers for floating systems are flexible risers and major portions of flexible risers in operation in the Norwegian offshore sector are unbonded flexible pipes.

1.3.1 Unbonded Flexible Riser vs. Rigid Steel Riser

Flexible risers are used for a range of functions: production risers for gas and oil, water injection, gas lift, gas injection, oil or gas export, test productions, etc. Flexible risers are also used for drilling and well maintenance. The flexible riser’s advantages include easier, cheaper and quicker installation due to their inherent flexibility and ability to be simply laid on the seafloor. The detailed advantages of unbonded flexible risers over rigid steel risers are summarized as follows:

Fast installation

Fatigue resistance

Thermal resistance

Flexibility

Corrosion resistance

Life cycle cost

New materials/structures

Hybrid solutions

Involvement in FEEDs and planning.

Disadvantages of unbonded flexible pipes include

High capital investment

High product cost

Submerged weight

Collapse resistance in deep water

Deep water thermal insulation

A comparison of four different types of risers (unbonded flexible riser, hybrid riser, rigid steel riser (SCR), and rigid titanium riser) is summarized in Table 1.2.

Table 1.2 Comparisons of four different risers.

1.3.2 Flexible Jumper vs. Rigid Steel Jumper

Flexible jumpers have been widely installed in subsea oil/gas fields because they are a robust solution for better accommodating design and changing field layouts. In addition, they have the following advantages over the rigid steel jumpers.

Flexibility in offshore operations;

No extra deck space required and can be stored in reels or carrousel;

Relatively tolerant positioning accuracy;

Easy connection to gooseneck;

Possible to reuse;

No metrology required during offshore operations;

No need for PLET and PLEM.

In addition to the above advantages, flexible jumpers may be cost competitive compared to rigid jumpers. A cost comparison between a 6-inch flexible jumper and a 6-inch rigid jumper is given in Table 1.3. The flexible jumper includes end fittings and goose necks while the rigid jumper includes clad pipe segments, 6 bends and end connectors.

Table 1.3 Cost comparison between flexible and rigid jumpers.

1.3.3 Flexible Composite Pipe vs. Rigid Pipe

Flexible composite pipes have been widely used in onshore oil/gas transportation applications due to their inherent corrosion resistance, quick installation, spoolability and other advantages compared to traditional rigid carbon steel pipes.

Table 1.4 lists the general differences between flexible composite pipes and rigid steel pipes in the design, construction, and operational phases. Flexible composite pipes are produced, transported and installed in reels with relatively light installation equipment, resulting in significant project acceleration and allowing the oil to generate cash flow early on. In addition, they have the potential to reduce the total cost of ownership due to lower CAPEX and OPEX.

Table 1.4 Differences between flexible composite pipe and rigid carbon steel pipe.

Cost is often a strong argument for project engineers when selecting either FCP or rigid steel pipes. The material, installation and operational costs for FCP are discussed below in a cost overview.

1.3.3.1 Material Costs

Material costs for FCP have a large spread and depend heavily on the application details. Sizing and pressure are the biggest cost drivers. FCP material costs vary between 75–225 USD/m for typical onshore applications and between 500–1500 USD/m for offshore applications.

1.3.3.2 Installation Costs

The onshore construction cost (including site preparation and construction costs) is around 600 USD/m for large diameter rigid steel pipes and around 360 USD for small diameter (<10 inch) rigid steel pipes. As a rule of thumb, the installation cost for FCP is estimated to be only one-third of the cost of rigid steel pipe installation, meaning it is about 120 USD/m. FCP requires a smaller construction crew, smaller right of way preparation and can be installed much faster. For offshore applications, the installation cost needs to be evaluated on a case-by-case basis. In shallow waters, it is cost effective to use standard subsea construction vessels temporarily equipped with FCP installation equipment.

