Finn Harald Sandberg, Norwegian Petroleum Museum
Combining good and pleasant accommodation with safe and effective access to work areas was a key requirement in designing the living quarters for the Draugen production platform. Bernt Bekke landed this job in 1988.
— The mess at Draugen. Photo: Shadé Barka Martins/Norwegian Petroleum Museum
He had considerable experience of creating such facilities from his previous job with Phillips Petroleum, and remained with the Draugen project until it came on stream in the autumn of 1993.
Most of the offshore platforms built with a concrete gravity base structure (GBS) in the 1980s were designed with a topsides put together from modular units.
The quarters module was then placed as far as possible from areas which would suffer the biggest consequences in the event of an explosion or fire.
That was particularly the case after the Norwegian Petroleum Directorate (NPD) insisted in 1976 that a safer design was needed for crew cabins and living areas on the Statfjord B platform.
The main rule was that these quarters had to be separated from the drilling and process areas by a solid fire- and explosion-proof wall.
To preserve the strength of this divider, solutions had to be found which ensured that it had a minimum of penetrations for conducting pipes, cables or ventilation channels.
In contrast, the Draugen topsides was to an integrated solution where the various functional areas were built into an overall framework of big steel girders. This presented big challenges, but also very special opportunities.
The architect’s role in shaping the working and living environment on an offshore production platform has a wider scope than for a building on land.
Not only is the space available very limited, but both efficient work spaces and comfortable leisure facilities must be taken into account.
Noise is a major problem, for example. And making sure facilities and suitable areas maintain both physical and mental condition is important. But space often blocks ideal solutions.
Safety considerations on the Draugen platform meant that specific requirements were set for the working environment – and these were stricter in many cases than in the UK sector.
Bekke discovered this fairly soon after being hired as the Draugen topsides architect, when visiting Shell Expro’s head office in London.
He had a tour of the various departments, and was presented with models of the living quarters on Shell’s UK platforms which differed markedly from designs he was used to off Norway.
Common areas such as canteen and lounges were often smaller, and four-berth cabins were largely the standard on British installations at the time.
Shell may have wanted to apply a UK standard, but chose not to challenge Norway’s requirements. These also included daylight in as many cabins as possible, although that was not an absolute.
Understanding the various work processes on the platform was important for Bekke to achieve good solutions which provided operators with positive working conditions.
Good access was significant for maintaining and servicing equipment components where this involved overcoming physical challenges in structural terms.
It was also important to reduce the possibilities for personnel developing repetitive stress injuries while engaged in routine work.
Assessing all operations and requirements for access to and expansion opportunities in spaces related to power supply, ventilation and other technical systems was important.
Similar consideration had to be given to activities in administrative areas, workshops and laboratories – and not least in the control room.
All in all, it was important that the architect knew what was going to happen in those parts of the topsides which did not deal with drilling or oil production/processing.
To avoid unnecessary penetrations in the fire/explosion-proof wall, a protected rest area had to be provided outside the living quarters in the utility section of the topsides.
This “dirty coffee shop” was a place where personnel could take a break without having to change out of their work clothes – as they had to do when entering the quarters.
Between each of the main meals, which were served every six hours, workers thereby had time for a cup of coffee and something to nibble in the shop.
With its location and design developed in close collaboration with the operations organisation, this safe space also had to incorporate washing and changing facilities.
Each of the spaces between the big support girders separating the main and upper mezzanine decks under the quarters turned out to offer very special opportunities for leisure activities.
Leaving out a mezzanine deck created space for a recreation room with such a high ceiling that various ball games could be played – unusual offshore, but no less popular for that.
Badminton represented one of the favourite activities on board to begin with, but was eventually replaced by indoor bandy, spinning and volleyball.
The width of the topsides provided room for proper dressing rooms and additional space for weight training. Such a popular off-duty area was not found on all offshore platforms.
In conjunction with the dressing room, a solarium and a sauna were also installed. The latter not least offered a way of dealing with the results of all the good food served on board.
One of Bekke’s jobs was to contribute to an attractive environment through artistic embellishment, and he served as secretary for the art committee.
This had a budget of NOK 2.5-3 million (an unusually large amount at the time), which was spent in two ways:
purchasing works from profiled artists in mid-Norway
works from a dedicated exhibition at the Stavanger Art Association, which the committee could pick out at will.
The walls of the stairwell up to the cabins were clad in a specially developed plastic laminate which told a story based on Norse mythology and old legends. This runs from the lowest floor to the highest. See the separate article about art in Draugen.
At the top of the accommodation block is the helideck, where Bekke had to ensure a safe and efficient solution for quickly checking passengers in and out.
He was also the contact with the civil aviation authorities to ensure that their requirements for safe transport of goods and people were met.
An offshore architect is also responsible for coming up with a design which permits efficient working and takes account of the psychosocial environment both at work and in free time.
