Design basis (a) Topics in Design Basis

The Design Basis will usually address the following topics:

• The client’s most important functional requirements

• Reference to rules, regulations, standards and specifications • Possible deviations from standards

• Design principles and limit states

• Temporary and permanent construction phases • Loads, load combinations and load factors • Material coefficients

• Materials and material parameters • General reinforcement detailing • Design assumptions and criteria • Design procedures and methods • Interface areas.

The above list only gives typical topics; it is not complete and should be supplemented as needed.

(b) Reference to rules, regulations, standards and specifications

The contents of the Design Basis document will be based on updated rules and specifications, but also refer to good practice developed by practical experience in the design of concrete platforms. The document also to some degree expresses the design philosophy of the project.

As an example, national rules and specifications used as main references for a Design Basis in Norway are found in (NPD, 1992), (NBR, 1990), (NBR, 1998) and (Statoil, 1992). Although (Statoil, 1992) is a company specification, it is often used as basis for the design, also by other clients. The client will then usually prepare his own supplementary specification which will be included in the design contract.

In addition to the above-mentioned national standards and specifications a varying number of recognised national and international specifications/standards, guidelines, research reports, and published articles will form the basis for the content of the Design Basis. Among the international standards for concrete, which are sometimes referred to, the following are mentioned: (CEB-FIP, 1993) and (CEN, 1991).

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If, at some point, the rules put forward in the Design Basis deviate from the rules in the references, this should be clearly stated. All documents used in the preparation of the Design Basis should be listed in a reference list.

(c) The client’s functional requirements

The client’s functional requirements are usually specified in a separate document, where the conditions for management and use of the structure are described. In addition to a reference to this document it would be practical to quote for example:

• Maximum and minimum deck weight with centre of gravity • Environmental loads

• Lifetime of the structure • Water depths at the field

• The orientation of the structure.

(d) Design principles

The structures are usually designed according to the partial safety factor method. Under certain conditions the safety of the structure may also be assessed on the basis of probabilistic methods with specified safety indices. This approach is mainly used in connection with special accidental loadings.

In special cases a testing of structural parts may also be applied. It should be explicitly stated in the Design Basis document if such methods are to be applied. The structures are usually designed in the following limit states:

• Ultimate limit state ULS

• Serviceability limit state SLS

• Fatigue limit state FLS

• Accidental limit state (progressive collapse) PLS.

Appropriate criteria are given for each limit state. FLS and PLS are according to international terminology special cases of ultimate limit states, but since fatigue and accidental actions are particularly important for offshore structures, it is found convenient to use special identifiers for these ultimate limit states.

(e) Lifespan phases

The lifespan of the concrete structure from the start of the construction to removal of the structure from the field may be divided in several phases. Statoil (Statoil, 1992) distinguishes between temporary and permanent phases. It may be convenient to subdivide as follows:

• Construction phase (temporary) includes the building of the structure. This phase is often further subdivided.

• Transport phases (temporary) include all transport of parts of the structure or the complete structure until it is at the field, ready for installation.

• Installation phase (temporary) includes the time for installation of the structure in its correct position according to the specification of the client.

• Operational phase (permanent) includes the time from completed installation to removal of the structure.

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Different sub-phases during construction will often be decisive for the design of traditional platform structures. These include floating out of dock or different floating phases during further construction and especially the almost complete submersion of the structure for deck-mating.

(f) Loads, load combinations and load factors

The loads acting on an offshore concrete structure have different characteristics. As an example, the Norwegian Petroleum Directorate (NPD, 1992) categorizes loads as permanent loads (P), live loads/variable functional loads (L), environmental loads (E), deformation loads (D), and accidental loads (A). Loads with categorization and detailed specifications concerning establishment of characteristic values will be provided by the client. As an example, see (Statoil, 1992).

Usually, permanent loads from self-weight and water pressure combined with environmental loads due to waves and wind will have a dominating influence on the design of the concrete structure. Determination of the characteristic environmental loads are based on observations at the site and the calculation of wind and sea states, according to (NPD, 1992) with 100 years return period. For serviceability limit state criteria or for temporary states shorter return periods are used, such as 1 year.

In order to determine the static equivalent design load on the basis of dynamic environmental loading, separate analyses are performed which take account of the stochastic variation of the loads and response of the structure. The designing wave load may therefore take different values for different parts of the structure.

1. If the loads and load effects can be decided with high accuracy, the Norwegian Petroleum Directorate may allow the use of load factor 1.2.

2. The load factor for permanent loads is set to 1.0, if this is unfavourable.

3. For deformation loads from prestressing, national or regional standards may prescribe other values.

4. In the accidental limit state (PLS) it is to be checked that the damage due to accidental loading remains local. After local damage the structure should still be able to resist defined environmental conditions without extensive failure, free drifting, capsizing, sinking or extensive damage to the environment.