1.3.3.3 Operational Costs

The largest drivers in the operational costs are the corrosion control methods (for example, using intelligent pigging and inhibitors). The amount of corrosion control needed is very much dependent on the service fluid (water cut, H2S, CO2, bacteria) and it is inherently difficult to estimate the associated costs. FCP is inherently corrosion resistant and although careful material selection for the potential wetted surface areas of the coupling is required, their operational cost is much lower compared to traditional rigid steel pipes. It is conservative to estimate half of the rigid steel pipe’s associated operational costs for FCP.

1.3.3.4 Comparison Example

A cost comparison between flexible composite pipes and rigid pipes for a 10 km long, 6-inch onshore production flowline with sour service is carried out in four aspects: material cost, coating cost, installation cost and operation cost. Table 1.5 lists the detailed costs for both rigid pipes and composite flexible pipes.

Table 1.5 Cost comparison between rigid pipe and composite flexible pipe.

The cost comparison between rigid pipes and FCP shows that although the material costs for FCP may be expensive, taking into account the reductions in the total installation costs of -46.5% and total cost of -49.0%, the cost of FCP is much lower. Even with a more expensive material cost, FCP is still cost competitive when compared to rigid steel pipes.

1.4 Failure Mode and Design Criteria

1.4.1 Unbonded Flexible Pipe

1.4.1.1 Failure Modes

The detailed knowledge of the potential degradation and failure modes for the intended application of unbonded flexible pipes is important so that it can be well addressed in the design. The failure of the flexible pipe due to pipe layer separation, leakage or pipe collapse will lead to the loss of pipe functionality and accidents involving transported fluid loss or blockage. The loss of pipe functionality is often the result of a sequence of damage events and/or time-driven degradation mechanisms and layer/component layer failures. The SureFlex- JIP reports by MCS Kenny provides representative historical data of the failure and damage of unbonded flexible pipes. The incidents involving flexible riser failure and damage are sorted by failure and are presented graphically in Figure 1.10. The data collected worldwide in 2010 are also shown in the figure for comparison. The most frequent failures are external sheath damage (35%), vent system anomalies (12%), ancillary device failure (8%) and carcass failure (7%). The elements included in ancillary devices include buoyancy elements, bend stiffeners, bend restrictors, interface to turret or deck, riser base, tether base, mid-water arch and its riser interfaces.

Figure 1.10 Failure and damage mechanism for flexible pipe [18].

Failure modes for the primary structural design of unbonded flexible pipes include:

Collapse of

Carcass and/or pressure armor due to excessive tension => Solution: increase thickness of the carcass strip, pressure armor, or internal pressure sheath (smooth bore collapse).

Carcass and/or pressure armor due to excessive external pressure => Solution: modify the configuration or installation design to reduce loads.

Carcass and/or pressure armor due to installation loads or ovalization due to installation loads => Solution: add an intermediate anti-collapse sheath (smooth bore pipes).

Internal pressure sheath in smooth bore pipes => Solution: increase the area moment of inertia of the carcass or pressure armor.

Carcass due to pressure buildup in multilayer pressure sheaths followed by rapid decompression. => Solution: Prevent pressure buildup and rapid decompression through operational procedures.

Pipe due to carcass pull out from end fitting resulting from lack of pressure sheath support => Solution: Design end fittings to assure support of the pressure sheath and carcass.

Carcass due to fatigue => Solution: Assure that the carcass is manufactured correctly to avoid fatigue loading.

Burst

Rupture of the pressure armor due to excessive internal pressure => Solution: Modify the design (e.g., change lay angle, wire shape, etc.).

Rupture of tensile armor due to excessive internal pressure => Increase wire thickness or select higher strength material if possible

Failure modes also include tensile failure, compressive failure, overbending, torsional failure, fatigue failure, erosion and corrosion. Detailed potential failure mechanisms and design solutions to the failure modes are shown in Table 1.4 of API RP 17B [8].

1.4.1.2 Design Criteria

The design criterion for unbonded flexible pipes is assigned to each relevant failure mode and its corresponding mechanism. For each different layer of unbonded flexible pipes a different design criterion is met. For the carcass layer, buckling loads due to external pressure, for the armor layer, stress utilization and for the polymer layers, strain distribution is the critical parameter in the design criteria.

