Wall Thickness Determination

pipeline must be strong enough to withstand internal pressure, external pressure, and installation stresses.

• Hydrodynamic Stability: The submerged weight of the pipeline must be sufficient to resist movement when subjected to wave and current loads. This evaluation is typically performed for both the installation environment (1-year return period environmental loads and an air-filled pipe) and

UNUttiVVA it H IN6ÎALLA HUN hhoctzdures the operating environment (20-year + return period environmental loads and a product-filled pipe).

• Route Selection: The pipeline route must be selected which will ensure the safety of the pipeline.

• Installation Procedure Selection: An installation procedure must be selected which is cost effective, simple, and reliable.

• Material Selection: The pipeline material must be selected which will ensure product quality, pipeline safety, simplicity of installation, and cost effectiveness.

• Pipe Joint Selection: A jointing procedure must be selected which is compatible with the pipeline product and the selected installation procedure.

• Internal and External Coatings: Internal and external coating systems must be selected which will protect the pipeline from corrosion and the product from contamination.

• Final Inspection, Hydrostatic Testing, and Startup: The pipeline should be visually inspected, cleaned of any internal debris, hydrostatically pressure tested, and filled before acceptance.

• Operation, Inspection, and Maintenance: Operation, inspection, and maintenance procedures should be identified during the design stage and provisions incorporated in the pipeline design (e.g., pressure relief valves, shutdown valves, purge fittings, pig launchers, etc.) to accommodate these requirements.

6.3.2 Pipeline Hydraulics

Design procedures for determining liquid flow in a pipeline are based on the Bernoulli theorem (or energy equation). For a simple single product line, the Bernoulli theorem can be used to express pressure losses in a pipeline (shown in Figure 6-16.)

The pressure losses shown in Figure 6-16 are determined by the required throughput, pipe diameter, pipe length, pipeline material, and ancillary fittings. The reader may refer to any standard fluid mechanics text for detailed procedures to calculate pressure losses.

6.3.3 Wall Thickness Determination

Factors governing the selection of the pipeline wall thickness are:

• Internal operational pressure

• External installation pressure

• Installation stress

• Submerged weight requirements

Although each of these factors must be analyzed prior to selecting a pipeline wall thickness, the most basic consideration in wall thickness selection is internal operational pressure. This selection forms the starting point for all other analysis.

The required wall thickness of a pipeline may be estimated by:

where:

D = pipeline diameter (in.)

Pj = internal operating pressure (lb/in.2)

S = maximum allowable hoop stress (generally 72 percent of yield stress)

Pressure loss Tank Farm

Pressure loss Tank Farm

Underwater Pipe Construction

I'm = Pout + Pressure loss due + Pressure loss due + Pressure loss due to friction in pipe to elevation to bends, valves, and fittings

Figure 6-16. Pressui

6.3.4 Hydrodynamic Stability

A submarine pipeline is subjected to hydrodynamic forces imposed by currents (steady state or tidal driven) and waves. Wave forces, in particular, can impose substantial loads on a pipeline even at depths of 30 feet or more below the sea surface. Hydrodynamic forces acting on a pipeline are depicted in Figure 6-17.

Determination of the required pipeline submerged weight to prevent movement of die pipeline is achieved by balancing the horizontal and vertical forces imposed on the pipeline. Computer programs are available from both Government and commercial sources to assist in this analysis.

6.3.5 Route Selection

Every pipeline installation should begin with a survey of the pipeline route (see Section 4.1). Particular emphasis during both the pre survey and the route selection survey should be placed on identifying losses in a pipeline.

manmade and natural hazards along the pipeline route. A final route selection is made after the site investigation and route survey is complete.

Pipeline route selection is a tradeoff between hazard avoidance and installation cost. The first priority in selecting a pipeline route is ensuring the safety of the pipeline. This is accomplished by selecting a route that avoids stationary hazards (e.g., anchorage areas, areas prone to mudslides, suspended spans over faults or rock outcrops, etc.).

Additional protection against movable hazards (e.g., bottom dragging fishing gear, small craft anchors, etc.) is provided to the pipeline by trenching, burial, or providing a riprap cover over the pipeline. Protection against wave and current forces is provided by routing the pipeline perpendicular to the bottom contours and providing additional stabilization (e.g., mechanical anchors, concrete weights, burial, etc.) in areas of high hydrodynamic forces.

Vertical wave inertia force A Hydrodynamic lift force

Thickness Submarine Walls

Weight of pipe coating

Weight of anodes or corrosion coating *

y Weight of pipe contents

Figure 6-17. Typical forces on a submarine pipeline.

After the risk of pipeline damage has been minimized by obstacle avoidance and cover techniques, route selection then becomes an exercise in finding the least cost (generally, the shortest) distance between the upstream and downstream ends of the pipeline. Whenever possible, shore crossings should be selected that are easily accessible and possess a uniform, gently sloping bottom profile.

6.3.6 Installation Methods and Equipment

Submarine pipelines may be installed using a variety of procedures and equip ment spreads. Several different installation procedures are often used on a single project. It is quite common to employ one method for installation of the deep water portion of the pipeline and another for the shore crossing and near shore area. Selection of the installation procedure(s) is determined on a case-by-case basis and is influenced by mission requirements, local site conditions, and the availability of equipment, personnel, and funding resources.

The major methods of submarine pipeline installation are summarized in the following sections and illustrated in Figure 618.

Pipe assembly

Pipe assembly

Rebound Hammer Data Sheet

Bottom Pall z

Water surface

Towing vessel-

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