Prepared for Shell U.K. Exploration and Production
by Rudall Blanchard Associates Limited
Brent Spar Abandonment - BPEO Assessment
15/12/94
2. STRUCTURE OF THE BPEO ASSESSMENT 4
3.SUMMARY INFORMATION ON THE BRENT SPAR
1.1. The Brent Spar was commissioned in 1976. It is a cylindrical buoy, moored to the sea bed by six anchors and is made up of oil storage tanks at the bottom, buoyancy tanks toward the middle and a topside containing the offshore tanker loading equipment. A full description is provided in Section 3 - 'Summary of Information on the Brent Spar'.
1.2. The costs of maintaining the Spar increased substantially in the period 1987 to 1990. In 1991, a review concluded that the work necessary to refurbish the facility to extend its operational life could cost over 90 million. The buoy would have to be out of commission for a 2 - 3 year period during the refurbishment. Given the age of the structure, the presence of a pipeline system for the export of crude oil and the substantial cost of refurbishment, it was decided that the Brent Spar should cease operations. It was taken out of commission in September 1991 after 15 years service.
1.3. The Brent Spar is presently classified as a "not normally manned installation" and, following decommissioning, a letter of limitation to its Certificate of Fitness prevents its use as a storage or tanker loading facility. This certificate expires in 1995 and if it is to be renewed, refurbishment would be required before that date. The operators, Shell UK Exploration & Production (Shell Expro) have recognised that the Spar is now obsolete and wish to abandon the structure, in accordance with all regulatory requirements.
1.4. Thirteen possible methods of abandoning or re-using the buoy were initially put forward for consideration, of which six were identified as viable options:
1.5. This document describes the assessment undertaken to determine the Best Practicable Environmental Option (BPEO) for the abandonment and disposal of the Brent Spar. The details of the BPEO assessment are described in Section 2 - 'Structure of the BPEO Assessment'. It includes consideration of technical feasibility, risks to health and safety of the workforce, environmental impacts, public acceptability and costs.
1.6. The BPEO Assessment demonstrates that the most appropriate action is to dispose of the Brent Spar at an authorised deep water disposal site, as it is the option of least technical risk, minimises exposure of the workforce to accidents, will have small but insignificant impacts on the environment and is economically the most attractive.
2.1. The document provides a detailed assessment of the feasible disposal options of the Brent Spar, together with a statement setting out reasons which have led to the conclusion that deep sea disposal is the Best Practicable Environmental Option.
2.2. This assessment is supported by a full set of engineering, safety and environmental study documentation, referenced in Section 12.
2.3. A short summary of the Brent Spar is presented in Section 3 describing the facility, its location, history and current status, and a brief overview of the UK legal framework.
2.4. Following an examination of all the feasible options, six were selected for further study and appraisal. These six were screened and a summary of the findings provided in Section 4.
2.5. The screening process described in Section 4 established that only two options presented realistic solutions. These are described in detail in Section 5 and the remainder of the report is given over to comparing and assessing the relative merits of these. The following aspects were looked at in detail:
Engineering Complexity (Section 6) - The difficulty of carrying out the engineering procedures considering the range of tasks to be performed, the locations in which they might be carried out and the type of vessels required;
Risk to Health & Safety of Workforce (Section 7) - The likelihood of serious injury and fatality in each option, based on an assessment of the tasks and the number of man hours dedicated to each task;
Environmental Impact (Section 8) - The range of impacts on the environment and resource users as a result of carrying out the option, including an assessment of the consequences of possible accidents;
Cost (Section 9) - The estimated gross cost of the short-listed options.
Consultation Process (Section 10) - Discussion of the process used to ascertain acceptability to the authorities and other interested parties;
2.6 Section 11 provides the final logic for selection of the Best Practicable Environmental Option.
3.1. DESCRIPTION OF THE FACILITY
3.1.1. The Brent Spar is a 29m diameter cylindrical buoy which floats vertically in the water with a draft of lO9m and a height above water of 28m. It was constructed using techniques similar to those of ship building and consists of a thin outer skin of 20 mm plate steel, stiffened by ribs and bulkheads. [FIGURE OMITTED]
3.1.2. The total weight of the structure is 14,500t made up of 6,700t of structural steel, 6,800t of permanent haematite ballast and lOOOt of equipment.
