Flexible Risers

Flexible Risers

Flexible risers trace their origins to pioneering work carried out in the late 1970s. Initially flexible pipes were used in relatively benign weather environments such as offshore Brazil, the Far East, and the Mediterranean. Since then, however, flexible pipe technology has advanced rapidly and today

File:Typical Buoyancy Modules.png

Typical Buoyancy Modules

flexible pipes are used in various fields in the North Sea and are also gaining popularity in the Gulf of Mexico. Flexible pipe applications include water depths down to 8000 ft, high pressures up to 10,000 psi, and high temperatures above 15 F, as well as the ability to withstand large vessel motions in adverse weather conditions.

The main characteristic of a flexible pipe is its low relative bending to axial stiffness. This characteristic is achieved through the use of a number of layers of different materials in the pipe wall fabrication. These layers are able to slip past each other when under the influence of external and internal loads; hence, this characteristic gives a flexible pipe its property of low bending stiffness. The flexible pipe composite structure combines steel armor layers with high stiffness to provide strength and polymer sealing layers with low stiffness to provide fluid integrity. This construction gives flexible pipes a number of advantages over other types of pipelines and risers such as steel catenary risers. These advantages include prefabrication, storage in long lengths on reels, reduced transport and installation costs, and suitability for use with compliant structures.

Flexible Pipe Cross Section

Two types of flexible pipes are in use: bonded and unbonded

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Typical Cross Section of an Unbonded Flexible Pipe

flexible pipes. In bonded pipes, different layers of fabric, elastomer, and steel are bonded together through a vulcanization process. Bonded pipes are only used in short sections such as jumpers. However, unbonded flexible pipes can be manufactured for dynamic applications in lengths of several hundred meters.Unless otherwise stated, the rest of this chapter deals with unbonded flexible pipes.

Carcass

The carcass forms the innermost layer of the flexible pipe cross section. It is commonly made of a stainless steel flat strip that is formed into an interlocking profile. The main function of the carcass is to prevent pipe collapse due to hydrostatic pressure or buildup of gases in the annulus.

Internal Polymer Sheath

The internal polymer sheath provides a barrier to maintain the bore fluid integrity. Exposure concentrations and fluid temperature are key design drivers for the internal sheath. Common materials used for the internal sheath include Polyamide-11 (commercially known asRilsan), high-density polyethylene (HDPE), cross-linked polyethylene (XLPE), and PVDF.

Pressure Armor

The role of the pressure armor is to withstand the hoop stress in the pipe wall that is caused by the inner bore fluid pressure. The pressure armor is wound around the internal polymer sheath and is made of interlocking wires.

Tensile Armor

The tensile armor layers are always cross-wound in pairs. As their name implies, these armor layers are used to resist the tensile load on the flexible pipe. The tensile armor layers are used to support the weight of all of the pipe layers and to transfer the load through the end fitting to the vessel structure. High tension in a deepwater riser may require the use of four tensile armor layers, rather than just two.

External Polymer Sheath

The external polymer sheath can be made of the same materials as the internal polymer sheath. The main function of the external sheath is as a barrier against seawater. It also provides a level of protection for the armor wires against clashing with other objects during installation.

Other Layers and Configurations

Besides the five main layers of a flexible pipe just discussed, there are other minor layers, which make up the pipe cross section. These layers include antifriction tapes wound around the armor layers. These tapes reduce friction and wear of the wire layers when they rub past each other as the pipe flexes due to external loads. Antiwear tapes can also be used to ensure that the armor layers maintain their wound shape. These tapes also prevent the wires from twisting out of their preset configuration, a phenomenon called birdcaging that is a result of hydrostatic pressure causing axial compression in the pipe. In some flexible pipe applications, the requirement for the use of hightensile wires for the tensile armor layers arises because of high-tensile loads, and yet the presence of a “sour” environment means that these wires would suffer an unacceptable rate of HIC/SSC. A solution to this situation is to fabricate a pipe cross section with two distinct annuli rather than one.

