Removal of Offshore Installations- Analysis of Necessary Development to Solve a Specific Task

Anders Hägglin
Dept. of Underwater Technology, Chalmers University of Technology
Intervention 87, San Diego

ABSTRACT
The need for thorough and extensive system analysis of advanced systems and complex operations, has grown steadily during the past decades. In areas such as space technology and aviation, the system analysis is a well integrated part of all development programs. This, has not been the case in the offshore industry, although the use of different analytic techniques are growing more frequent today.

This paper uses decommissioning of offshore installations as a case for system analysis of a complex operation, where there exists no or little previous experience. The paper concentrates on the underwater activities of the operation and the possible worksystems.

INTRODUCTION
Decommissioning and removal of offshore installations are currently becomming popular subjects for discussion among authorities and different experts tied to the offshore industry. The oil companies, who should be most interested, has this far stayed in the background.

Removal operations are complex from both legislative and technical point of view. Also, the views from those concerned are far from unanimous. One side claims that, this is a huge new market that is measured in billions of USD and that will demand an extensive technical development for its completion. The other side considers offshore platforms to be pieces of scrap stuck on the seabed, causing an obstruction.

This new field of business is not even new. Platforms have been removed for a number of years, however only from shallow waters.

LEGAL REQUIREMENTS FOR REMOVAL OF OFFSHORE INSTALLATIONS
When the development of the oilfields in the North Sea started, the removal of the installations was controlled by the "Geneva Convention on the Continental Shelf 1958" article 5.5, stating that everything installed should be removed.

Later, the "United Nations Law of the Sea Treaty" and its article 60.3 gave a more flexible meaning to offshore installation removal. It prescribes that any installation that is abandoned or disused, shall be removed to ensure safety of navigation. The removal must also give due regard to fishing, protection of the marine environment and the rights of other states. This article is, however, not ratified by enough states, to be regarded as international law.

In the USA today, what goes in comes out (1). In the UK interpretations of UNCLOS have led to the "Petroleum Act 1987", that in its turn has led to different demands on removal in different parts of the British sector of the North Sea (1-2).

A proposal provided by the British government, makes it clear, that all the installations in the southern sector of the North Sea will have to be totally removed. North of the 58th Parallel the required depth of clear water is 75 meters (1-3). The offshore industry have submitted a document to the UK government, with a proposal to provide a clear depth of 40 meters, with certain exception provisions (3-4).

RELEVANT EVENTS IN A PLATFORM REMOVAL OPERATION
Main alternatives are total removal, partial removal and toppling on site. To leave the installations in-situ is not regarded as an alternative. The operation is also depending on, whether it is a jacket structure, a gravity structure or a pipeline that is to be removed.

A plausible sequence of events in the case of the removal of a jacket structure may look like this (4-6):

-Prepatory engineering and planning

-Shotdown of production

-Well plugging and abandonment

-Decommissioning and cleaning of the production facility

-Topside deconstruction and lift-off

-Jacket deconstruction and lift-off

-Seafastening, transportation and disposal

-Clearing of adjacent bottom area

-Pipeline removal

Preparations and planning for a large and extensive projects as the removal of an offshore platform, will start years in advance. This type of project is in no way new to either oil companies or contractors (4-5, 7). Projects with longer preplanning stages have been successfully performed.

Shutdown of production is a routine operation, carried out on a yearly bases in connection with maintenance and repair. Well plugging and abandonment of wells are operations that are performed regularly by drilling contractors. Decommissioning and cleaning of the process facility will be a part of the operation, where demands from authorities and classification societies will guide the work (2). Close inspection of the process facility will take place, before the removal work will be allowed to begin.

When it comes to deconstruction of the jacket structure, a complicated operation is at hand. Again, this depends on if the entire structure is to be removed and on the capacity of the lifting vessel. By using some system to devide the structure into manageable parts, the dismantling of the structure can take place (4-5, 7). The behavior of a jacket structure during the dismant- ling face is not known, although it can be predicted to some extent. Also, there are no reliable methods for the parting of the really large structures (7).

Seafastening and transportation of the remains of the offshore installation, are routine operations for companies taking on such contracts (4). If everything is to be taken back to shore, scrapping will take place there. If the structure and topside will be dumped in the sea, special dumping barges will be required (4-6).

Cleaning of the bottom area from scrap and debris, will probably take place even if the structure is partly or totally left on the bottom. Lots of things are found around offshore platforms that have no natural reason for being there (5). Loose items travel far under water and can cause problems for fishermen and their equipment (8).