3.1.3. The upper section consists of a helideck, crane, tanker mooring/loading boom and accommodation for 30 people.
3.1.4. The top of the lower section is made up of twelve buoyancy tanks. Below this, the main storage area is divided into six tanks which extend to the base of the buoy. The total storage capacity is 300,000 barrels of oil. At the base of the buoy there is a sealed compartment containing the permanent ballast. which is composed of haematite embedded in concrete.
3.1.5. During operations, the buoy was used to store oil from Brent 'A', and also acted as a tanker loading facility for the whole Brent Field. It was the sole route for the export of crude oil until the Brent System Pipeline was commissioned in 1978. After this, it continued to be used as an alternative to the pipeline system.
3.1.6. The Brent Spar is held on location by a six leg catenary mooring system. Each mooring line is made up of a 285m length of 4" chain, one end of which is attached to the buoy and the other to 800m of 3.5" wire length, connected directly to the anchor block. [FIGURE OMITTED]
3.1.7. The manifold, located on the sea bed directly below the Spar. is now isolated and will remain in place after removal of the Spar. It acts as a junction in the pipeline between Brent 'A' and 'B' platforms. It will operate as a fully maintained sub- sea facility. [FIGURE OMITTED]
3.2 LOCATION, HISTORY AND CURRENT STATUS
3.2.1. The Brent Spar was constructed in a horizontal position and was installed in the Brent Field in 1976. The Brent Field is located in UK Block 211/29 in the North Sea, some 190 km ENE of Shetland. The Spar is located at Latitude 61 deg 03'14.7" N, Longitude 01 deg 40'04" E, approximately 2.9 km from the Brent 'A' platform and 1.9 km from Brent 'B' platform. The water depth at the Spar location is 140m.
3.2.2. During a nine day operation in a Norwegian fjord, the Spar was upended by the gradual ballasting of the storage tanks. Despite the care taken during this operation, subsequent analysis has indicated that the buoy was overstressed to some extent by the pressures it experienced . [FIGURE OMITTED]
3.2.3. In January 1977, two of the main storage tanks were ruptured by the accidental build up of differential pressures, which were in excess of design limits, between the inside and outside of the tanks. The subsequent repairs were conducted only to maintain structural integrity of the buoy not to make the tanks water tight. The tanks were not used again for oil storage and remained filled with sea water.
3.2.4. ln 1978, the Brent System Pipeline to Sullom Voe Terminal was commissioned and became the main export route for the Brent crude. After this, the Spar continued to be used only as an alternative export.
3.2.5. Between 1987 and 1990, the maintenance costs of the facility increased substantially. In 1991, a review of the refurbishment requirements concluded repair costs of over 90 million were necessary to extend the buoy's life and the facility would be out of commission for two to three years. Given the age of the structure, the cost of refurbishment and the presence of the Brent System Pipeline, it was concluded that the facility should be withdrawn from service.
3.2.6. The Brent Spar was taken out of commission in October 1991, during which time the undamaged storage tanks were emptied of crude oil and filled with sea water. The process pipework was flushed through with sea water and the storage tank oil/water interface emulsion and slops were pumped into the final shuttle tanker. All buoyancy tanks were emptied and all valves, watertight hatches and doors were shut to prevent flooding. Items of loose equipment, including fire fighting equipment, life saving appliances and spares were removed. In November 1991, the manifold was isolated and the flexible risers were removed.