Table 1.6 lists the failure modes and the allowable design criteria defined in Table 8 of API 17J [6] for all service conditions:

Table 1.6 Failure modes and design criteria for unbonded flexible pipes [8].

Normal & extreme operation

Abnormal operation

Installation

Factory acceptance testing (FAT)

The FAT requirements are specified in the FAT section of API 17 J and includes

Gauge test

Hydro test

Electrical continuity/resistance

Gas venting test

The purpose of the hydrostatic pressure test is to demonstrate if the pipe can hold a pressure above a desired level or to identify the latent defects in the pipe. The minimum hydrostatic test pressure for flexible flowlines and subsea jumpers is 1.3 times that of the design pressure. For all other applications, including flexible risers and topside jumpers, the minimum hydrostatic test pressure is 1.5 times that of the design pressure. After the 24-hour period, if the pressure has not dropped more than 4% the pipe is considered to have passed the hydrotest.

1.4.2 Flexible Composite Pipe

1.4.2.1 Failure Modes

The failure data for flexible composite pipe systems reveals the following causes of failure:

Damage resulting from installation;

Corrosion of associated steel fittings;

Pipe failure;

Damage by others (third-party damage);

Mechanical failure of the valves or fittings.

The most common failure modes and associated failure mechanisms for the FCP itself are listed in Table 1.7 [13].

Table 1.7 Failure modes and failure mechanisms for FCP.

Kinking can occur during installation of FCPs and requires careful unreeling during construction. When FCPs are installed as a liner through an existing steel carrier pipe, supporting the spoolable pipe where it enters and exits the steel pipe is of primary importance. The steel carrier pipe behaves similarly to a solid and settled area of ground. However, the area where the composite pipe exits is subjected to a new and varying soil settling that could lead to failure at the entry/exit areas of the carrier pipe.

1.4.2.2 Design Criteria

The design criteria for flexible composite pipe systems is defined in API 15S for all service conditions to withstand the most severe anticipated conditions during the installation and service life of the system. Typical local loads include pressure, axial loads, bending loads, torsion and vacuum (for unbonded FCPs). The local loads are obtained from global system analyses due to the following global loads:

Pressure loads

Functional loads

Environmental loads

Accidental loads

Pressure Rating

For various flexible composite pipe products, different pressure ratings are available that depend on the product type and pipe diameter involved. In some cases, pipe manufacturers may be capable of producing special-sized or pressure-rated pipe that may not be listed in their standard product literature.

The structural layer including any bonding agents should sustain its integrity throughout the lifetime of the pipe under the given service conditions. The manufacturer should provide test data that demonstrates the short-term and long-term load-bearing capabilities of the layer and the temperature capabilities, required fluid compatibility and aging characteristics of all employed materials. The pressure rating of the pipe body should be verified using long-term rupture regression tests under constant pressure at a qualification test temperature.

Due to the burst failure mode of FCP, designs should be based on short-term burst strength (STBP). The manufacturer should determine the Nominal Pressure Rating (NPR) through short-term hydraulic burst strength tests with a safety factor. For metal reinforced thermoplastic pipe designed according to API 17J, a minimum safety factor of 2.5 may be used. The burst pressure should be greater than or equal to the MPR (maximum pressure rating) divided by the design factors.

Maximum Allowable Operating Pressure

The maximum allowable operating pressure (MOP) is determined by the following equation

(1.1)

Graphic

where MPR is the maximum pressure rating determined at the maximum design temperature and minimum 20 years design life of the pipeline. Ffluid is the service fluid factor that accounts for the effects of the transported fluid on the pipe material to ensure the fluid does not reduce the pipe’s MOP maintaining capability over the design life. The guidelines of this service factor for fluids are as follows,

use a Ffluid of 0.67 or less for all gas services;

use a Ffluid of 0.80 or less for all hydrocarbon liquid and multiphase services;

use a Ffluid of 1.0 or less for all water services.

