EA50JG Offshore Structural Design – Jacket Platforms
9 Finite Element Analysis 9.1 Introduction
Structural analysis is the process of determining the action effects in a structure or structural component in response to a given set of actions. Structural analysis is required to demonstrate that the design of a platform. satisfies the relevant design code.
Action effects required for the design of jacket structures typically include the following:
● Internal section forces, which shall not exceed the strength of the section (checked using member strength checks);
● Support reactions, from which the required foundation capacity can be determined;
● Displacements and vibrations, which shall be within acceptable limits for operation of the structure;
Various calculation methods may be used for the determination of action effects in response to a given set of actions. These include, but are not limited to, hand calculations and computer methods, such as spreadsheets and finite element analyses (FEAs).
9.2 Types of analysis
Different analyses that may be required in the design of a jacket structure are discussed in the sections below. The applicability of different analysis types for checking the design conditions described in Lecture 5 are shown in Table 9.1 below.
Table 9.1 Applicability of Different Analysis Types
9.2.1 Static/Quasi-Static Linear Elastic Analysis
Static analysis is appropriate when dynamic effects are minimal and can be assumed to be covered by either: the partial action and resistance factors (LRFD) or the applied safety factor (ASD).
Quasi-static analysis is applicable when dynamic effects can be assumed to be approximately uniform throughout the structural systems and so small that one static analysis or a series of static analyses, with a small correction for dynamic effects can adequately account for the dynamic response. The correction for the dynamic response is often applied as a dynamic amplification factor (DAF) on the applied loading.
Linear analysis of offshore jackets can be carried out using a wide range of different software packages. Many offshore specific packages exist that include modules to calculate wave kinematics and member hydrodynamic forces and to solve for pile head displacements.
9.2.2 Natural Frequency Analysis
A natural frequency analysis is required to calculate the natural frequency and period (period=1/frequency) of a platform. This gives an indication of whether dynamic behaviour will be significant.
Structures for which dynamic behaviour is significant are generally referred to as dynamically responding structures. Redundant, multi-legged fixed structures (e.g. jackets, towers, etc.), with fundamental natural periods or having one or more components with natural periods greater than 2.5s to 3s usually respond dynamically to wave action during sea tow or in-place situations. For other types of structures, such as mono-towers and caissons, dynamic behaviour can be significant even with natural periods of 1s or less.
To calculate the natural period of the platform a reasonably accurate structural model, including both stiffness and mass is required. Dynamic behaviour is likely to be significant if any natural frequency, particularly the fundamental frequency, is similar to the frequency of an excitation (typically the wave frequency).
9.2.3 Dynamic Linear Analysis
When dynamic response is considered significant (typically if there is interaction between the natural period of the structure and the period of the loading), the structural system should be designed and analysed for dynamic behaviour. For a dynamic linear analysis an accurate structural model including both stiffness and mass is required. The type of analysis is governed by the form. of applied actions:
● Steady state analysis in response to harmonic actions, as required for spectral analysis;
● Transient analysis in response to arbitrary time-history actions, as can be required for accidental situations and non-linear actions due to waves or earthquakes.
For both types of analysis, the behaviour of the structure and the foundation are assumed to be linear elastic.
9.2.4 Non-Linear Analysis
The collapse of a space frame. structure usually results from progressive failure of its components, in particular its primary members and/or joints. Linear analysis can be used to check the strength of the structural components against the applied loading, however to investigate the redistribution of internal forces following a component failure, and the prediction of collapse behaviour a non-linear analysis is required.
Non-linear analysis can be used to account for three forms of non-linearity:
● Geometric non-linearities occur if a structure experiences large deformations under the applied loading. The changing geometric configuration can cause the structure to respond nonlinearly.
● Material non-linearities occur when a material is stressed beyond its yield point and begins to behave plastically.
● Contact non-linearities occur when deformation of the structure results in a change to the structures boundary conditions.
Non-linear analysis may be required if a structure is subjected to abnormal environmental actions due to wind, wave and current or an earthquake, or to accidental actions from ship impact, fire or explosion, and when a linear analysis predicts:
● Displacements of a magnitude that are likely to cause second order (P-Δ) effects,
● Joint failure,
● Member buckling, and/or
● Stresses that exceed the yield strength of the material,
For these cases non-linear analysis may be required to justify that the overall structural integrity of the platform. is not impaired.
9.2.5 Reliability Analysis
Structural reliability analysis can be used to calculate the probability of failure of a jacket structure that does not meet the required acceptance criteria when analysed using a conventional linear or non-linear analysis. Reliability analysis is often used to reassess existing jacket structures (often designed to earlier now superseded design codes). In general reliability analysis methods should not be required for the design of new structures.
9.3 Analysis model
The analytical models used in offshore engineering are in some respects similar to those adopted for other types of steel structures. Only the salient features of offshore models arepresented here.
The same model is used throughout the analysis process with only minor adjustments being made to suit the specific conditions, e.g. at supports in particular, relating to each analysis. An example of a jacket structural analysis model is shown in Figure 9.1.