3.2.7. The Spar is presently classified as a "not normally manned installation" and a letter of limitation to its Certificate of Fitness prevents its use as a storage or tanker loading facility. This certificate expires in 1995 and, if it is to be renewed, a detailed structural integrity survey and refurbishment programme would be required before this deadline. Page 6
3.3. INVENTORY OF MATERIALS
3.3.1. The bulk of the structure is composed of structural steel (approximately 7,700t) and haematite embedded in concrete (some 6,800t). Inventory items have been classified in accordance with the ' Convention for the Protection of the Marine Environment of the North East Atlantic 1992" (Oslo and Paris Commissions - OSPARREV). This Convention regulates substances and materials whose disposal at sea is controlled and are referred to as "Regulated Substances". [FIGURE OMITTED]
3.3.2. Equipment throughout the structure contains small quantities of heavy metals (in metallic form) as an integral part of the materials of construction. The sacrificial anodes contain an estimated 28.7t of aluminium, 10.2t of zinc metal and minor quantities of cadmium (8 kg), lead (0.6kg) and mercury (0.1kg). Electric cables contain some 13.5t of copper and paint on the structure includes some 3.5t of zinc. Small quantities of lead (2.5kg) and of nickel (3.5kg) are contained in remaining batteries. Small quantities of other materials are also contained in some of the equipment. Traces of PCB (<20ml) may still remain in the two transformers, although PCB-containing transformer fluids were replaced some years ago. Synthetic materials and plastics are also located on the structure (eg. in fittings in the accommodation/control room and cable insulation) but the exact quantities are not known (AURIS,1994,1; Metocean 1993, 7, 8&9).
3.3.3. The oil storage tanks contain some 48,000m3 of sea water together with an estimated 100t of oily sludge at the bottom. A 1991 analysis indicated that the oily sludge contains an estimated 9.2t of oil and a number of heavy metals, including cadmium (5.8kg), chromium (2.1kg) copper (42.9kg), nickel (3.9kg), lead (8.9kg), zinc (87.4kg), arsenic (0.3kg), and mercury (0.2kg). The remainder of the sludge is composed of a mixture of sand and scale. The walls of the storage tanks are also coated with an estimated 41.3t of hydrocarbons, in the form of thin layer of oil and wax. The sea water has not been analysed in detail and it has been conservatively assumed that it will contain hydrocarbons (up to 40ppm) from the residual oil in the tanks, together with zinc (up to 12ppm) and aluminium (up to 19ppm) from the internal anodes (AURIS, 1994,1)
3.3.4. Scale is commonly found in oil processing facilities throughout the world. In many areas scale may be contaminated by small amounts of naturally occurring radioactive salts from the oil reservoir formation to form low specific activity (LSA) scale. LSA scale is present in the sludge (with average activity of radium-226 and actinium-228 of 4.5 and 3 Bq g-1, respectively) in the storage tanks and as hard scale (with average activity of radium-226 and actinium-228 of 17.6 and 15.2 Bq g-1, respectively) in the internals of the pipework. Conservatively, taking the total mass of sludge to be 1OOt and the hard scale to be 30t, the total mean activity of the LSA scale is 11.96 GBq (AURIS, 1994,1; ICI Tracerco, 1993,5&6).
3.4. REGULATORY FRAMEWORK
3.4.1. The abandonment of the Brent Spar will comply with all United Kingdom legislation. The following diagram summarises the regulatory framework relating to the abandonment of structures on the United Kingdom Continental Shelf. [diagram omitted]
4.1. INTRODUCTION
Thirteen options for abandoning or re-using the structure were considered in the preliminary study (McDermott, 1993, 10). These have been screened to identify those most suitable for detailed study. Four abandonment and two re-use options remained after this process and these are examined in this section. The table below briefly summarises the six options considered and the reason for their selection.
