Section 9 Strength

9.1.1 In general the strength of the pipeline and riser is to be determined from a three-dimensional finite element method. Only if it can be demonstrated that other methods are adequate, will they be considered.

9.2.1 The design of the pipelines is to be analysed and the resultant stresses determined. The loading combinations considered are to represent all modes of operation so that the critical design cases are established.

9.2.2 All loads applicable to the design (see Ch 1, 8 Design loadings and environmental criteria) are to be fully covered in the loading combinations.

9.2.3 The system is to be divided into two zones which contain components at or near the platform, and the pipeline distant from the platform respectively. Zone definitions are shown in Table 1.9.1 Zone definitions as distances in metres from the platform.

9.2.4 A fully representative number of design cases are to be defined, each of which should be associated with appropriate environmental conditions and allowable yield ratios. Yield ratios will depend on the zone in which the component is located, and refer to minimum specification yield stress defined as the 0,5 per cent proof stress. The design cases are to cover all critical aspects of pipeline installation, testing and operation.

9.2.5 For any particular location, two stress intensity calculations will be required, as follows:

1. Hoop stress. Calculations are to be made utilizing the minimum specification wall thickness less corrosion allowance as appropriate.

2. All axial stresses arising from end load, bending moment, shear and torsion are to be combined with hoop stress to given an equivalent stress based on the Mises-Hencky criterion to conform with specified yield ratio limits. For this purpose, nominal section dimensions may be used.

9.2.6 Required design cases, together with environmental and functional conditions and associated allowable yield ratios are given in Table 1.9.2 Pipeline and Riser Design Criteria. Other cases may be required in specific applications.

9.2.7 Specific aspects of riser and pipeline system behaviour which are to be considered include:

1. Pipeline static analysis.

   • The pipeline is to be checked under conditions of full axial restraint for yielding at all points away from seabed discontinuities, and the expansion loop area.
   • Elsewhere, a complete stress analysis will be required for all sea bed features, including crest, trough, prop and freespan.
   • Proper account must be taken of the effect of axial pressure and wall forces on bending moments.

2. Vortex shedding response:

   • The effects of vortex induced oscillations are to be accounted for at all locations where free spans can arise.
   • The effect of axial compressive forces on natural frequency is to be included.
   • The restraining effect of external spans, and relief due to wave and current directionality may be included provided that sufficient environmental data is available.
   • For riser spans, a boundary restraint midway between fixed and simply supported may be assumed in calculating natural frequency.
   • In all cases, the effect of vortex shedding on fatigue life is to be checked.

3. On-bottom stability:

   • The lateral stability of pipelines which are not buried and are subject to wave and current action is to be verified.
   • Water particle velocity one external diameter from the seabed is to be taken to act on the pipeline for the calculation of lift and drag.
   • Sea bed friction will be required to provide a 10 per cent margin of stability when the pipeline is subjected to combined significant wave and current.
   • Lateral movement will be allowed under an extreme wave. However, it is to be demonstrated in submitted calculations that this movement does not lead to stresses exceeding the limits laid down in case 5c, in Table 1.9.2 Pipeline and Riser Design Criteria.

4. Buckling:

   • Local and overall buckling of the pipeline is to be checked for all locations and loading conditions for which free spans may arise. The worst combinations of axial and lateral loading are to be considered.
   • For laterally-restrained (i.e. trenched or rock-dumped) pipelines, the resistance to upheaval buckling is to be considered.
   • This will involve a detailed consideration of possible seabed imperfections, together with the downward restraining effect provided by soil or rock cover.
   • In the event that this cover is shown to be critical, procedures are to be established which will verify the construction and future maintenance of this cover. These effects are especially important for lines designed to operate at elevated temperature.
   • Proposals for the use of buckle arresters should be submitted for consideration.

5. Riser stress analysis:

   • A detailed analysis to the riser, including interaction with pipeline and expansion loop is to be carried out. This is to take account of thermal, hydrodynamic, gravity and pressure effects. Modelling is to describe riser geometry and stiffness, and soil interaction, including loss of contact.
   • Riser clamp forces are to be determined, and strength checks carried out.

9.2.8 The pipeline/riser system is to be designed such that under transient operating conditions the maximum allowable operating pressure may not be exceeded by more than 10 per cent.

9.2.9 Pressure calculations for pipelines are to be carried out using the minimum specified wall thickness, i.e. the smallest wall thickness which could be used and still meet the requirements of the material specifications. See Ch 1, 9.2 Structural analysis and stress calculation 9.2.5 regarding corrosion allowance.

9.2.10 Pig trap stresses. The pig trap is to be considered as part of the riser and associated equipment and as such hoop stresses are not to exceed 60 per cent of the specified minimum yield stress of the material.

9.2.11 The riser supports are to be designed to meet suitable structural design codes. For example API RP 2A — Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms.

9.3.1 The effect of notches, stress raisers and local stress concentration is to be taken into account in the design of load carrying elements.

9.4.1 Fatigue damage due to cyclic loading is to be considered in the design of the pipeline. The cyclic loading due to internal (contents) pressure fluctuations and external environmental loadings are to be taken into account.

9.4.2 The extent of the fatigue analysis will be dependent on the mode and area of operations.

9.4.3 The minimum design fatigue life of a pipeline is to be 3 times the specified operational life.

9.5.1 Where plastic design methods are employed, the load factors will be specially considered.

9.6.1 All yield ratios derived from stress analysis calculations are to conform with the values specified in Table 1.9.2 Pipeline and Riser Design Criteria.

9.6.2 In the absence of other specified requirements by LR, the requirements of Code IP6 — Institute of Petroleum, Model Code of Safe Practice in the Petroleum Industry Petroleum Pipelines and the standards of the American Institute of Steel Construction are applicable.

9.6.3 Permissible stresses in materials other than steel will be specially considered.