The concept definition phase

2.7.1 Concept report

The goal of the concept definition phase (the concept evaluation phase) is to work out a Concept Report, which in turn forms the basis for the detail design and the fabrication contract.

The concept report shall describe the concept in sufficient details to avoid major changes in the successive detail design phase. Furthermore, the concept shall be robust enough to accommodate minor technical changes in the design basis. The concept shall also have built in a certain “forgiveness” to allow for minor inaccuracies. It is, furthermore, needed to emphasise that weight control shall be an important parameter for those working with the topsides, and major changes in the topsides weight must be avoided. For concrete structures it is also of importance that uncertainties in weight, due to the amount of reinforcement and to water absorption in concrete, are included in the concept definition phase. This is to assure that the platform has acceptable floating stability during all temporary phases, and for floating concrete structures also during the operation phase.

The concept report shall also describe important principles for the detail design, and these principles are assumed to be unchanged throughout the construction phase. For instance, the extent of vertical post tensioning of the concrete shafts represents a principle which is defined during the concept definition phase. Other principles of importance which are suggested to be included in the concept definition phase are:

• that the geometry of the concrete is correct. Changes to the geometry (e.g. wall thickness) could lead to large consequences in the detail design phase

• that the concrete quality C70 (for normal density concrete) or LC60 for LWA-concrete (light weight aggregate concrete) are the maximum values applied during the concept definition phase. This will give the project some reserves in highly stressed areas. It is, for example, particularly unfavourable if the amount of reinforcement is significantly increased during the later phases due to the fact that shell walls are too thin in the early phases. It is also very costly

to build a structure where there is barely room enough for placing the required amount of rebars as rebar placement and compaction of the concrete then would become very demanding

• that all openings towards the sea have double barriers in case of damage

• that the lower dome of the drillshafts shall not be penetrated if a conductor-pipe is lost during the construction phase. This applies to the cases when the structure is floating with no damage stability and the lower dome is exposed to dropped objects

• that (during the concept definition phase) it should be aimed at avoiding membrane tension in the concrete walls throughout the section when the structure is to be exposed to the design wave (ultimate limit state with load coefficient of 1,3). This will contribute to a robust structure with only minor cracks and an acceptable fatigue life.

2.7.2 Technical work

The technical work during this phase will be comprehensive, as indicated in Appendix A (Discipline Activity Model for Design of Offshore Concrete Structures); see also (Fjeld and Morely, 1983) and (Tjelta, Aas, Hermstad and Andenæs, 1990). Some special problems which shall be evaluated, are as follows:

• load analysis with assessment of whether special phenomena may be present due to the geometry of the structure

• the need for model testing

• geotechnical conditions and geotechnical stability during installation and operation

• analyses of all phases during construction and installation, specially emphasising floating stability and requirement for stability in case of damage

• designing of all critical sections, with calculation of section forces and moments, including amount of rebars and detailing of the need for pre stressing (see Chapter 3 and Chapter 5).

For structures of the Condeep type, the geometry in the tri-cell areas and other joints are to be designed in detail.

• element analyses of critical sections like top of shaft (see Chapter 4)

• the safety of the platform’s mechanical systems e.g. ballast systems and the oil company’s mechanical systems (flowlines from wells, risers, oil storage system etc.)

• choice of bulkhead sectioning and analyses of floating stability during operation. For floating platforms, specially emphasising how the platform can be designed with acceptable spare buoyancy to allow for some topside load increases in the later phases.

For fixed and floating structures, respectively, the following calculations shall be discussed in depth:

Fixed concrete structures:

• Floating stability, including level of ballast water in all essential stages of fabrication

• Global wave forces, including the requirements for calculation of forces in the shaft base (Morison wave load analysis with Stokes 5th order wave theory)

• Dynamic calculations, including demonstration of the natural response period if “springing”

and “ringing” (transient dynamic response) can give a substantial contribution to the global wave forces

• Geotechnical evaluation, both of geotechnical stability as well as of soil structure interaction in conjunction with dynamic calculations, including analysis of soil pressure distribution under the foundation during various load applications Furthermore the possibility of soil erosion shall be assessed.

Floating concrete structures:

• Floating stability, including estimate of the level of ballast for all essential stages of fabrication and operation

• Detailed hydrodynamic calculations of wave forces as well as global responses. In addition, it should be noted that certain stages in connection with fabrication can lead to special design forces in parts of the structure

• Dynamic design, including identification of the various natural response periods where

“springing/ringing” can contribute to a substantial part of the global wave forces

• Geotechnical calculations in connection with the mooring systems

• Analyses of the water tightness of the structure to assure that the structure can be safely operated throughout its entire design life.

The oil company normally gets most of this work performed by experienced consultants/

contractors. Concerning development of offshore concrete structures, it is essential that the contractors tendering for construction are allowed to develop competitive structures adopted to their own construction facilities. Thus, the oil company can obtain competitive bids from several competent tenderers.

2.7.3 Quality assurance and verification

It is assumed that the work performed during the concept development phase has been subject to the proper quality assurance, see Appendix A (Discipline Activity Model). The principles described here are used both by the oil company and the external contractors involved.

In addition it is relevant to use external consultants to perform verification of parts of the documentation. For this phase it is recommended to channel all communication related to verification through the oil company (see Chapter 7).

There is also a need for a rough risk analyses during the concept definition phase to identify whether the concept has built in major risks during the phases of construction or operation related to:

• loss of human life

• damage to the environment

• loss of values.

Thus, risk reducing efforts can be incorporated. Special emphasis should be made to focus on the following risk elements during the construction phase:

• risk of falling objects

• risk of uncontrolled ingress of water due to pipe failure or malfunctions of the ballast system

• risk of collisions

• risks during towing and installation.

In addition, the accidental design loads applied in the operation phase are to be determined.

2.7.4 Schedules and budgets

An important part of the concept definition phase is to establish detailed schedules and budgets for the development project.

2.7.5 Competence requirements

The concept report represents the final basis for a successful performance of a development project. Consequently, it is of the highest importance that all parties involved use their most experienced engineers and make them responsible for the project’s concept evaluation phase.