That calls for good communication with those who will be spending a lot of time on board, as well as detailed knowledge of the work processes and routines.
Source: Bernt Bekke interviewed by Finn H Sandberg on 25 August 2017.
Published October 9, 2018 • Updated October 11, 2018
Operator Norske Shell’s main alternatives up to a final decision on the plan for development and operation (PDO) of Draugen were the concrete monotower or a floating unit.
Studies found that either a semi-submersible similar to a drilling rig or a tension-leg platform (TLP) would be the cheapest option.
But the final choice was determined by costs associated with operating and maintaining the support structure. A Condeep monotower made it possible to retain the basic topsides configuration without a new round of design and planning work.
The integrated approach yielded a very compact topsides solution with an efficient relationship between weight and capacity.
With a footprint of 78 by 48 metres, including the external gangways, the topsides measure 32 metres in height from the top of the GBS shaft to the upper surface of the helideck.
The derrick adds almost 50 metres more, and the total height from the bottom edge of the GBS skirts to the derrick tip was about 370 metres.
Safety was a key factor from the very start in developing the platform concept, and the threat of explosion in its various areas came to have a big impact on the final solution.
In particular, the choice of an open truss construction and the use of floor gratings rather than deck plating reduced possible overpressures in the event of an explosion.[REMOVE]Fotnote: ockbain, G., Jermstad, A. (1990). Design of the Draugen Topsides for the Effects of Gas Explosions. Paper presentert på OTC 6477. Houston, Texas.
That also made an important contribution to keeping weight down. When the platform came on stream, the topsides had a dry weight of about 18 500 tonnes.
To optimise the topsides design, Kværner Engineering and Shell commissioned special calculations from the Christian Michelsen Institute in Bergen.
These utilised the flame acceleration simulator (Flacs) – a special computer programme developed at the research facility a few years earlier for several large international oil companies.[REMOVE]Fotnote: Gexcon. (2018). Flacs software. Hentet fra https://www.gexcon.com/products-services/FLACS-Software/22/en
Achieving the output planned for the first year depended on being able to drill and produce oil in parallel through the single platform shaft. That was verified in a separate study.[REMOVE]Fotnote: SikteC A/S. (1991. august). Draugen GBS Shaft Safety Study – Management report. Report no. ST-91-CR-018-01.
The drilling equipment was integrated in the topsides, with the derrick movable so that it could be positioned over the relevant well slot. This structure was removed in 1997.
Work on preparing the rig for removal began on 10 April that year, and the last of 26 heavy lifts was completed on 10 May exactly a month later.[REMOVE]Fotnote: Shell UP. (1997). no 5, June: 14.
Injecting water to maintain reservoir pressure was necessary from the start. This liquid has to be entirely free of harmful substances to avoid damaging the oil and gas resources.
Integrated with other seawater systems for cleaning and cooling, Draugen’s water injection system on includes filtration, deoxidisation, pumping, sterilising and chemical treatment.
Seawater enters the shaft at a depth of about 70 metres, where its quality is more than good enough for injection. Harmful seasonal plankton blooms occur much nearer the surface.
As an extra safeguard, a simple filtration system has nevertheless been installed at the platform intake. Chlorine is added to kill all possible organic material in the flow.
Oxygen is removed by passing the water through a vacuum chamber and, as a final precaution, chemicals are added to remove the last residues of undesirable components.
Main process equipment
The main process equipment on the topsides is intended to meet the requirements set for producing, processing and exporting oil and gas. See the separate article for details of the process.
This is a general term for all the systems required to operate the platform which are not directly involved in oil and gas production.
That includes direct process support as well as equipment for power generation and for living and working on a platform, such as safety, process control, heating and ventilation, and communication equipment.
Safety and security
Systems in this category are required for notifying and executing actions to prevent or reduce major or minor damage. They also include systems for emergency evacuation or for retrieving people who have fallen into the sea.
These are intended to warn of incidents which require a coordinated commitment by all personnel to saving life and maintaining platform safety.
Alarms are given over the public address system, either as a signal or as a verbal announcement. Other alarms were located in areas which used to be protected by halon. A combination of sound and flashing lights, they gave personnel 10 seconds to leave.
This is intended to permit speedy evacuation of personnel from the platform in an emergency, or to retrieve people who have fallen overboard.
covered free-fall lifeboats
covered rafts which inflate on contact with the sea
man-overboard boats (MOBs)
chutes for evacuation to the sea
personal survival suits.
Fire and gas detection
All areas of the platform are fitted with fire and gas detectors. Should fires or leaks be registered, the following actions are initiated automatically:
fire pumps start
sprinkler/deluge systems are initiated (halon has been phased out)
fire dampers in the ventilation system are closed
emergency shutdown (ESD) of the platform is initiated.