Fcyclic is the cyclic pressure service factor and addresses service conditions that can be considered to be neither static nor fully cyclic. All pressure applications have some expected level of cyclic pressure fluctuations. This can be in the form of on/off pump cycles, up/down strokes from a pump jack, high frequency pressure pulsations of positive displacement pumps or other services with known fluctuations in pressure. If the number of cycles is less than 7000 or ΔP/NPR is less than 6%, the service is considered static.

The procedure that establishes the MOP is illustrated in Figure 1.11, where data from multiple long-term pressure tests are plotted on a log-log graph. Fsn is the factor for service conditions and includes the service fluid factor and cyclic pressure factor. The service conditions are, but not limited to, cyclic, chemical aging, installation factors, degree of reliability selected, etc.

Figure 1.11 Procedure to determine the LCL, MPR and MOP [13].

The MPR is derived using statistical calculations for nonmetallic reinforced pipes and includes a design factor (Fd) and Fsn to determine the MPR and the MOP. The design factor is a service factor that accounts for small variations in the material, installation, or operating parameters, and ensures these variations do not cause the pipe’s actual capabilities to be exceeded. The design factor Fd for nonmetallic reinforced pipes is applied to the lower confidence limit (LCL) at the design life to determine the MPR at the qualification temperature. The MPR for nonmetallic reinforced pipes is calculated as follows,

(1.2)

Graphic

where Fd = 0.67 is recommended by API 15S based upon the following conditions:

maximum design temperature is equal to or less than the qualification temperature

fluid service is water

loading is static

installed in full compliance with the manufacturer’s recommendations and requirements

The LCL curve is constructed so that there is a 97.5% probability that the pipe’s actual mean pressure versus time-to-failure regression line falls above the curve. The LPL curve is constructed so there is a 97.5% probability that a single test specimen’s pressure will fall above this curve. The LCL of the set of burst specimens is calculated using Student’s t-distribution.

For a steel reinforced pipe, the MPR is calculated as follows,

(1.3)

Graphic

where Pburst,min is the calculated minimum burst pressure based on the minimum cross-sectional area and minimum mechanical properties of the reinforcement. A maximum design factor Fd = 0.5 is employed for steel reinforced pipes based on the following conditions [5]:

the fluid service is water;

the loading is static; and

installed in full compliance with the manufacturer’s recommendations and requirements.

Long-term loads

Long-term loads including axial loads are defined over an observation period and correspond to the entire design life or to a part of the design life. Long-term cyclic loads are specified as load sequences in terms of mean loads and amplitude. All load effect fluctuations, e.g., stress or strain fluctuations imposed during the entire design life should be taken into account when determining the long-term distribution of stress or strain ranges for obtaining the fatigue load effects.

1.5 Limit State Design

Rigid pipeline designs have been successfully carried out using limit state design (LSD) with reliability-based techniques that provide a consistent treatment of uncertainties. The load resistance factored design (LRFD) method was introduced in DNV-OS-F101 as a design basis for the given structural limitations. This method incorporates uncertainties in the design into an approach that involves partial factors of safety. These uncertainties are grouped together as either partial loads or material factors. The partial safety factors are associated with characteristic loads and resistance effects. The basic approach of the limit state design method consists of recognizing the different failure modes related to each functional requirement and associating each mode of failure with a specific limit state beyond which the pipeline no longer satisfies the functional requirements. Each limit state is related to the failure mode and its anticipated consequences. The use of limit state designs for flexible pipes is more complicated than for rigid pipes due to the larger number of potential failure modes. The appropriate limit states should be selected.

1.5.1 Limit States

The limit state design of pipelines is a rational design method based on the concept of a limit state, a condition that limits the continued safe operation of a pipeline. The limit state is exceeded when the response of the pipeline to loading is unacceptable. Each limit state divides the pipeline into two states, a safe state and the failed state. The following four limit states are checked in the pipeline’s design [2]:

Ultimate limit state (ULS): Total collapse or failure of the structure or component that threatens human life or the environment is categorized as the ultimate limit state. The flexible pipe may experience loss of structural integrity if the limit state is exceeded.

