DESIGN METHOD ADDRESSES SUBSEA PIPELINE THERMAL STRESSES

Written by : J. C. Suman, Sandor A. Karpathy

Managing thermal stresses in subsea pipelines carrying heated petroleum requires extensive thermal-stress analysis to predict trouble spots and to ensure a design flexible enough to anticipate stresses and expansions.

Explored here are various methods for resolving predicaments posed by thermal loads and resulting deformations by keeping the stresses and deformations in the pipeline system within allowable limits.

The problems posed by thermal stresses are not unique; the solutions proposed here are. These methods are based on recent work performed for a major Asian subsea pipeline project currently under construction.

MAINTAINING VISCOSITY

For crude oil to flow through any pipeline under design pressure, it must maintain an optimum viscosity that depends on temperature and decreases proportionally as the fluid loses heat.

Transporting crude oil of high viscosity through subsea pipelines can be particularly difficult. To ensure an efficient flow, pipelines are often insulated to reduce heat loss and to maintain optimum viscosity.

But insulated marine pipelines pose numerous problems.

• The insulation must be protected from the hostile marine environment during its entire operational life.
• Provisions to protect the insulation from damage during pipeline installation are especially important.
• Moreover, the line must be engineered for and employ certain mechanisms to handle thermal expansion of the line that results from the temperature differential between the ambient water temperature and the design temperature of the fluid in the pipeline.
• Fabrication and installation scenarios consider cost-effective installation of the pipeline from conventional marine lay vessels.

Depending upon the size, length, and layout of the pipeline and the terrain of the ocean floor, restricting the expansion of the pipeline can lead to excessive loads on the line.

These loads generally act on pipeline connections adjacent to platforms and pipeline end manifolds (PLEMs) or at subsea tie-ins. Design must consider both thermal growth and the resulting forces to ensure that the stresses in the pipeline components are within allowable limits.

Situations in which marine pipelines are exposed to cold ambient water temperatures, while the transported product requires heat to maintain normal flow, need insulation to control heat loss.

Although insulation keeps the heat loss to a minimum, thermal expansion of the pipeline and resulting thermal loads must be contended with.

Managing these expansions and stresses is a major problem associated with marine pipelines that carry heated products. This problem is discussed in two parts here.

The first part deals with the behavior of a finite pipe segment under thermal expansions. The second part deals with the management of the thermal expansions and resultant stresses.

Additionally, various techniques will be discussed for transferring stresses within the pipeline system to achieve a cost-effective pipeline design.

ASIAN CONTEXT

Several pipelines are currently proposed for the Pearl River basin off the coast of China. There, the crude oil is generally highly viscous (5-500 cSt, depending on temperature) and the ambient water temperature (45- 55 F.) is relatively cold.

As the oil leaves the platform, its temperature quickly drops in the colder water if the pipeline is not insulated. This drop in temperature decreases the viscosity of the crude which in turn increases the required pressure rating of the pumps.

Once the oil temperature drops below its gel point, the temperature at which the crude becomes too viscous to maintain a normal flow, operation of the pipeline becomes difficult. The insulation reduces heat loss and ensures that the product viscosity is maintained within operational limits.

During shut down, the product may remain inside the pipeline for some time and begin to gel as the temperature gradually declines. The thicker the crude becomes, the more pumping pressure will be required to start it moving again. This translates directly into operational costs.

Should the crude temperature drop below its gel point, the entire pipeline system may be plugged with a solid mass of crude and pumping re-start may become virtually impossible.

Higher design pressures would require the use of higher strength steels and possibly thicker wall for the pipe. These constraints define the overall economics of a pipeline system.

The temperature decay period required for the crude to cool down to its gel point is determined by the insulating capability of the pipeline. The better the insulation, the longer period required for the crude temperature to decline.

The longer decline period allows more time to re-start the product flow or clear the crude from the pipeline before it gels.

The important rate of heat loss depends solely on the insulating properties of the pipeline. Operations personnel require maximum time to repair equipment and to restart product flow should problems arise.

Heat tracing along the length of the pipeline can ensure that the product stays at the optimum flow viscosity. This tracing involves use of an additional pipeline, smaller than the carrier pipe, carrying hot water. It usually is attached to the carrier pipe. This hot water continually warms the product pipeline and ensures that a constant temperature is maintained.

Pipelines with heat tracing are generally difficult to construct and add to operating costs. Also, damage to the heat tracing line may require shutting down of the product pipeline and expensive repair costs.

Another method of controlling heat loss in the pipeline is to wrap the outside of the pipe with insulating material. This approach is acceptable only if the insulation remains impervious to the marine environment during the entire design life of the pipeline.