Fire extinguishing system
This protects personnel, structures and equipment throughout the platform (including the shafts). Two types of system are installed:
wet, using water or foam
dry, using powder (and halon earlier).
The wet system is supplied by the fire water pumps installed on the service deck. Fixed foam extinguishers are positioned in areas with a high risk of oil fires.
Halon was originally used in technical spaces which contain much electrical and electronic equipment, but has been phased out. A large powder system has been installed in connection with the helideck.
In addition, fire extinguishers – both carbon dioxide and powder – have been positioned for quick response throughout the platform.
The platform needs a lot of water for various purposes. Its sea and service water system is designed to supply all the liquid required for drilling, injection and ventilation systems, and to produce fresh water.
Separate systems are installed for fire and ballast water, while supplies for flushing are used to help wash sand out of the vessels used in the separation process.
Fresh water is produced from seawater with a maximum chlorine content of two parts per million (ppm). This is distilled in three evaporators.
Output from that process is cooled before being pumped via two units which regulate its acidity (pH) into storage tanks, which can also receive desalinated or potable (drinking) water pumped from supply ships.
Most of the fresh water is used for drinking, with some also consumed by cleaning and cooling. Potable water is supplied to the living quarters and selected areas elsewhere on the platform.
Desalinated water is pumped from one of the storage tanks via ultraviolet sterilisation units to consumer tanks located in the roof spaces of the living quarters.
Also called raw fresh water, desalinated service water is fresh water of secondary quality stored in a tank on the cellar deck and distributed by pumps for cleaning, drilling and refilling coolant water.
Used in the coolers for gas and recovered oil, coolant water is a mix of three parts fresh water from the distribution system for desalinated water and one part monoethylene glycol from the glycol system, giving a freezing point of -12°C. A small amount of corrosion inhibitor is added.
Hot water is produced to provide a reliable heat source with a constant temperature for the following applications:
desalination of seawater in the evaporators
heating and ventilation systems (except in the living quarters, which have electrical heaters)
supplies of coolant water to the circulation pump for hot medium.
Steam is produced in a generator with a pressure of eight bar for cleaning process vessels and for various other types of cleaning. The steam generator is a heat exchanger.
Heating, ventilation and air conditioning
These functions are split into two separate systems, covering the production area and the living quarters respectively.
The system for the production area is designed to deliver air at a specified temperature and pressure to the platform’s areas. This is intended in turn to reduce risk and accidents in spaces where fire and explosion are hazards (see compressed air below).
Provision of such air is crucial for safe operation of the platform. Should the system fail for any reason, the process plant must be shut down immediately.
Heating and ventilation of the living quarters involve a completely separate system, which functions in the same way as an installation in a normal building on land.
The heating medium system
serves as a heat source for:
the circulating hot water system for desalination of seawater and space heating, with the exception of the electrically heated living quarters
superheating of sludge
The heat source is a refined paraffin circulated to the user sites, where it is heated in furnaces over an open flame and in three recovery units for waste heat.
Air conditioning in the living quarters serves cabins, recreation areas and the galley. Located in the ventilation room on the service floor, it sucks in fresh air and delivers it at a predetermined pressure, temperature and humidity to the whole living quarters.
A helideck heating system keeps the deck free of ice, maintains the temperature of the fuel gas and the process gas piping to prevent formation of condensate and hydrate (hydrocarbon ice) respectively, and prevents the fire and injection water systems from freezing.
Heating cables are located in channels under the helideck, with electrical heating strips installed externally on piping. These activate automatically if the ambient temperature drops below 5°C.
In a process facility where explosive gases could build up, electrical instruments and spaces containing such equipment must be kept at a pressure above the surrounding plant.
This is intended to prevent gas from entering and being ignited by electrical sparks. A dedicated system provides a reliable source of clean compressed air for instrument and working atmospheres.
This system collects all sewage and waste water for treatment and subsequent discharge to the sea. Most of the sewage comes from toilets, showers, washbasins, kitchen sinks and washing machines in the living quarters.
It is conducted by gravity and negative pressure to septic tanks. A filter removes solid particles, which are then sent to mills for grinding to a liquid sludge.
All bacteria in the sewage – particularly coliforms – are killed by chlorine injection before treated waste is discharged to the sea 10 metres below its surface. If necessary, raw sewage can be discharged to a barge through a hose connection for disposal on land.
Two types of fuel are needed on the platforms – helicopter (aviation) fuel and diesel oil for power generation and other specialised machinery.
Supplies are brought in by ships equipped with special tanks for helicopter fuel. These can also pump diesel oil directly via hoses to special storage tanks in one of GBS cells.
This system distributes various types of lube oil to the main systems through a permanent piping network.
From the filling (tote) tanks, they are piped via lube oil distribution tanks to the most important consumers – gas turbines, generators, water injection pumps and fire pumps.
