MAIN STRUCTURE DESIGN 1 Introduction

Some major topics in topside structural design are reviewed below.

2.2 Main Structure-Portal Frame Design

A portal frame design has been used in recent major projects in the Dutch sector such as Amoco P15, Placid K12 [5] and Penzoil L8.

The main girder/column joint, as shown in Figure 1, is very important in determining the height. It is most practical to position the longitudinal and transverse main girder flanges at the same elevation.

Haunching of the transverse main girder, which is more lightly loaded-in-plane, however is not an option as these girders become highly loaded during transport.

The severe restraint of welding a tubular in a diaphragm requires the selection of TTP steel for the column section.

Due to the high importance of the diaphragm plates in the overall integrity of the structure and the welding constraints on the web plates in between, TTP-steel is chosen also for the diaphragm.

Another option is to weld the girders directly onto the unstiffened can section of the column.

The assessment of ultimate resistance as well as fatigue strength has been the subject of recent research (see Lecture 15A.12).

Further improvement of the theoretical and experimental background is required. For lighter loaded truss structures, this non-stiffened type of joint has been used successfully.

A third solution is to weld the girders directly to the can section of the column, which is internally stiffened by rings. Its most severe disadvantage is the difficulty of inspecting the column interior.

The disadvantage of both direct girder-column joints is that the girder sizing is governed by the very high moments at the column/beam transition point.

Cast steel nodes form an alternative to the welded designs.

Member selection for portal frame structures with increasing section module usually includes:

• 300 mm wide rolled beam.

• 400 mm wide rolled beam.

• 450 mm /460 mm wide rolled beam.

• castellated beams fabricated from rolled beams, giving a height 1,5 times the original beam height

• built-up girders fabricated from rolled beam T-sections with a web plate welded-in-between

• plate girder

The plate girder of course provides the greatest flexibility for design, material selection and procurement, though its cost per tonne is approximately twice that of a rolled beam.

2.3 Main Structure-Truss Design

Most offshore structures of moderate size have been provided with a truss-type structure.

Typically such trusses consist of rolled beams as chords and tubulars as diagonals.

Truss design requires several choices which affect the structural efficiency and have impact on other disciplines:

• number and configuration of braces

• falling or rising braces

• intermediate load carrying of chords

• presence of external moments on joints

• braces: tubulars or H-rolled sections

• chords: rolled section or plate girders

• truss joints: locally reinforced chord or prefabricated node section

Figure 2 shows different arrangements of braces (basically N or W-type) obtained by variation of the number of nodes. It should be kept in mind that all diagonals and verticals form obstructions for piping and cable routings of all kinds.

For the transverse trusses, transparency is even more important, especially near the well area. The number of members required should therefore be reduced to a minimum.

Providing a W-truss with light verticals should be evaluated against choosing a heavier chord section.

If a joint, e.g. at the top deck, is subject to severe moments due to lifting, ventstack, or crane pedestal for example, much of the bracing stress would result from unintended bending.

Generally the deck leg restraint creates a similar problem in the lower deck. An evaluation should yield a preferred location therefore for the node of the end brace.

The truss deflects under its vertical load which leads to restraint of the chord in the column and to bending of the chord. Both effects can quite severely effect the efficiency. The chord section should be kept compact therefore and not given too much height.

Tubulars (circular, square or rectangular) or rolled sections can be chosen for the braces.

The choice depends primarily on the loads and the chord width. A chord width of 300mm can accommodate a 10 in. brace only. Thus a wider chord flange is preferred.

2.4 Main Structure-Stressed Skin Design

A third major structural option is the stressed skin concept, where full height plate walls take the function of the truss or the frame.

Modules for living quarters are frequently built to this concept. Other types of modules have not been built with stressed skin since the obstruction they cause during construction is severe.

For smaller stressed skin modules, trapezoid corrugated plate can be used to provide a wall in a frame of square hollow sections.

For bigger modules, flat plate stiffened with through-stiffeners is used for the walls.

The detailed design can only be made with a clear plan for assembling the module which shows the panels that must be prefabricated.

2.5 Non-Load Bearing Walls

Blast or fire walls are provided in offshore platforms. Due to their function full welding to the main structure is often unavoidable, see Figure 3a.

Special attention is required concerning:

• the capability of the walls to comply with the deformation of the main structure during load-out, sea transport, lifting and in-service.

• the strength of welds to the main structure being stronger than the plate to avoid rupture and potential crack initiation of the main structure.

One solution is to provide a flexible detail, see Figure 3b and 3c, with stiffeners falling short.

2.6 Crane Pedestals

Crane pedestal, are discussed briefly below.

It is structurally economical to put the crane pedestal on top of a main column. For a truss type the main structure will be close to the platform periphery so a moderate length of crane boom is sufficient.

For a portal frame type with columns closer to the outer periphery, the pedestal requires a special column in order to avoid using a crane with large boom length. Figure 4 depicts such a solution.

The functions of the main structure with respect to the crane pedestal are:

• to provide torsional support preferable at top deck level

• to provide lateral restraint at top deck level

• to provide lateral restraint at the lower end of the pedestal

• to provide vertical support, preferably at the lower end of the pedestal.

Bending restraint by deck beams and/or main structure girders is not required and should be reduced where possible. Torsion caused by slewing of the crane should preferably be resisted by the floor plate, the stiffest element.

It has become practice to include the tapered top section of the pedestal in the supply package of the crane. The top section contains the large flange for the slewing bearing.

Fatigue due to crane operations is a design criterion and requires careful detailing of the pedestal and the adjoining structure.

3. ANALYSIS OF DECK STRUCTURES