OPTION 1 Horizontal Dismantling
Remove the topsides and transport ashore for mechanical breaking and disposal. Deballast the remaining hull to the horizontal by emptying storage tanks and tow to a suitable inshore site for transfer to a transport barge. Transport the hull to a suitable onshore for decontamination, mechanical breaking and disposal
Provides a feasible method albeit complex to bring the buoy ashore for mechanical breaking and disposal without depth restrictions at the landing site (Reverse of installation)
OPTION 2 Vertical Dismantling
Tow the Spar intact and in the vertical to altered deep water site. Partially decontaminate and dismantle the structure by cutting horizontally into sections. Transport each section to shore for total decontamination, mechanical breaking and disposal
Provides a feasible way to bring the buoy ashore for mechanical breakage and disposal but with depth restrictions at vertical dismantling site (Reverse of construction)
OPTION 3
In-Field Disposal
Sink the Spar intact at or near its present position in the Brent Field
Simplest abandonment option
OPTION 4
Deep Water Disposal
Tow the Spar intact to an authorised deep ocean disposal site and sink it
Logical alternative to Option 3
OPTION 5
Refurbish and Re-Use
Carry out refurbishment of the Spar to make it fully operational at an alternative site
Alternative to abandonment if a suitable use or buyer could be found
OPTION 6
Continued Maintenance
Carry out the minimum of maintenance and repair to keep the buoy in its existing condition at its present site
Possible option to delay abandonment and combine it with other field abandonment programmes
4.2. ASSESSMENT OF OPTIONS
4.2.1. INTRODUCTION
Each option was assessed against the criteria specified in Paragraph 2.5. The findings of the screening study are summarised in the text below:
4.2.2. OPTION 1 - HORIZONTAL DISMANTLlNG
This option involves removal of the topsides, repair of the damaged tanks, rotation of the buoy to the horizontal, transfer to a cargo barge, transport to shore and onshore dismantling (AURIS, 1994, 1&2; McDermott 1993, 10&11).
CRITERIA: Engineering complexity
FINDINGS:
involves extremely complex operations to upend the structure and transport it ashore but provides a feasible method of bringing the buoy ashore for breaking and disposal
the most critical operation is upending the buoy to the horizontal to reduce the draft sufficiently to permit it to be towed to a suitable breaking and disposal location in the UK
the complexity of operations provides high potential for unplanned events (eg. accidental flooding of tank(s) during upending/horizontal tow resulting in aborting the operation or sinking of the structure)
CRITERIA: Safety and Risk FINDINGS:
significant exposure of the workforce to hazardous operations there would be occupational exposures to hazardous materials during decontaminating and breaking the buoy under controlled onshore conditions; these would be less than the same operations in Option 2
CRITERIA: Environment and Resource Use FINDINGS:
negligible risk to environment through planned operations although more sensitive inshore environments exposed to risks from unplanned events high potential for unplanned events, at worst premature loss of buoy (particularly in shallow inshore waters) could have significant localised impacts particularly to other users of the sea (eg. shipping and fishing activities
CRITERIA: Acceptablilty
this option considered to be acceptable to the authorities and other interested parties
CRITERIA: Cost
FINDINGS:
high comparative cost - Ѓ46.0 million
This option was considered the most feasible onshore disposal method and was put forward for detailed consideration.
4.2.3. OPTION 2 - VERTICAL DISMANTLING
This option involves in-situ repair to the damaged tanks, a tow to a deep water inshore site. removal of the topsides and vertical dismantling the hull in sections, transferring the hull sections ashore and finally breaking the structure/disposing of the materials onshore (AURIS, 1994,1&2; McDermott, 1993, 10).
CRITERIA: Englneerlng Complexity
FINDINGS:
involves slightly less complex operations than Option 1
requires deep water site as the draught of the buoy is 109m; the only suitable UK deep water site identified was Loch Kishorn on the west coast of Scotland which has a draught restriction of c80m and would require deballasting of the structure making the operation more complex, increasing the risks and further increasing the costs.
alternative deep water sites were investigated in Europe but were eliminated due to regulatory constraints
the most critical operation Is maintaining stability/ballast control during vertical cutting/sectioning of the structure hence the requirement to conduct the operation in sheltered inshore waters
less complex operation than Option 1 but still high potential for unplanned events (eg loss of hull during sectioning)
CRITERIA: Safety and Rlsk
FINDINGS:
significant exposure of the workforce to hazardous operations exposures during decontaminating and dismantling the buoy would be greater (ie. offshore confined spaces) than the same operations in the controlled onshore conditions provided in Option 1
CRITERIA: Environment and Resource Use
FINDINGS:
negligible risk to environment through planned operations although more sensitive inshore environments exposed to risks of unplanned events
high potential for unplanned events including loss of containment of contaminants during vertical sectioning or at worst premature loss of buoy in inshore waters which could have significant localised impacts particularly to other users of the sea (eg. shipping and fishing activities)
CRITERIA: Acceptabillty
FINDINGS:
this option considered to be acceptable to the authorities and other interested parties
CRITERIA: Cost
FINDINGS:
high comparative cost - Ѓ44.0 million
This option had no advantage over Option 1 and was dismissed as no suitable deep water sites were identified.