Other types of oils/lube oils are also required on board, but the level of consumption does not warrant a fixed distribution system for them.
A great many different chemicals and chemical compounds are used on the Draugen platform for such purposes as separating oil and water.
Other applications include inhibiting or breaking down oil droplets in the produced water (emulsions), which is separated from the crude oil flow.
Chemicals also prevent or stabilise foaming, or inhibit hydrate (hydrocarbon ice) formation, bacterial growth or corrosion.
These substances are shipped out to the platforms on supply vessels. Among the commonest are the following.
Methanol is used to prevent the formation of hydrate plugs in pipelines, which can halt liquid flow. When gas contains small quantities of water, ice-like clumps can form under special pressure and temperature conditions.
Glycol primarily serves an agent for removing water from rich gas because it acts as an efficient absorber of water. It is also used in coolant systems to reduce the freezing point to -12°C.
Chlorine can be added to seawater to prevent the growth of bacteria in pipelines and ballast water, seawater and fire water systems. Sodium hydrochlorite (NaOCI) or bleach is used to kill unwanted organisms.
Corrosion inhibitor is added to prevent internal corrosion in piping and tanks. The substances used are usually based on organic compounds which form a protective film on metals.
Bactericides are deployed to control the growth of bacteria in water and hydrocarbons. The most serious problem for oil and gas production is provided by sulphate-reducing bacteria which develop hydrogen sulphide (H2S). This substance is not only toxic but also both explosive and extremely corrosive.
Anti-foaming agents are used to prevent foaming in the main process, and are injected ahead of the separator tanks in order to ensure that separation of water, oil and gas is as efficient as possible.
Transfer and metering systems
Crude oil is transferred from the storage cells to loading buoy via a discharging system which includes powerful pumps installed on top of the shaft.
The transfer then passes via smaller export pumps and the fiscal metering system, which measures the quantity being exported before entering a dedicated flowline system.
Accurate fiscal metering is important, since the licensees must feel confident that the quantities registered are correct and the tax authorities also have to be convinced. A special system therefore conducts regular checks.
Electricity required to operate the platform can be generated by three gas turbines which each have the capacity to meet 50 per cent of maximum power requirements on board.
This means that, if one turbine is temporarily out of operation because of repairs and maintenance, sufficient capacity remains to keep the platform running.
Mains electricity is supplied by three 19-megawatt generators as a 13.8 kilovolt, three-phase 60 Hertz current. The generators are driven by gas/diesel turbines.
Emergency power is supplied by three 1.18 MW generators which start up automatically and connect to a 6 kV panel. If both main and emergency power systems fail, supplies of alternating and direct current will be maintained by batteries.
Electricity for the living quarters comprises the normal supply of alternating current, and emergency supplies of both alternating and direct current.
The normal supply is used for air conditioning, galley equipment, heating, hot water, laundry, lifts, lighting, refrigerators, waste units and ventilation.
Emergency supplies are used to maintain necessary lighting and electronic equipment.
Monitoring and control
The monitoring and control systems on Draugen are located in a central control room (CCR), and ensure an efficient, safe and reliable automated process. Placing the whole monitoring system in the CCR reduces personnel exposure to the production area.
Process control monitors and controls all systems on the platform to ensure that hydrocarbons can be produced as safely as possible. The main functions involve maintaining a check on:
data transmission between the production system and the control-room terminals
analogue operating commands
automatic switches and logical sequence control commands
All this information is monitored from the CCR. Printers and displays for alarms and trends are also concentrated there to provide the operators with a good and accurate picture of conditions at all times.
Safety monitoring is a system intended to handle “all” aspects of safety on the platform. Field instrumentation and sensors for fire and gas alarms monitor the whole process and every area.
The purpose of the system is to initiate production ESD when the process monitoring system fails to handle the problems which might occur.
In principle, it comprises two sub-systems.
Process shutdown monitors the process and shuts it down if control is lost, and thereby prevents the plant being operated in a hazardous manner – under pressures higher than the tanks are designed to handle, for example.
ESD is initiated if hazardous conditions arise – such as a gas leak or a fire. This system receives signals from fire and gas detectors as well as from manual alarms.
The metering system for production and consumption meters the quantity of gas and oil exported from the platform as well as the amount of consumption and fuel gas used internally.
This system attracts great attention from all levels of the organisation, since its measurements form the basis for the revenues generated and the tax to be paid on output.
Systems for telecommunications and pollution control have been constantly replaced and improved in line with technological advances and changing regulatory requirements.
The loading/discharging system is designed to handle supplies brought in or taken away by sea.
Cranes on the platform are used for:
lifting from or discharging to supply ships
maintenance and construction lifting over the whole platform and in the equipment shaft
handling pipes and equipment.
Equipment for bulk handling is used to transport, handle and store various liquids, powders, gases and chemicals required for the platform’s process system and utilities. These products are brought out by supply ships and transferred to the platform either in tanks or via hoses.
