Four Generations of Fire- and Blast-Resistant Walls on Offshore Installations
This paper presents an overview of the development of fire- and blast-resistant walls for offshore installations and introduces a fourth generation, or type, of wall. This thoroughly tested and certified type was developed as a response to the continual quest to further optimize the cost and weight of offshore topsides, thus allowing operators and designers to accommodate more equipment and higher field yield.
The design of offshore installations is based on a variety of factors. This paper aims to explain dominant design preferences in the industry and review the key criteria that should be considered in order to optimize fire- and blast-resistant wall designs, with the hope that this may facilitate further wall optimization based on specific performance requirements. The intention of the paper is to provide the professionals who consider fire- and blast-wall designs for offshore installations with an objective viewpoint, allowing them to improve design for additional weight and cost savings.
It was not long in the evolution of offshore drilling that companies began to choose steel over wooden drilling structures, recognizing the metal’s greater structural capabilities for rigs and its lower costs over the life of the well. At that time, rigs were designed by shipbuilders and were therefore monocoque structures. Eventually, naval architects adopted the container design. This corrugated design had a positive effect on the weight of offshore platforms.
Both types of walls are currently still in use. However, in the 1970s, companies began to experiment with lighter-weight cladding constructions to reduce weight and lower production and installation costs. Fire-resistant cladding panels have been in use onshore since the 1980s. These panels comprise two light-gauge metal panels with insulation in the middle; they were assembled on-site. Recent technology, however, enables fully prefabricated and certified sandwich panels to be delivered to the site and installed in one operation.
A nonscientific research study conducted in 2010 indicated that 85% of lead design engineers originally graduated as naval architects. There is, however, a growing number of lead design engineers who originally graduated as mechanical or structural engineers.
Shortly after the Piper Alpha disaster in 1988, offshore safety regulations were tightened; those relating to fire-resistance tests became more stringent. Different types of construction elements had to be tested separately. The strength of steel diminishes as temperature increases. At temperatures above 800°C, steel loses approximately 90% of its strength. The experiences of Piper Alpha and Deepwater Horizon demonstrated that steel indeed buckles when subjected to large hydrocarbon fires, and even more so when structures are load-bearing. If walls are used to support the structure itself, or heavy equipment, these might buckle within the time defined by their fire rating.
There are essentially two ways to prevent premature buckling. The first is to make sure that in the event of a fire, the imposed loads are immediately released by the use of fixings that rapidly fail at the beginning of a fire (e.g., at around 200˚C). An alternative is to ensure that the temperature of the load-bearing steel will be well below the limit defined by the required load-bearing capacity. This implies that the steel needs to be isolated from the expected fire side.
There is, however, a third option: the use of nonload-bearing walls. This ensures that the response of the walls during laboratory tests is as close as possible to that exhibited during actual fires. Loads from equipment items hung on the walls can be transferred back to the wall-support structure and to the decks, thus ensuring that the wall itself will not buckle in the event of a fire and will keep the “cold side” safe.
The first type of offshore wall, the stressed-skin type, originated in the shipbuilding industry. The second type, the corrugated-skin type, originated in the transport industry. The third type, the bolted built-up type, originated in the building industry. Finally, the fourth type, the sandwich-panel type, consists of prefabricated modules and can be viewed as a progression of technologies.
Although wood can still be found offshore on some helidecks, it has not been used on newly built structures for many decades for obvious reasons: fire resistance, maintainability, and lifespan. However, the other four wall types are still in use. Even today, many offshore structures are built as stressed skin, and large modules especially tend to be designed as monocoque structures using load-bearing walls, executed either as stressed or corrugated skins. Very large modules are too heavy to lift, transport, or install if built as monocoque structures. Floating platforms, however, do not have such lifting or transport issues. Figs. 1 through 4 show the types of walls used on offshore installations.
Type IV walls do show a resemblance to internal walls for living quarters, where B-rated panels can span up to a maximum of approximately 3200 mm and are usually used to create rooms and hallways within monocoque structures. Such B-rated walls, however, do not meet the required criteria to be considered as an alternative for A- and H-rated wall systems, which must be able to span over 6000 mm.
Design Philosophy for Offshore Structures. There are three different dominant types of offshore structures: fixed structures, subsea structures, and floaters. Before the start of the design, a number of criteria should be taken into consideration, including the location and function of the platform, field-development options, and transport and installation of the structure. During the feasibility study phase of any offshore project, design decisions are made regarding the type of offshore structure. In the early front-end-engineering-design (FEED) phase, the project group has to decide on the type of structure (i.e., a monocoque shipbuilding structure or a framed, industrial-type structure). A direct consequence of such a decision is that it limits options with regard to the type of fire walls to be used, because Types III and IV are nonload-bearing walls requiring a framed steel support structure.
Hybrid structures do exist, and it is possible for Type II walls to be bolted and act as nonload-bearing walls. Type IV walls can also be used on monocoque structures in which some of the walls are nonload-bearing. In general, the weight of nonload-bearing walls can be decreased by using Types III or IV walls. For the purposes of this paper, however, hybrid structures are not considered.
Monocoque or Framed Construction?
The four types of wall can be allocated to the two dominant philosophies of structural engineering: the use of stressed skin or corrugated skin (Types I and II) provide the walls with load-bearing capacity, thus resulting in a fully welded monocoque structure, whereas Types III and IV are nonload-bearing walls and thus require a load-bearing support structure.
Changing a design from a monocoque to a framed construction is a radical change that will only be carried out when there is a fundamental need. An important question to ask is why new offshore platforms tend to have a framed construction. Over the past decades, the optimization of field yield has led to an increasing demand for additional, heavier equipment. When adding equipment to an existing platform, weight is always a key parameter. This is the case for high-yield platforms and even more so for the recently introduced offshore wind high-voltage direct-current (HVDC) transformer platforms. These transformers require ample space; early FEED studies of a 100×40×26-m HVDC platform demonstrated that the originally adopted monocoque design was too heavy to be installed in the shallow waters of the German Bight. The quickest solution to reduce weight was to change the design from monocoque to framed construction and to use Type IV sandwich panels as a fire-rated external envelope and also for internal partitioning. Sandwich panels are typically produced on a continuous production line, and are tested and certified according to strict guidelines.
The complete paper provides a detailed overview of the relevant criteria for the design of offshore installations in order to assist design engineers in addressing all functional requirements during preliminary design studies, detailing the advantages and disadvantages of each type of wall design. Tradeoffs are addressed; these can be optimized. The decision model underpinned by such criteria can be a useful tool for design engineers; this, also, is provided in the complete paper. Hard criteria are mandatory, whereas soft criteria provide scalable value to the project. Some of the criteria are quantified in offshore standards and well-known calculation models.
Hard Criteria. Resistance (blast, fire, fire post-blast, wind, impact); structural integrity/free span; load-bearing capacity; acoustic insulation; and tightness (weather and air)
Soft Criteria. Weight savings; production and installation cost savings; applicability to arctic environments; thermal insulation; design life; corrosion resistance; free span; load-bearing capacity during fire; transport and installation absorption capabilities; allowance for penetration; maintenance and sustainability.
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01 June 2018