Open-celled foam materials provide good insulation properties, but open cell also permits water to infiltrate the insulation and thereby negate the insulating capability of the material.

Although closed-cell foam materials are also available, they are relatively expensive, and eventually even closed-cell foams will become saturated.

Insulating material on the outside of the pipe will lead to a larger effective pipe diameter.

An increase in pipe diameter leads to increased buoyant force and will affect the hydrodynamic stability of the pipeline system.

In addition, careful consideration must also be given to the construction methodology used to install the insulated pipeline to protect the insulation from high stresses imposed by the rollers of the lay-barge stinger.

EXPANSIONS AND LOADS

The temperature of product flowing through the pipeline is usually much higher than the original temperature of the pipe. As the pipe temperature increases, the pipeline starts expanding longitudinally.

The magnitude of this expansion depends on the temperature differential (AT) between the initial temperature of the pipe (before the heated product was introduced) and the maximum temperature it attains. The total expansion of the pipe can be calculated with Equation 1 (see accompanying equations box).

Should this thermal expansion be restrained, large forces will be generated within the piping components. These forces are a direct function of the cross sectional area "A" of the pipe and are related by the general formula shown in Equation 2.

Solving Equation 2 yields Equation 3.

Management of thermal forces and expansions in the pipeline design must be careful, It is the inherent property of metals to expand when subjected to heat.

The effect of differential expansion between components is an integral part of the pipeline system design. Despite all restraining efforts, the pipeline will expand when subject to heat.

PIPE-IN-PIPE

One alternative to insulating marine pipelines and dealing with the thermal expansion problem is to utilize a "pipe-in-pipe" design (Fig. 1).

In this system the inner pipe, known as the "carrier" pipe, carries the crude oil. The carrier pipe is encased in an outer pipe, known as the "jacket."

The annulus between the two pipes is filled with insulation, usually a polyurethane foam of relatively low density. The primary function of the jacket pipe is to provide protection for the insulation.

Marine pipeline systems are generally constructed of 40-ft segments of pipe, a constraint usually dictated by the capabilities of the lay vessel. A pipe-in-pipe pipeline consisting of doublepipe construction can be prefabricated, corrosion coated, and weight coated in the pipe yard similar to a standard single-pipe pipeline.

The individual segments can then be welded on the lay barge into a continuous pipeline during pipe lay operation.

(Editor's note: A configuration similar to this one has been developed for Total Oil Marine plc's Dunbar field in the U.K.'s North Sea sector,OGJ, May 17, p. 61.)

In the pipe-in-pipe design, the jacket pipe and the carrier pipe are connected to each other near the end of each pipe segment by a "donut plate." The jacket pipe and the carrier pipe must be connected to each other at the ends, for the sake of structural stability and water tightness to protect the insulation.

This donut-plate connection becomes the single most important element in the double-walled pipeline system design, when the effect of the thermal loads are considered. Forces generated by the thermal expansion of the carrier pipe are resisted by the donut plates at either end of the pipeline segment.

Management of thermal forces and expansions must be undertaken carefully. Because the two pipes are connected to each other and subjected to different temperatures, each will resist the expansion of the other.

The resulting differential expansions will generate large internal forces in each pipe. While the carrier pipe is subjected to thermal expansion as a result of the hot flowing product, the jacket pipe will not expand because it lies in ambient water temperature.

Stress analysis of the pipe segment must include the jacket pipe, the carrier pipe, and the donut plate as a unit, and is quite complex.

During its thermal expansion, the carrier pipe exerts forces on the donut plates. The donut plates in turn transmit the force to the jacket pipe (Fig. 2).

The nature of the force in the carrier pipe is compressive whereas it is tensile in the jacket pipe. The aspect ratio of the donut plates must be designed so that the force exerted by the carrier pipe is transferred directly to the jacket pipe in shear.

Bending moment is also generated at the inner and outer edges of the donut plates, at the jacket and carrier-pipe joints. The magnitude of this bending stress is controlled by varying the dimensions of the various elements at the joint.

Forces and moments in the donut plate can be calculated with equations derived from plate-bending theory. More sophisticated analysis can also be performed using the finite-element method.

Although the thermal forces can be viewed as a problem by the design engineer, several distinct benefits are derived from this thermally loaded structure.

The expansion of the carrier pipe is restrained at both ends by the donut plates. This restraint induces compressive forces in the carrier pipe and tension in the jacket pipe.

The tension in the jacket pipe leads to an increased allowable free span length of the pipeline and also reduces the overall expansion of the entire pipeline.

Additionally, the resulting tensile force in the jacket pipe adds to the buckling capacity of the jacket pipe. These forces reduce the required jacket pipe wall thickness, leading to substantial material cost savings.