Tanks and other bulk containers are lifted by crane from the supply ship to the platform’s storage area on the open deck. Liquids used in large volumes are transferred via permanently installed piping to the fixed storage tanks. Empty tanks are discharged for return to land.
Diesel oil, fresh water, barytes, gel and cement are transferred to the platform’s bulk storage tanks with the aid of hoses lowered to the supply ship.
This part of the platform provides accommodation, recreation and catering for all operational phases. About 60 people will be on board during normal operation, but full service can be provided for 150.
To give the best protection against possible incidents in the well area, the quarters are insulated from the rest of the topsides by a high-performance fire and explosion wall.
This covers the full width and height of the quarters, while a similar wall separates the production section from the area where the wellstream reaches the topsides.
Read more in a separate article on the role of the architect.
Construction of the Draugen platform was in full swing at this time. The immediate question was whether the same fate could befall its GBS.
Was there a weakness in the Condeep design which had so far gone undetected or failed to show up, or was this unknown fault confined to the Sleipner A structure?
The sinking was heralded at 05.49 with a loud report from inside the GBS, followed by two smaller bangs, and a substantial leak was immediately registered.
Continuous measurement by the control room established that almost 1 000 tonnes of water per minute were flowing into the drilling shaft.
The ballast pumps were immediately started, but the was much greater than they were designed to handle. About 18 minutes later, the GBS went to the bottom. That was long enough for the 22 people on board to escape without injury.
On its way down, the GBS imploded and all air in the storage cells was expelled. That created a tsunami effect so powerful that two tugs lying close to the structure lost sight of each other.
Sleipner A hit the seabed with great force in just under 200 metres of water.[REMOVE]Fotnote: Intervju med Einar Wolff, NCs prosjektdirektør for Draugen. Tremors from this impact were measured by several seismological stations (figure 2).
An investigation of the remains revealed that the GBS was completely destroyed, with remains scattered over a wide area in 170-220 metres of water.
Appointed almost immediately after the incident, a commission of inquiry led by operator Statoil sought to identify the cause of the sinking.
Builder Norwegian Contractors (NC) also established an internal investigation, which reached the same main conclusion as the other group.
This was that the concrete wall fractured because the connection points between the cells – known as the tricell (figure 3) – had been insufficiently dimensioned.
That in turn reflected errors which had arisen in the finite element analysis of the structural strength of the GBS.
NC commissioned a full-scale model trial which checked eight test sections. This work confirmed the causes identified by the engineers.[REMOVE]Fotnote: Steen, &., & Norwegian Contractors. (1993). På dypt vann : Norwegian Contractors 1973-1993. Oslo: [Norwegian Contractors].
The Sleipner GBS sinking also touched off a small earthquake at Shell, but actually had few consequences financially or in terms of progress.
As early as the day of the incident, NC representatives visited Shell’s offices to explain that it was probably caused by under-dimensioning of tricells.
That in turn reflected inaccurate use of computer programmes at consultant Olav Olsen. Worse – the same error had been made on the Draugen GBS.
The news came as a powerful blow to Sigbjørn Egeland, Shell’s lead manager for construction of the Draugen GBS. “Until then, I’d had boundless confidence in NC,” he says.[REMOVE]Fotnote: Intervju med Sigbjørn Egeland, Shells hovedansvarlig for betongunderstellet til Draugen.
Shell initiated independent analyses, both in-house and externally, while work on the GBS was put on hold until the results were available.
After about three months, Shell got the same answers NC had obtained through its model trials and the construction process could be resumed.
As a result, the impact of the sinking was limited. The really significantly financial consequences arose from the ringing problem (see separate article).
“The principal effect of the Sleipner incident was that I lost a lot of my confidence in the supplier,” recalls Egeland. “From then on, it became necessary to test all the important conclusions with independent experts.”
Where seabed soil strength was also weaker – on soft clays, for example – the foundation challenge became fairly considerable.[REMOVE]Fotnote: Alm, T., Bye. A,, Sandvik, K. & Egeland, S. (1995). The Draugen Platform and Subsea Structures, Installation and Foundation Aspects. Paper presentert på OTC 7670-MS OTC Conference.
Olav Olsen, Norway’s leading expert on shell structures, regarded the skirt pile as a tool for overcoming this problem.[REMOVE]Fotnote: Steen, &. (2002). Den frie tanke : Om kreativ frihet og en ledende norsk ingeniør. Lillestrøm: Byggenæringens forl. It surrounded and enclosed the weakest top layer of clay and transferred the load to deeper and more stable levels.
Clay also has a tensile strength which can help to counter the burden of short-term loads such as breaking waves.
A steel skirt had been incorporated beneath the storage cells on a number of earlier Condeep concrete platforms to ensure stability. The steel skirts are suitable for sandy soils.