Removal of pipelines has not been discussed as thoroughly as the removal of platforms. Bringing the pipelines to shore in sizeable parts will take some time. This is much depending on if the ope- ration will be the reverse of the laying operation, or if extensive underwater activity is necessary (9-10). Some people argue that the pipelines very well can be cleaned, plugged and left where they are. There are of course people with an opposit view.

The large concrete gravity platforms represents an operation of certain magnitude and with a substantial number of unknown factors. To demolish the Condeep platforms on site is said to be hard, if not impossible with awailable technology. The constructor, Norwegian Contractors, claims that their platforms can be brought out in the reverse way in which they where brought there. That is, to refloat the platforms and tow them away (11-12).
It is, however, far from certain, that the piping under the platforms will be in condition to flush water under the platforms, to eliminate the breakout force between the platform and the clay. If that force can not be controlled, the refloating will end with the champange cork effect (4, 6-7, 13-14).

RE-USE AND ALTERNATIVE USE OF OFFSHORE PLATFORMS
Re-use of platforms has become a new bransch on the offshore industry tree. In the Mexican Gult area some 25 platforms have been re-certified for re-use. It is even said in the media, that smaller platforms are being held intact in stock on a speculative basis (15).

Most offshore platforms are specially designed for the area in which they stand, or for the topside equipment depending on reservoir behavior etc. This puts restraints on the re-use of the jackets (4, 6). Larger jackets, such as Cognac in the Mexican Gulf and Magnus in the North Sea, will never be re-used.

Alternative use of platforms have been discussed. It is though hard to envision how any activity can take the costs of running those structures, even if the capital cost is excluded from the bill (6).

ENVIRONMENTAL AND FISHING CONSIDERATIONS
There are more than ten times more fish around the platforms, compared with the open sea. The number of individuals is high as is the number of different species. Exactly what attracts the fish is not clear, but apparently they like it there.

The physical presens of the installations provides shelter for the individuals and an increased organic production. This do not, however, explain the high number of individuals, since it well exceeds what the organic production can account for (8).

Rigs to reef programs exists in the Mexican Gulf and is discussed in the UK. However, an optimistic estimation of the possible increase in production of fish of economical value, due to the presence of the platforms in the central and nothern part of the North Sea, will give an increase of 100 tonnes. This should be compared with the yearly catch in the UK sector of the North Sea, which is 350000 tonnes (4, 8).

Another question of interest is the area from which the fishermen are prohibited. That is the safety zone of a 500 meter radious around all platforms. Today that area represents 0.05%, or 31 square kiliometers of the central and northern sector of the UK North Sea (8). If the platforms are toppled in-situ, that area will be reduced and there will be a very accurate position of the structure.

As a comparison to the 40 or maybe 50 platforms that can be dumped in this specified area, there are about 7500 wrecks of which the exact position is known only of 2500 (6, 8).

Artificial reefs for sportfishing have been successful and the concept is working. The structures are there and they are more valuble as a sanctuary for fish and their breed, than they are as metal scrap.

The oceans must be protected from all possible pollution. At the same time we must be realistic in this very subjective debate. Concerning the central and northern parts of the North Sea and its 40 to 50 platforms, the possible and hopefully well controlled amount of pollution, will be very small compared with the pollution, in the same area, from shipping and from land based sources (3, 8).

THE POSSIBLE MARKET
The number of different platforms in the North Sea is about 230. Depending on choice of removal option, the cost for the removal of those platforms have been estimated to between 10 and 20 billion USD (3, 16). Compared with the Mexican Gulf, there are totally different conditions, mainly concerning the depths. American estimates states that the 5500 platform will be removed at about the same cost as the low prediction for the North Sea, or 7.5 billion USD (2-3, 16-18).

Resent figures for the central and northern parts of the UK sector of the North Sea states that the cost for removal will be 30% less in that area (2-3, 17). This shows, that as the oil companies increases the attention on the subject, figures on the costs will become more accurate. It is expected that the estimated costs will fall as the operational and technical knowledge is improved.

There are differencies in the interpretation of the regulations concerning platform removal between USA, UK and Norway. The most obvious distinction between the countries in this matter, is the tax reduction system. US governmental subsidization for platform removal is limited to 38%. The corresponding figure for the UK is about 70% and for Norway as high as 90% (17).

A large part of the costs for decommissioning and removal, will be rates for heavy lift vessels and other offshore support vessels. It is possible, that those vessels will account for as much as 50% of the total budget (4, 6-7). It is obvious from this prediction that an effective method of minimising the expenditure, is to minimize the use of those vessels.

EXISTING UNDERWATER WORKSYSTEMS
Operational worksystems today are saturation divers, ROVs, one manned monobaric vehicles and atmospheric diving suits (ADS).