Serviceability limit state (SLS): The state is related to failure modes where human risks or environmental risks are not an issue. This state is not associated with catastrophic failure but reduces the operational capability or utility of the pipeline. If the limit state is not satisfied, the pipeline will not meet its functional requirements such as partially blocking the flow or preventing pigs from traveling along the pipeline due to a change in the local ovalization.

Fatigue limit state (FLS): This is a ULS condition accounting for accumulated cyclic load effects.

Accidental limit state (ALS): This is a condition that, if exceeded, implies loss of structural integrity caused by an accidental load.

1.5.2 Reliability-Based Methods

The basic approach of the limit state design method consists of recognizing the different failure modes related to each functional requirement and associating each mode of failure to a specific limit state beyond which the structure no longer satisfies the functional requirement.

The design analysis consists of associating each failure mode with all the possible failure mechanisms. The fundamental principle of the LRFD is to verify that the characteristic factored design loads (Ld) do not exceed the factored design resistance effects (Rd) for any of the considered failure modes:

(1.4)

Graphic

where the factored design load Ld and the factored design resistance Rd are expressed as follows:

(1.5)

Graphic

(1.6)

Graphic

(1.7)

Graphic

Where gL is the load effect factor and gr is the safety class resistance factor and are determined using the risk and reliability methods to provide a target reliability level.

The design equations are formulated in the partial safety factor method format where partial safety factors (load factors and resistance factors) are applied to the load effects (characteristic load values) and to the resistance variables (characteristic resistance values) in the design equations, thereby accounting for possible unfavorable deviations of the basic variables from their characteristic values. Figure 1.12 illustrates the relationship of the partial factors of the characteristic loads and resistances in the limit state design. For example, the characteristic load and resistance may be stresses caused by an applied hoop stress, sh and the pipeline yield strength sy represented by statistical distribution. The probability density function for the yield strength is found by a statistical analysis of measured test values in the pipe mill certification records. The mean value and variability of the wall thickness and diameter are found in the pipe delivery records. If the load is larger than the resistance, the system fails.

Figure 1.12 Partial factors in limit state design.

The safety classes are based on the consequences of failure when the mode of failure is related to the ultimate limit state. Different safety classes (low, medium and high) may be defined for different parts of the FCP system. The service classes are based on the frequency of service interruptions or restrictions caused by the modes of failure related to the serviceability limit state. The operator should specify the service class to which the structure is designed to.

A structural reliability analysis (SRA) may be used to determine the partial safety factors of the FCP system. The procedure and the calibrated method are detailed in Chapter 4 of this book.

References

1. S. Berge and A. Olufsen, Handbook on Design and Operation of Flexible Pipes, SINTEF Report STF70, A92006, 1992.

2. Q. Bai, and Y. Bai, Subsea Pipeline Design, Analysis and Installation, Elsevier Science Ltd., 2014.

3. S. Antal, T.Nagy, A. Boros, Improvement of bonded flexible pipe according to new API standard 17K, OTC 15167, 2003.

4. DNV, Thermoplastic Composite Pipes, DNVGLRP-F119, Det Norske Veritas, 2015.

5. API, Qualification of Spoolable Reinforced Plastic Line Pipe, API RP 15S, Second Edition, American Petroleum Institute, 2016.

6. Y. Zhang, B. Chen, L. Qiu, T. Hill, and M. Case, State of the art analytical tools improve optimization of unbonded flexible pipes for deepwater environments, OTC 15169.

7. P. Boschee, Best practices for flexible pipe integrity evolve, Oil & Gas Facilities, 2012.

8. API, Recommended Practice for Flexible Pipe, API RP 17B, Fifth Edition, American Petroleum Institute, 2014.

9. ISO, Petroleum and natural gas industries — Design and operation of subsea production systems —Part 11: Flexible pipe systems for subsea and marine applications, ISO 13628-11, 2007.

10. API, Specification for Unbonded Flexible Pipe, API Specification 17J, Furth Edition, American Petroleum Institute, 2014.