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General requirements concerning loads, and especially which types of accidental loads are to be considered (ship collisions, falling objects, explosions, fire, earthquake, loss of internal pressure, erroneous trimming of the ballast, etc.) are to be given in the Design Basis. Furthermore, the extent to which analysis of the consequences of local damage is required, with possible flooding of parts of the cell structure, must also be specified.

Design load combinations and load factors should be based on analysis of the uncertainty of the load effects from combinations of typical loads acting on offshore structures, and will be specified in national or international regulations, or by the client. An example is found in Table 5.1.

(g) Material safety factors

Material safety factors will also be specified in national or international regulations. As an example, material factors used for offshore structures in Norway are given in Table 5.2.

In (Statoil, 1992) material factors according to Table 5.2 are specified, but it is required that factors according to national standards shall be applied when they are higher.

(h) Material properties

High performance of the construction materials is generally required. For concrete in particular, low permeability is needed to satisfy the required durability and water-tightness of the structure. These requirements result in demands for low water/cement ratios and corresponding high concrete strength.

The importance of the self-weight of temporarily or permanently floating structures will often result in a demand for a high strength/density ratio of the concrete. Strength classes (according to NS 3473 (NBR, 1998) C65-C75 for normal density concrete and LC55-LC65 for lightweight aggregate concrete (where numbers refer to 28 days cube strength) are often used. The tendency during the development of offshore concrete structures has been increased strength/density ratios. The above strength classes are in principle covered by NS 3473, but it is recommended and partly required, that the mechanical properties are determined by testing. Prior to application of new concrete types, the testing should include not only the usual mechanical properties, such as the ratio of the compressive strength of cubes and cylinders, E- modulus, creep coefficients, stress/strain diagram and tensile strength, but also fracture mechanics properties. To some extent, it is also recommended to test reinforced structural elements to verify the expected composite action of a new concrete type with reinforcement in terms of anchorage, shear strength, etc.

The strength classes according to NS 3473 are defined on the basis of such pre-testing with possible adjustments of the characteristic material properties to be used in design.

Similar specifications and documentation of the characteristic properties are required for reinforcement and prestressing steel.

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(i) Reinforcement principles

The Design Basis document should outline the basic principles for the reinforcement system in order to ensure a unified detailing of the reinforcement in the whole structure. This will typically comprise the required minimum reinforcement, maximum spacing, standard bending diameters, methods and standard dimensions of splices and anchorages, limitations of maximum reinforcement density; use of bundled bars, etc. The prestressing system with standard cable dimensions will also be specified in the Design Basis.

(j) Design assumptions and criteria

The main parts of a concrete platform are classified in a high safety class taking into account that a failure situation may result in catastrophic consequences with high risk for loss of human lives. The extent of control measures is to be evaluated especially. Regarding control of the design and construction, reference is made to Chapters 6 and 7. Examples of relevant specifications are:

For the serviceability limit state:

• design exposure class (with corresponding concrete cover and crack widths) • structural requirements to ensure strict water tightness

• criteria concerning vibrations and displacements, especially for shaft structures.

For the fatigue limit state: load distribution spectra and lifetime factors.

For dimension tolerances:

• thickness of each structural part

• deviations from the intended centre line of the components • concrete cover

• position of the reinforcement.

• deviations from the ideal middle plane of the structure (as a basis for the design of slender shell structures).

(k) Design procedures and methods

Procedures and methods which are not uniquely described in the reference documents should be specified in the design basis. Examples may be:

• Load effects calculated by linear finite element analysis may be used as the main basis of design of the concrete structure

• Detail design of regular sections of the structure performed by automatic post-processing of the analysis results

• The method to be used in transverse shear capacity control • Effect of water pressure in cracks

• Practicable simplifications and approximations for design for restraint forces due to imposed deformations

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• Additional design tasks which are not naturally included in the general design of the structure, like embedded steel structures, crane support structures, tube penetrations, temporary block-outs, etc.

• interface areas to other main parts of the concrete structure and to other design disciplines requiring procedures regarding management of interface information and co-ordination. Practical experience has shown that it is a considerable challenge to handle all the information necessary to satisfy all the different requirements in such interface areas, and that this is a source of possible design errors.

An example of the last item is that an offshore oil production platform will be equipped with a large number of tubes which are originally planned and designed by designers in other disciplines. The connection of such tube systems to the concrete structure will often require specific limitations of the load effects (deformations) in certain regions of the concrete structure. The technical solution to such problems should be worked out in close co-operation between process equipment and concrete structure designers to ensure that the combined structure performs as planned and the integrity of the concrete structure is taken care of.

5.2.3 Design briefs