Saturation diving technique is today used for operational diving down to 300 meters depth. Dives to such extreme depths are rare and very hazardous. A more reasonable diving depth is 200 meters but even there the conventional diving equipment, has substantial shortages.

ROVs have taken over a large part of general inspection work offshore. Close inspection is still performed by divers inspite great efforts to develop inspection technology that can be operated from ROVs. Larger and more capable ROVs have captured niches of underwater work, such as drilling support, but the saturation diver, is the main worksystem.

Manned vehicles have become less important over the years. Onemanned systems, have though managed to defend a position, by performing specieal tasks in an efficient way. Lately the interest for large manned vehicles have increased again. In Norway several concept studies of large manned submersibles for underwater intervention are under way. The successful salvage of the remains of the space shuttle Challenger showed, how manned systems have a large potential under certain conditions.

UNDERWATER TASKS AND METHODS RELEVANT TO PLATFORM REMOVAL
Underwater tasks relevant to platform removal are, e.g. inspection, attachment of lifting and buoyancy devices, cutting of piles and platform legs and finally bottom clearance (4-7).

Different types of inspection are performed through the entire removal operation. First, the general condition of the platform is examined, including flooded member detection. Then points for cutting and attachment of lifting devices are identified. Later the cutting and lifting operations are supervised and the bottom is scanned for debris. Most of those inspection tasks can be performed by ROVs and divers (4-5, 7). However, the supervision of the cutting and lifting operation will require other means (7).

Attachment of lifting devices will depend on the size and weight of the pieces and if conventional slings, welded padeyes or dedicated lifting devices are used. The capacity of the chatered lifting vessel and whether the platform is partly or totally removed, also have an influence.

If welded padeyes are decided on they can be placed inside the platform legs just below a cut, so that they can be used to lift the next piece (5). If it is possible to get access to the entire length of a leg, all padeyes can be welded in dry enviroment before the cutting starts.

Padeyes may also be attached by using wet welding if such procedure qualify. Smaller pieces can be lifted with conventional slings. The use of dedicated lifting devices such as clamps, might appear, if it is a well integrated part of the lifting operation (5, 7).

This part of the removal operation of course depends on if the entire platform is to be removed, or if parts of it will be left on the bottom. Weight control engineers, underwater contractors and lifting vessel operators will have problems, when deciding on the size of the pieces of a jacket structure. If a large number of structure members are flooded, the water will add substantially to the weight (6-7).

Attachment of buoyancy tanks can be done by clamps or wet welding. However, any form of removal involving extra buoyancy systems for towing, will be more expensive than sectioning for removal and disposal (4).

Cutting of piles and platform legs can be done by explosives, thermal cutting and mechanical cutting (4-7). There are also other techniques being developed e.a. water jet techniques and chemical methods (19).

If possible it is prefered to make the cutting from inside legs or piles (4, 7, 19). A lot of preparatory work can be done in the dry and it is easier to work by remote control, compared with the outside. It is also a great advantage when the piles are cut below mudline. A disadvantaged is that the piles will generally stick to the structure, because their diameter increases, so that they can not be lifted out separately (18).

Explosives is a favourable technique for cutting, but it takes an expert to succsessfully accomplish a large operation (7). Explosives are at extensive use all over the world, however, from time to time, restrained from use due to complaints from enviromentalists and neighbours fearing for ships and installations (19).

Removal operations in the Gulf of Mexico have been largly based on the use of explosives. Unfortunately those operations have been performed by smaller companies going for the cheapest solu- tion, that means large charges (7). Damages to wild life have been recorded, that have led to a stop in using explosives underwater, in some areas, for some time (20).

Main advantages with explosive cutting are low costs, speed of operation, simplicity, ability to perform simultaneous cuts and the remote activation of the charges. Disadvantages are the shock wave and its potential danger to wild life, installations and surface vessles (7, 19). There is also a psycological disadvantage, in the fear most people have regarding explosives in their vicinity.

In thermal cutting today, the cutting rod is the prefered tool. This is a rod filled with iron wire and flushed with pure oxygen. After electrical ignition the rod can be used to cut all materials. This is a system operated by divers. Remotely operated thermal cutting systems are beginning to appear on the market, but this far they do not have any tracrecord.

Mechanical cutting systems are generally hydraulic power tools. These are widely used onshore and are regarded as reliable systems, in the underwater enviroment. Their main use offshore is not cutting but to, e.g. prepare tubular ends before connection. Different kinds of saws are also suggested as cutting tools.

Abrasive water jets have been used for a long time in onshore industry. An abrasive is fed through a nossle to a high pressure water stream. Such systems are being modified and developed for underwater use and will soon be operational (21).