Gullfaks C was the first of these installations to feature an integrated concrete skirt, which thereby formed an extension of the cylindrical cell above and naturally had to cope with the loads placed on it.
But questions were posed from certain quarters about the suitability and strength of these skirts, including a British consultant who warned Gullfaks C operator Statoil against them.
Olav Olsen’s company invited this critic to a meeting where a Statoil representative was also present along with Olsen himself and his son Tor Ole Olsen.
The British visitor had prepared well, bringing with him a suitcase full of modelling clay and the cut-off bottoms of plastic bottles (which had appropriately held various oil products).
By pressing a bottle base into the clay, he was able to demonstrate that the skirts deformed severely. The side of the bottle bent as the pressure increased.
With a doctorate on shell structures and the insight and experience conferred by many years of development work, Olsen immediately saw that the plastic bottles lacked an important component – stiff wall-bottom connection.
He demonstrated this by taking one of the cut-off bottles, filling clay into the bottle bottom to stiffen the wall and then pressing it down on more clay. The skirts (bottle sides) now retained their shape.
Olsen had thereby used the critic’s own toolkit to demonstrate the applicability of the skirts, and the British consultant’s warnings were silenced.
The Gullfaks C, Draugen and Troll A platforms all incorporate long skirt piles, and all have operated on the Norwegian continental shelf for more than 20 years.
The skirts on Draugen are 16 metres high and penetrate nine metres into the seabed. Their construction in the dry dock at Hinna in Stavanger is clearly illustrated in figure 2.
Uncertainty over whether this would also occur on the Draugen platform prompted hectic activity at both operator Shell and Norwegian Contractors, builder of the concrete gravity base structure (GBS).
Put simply, the ringing effect described oscillations which arose in the tethers mooring Heidrun’s floating concrete support unit to the seabed when it was subject to large and uneven wave forces.
In the Draugen case, the concern was that a similar phenomenon might cause deflections in the single topside support shaft which increased loads on the concrete and reinforcement.
Slipforming of the Draugen monotower was actually halted while model trials were conducted to assess how important the ringing phenomenon might be.
A rather more comprehensive and technical explanation is that this represents a non-linear effect which arises when steep waves strike the vertical surfaces on a structure.
This is a short-lived reaction which particularly affects the splash zone – the area alternately above and below water as waves rise and fall.
The effect gets its name because the oscillations created are similar to those experienced when a church bell is set in motion.[REMOVE]Fotnote: Intervju med Eivind Wolff, NCs prosjektdirektør for Draugen-prosjektet. Av Finn Harald Sandberg, Norsk Oljemuseum.
Since the hydrodynamic mechanism which causes ringing is still not fully understood, model trials continue to be recommended to determine the relevant load.
Such tests were initiated for the Draugen platform at a specialist lab in Denmark as a result of the Heidrun experience. Calculations also sought to simulate what might happen with the Draugen structure under the relevant conditions.
The platform’s slim support shaft is topped by a transition piece which converts a circular cross-section into a square shape. That places four big vertical surfaces above the sea surface and immediately beneath the topsides.
Model testing showed that when a particularly high (100-year) wave hit such a surface, the shaft experienced an extra loading which transmitted extreme forces – up to 70 per cent greater than those previously calculated – down its structure.[REMOVE]Fotnote: Intervju med Eivind Wolff, NCs prosjektdirektør for Draugen-prosjektet. Av Finn Harald Sandberg, Norsk Oljemuseum.
The immediate problem on Draugen was overcome by extending the circular section of the shaft a further four metres, so that the big surfaces were less exposed to high seas. That also helped to resolve the problem of waves hitting the base of the topsides.
It transpired that the predicted impact was smaller than had been feared after the initial model tests on Heidrun. Two reasons accounted for this.
Ringing reduces with larger dimensions. A model will accordingly show a much bigger effect than a full-scale structure. This primarily reflects the impossibility of simulating the real speed of shock waves in model trials.
The effect on a cylindrical or conical tower will also be smaller because the waves strike a smaller surface (decrease with reduced diameters) than with the rectangular areas found in parts of the Heidrun support structure. In addition, Heidrun’s four tethers produce bigger oscillations than the single shaft used on Draugen.[REMOVE]Fotnote: Intervju med Torvald Sande. Abv Finn Harald Sandberg, Norsk Oljemuseum.
Slipforming the concrete monotower for the Draugen GBS had been halted for three months in anticipation of these results, but this structure was not on the critical path anyway.
A bigger problem was presented by the cost of the required changes. Not only were more steel and concrete needed, but the internal outfitting of the shaft was affected by the added height.