4.2.3 OPTION 3 - IN-FIELD DISPOSAL
This option involves clean-up of the topsides, placement of explosives, a short tow to the disposal site and sinking of the structure (AURIS 1994, 1 & 2).
CRITERIA: Engineering Complexity
FINDINGS:
technically the simplest option
lower potential for unplanned events than the other abandonment options
CRITERIA: Safety and risk
FINDINGS:
minimal exposures of workforce to hazardous operations regarded as safest option
CRITERIA: Environment and Resource Use
all materials released to North Sea environment with potential for local impacts
long term restriction to access by resource users (notably fishing activities in the area) through exclusion zone at wreck site
CRITERIA: Acceptability
FINDINGS:
this option was not considered to be acceptable to Shell Expro or permissible to the authorities given the alternatives
CRITERIA: Cost
FINDINGS:
no costs estimated though undoubtedly the cheapest option
This option was dismissed on the grounds that this option would not be acceptable given the alternatives for abandoning the structure.
4.2.5. OPTION 4 - DEEP SEA DISPOSAL
This option involves clean-up of topsides, placement of explosives long tow to the disposal site in the North East Atlantic and sinking of the structure (AURIS 1994, 1&2; Global 1993,4).
CRITERIA: Engineering Complexity
FINDINGS:
relatively simple option
most complex operation is the long tow to the dump site
lower potential for unplanned events compared to the onshore disposal options
CRITERIA: Safety and Risk
FINDINGS:
comparatively low exposures of workforce to hazardous operations
CRITERIA: Environment and Resource Use
FINDINGS:
impacts through planned operations would be small and localised at a deep water site authorised for disposal of such materials medium potential for unplanned events during tow at worst premature loss of the buoy would have localised impacts on the environment and resource users, the significance to the latter depending on location
CRITERIA: Acceptability:
FINDINGS:
this option considered to be acceptable to the authorities and other interested parties
CRITERIA: Cost
low comparative costs - Ѓ11.8 million
This option was a preferred abandonment option on the basis of relative technical simplicity, low risks to the workforce and costs and was considered for further appraisal
4.2.6. OPTIONS - REFURBISH AND RE-USE
This is a re-use option rather than abandonment and is dependent on finding an alternative use, refitting the buoy offshore or at a suitable deep water site and then towing the structure to its eventual end location (McDermott 1993, 10).
CRITERIA: Engineering Complexity
FINDINGS:
complexity would depend on the re-use requirement extensive engineering would be required over a long period (c.2 years) to refurbish and refit the buoy to current standards
CRITERIA: Safety and Risk
FINDINGS:
comparatively low exposures to hazardous operations envisaged but will depend on re-use requirements
CRITERIA: Environment and Resource Use
FINDINGS:
very low impacts
CRITERIA: Acceptability
FINDINGS
this option considered to be acceptable to the authorities and other interested parties
the most critical factor was that no alternative users or buyers were found
CRITERIA: Cost
FINDINGS:
very high (up to Ѓ90 million depending on re-use requirements) but may be partially offset by sale value
This option was dismissed on the basis that no alternative users or buyers were found.
4.2.7. OPTION 6 - CONTINUED MAINTENANCE
This is a re-use option rather than abandonment and would involve continued maintenance of the structure to maintain the Certificate of Fitness until the structure was eventually abandoned (AURIS 1994, 1&2).