A possible alternative to the pipe-in-pipe segment design is a prefabricated end joint manufactured by third-party suppliers (for example, Snamprogetti of Italy; Fig. 3).

This prefabricated unit is essentially similar to the joint with donut plates. But stress distribution in the prefabricated units is different, and the pipeline segment fabrication requires considerable control of manufacturing tolerances and welding procedures.

The design engineer must keep economics and constructability in perspective when choosing the right fabrication method.

EXPANSION MANAGEMENT

Thermal expansion in marine pipelines cannot be eliminated completely. The pipeline will expand, no matter how small the growth. Restrictions of the expansion of the pipe are confined by allowable stress limitation; of the various components of the pipeline system.

The donut plates described do not restrict the expansion of the carrier pipe entirely. Some expansion is anticipated because the compressive force in the carrier pipe must remain within the critical buckling load, and the combined stresses must also be kept below allowable values.

It is unnecessary to restrict the thermal expansion of the pipeline entirely. The magnitude of thermal expansion allowed in the pipeline system, however, must be managed in such a way that it is safe, economical, and allows uninterrupted operation of the pipeline system.

The following methods are suggested to manage the expansion and resulting stresses in a marine pipeline system.

DOGLEG CONFIGURATION

The "dogleg" configuration (Fig. 4) is useful for managing thermal expansion at the base of a riser, at a PLEM connection, or at any subsea tie-in location.

The total expansion of the pipeline end must equal the allowable horizontal deflection of the dogleg and the allowable torsional deflection of the riser.

Horizontal deflection of the dogleg is determined by application of principles of applied mechanics, as explained previously. The dogleg will deflect as a typical cantilever beam, fixed at the base of the riser.

Deflection of the cantilever is calculated as shown in Equation 4. The torsional deflection is determined by the actual twisting of the riser over a predetermined length. The relationship for the torsional deflection is expressed by Equation 5.

The stresses are optimized in the pipeline segment and the riser by controlling the stiffness of the respective members. The advantage of using this form of connection is that the actual pipe segment can be utilized to take up the thermal expansion. Depending on the required member lengths and allowable space at the base of the riser, this connection offers a simple, efficient method to control thermal expansion.

"U" LOOP

The "U" loop connection combines two "dogleg" configurations set back-to back (Fig. 5). The loop absorbs the thermal expansion of the pipeline by deflecting all three members that constitute the U.

Dimensions of the U-loop can be determined by analyzing it as a frame, subject to displacements at the ends. The magnitude of the displacement at one end is theoretically equal to half the allowed thermal expansion of the pipeline.

The advantage of a U loop over a dogleg design is that generally the loop allows the use of shorter dimensions and in some cases the constructability and installation of the unit are simplified.

In theory this configuration will function as designed so long as the resistance on all of the components is uniform. In a marine environment, where the consistence of the soils cannot be controlled, however, soil resistance on the various components will vary. This variance causes the components to be loaded inconsistently, resulting in unpredictable stress levels.

DYNAMIC CONNECTIONS

With the dogleg and U loop configurations, the thermal expansion of the pipeline is taken primarily as bending stress.

Dynamic balls or joints, such as those manufactured by Cooper Oil Tools, Houston, relieve the thermal expansion by controlled deformation of the joint (Fig. 6). The ball joint stay's dynamic (movable) during its entire design life and allows the pipeline thermal expansions to take place.

These joints are capable of an angular movement of as much as 10 from their centered position while allowing unobstructed flow of the product as well as normal pigging operations.

The most significant advantage of using these joints is the stress-free operation of the pipeline under thermal expansion. Virtually no loads are generated as a result of the thermal expansion of the pipeline when swivel joints are used.

A few of these dynamic joints have been used in marine environments for as long as 20 years and are known to have functioned relatively problem free. Many operators are concerned with leakage integrity and long-term reliability, however, and are not ready to use them extensively.

PRESTRESSED COMPONENTS

Sometimes reduction in thermal stresses and thermal expansions may be achieved by prestressing the pipeline during installation.

This is accomplished by predeflecting a U loop or dogleg configuration equal to the magnitude of the expected expansion, but in the opposite direction. During operation, when the pipeline is heated and expands, the predeflection is dissipated and the pipeline system comes to a virtual stress-free condition.

This alternative is generally feasible where pipeline components expand only in a single direction and seldom return to their original state.

If the prestressed components are allowed to sit too long without subjecting them to thermal loads, however, there will be a gradual redistribution of the preapplied loads, and the intended preload will be negated. This can easily happen should system start-up be delayed after installation.

Source : http://www.ogj.com/articles/print/volume-91/issue-35/in-this-issue/pipeline/design-method-addresses-subsea-pipeline-thermal-stresses.html