Chemical methods may be used to cut steel with sufficiant speed. R and D programs concerning chemical cutting are under way and may offer advantages, compared to explosives (19).

Bottom clearance can be done by using bottom trawls or by simply picking up the items from the sea floor.

NECESSARY DEVELOPMENTS OF UNDERWATER WORK SYSTEMS AND METHODS
Identified tasks to be preformed are inspection, attachment of lifting devices, cutting of piles and platform legs and bottom clearance.

With existing work systems and operational methods all necessary inspection work, including flooded member detection, can be performed. It will, however, be hard to successfully perform, an overall inspection of an extensive simultaneous cutting operation. The supervision of cutting and lifting operations may require other means of imaging and sensor technology, due to poor visability or problems to survey simultaneous events (4, 7). Attachment of lifting devices on the underwater structure, will depend on size and weight of the pieces to be lifted and on the capacity of the lifting vessel. The attachment of lifting devices, can not be regarded as an area, where technical developments are essential to the successfull completion, of the operation.

The cutting of underwater structures will probably be done with explosives. If the use of explosives is controled in some way and the transportation and storage can be done in an acceptable manner, the advantages compared to other methods will be obvious. Explosives are cheap, fast, comparatively simple, remotely activated and facilitates simultaneous cuts (7). Finding suitable places for charges in or on legs and piles can, however, prove to be difficult.

There are areas where there are great needs for improvement and development. Such areas are charge refinement, shock wave monitoring and inhibition, remote placing of charges and a reliable multiple detonation system, probably using no detonation cord (7, 19).

The refinement of linear shaped charges will lead to smaller charges providing better cuts. Other explosive cutting methods are developed, e.g. shock wave focusing techniques. Smaller charges also give a smaller shock wave. The control of the shock wave is essential to the structure being cut and to the heavy lifting vessel and barges positioned close to the structure. Development programs on shock wave monitoring and inhibition are currently running (19).

Remote placing of charges presents a variety of problems. Merely the size and weight of a linear charge used to cut a pile with a two meter diameter, is a big problem. Apart from that, all water between the charge and tubular must be removed, otherwise it will not function. Existing ROVs and saturation divers will have great difficulties, handling such large devices (7).

If a large number of charges must be detonated at the same time, a reliable and practical multiple detonation system must be provided (7). Such a system will probably not use any detonation cord of conventinal type. When placing charges on large jackets, hundreds of meters of detonation cord must be used. If at any time, pieces of cord cross each other and cuts each other off, or gets entangled in the diver who pulls them out, there will be a malfunction in the cutting operation.

To remove a jacket structure with explosives, a well integrated system comprising charge, multiple detonation and a method to place the charge in its position, must be developed (7). It is essential that this is a complete system, not just bits and pieces put together on the work site.

The clearence of the adjacent bottom area can be done effectively with existing systems.

DISCUSSION AND CONCLUSIONS
At this superficial consideration the estimated costs and suggested engineering obstacles seems exaggerated. It appears to be necessary to provide a new approach to offshore platforms. The men in the offshore industry must get used to the fact that platforms which have served their time, are no longer high technology and neither is their removal.

Since time is on the side of the remover, he can plan for his operation properly and use the most cost effective methods available. Due to the fact that North Sea platforms will stand for several years after being abandoned, closely sited platforms may be removed in one operation, even if there are some years between their closing.

A successful decommissioning and removal program must deal with three important points. Firts, the enviromental considerations that not only complies of the discharge of petroleum products and the dumping of the remains of the installations. No matter what arguments are brought forward, the voices of the enviromentalist groups will be loud. Secondly, the management responsabilities will be huge and the demand for proper and realistic planning immense. Finally, there is need for a simple, reliable and cost effective method for cutting piles and platform legs.

When planning for a platform removal the choise of cutting technique may well be disregarded and given a low priority due to such important details as, economical analysis, legislation, enviromental consideraitions etc. This can be a big mistake. The cutting method, or combinaiton of methods chosen, may prove to be the key to the success of the whole operation.

A substantial part of the budget for a decommissioning and removal operation, will be the different offshore vessels used. The planning for their effective use will be very important. The capable contractors for a removal operation already exists, but they will not succeed in bringing the Magnus platform to shore, if that is their first contract. Thousands of smaller platforms will be removed before the mighty North Sea jackets and Condeeps are to be taken out. They will represent a challenge, a challenge that will be overcome by proper planning.

What actually happens to the platforms when they reach the end of their operational life, will depend more on political and financial considerations, than engineering practicalities.

REFERENCES
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