9.3.1 Beam models
Members
The structural analysis model of a jacket predominantly consists of beam elements representing the axial, bending, shear and torsional stiffness of the structural members. In some cases special modelling arrangements (either using shell elements or equivalent sections) are used to represent pile clusters and large diameter members provided for storage or flotation.
In addition to its geometrical and material properties, each member is characterised by hydrodynamic coefficients, e.g. relating to drag, inertia, and marine growth, to allow wave forces to be automatically generated.
The structure shall be modelled in detail and should include the primary and secondary structures, conductors, and appurtenances to ensure that action effects are accurately predicted. If this is not possible, the necessary detail of the model shall be prioritized as follows, in the order given:
1. Primary Structure;
2. Secondary Structure (conductor supports and framing);
3. Components provided for Temporary Conditions (launch framing, mudmats etc.);
4. Conductors;
5. Appurtenances.
When the structural contribution of any component is neglected, the self-weight, buoyancy and hydrodynamic actions on the component shall still be included in the model.
Figure 9. 1 Jacket structural analysis model
Joints
Each member is normally rigidly fixed at its ends to other elements in the model. If more accuracy is required, particularly for the assessment of natural vibration modes, local flexibility of the connections may be represented by a joint stiffness matrix.
For typical jackets, depending on the diameter of the chord, the length between the physical end of the brace stub and the centre line of the chord can be significant, and can affect the calculation of member end forces and stresses, weights, masses, hydrodynamic and hydrostatic actions. In such cases, it is customary to model the length of braces between the outer surface of the chord and its centre line as rigid connections (as shown in Figure 9.2); joint flexibility of brace and chord connections is thus neglected.
Figure 9.2 Joint rigid link definitions
9.3.2 Foundation model
The stiffness of a piled foundation generally displays non-linear characteristics. The foundation should be modelled and analysed using non-linear soil p-y, t-z and Q-z curves as described in Lecture 7. It is important to ensure compatibility between the forces and displacements at the pile heads calculated with both the non-linear pile model and linear jacket model. To achieve this the pile stiffness in the jacket model is usually represented by an equivalent load-dependent secant stiffness matrix; coefficients are determined by an iterative process where the forces and displacements at the common boundaries of structural and foundation models are equated. This matrix may need to be adjusted to the mean reaction corresponding to each loading condition.
9.3.3 Topsides
For structures where the stiffness of the topside and jacket do not interact significantly the jacket and topside can be modelled separately. If separate models for the structure and the topsides structure are used, the stiffness of the topsides structure and its interface with the structure should be modelled in sufficient detail to allow its self-weight and applied actions to be calculated and applied to the jacket support points.
Where the structure and the topsides structure interact significantly, a combined model of structure and topsides structure should be used.
9.3.4 Conductors
Conductors can be modelled as beam elements with appropriate releases at the guide frame support points. At typical guide locations the conductor should be free to move axially and rotationally with only lateral support. For jacket structures, the deadweight of conductors is usually self-supported. Care should betaken that appropriate releases are included to ensure that the conductors do not transfer topside loads to the seabed.
9.3.5 Appurtenances
The contribution of appurtenances (risers, J-tubes, caissons, boat-fenders, etc.) to the overall stiffness of the structure is normally neglected.
They are often therefore analysed separately and their reactions applied as loads at the interfaces with the main structure.
9.3.6 Plate models
Integrated decks and hulls of floating platforms involving large bulkheads are described by plate elements. The characteristics assumed for the plate elements depend on the principal state of stress which they are subjected to. Membrane stresses are taken when the element is subjected merely to axial load and shear. Plate stresses are adopted when bending and lateral pressure are to betaken into account.
9.3.7 Loadings
Functional loads
Functional loads consist of:
● Deadweight of structure and equipments.
● Live loads (equipments, fluids, personnel).
Depending on the area of structure under scrutiny, live loads must be positioned to produce the most severe configuration (compression or tension); this may occur for instance when positioning the drilling rig.
For dynamic analysis all applied functional loads must be converted to masses.
Environmental Loads
Environmental loads consist of wave, current and wind loads assumed to act simultaneously in the same direction.
In general eight wave incidences are selected; for each the position of the crest relative to the platform. must be established such that the maximum overturning moment and/or shear are produced at the mudline.
In general, environmental loading from the platform. orthogonal directions (platform. North, East, West and South) will maximise the loading in the jacket bracing. Environmental loading from diagonal directions will maximise loading in the jacket legs and foundations. When analysing diagonal directions the wave approach angle should be modified to minimize the lever arm between the jacket legs resisting the applied overturning moment as shown in Figure 9.3.
Figure 9.3 Diagonal wave approach directions
References
[1]. API-RP 2A-WSD: Recommended practice for planning, designing and constructing fixed offshore platforms. American Petroleum Institute 21st Edition. Errata and Supplement 2 October 2005.
[2]. International Standard, ISO 19902-2007, Petroleum and natural gas industries - Fixed steel offshore structures, 2007.
[3]. NORSOK, NORSOK STANDARD N-003 Actions and Action Effects, Edition 2 September 2007.
[4]. DNV: Offshore Standard DNV-OS-C101, Design of Offshore Steel Structures, General (LRFD Method), April 2011.
[5]. ESDEP, WG 15A : Structural Systems: Offshore, 1993