Flexible Riser Design Analysis

The design document should include the following, as minimum requirements:

• Host layout and subsea layout;
• Wind, wave, and current data, as well as any vessel motion that is applicable to riser analysis;
• Applicable design codes and company specifications;
• Applicable design criteria;
• Porch and I-tube design data;
• Load case matrices for static strength, fatigue, and interference analyses;
• Applicable analysis methodology.

Several types of analyses have to be carried out when conducting the design analysis for a flexible riser, including these:

• Finite element modeling and static analysis;
• Global dynamic analysis;
• Interference analysis;
• Cross-sectional model analysis;
• Extreme and fatigue analysis.

End Fitting and Annulus Venting Design

End Fitting Design and Top Stiffener (or Bellmouth)

The end fitting design is a critical component of the global flexible pipe design process. The main functions of the end fitting are to transfer the load sustained by the flexible pipe armor layers onto the vessel structure, and to complement the sealing of the polymer fluid barrier layers. The most severe location for fatigue damage in the risers is usually at the top hang-off region. The riser is protected from overbending in this area by either a bend stiffener or a bellmouth. Detailed local analyses for the curvature or bellmouth are carried out using a 2D finite element model.

Annulus Venting System

To prevent the buildup of gases in the annulus due to diffusion, a venting system is incorporated into the pipe structure to enable the annulus gases to be vented to the atmosphere. Three vent valves are incorporated into both end fitting arrangements of a pipe. The vent valves are directly connected to the annulus and are designed to operate at a preset pressure of about 30 to 45 psi.

File:Typical End Fitting System.png

Typical End Fitting System

The vent valves in the end fitting arrangement located subsea are sealed to prevent any ingress of seawater into the annulus.

Integrity Management

This section deals mainly with risk assessment and integrity management of flexible pipes . A recognized methodology for formulating an integrity management plan involves carrying out a risk assessment and determining the risks inherent to the use of flexible pipe. Once risks have been determined, specific integrity management measures can be identified to mitigate these risks.

Failure Statistics

It is important to determine actual failure mode statistics from operational statistics in order to overcome the failure or damage efficiently.

Risk Management Methodology

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System Failure Mechanisms

File:Comparison of Steel and Flexible Pipe Failure Statistics.png

Comparison of Steel and Flexible Pipe Failure Statistics

Risk is often quantified as an integer that is the product of two other values, which are known as a probability of failure and a consequence rating. A risk management should take all possible failure modes into consideration, through an analysis of failure drivers (such as temperature, pressure, product fluid composition, service loads, and pipe blockage or flow restriction) and general failure modes (such as fatigue, corrosion, erosion, accidental damage, and ancillary equipment).

Failure Drivers

Five failure drivers contribute to the failure of a flexible riser system. The identified five failure drivers are:

• Temperature;
• Pressure;
• Product fluid composition;
• Service loads;
• Ancillary components.

Failure Modes

Failure modes for flexible riser systems include the following:

• Fatigue;
• Corrosion;
• Erosion;
• Pipe blockage or flow restriction;
• Accidental damage.

Integrity Management Strategy

The integrity management system establishes and maintains a database for design data and field operation data, including:

• Design basis and main design findings;
• Manufacturing data relevant for reassessment of risers;
• Operating temperature and pressure with focus on temperature fluctuations;
• Fluid compositions, injected chemicals, and sand production;
• Riser annulus monitoring and polymer coupon test results;
• Sea-state conditions, vessel motions, and riser response.

Inspection Measures

Inspection measures include:

• General visual inspection (GVI)/close visual inspection (CVI);
• Cathodic protection survey.

Inspection and Monitoring Systems

A typical inspection and monitoring system includes:

• Polymer monitoring: online, offline, topside, and subsea;
• Annulus monitoring: vent gas rate, annulus integrity;
• Riser dynamics: tension, angle, and curvature;
• Steel armor: inspection method, magnetic or radiograph;
• Use of existing sensors and pressure and temperature sensors.

Testing and Analysis Measures

Testing and analysis measures include:

• Coupon sampling and analysis;
• Vacuum testing of riser annulus;
• Radiography.

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