Ringing is described as a transient response – in other words, it increases very rapidly and then declines rather more slowly depending on how the oscillations damp down. The oscillations are caused by a powerful load impulse, which acts like slamming (the same effect as a hard punch). But this is not the same as the effect caused in air by shock waves (such as aircraft breaking the sound barrier). It is manifested primarily by extreme values – like 100-year waves – although these also contribute to fatigue. Ringing is not significant for fatigue since powerful episode are infrequent. Næss, A. & Moan, T. (2013) Stochastic Dynamics of Marine Structures.
Kumar, A. & Kim, C. H. (2002). Ringing of Heidrun TLP in High and Steep Random Wave. International Journal of Offshore and Polar Engineering vol 12, no 3.
Fames, K. (1996). Computer Efficient Ringing Analysis of the Heidrun TLP. Paper presentert på Sixth International Offshore and Polar Engineering Conference, 26-31 May. Los Angeles, California, USA.
Four options remained for further consideration in May 1986. These were:
a fixed concrete platform with integrated topsides
a semi-submersible platform with attached storage ship
a weather-adapted monohull production floater with possible oil storage and offloading (FPSO)
two converted drilling rigs.
This quartet was then subjected to a year of further detailing and comparison with the implementation plan, the economics of the project, general operations optimisation and key uncertainties with the proposed technology. All the options had to satisfy the same operational assumptions in terms of production capacity, wells and transport capability.
While Shell considered various solutions – floater, steel jacket (support structure), gravity base structure (GBS) and so forth – Norwegian Contractors (NC) promoted its concrete option.
After all, this construction company could point to great success during the 1970s and 1980s with its Condeep design, not least on the Statfjord and Gullfaks fields. NC construction manager Dag N Jensen recalls the process:
Then Shell called me – I think it was Serge Leijten – to say that they had now decided that a semi was the best option. We, in other words NC, could not give up that easily, so I told Shell there and then that we had a better solution and agreed to meet the company in Stavanger at 08.00 the following day. I talked with Tor Ole Olsen at the Dr Techn Olav Olsen consultancy, and we agreed to create a monotower platform. Olav Olsen calculated and calculated, Tor Ole put the drawings in my letterbox during the night and I took the first flight to Stavanger the next day to present the drawings to Shell with some verbal cost and planning estimates. This aroused such great interest that the company revised its choice of concept, asked for further documentation, and ended up going for a GBS.[REMOVE]Fotnote: E-mail fra Dag N.jensen 26.04.2016
In other words, this solution represented an improved but not final version of the concept presented in the plan for development and operation (PDO) of Draugen. While a monohull FPSO basically represented the cheapest option, a fixed structure was recommended because it best satisfied Shell’s stated reliability and storage requirements. An economic analysis also showed that a fixed installation yielded the best internal rate of return on the huge investment involved.[REMOVE]Fotnote: Draugen Field Plan for Development and Opreation Appendix VI Concept Selection
The cheapest of the fixed concrete GBS solutions, ranging from one to four support shafts, was the monotower. This also provided sufficient support for the topside design and room for 10 wells to be conducted to the process facilities.
At the same time, it became important to demonstrate that drilling wells and producing oil simultaneously through a single shaft fell within the required safety margins.
Calculations indicated that the risk of accidents with a monotower solution did not differ significantly from alternative solutions with several shafts.
In addition, these assessments showed that the level of risk for simultaneous drilling and production was the same for the various designs.[REMOVE]Fotnote: Draugen GBS Shaft Safty Study (report No. ST-91-CR-018-01 SikteC A/S august 1991
The concrete structure was required to support a topside weight of 22 000 tonnes and to store a million barrels of crude oil.
Submitted in September 1987, the PDO for Draugen was approved by the Storting (parliament) on 19 December the following year.
Shell wanted to try to avoid NC – the sole supplier of concrete GBSs to date – being its only option. An attempt was therefore made to create competition.
Peconor, a group comprising several companies including Sweden’s Skanska, was accordingly invited to submit a bid for building the structure.
The original version was presented, but the consortium was challenged to come up with an optimised design (as NC had already done). It transpired that the final bid from NC was far below the Peconor offer, allowing the former to retain its monopoly of building large concrete platform support structures.
The concept of a monotower GBS was not entirely new. It had been proposed as early as 1975 for Norway’s Heimdal field in the North Sea, which was eventually developed with a jacket solution.
Once the PDO had been submitted, work began on optimising the GBS. This found that cylindrical components could be built with larger diameters. The earlier view had been that such shell structures could not have a diameter above about 30 metres. Any increase meant it would be unable to float on the shaft alone while building lower storage cells with sufficient volume.
In addition, the length of the skirt piles (the part of the GBS which would penetrate into the seabed) was increased in order to improve the structure’s resistance to wave motion (see separate article).
So the big monotower provided the buoyancy needed by the platform at tow-out to the field while consuming considerably less concrete in terms of both volume and weight.