11. API, Specification for Bonded Flexible Pipe, NSI/API Specification 17K, Second Edition, American Petroleum Institute, 2005.

12. API, Specification for Flexible Pipe Ancillary Equipment, API Specification 17L1, First Edition, American Petroleum Institute, 2013.

13. API, Recommended Practice for Flexible Pipe Ancillary Equipment, API RP 17L2, First Edition, American Petroleum Institute, 2013.

14. API, Evaluation Standard for Internal Pressure Sheath Polymers for High Temperature Flexible Pipes, API Technical Report 17TR1, First Edition, American Petroleum Institute, 2003.

15. API, The Ageing of PA-11 in Flexible Pipes, API Technical Report 17TR2, First Edition, American Petroleum Institute, 2003.

16. DNV, Composite Risers, DNV-RP-F202, Det Norske Veritas, 2010.

17. DNV, Composite Components, DNV-OS-501, Det Norske Veritas, 2009.

18. MCS Kenny and WGIM, State of the art report on flexible pipe integrity, Doc.ref.no. 2-4-5-013/SR01, Rev.02, Published by Oil and Gas UK under code OP010, August 2010.

Chapter 2

Materials and Aging

2.1 Introduction

Flexible pipes allow large deflections without a significant increase in bending stresses because they are a composite of layered materials. Due to the rather complicated structure configuration of flexible pipes where materials with different properties interact, a large number of failure modes are possible during installation and operation. Many of these failure modes are related to the material properties and are the key parameters when performing design analysis and verifying whether the design criteria are fulfilled [1].

For the integrity and fitness for purpose of flexible pipes at the design conditions, the chemical components’ effects on the liner materials (polymer) in the service environment at the design temperature should be assessed based on testing and experience to predict the aging or deterioration of the polymer under the influence of environment. As a minimum, polymer aging estimates should consider temperature, water cut and pH of the water. Special attention should be given to deplasticization, loss and/or degradation of additive formulation components, fluid absorption and changes in the dimensions of the pipe.

In this chapter, the materials of different layers of flexible pipes (including unbonded flexible pipes and flexible composite pipes) are described including function and structure. The material’s aging problems are also discussed.

2.1.1 Unbonded Flexible Pipes

Unbonded flexible pipes are a key part of the subsea oil and gas production system and have been used in many applications with different temperature and pressure ranges such as production risers, water or gas injection, drilling, in-field flowlines and jumpers. Depending on the specific application, the demands including dimensions, strength, flexibility, chemical compatibility, temperature tolerance and other properties may vary within wide margins. The configuration of the pipe cross section is designed to satisfy the load requirements using different material layers. Figure 2.1 shows a typical cross section of an unbonded flexible pipe that includes six material layers of steel and polymer [2]. The number and construction of the layers depend largely on the diameter and pressure rating of the pipe; unbonded flexible pipes with a large diameter for high pressure applications may have up to 20 layers in the cross section. Each layer has a specific function and interacts with the other layers [3]. The combination of materials used for a given flexible pipe design depends on the specific application. The materials and functions of the different layers of a typical unbonded flexible pipe are described below:

Carcass

The carcass is the innermost layer of a pipe and the only metallic component that is in direct contact with the transported bore fluid. The carcass is made from stainless steel strips in a continuous process onto a mandrel. Typically, AISI 304, 316, or A316L stainless steel are used when they are compatible with the chemical constituents of the transported fluids, depending on the corrosion resistance and weldability requirements. Sometimes, duplex steel (UNS 31803) is used when high corrosion resistance is required.

Liner

The liner is the sealing layer exposed to the bore fluid and is an extruded thermoplastic layer over the carcass. The liner limits the upper service temperature of the flexible pipe. The material of the liner depends on the design conditions; the following three generic classes of materials used are:

High density polyethylene (HDPE) and cross-linked polyethylene (PEX)

Polyamide (nylon) (Rilsan PA11 or PA12)

Poly vinylidene fluoride (PVDF)

Figure 2.1 Typical cross section of unbonded flexible pipes.

A major criterion for the selection of the liner material is the design temperature.