The selection of the concept for the topside structure is the second step in the development of a structural system. The two possible basic alternatives: a truss type (Figure 4) or a portal-frame type without braces (Figure 3), are compared in Table 1. Table 1 Comparison of concepts for main jacket-based structures
No. Item Truss type Frame type
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Discipline non-interference Flexibility during construction Flexibility during operation Automated fabrication Construction depth Inspection Maintenance Weight of structure Strength reserve
Potential for high strength steel Structural CAPEX Platform CAPEX - - - - ++ - - + + + + + ++ ++ ++ ++ 0 ++ + 0 ++ ++ + ++ Note: ++ denotes greater benefit
-- denotes greater disadvantage
The selection of the topside main structure concept, truss or portal frame, is linked with the decision of the position of the longitudinal structure in the cross-section. In a 20-25m wide deck, trusses will generally be arranged in longitudinal rows: centre line and both outer walls (Figure 6).
In such decks, however, portal frames will be arranged in 2 longitudinal rows, approximately 14-16m apart, allowing floor cantilevers of approximately 5m (Figure 3).
3.2 Selection of Topsides for Gravity Based Structures
Topsides of gravity based concrete structures (GBS) are quite different from the jacket based topsides, see Lecture 15A.1. The topside structure is an important element in the overall portal-type system. Gravity based substructures have been built with one to four shafts. A rectangular or a T-arrangement of four shafts has been adopted. The basic form is a modularized topside with a grid of heavy box girders.
A few elements only of the GBS-topside structural design are indicated below:
due to portal frame action, the deck is subject to fatigue; a design case difficult to control in topside design.
equipment lay out optimization, piping and cable routes, logistic and emergency routes require many big openings and perforations of plate walls, thus creating stress concentrations.
attachment of secondary structures and of equipment/pipe/cable supports to the main structure must be strictly controlled, to avoid fatigue problems.
the connection area with the concrete shaft must provide the transition from circular (shaft) to square (deck). It accommodates high strength anchor bars, temporary crushing devices for deckmating, and requires tolerances on deck and substructure dimensions.
inspection and repair options must be planned carefully, especially as fatigue may occur.
The material used at present is high strength steel typically of 355 MPa yield stress. There is a trend to use higher strength steel (420-460 MPa).
3.3 Floor Systems
The concept for the floor-system in offshore structures is conventional: hot rolled beams, typically at 1000-1200mm centres, are covered by a chequered or flat steel plate 6-10mm thick.
conventional steel floor
steel grating (bar-type or plate type)
aluminium floor system
orthotropic deck in steel
corrugated steel plate
The conventional steel floor contributes approximately half of the weight of the steel structure of an offshore deck. Steel gratings, especially with the plate type, could gain increased application as their weight per sq.m. is attractive. Aluminium has attracted much interest recently; current development in Norway will show its real potential.
Orthotropic decks in steel have found application in helidecks. Further study is required to assess their actual feasibility for floors of offshore modules.
Corrugated steel plate (approximately 1-3mm thick) as sub-flooring has been used in living quarters.
In summary, the floor concept used for a typical floor of an offshore deck of a module is a conventional steel floor or steel grating.
3.4 Floor Panel Concept for Conventional Steel Floor
The floor panel, defined as the assembly of the floor plate and the stringer, can be connected to the overall structure in two ways:
stacked: stringer over the top of deckbeams.
flush: stringer welded in between deck beam, with top flange in one plane. It is practically impossible to change from the flush to stacked arrangement in a later phase of the design.
All elevations and overhead clearances are involved in the choice of arrangement. Clearances are very important for equipment height, pipe routing, pipe stress, cable routing, etc. The single most important structural aspect is the amount of prefabrication that can be carried out away from the main fabrication yard. The cost is also a very important factor. 3.5 Floor Stabilization Concept
The deck structure requires lateral stabilization of each floor with respect to:
lateral instability of beams
horizontal forces, e.g. wind, pipe reactions, sea transport
horizontal components of permanent braces
horizontal components of temporary braces, e.g. seafastening
horizontal components of sling forces
module skewing during installation.
There are essentially two options for floor stabilization:
provision of separate underfloor horizontal bracings
allocate the stabilization function to the floor plate.
There is a clear preference for the stabilization by the floor plate. Where underfloor bracing is adopted, there are two configuration options (see Figure 11). The rhomboid solution should be chosen for the upper deck, due to congestion at the column by the padeyes for lifting. The underfloor bracing under a plate floor does create a very unclear structural situation. The bracing is assumed completely to perform the stabilizing function, but in practice the floor plate is much too stiff to allow that. It is common practice in the structural analysis for underfloor bracing to neglect completely the floor plate.
4. DECK FLOORING DESIGN