Another advantage was that the water depth along the tow-out route was very favourable in relation to the GBS height, allowing a relatively low freeboard during the operation. That ensured good stability and safety.[REMOVE]Fotnote: Steen, &. (2002). Den frie tanke : Om kreativ frihet og en ledende norsk ingeniør. Lillestrøm: Byggenæringens forl.: 110-112.
This followed an initiative by Professor Rolf Lenschow at the department for construction technology of the Norwegian Institute of Technology (now the Norwegian University of Science and Technology). He proposed to the Norwegian Concrete Association at the beginning of January 1994 that Olsen should be nominated for this honour.
The association submitted an official application on 13 January to Belgium’s Association of Ghent University (AIG) Foundation, the secretariat for the award jury.
Established in 1959, the Gustave Magnel medal goes to the designer of a structure in reinforced or prestressed concrete which is regarded as both important and innovative. It was created to honour Magnel, who had developed a new method for prestressing concrete during the Second World War, and is awarded every five years to a design and its originator.[REMOVE]Fotnote: Steen, &. (2002). Den frie tanke : Om kreativ frihet og en ledende norsk ingeniør. Lillestrøm: Byggenæringens forl.
Gustave P R Magnel was born in 1889 and graduated from Ghent University in 1912. He emigrated to the UK during the First World War, but returned to the university in 1919 and established his own laboratory for concrete research in 1926.
Appointed a professor in 1937, he authored a number of books and more than 200 professional papers as well as teaching until his death in 1955.
He also practised as a engineer/consultant. Among other jobs, he had principal responsibility for the design and construction of America’s first prestressed concrete bridge – the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania.
It became known in early September 1994 that the gold medal was to be awarded to Dr Olsen for his contribution to the Draugen platform.
The presentation ceremony on 25 November noted the recipients long and significant commitment to the use of reinforced concrete in shell structures.
Particular emphasis was given to his involvement in the creation of the Condeep GBS concept, and particularly his development of skirt piling.[REMOVE]Fotnote: Omtalen ved tildelingen 25.11.1994
Although Troll A is the tallest GBS, many people regard the Draugen platform as an equally impressive technological masterpiece.
It was the first fixed offshore production facility beyond the northern boundary of the North Sea, and is perhaps one of the boldest but also most elegant of the Condeep solutions.
With its slim conical monotower support column, it ranks in addition as a very efficient and attractive structure.
The contract for the platform was signed with Norske Shell in the autumn of 1989, when Dr Olsen was 76 years old and still worked every day in his own consultancy (which continues to bear his name in 2018).[REMOVE]Fotnote: Steen, &. (2002). Den frie tanke : Om kreativ frihet og en ledende norsk ingeniør. Lillestrøm: Byggenæringens forl.
Among other winners of the Gustave Magnel medal, particular mention should perhaps be made of Germany’s Fritz Leonard, who received the honour in 1968.
He is also the designer of the Helgeland Bridge, completed in 1991, which was acclaimed by engineering weekly Teknisk Ukeblad as Norway’s most beautiful bridge in 2010.[REMOVE]Fotnote: Øderud, H.T. (2013). Helgelandsbrua. Store norske leksikon. Hentet fra https://snl.no/Helgelandsbrua
The most recent winner is William Baker, who received the medal in 2014 for his unique design and development of special structural elements which permitted the construction of the Burj Khalifa, the 828-metre Dubai skyscraper ranked as the world’s tallest building.[REMOVE]Fotnote: Burj Khalifa. Facts & Figures. Hentet fra http://burjkhalifa.ae/en/the-t.
Finn Harald Sandberg, Norwegian Petroleum Museum
A new series of banknotes was issued by African oil producer Angola in 1995. Why and how Draugen featured on one was a big puzzle for operations head Gunnar Ervik when he was interviewed by Trondheim daily Adresseavisen 10 years later.
The banknote was for 50 kwanzas (AOA), corresponding to about 2.25 Norwegian kroner at the official exchange rate prevailing in September 2016. Its real value was about a third of this.
“Norwegian oil interests are strongly represented in Angola,” the newspaper noted. “Norway also has diplomatic relations with a country which has rich oil and diamond resources but was characterised until recently by internal strife.
“The Norwegian flag generally follows in the oil industry’s wake. Statoil and Hydro have established a Norwegian oil colony in Angola with investment in the Girassol field, and Norwegian government ministers concerned with oil pay it visits.”
A representative of the Money Museum in Angola has provided the following explanation: “Angola has been involved in oil production since 1973 and this is currently the country’s most significant source of revenues. Oil earnings represented about 90 per cent of Angolan export revenues.”
Perhaps an explanation, but …
Two other banknotes have also been issued with oil industry motifs. One worth AOA 10 000 (NOK 450) features a drawing which resembles the platform on Angola’s Girassol field, where Statoil has an equity stake.
The other, worth AOA 500 (NOK 23), bears drawings of a semi-submersible oil rig and a drill floor scene.