Main topsides layout

Most platforms on the NCS are multifunctional – in other words, facilities which combine areas for processing, wellheads, drilling, risers, utilities, quarters, lifeboats and helicopters.

See Figure 6-1. Such platforms operate with large quantities of flammable liquids, explosive gases under high pressure and heavy machinery, all positioned in small areas and often combined with living quarters. This requires safety considerations and systems to be included early in the planning phase. Although major accidents are infrequent statistically, the preconditions for their occurrence are constantly present.

An important goal is to prevent accidents escalating, so that personnel beyond the immediate vicinity of the accident site are not harmed. Load-bearing structures must continue to function until evacuation has been completed.

Figure 6-1. Functional areas on integrated platforms. (Source: Acona)

125 Main areas

Safety considerations make it appropriate to divide a topsides into main areas separated by fire/explosion-proof barriers or enough physical distance to prevent escalation. Depending on whether they are defined as main areas, these sectors must have an equipment

placement and layout which contribute to good inherent safety properties and reduce risk associated with potential hazards and accidents. In many cases, such divisions will also be appropriate in construction terms. Main areas will sometimes correspond to modules which can be built in parallel at different yards.

Spaces which are significant for combating accidents must be positioned as safely as possible – in other words, in quarters or utility areas – and safe areas must remain intact until the platform has been evacuated. At least one escape route must be provided from each area where personnel can stay until evacuation of the safe areas and rescue of personnel are completed.

Utilities and transport routes must be designed for efficient and prudent materials handling and personnel traffic. Materials must be handled as far as possible by mechanical systems and technical aids

The choice of platform support structure is significant because its geometry guides and restricts topsides layout. Moreover, stability and motion characteristics must be taken into account on floaters. Some concepts include an integrated oil store and system for offshore loading of crude. These elements have a big impact on safety.

Protecting wells and risers against external loads, such as ship collisions and drifting/

dropped objects, must be assessed. Wells and risers may have to be positioned behind large structural elements or dedicated protective shielding to minimise risk.

Examples of solutions, specific requirements and recommendations

The wellhead area has generally been regarded as the most hazardous. The ideal approach is therefore to use separate wellhead platforms, but the additional cost of such structures cannot be justified in most developments by the risk reduction achieved through a solution of this kind. Multifunctional platforms are therefore the norm on the NCS.

On such installations, the wellhead area is usually placed as far as possible from the living quarters and areas with emergency equipment and functions, and is separated from utilities and process areas to reduce the consequences of a blowout. It should also be positioned so that external support for firefighting is possible.

The less hazardous utilities area is intended to function as a barrier between Ex areas and the living quarters. Ensuring good access to areas and equipment is important for achieving efficient manual firefighting, both from the platform and through external support.

As little HC piping as possible should be led to or through utilities areas, and flanges must be avoided. One flange connection can be placed in each fuel line to internal combustion engines in the utilities area. HC piping cannot be led into the quarters. Nor may liquid piping of any kind be led through electricity, instrument and control rooms. Passage of HC piping into emergency equipment areas must be restricted to supply lines for diesel engines powering emergency equipment in the area. Where passive fire protection is used, it must be designed so that relevant structures and equipment have adequate fire resistance in relation to their load-bearing capacity.

No specific rule determines how the main functional areas should be laid out, since the overall risk picture for each facility will determine the optimal solution. Nevertheless, a couple of standardised solutions have developed on the NCS. See Figure 6-1.

a) This is the most widely used solution. The drilling and well area (or the riser area) lie as far as possible from the quarters area, on the basis of assessments that this is a zone with a high risk of fire/explosion. This layout also offers practical advantages relating, for example, to materials handling and an external contribution to

firefighting. The utilities area provides a buffer zone between the quarters and process areas.

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b) This solution is used in the Sleipner A and Troll A gas platforms. They have large gas processing and export capacity – in other words, possess a high fire/explosion risk associated with gas treatment. A priority for these installations was to ensure the largest possible distance between the quarters area and the area for gas treatment and the gas export risers. The utilities area lies as a buffer zone between the quarters and the drilling and well area.

Comments on Goliat, Aasta Hansteen and Ivar Aasen

As described in other chapters, Goliat, Ivar Aasen and Aasta Hansteen are based on three different concepts:

• Goliat: floating buoy with shallow draught – circular cylindrical construction – Sevan design

• Ivar Aasen: conventional fixed steel jacket

• Aasta Hansteen: buoy with deep draught – circular cylindrical construction with truss structure at bottom – Spar design – truss Spar.

The Ivar Aasen concept is the commonest platform design both on the NCS and internationally.

Goliat’s concept has been used for various applications internationally, but this is the first time it has been applied to the NCS and is also clearly the largest platform of its type.

The Aasta Hansteen concept has been used internationally, largely in the Gulf of Mexico, but this is its first application on the NCS and it ranks moreover as the largest platform of its kind. Unlike earlier Spar platforms, it has integrated condensate storage.

Of these three, only Ivar Aasen has a wellhead area (the two other have only subsea wells).

None of the platforms have permanent drilling facilities, but a chartered drilling rig linked by a gangway to the production platform is used on Ivar Aasen.

Figure 6-2. Area division on Goliat, Ivar Aasen and Aasta Hansteen. (Source: Acona)

Design of the topsides is governed by the type of support structure and considerations of construction, operation and safety. See Figure 6-2. Very different construction methods were used for the three projects:

• Aasta Hansteen: an integrated topsides ready-built as a single unit and mated at shore with the aid of barges

• Ivar Aasen: a topsides assembled from large modules lifted into place offshore

• Goliat: a topsides comprising a large number of small modules and sections lifted into place at the yard.

Nevertheless, the division into main areas with fire/explosion barriers and physical distance to prevent escalation is by and large the same on the three platforms. The review has shown that handling of safety in the early phase by and large utilised the same methodology, regardless of operator and development solutions. This shows that the principles and methodology enshrined in regulatory requirements and standards are well understood and implemented in the industry.

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