CRITERIA: Engineering Complexity
FINDINGS:
complexity of engineering work required would increase over time as main structural components exceeded their design life and required replacement
extended period of ad hoc repairs would make ultimate disposal more difficult
CRITERIA: Safety and Risk
FINDINGS:
comparatively low exposures to hazardous operations envisaged
CRITERIA: Environment and Resource Use
FINDINGS:
very low impact
CRITERIA: Acceptability
FINDINGS:
this option considered to be acceptable to the authorities for a specified period of time only
CRITERIA: Cost
FINDINGS:
c. Ѓ5 - 6 million initially plus annual maintenance which will increase with time
This option was dismissed on the basis that Shell Expro do not wish to incur continued maintenance costs when they do not foresee any future use for the structure.
4.3. SUMMARY OF SCREENING FINDINGS
The main findings and conclusions of the screening studies are highlighted below: .
Option 1: Horizontal Dismantling; feasible onshore disposal option; option put forward for further consideration
Option 2: Vertical Dismantling; no advantage over Option 1, limited availability of suitable deep water dismantling sites in U.K; option dismissed
Option 3: In-Field Disposal; unsuitable given the alternative options available; option not acceptable to Shell Expro or the regulating authorities
Option 4; Deep Water Disposal; feasible offshore disposal option; option put forward for further consideration
Option 5; Refurbish and Re-Use; no potential for sale or re-use was found for the structure: option dismissed
Option 6; Continued Maintenance; Shell Expro do not wish to incur continued maintenance costs when no future use for the structure is foreseen; option dismissed
Only two options were considered suitable for detailed review; Option 1 - Horizontal Dismantling and Option 4 - Deep Water Disposal. These two are discussed in greater detail in the remaining sections of this document. Page 15
5.1. DESCRIPTION OF HORIZONTAL DISMANTLING OPERATIONS
5.1.1. -The onshore dismantling option would involve a number of sequential and concurrent operations encompassing a wide range of technical activities and operating environments (AURIS, 1994, 1&2; McDermott, 1993, 11).
5.1.2. The most critical stage of this option would be the rotation of the buoy in the water to a horizontal position. This procedure is called "reverse upending" and would be accomplished by the controlled removal of water (deballasting) from the oil storage tanks. This operation has to be carried out offshore, in water of sufficient depth to accommodate the draft of the buoy (i.e. 102 m after topsides removal). [FIGURE OMITTED]
5.1.3. The first step in the disposal process would be to carry out detailed internal and external surveys of Brent Spar to determine its present structural condition. Temporary systems such as power, lighting, life support (ventilation), fire fighting and safety equipment would have to be installed to permit personnel to work inside the buoy safely.
5.1.4. After disconnecting the internal pipework, bulkheads and decks across the cutline, the outer structure would be cut using automated equipment and 1570t topsides lifted onto a cargo barge using a heavy lift vessel.
5.1.5. The two damaged storage tanks would be repaired by divers using welded patches to make them leak-tight. The reverse upending control system consisting of valves, pumps, pressure transducers and pipework would be fitted to the storage tanks. Two of the six anchor lines would be removed completely. The remaining four would be retained to form tow line attachments and emergency lines.
5.1.6. The buoy would be towed to a reverse upending site, located some 80 km north-west of the present site, to ensure the operation was carried out well away from existing installation and pipelines. The tow would take approximately one day.
5.1.7 At the upending site, the buoy would be stationed in a pre-installed mooring system, together with a heavy lift vessel, work barge and tanker. Once all connections to floating hoses were made, the contaminated water would be pumped from the storage tanks across to the tanker via the barge, thus deballasting the buoy. An inert gas mixture of nitrogen and carbon dioxide would replace the water in tanks. The sequence and rates of deballasting would be controlled to minimise differential pressures on the emptying tanks, and as far as possible to orientate the damaged tanks out of the water. The pumping equipment would need to be disconnected prior to the tow, and it would therefore not be possible to remove any water that subsequently leaked into the tanks.
5.1.8. The buoy would then be towed in its horizontal attitude to a sheltered site, such as Scapa Flow, for loading onto a submersible cargo vessel. This operation is necessary to reduce the draught sufficiently to enable the structure to be towed to a suitable dockside and to provide a stable and horizontal platform for dismantling of the hull prior to transfer ashore (see Paragraph 5.1.11).
[FIGURE OMITTED]
5.1.9. On arrival at the loading site, the buoy would again be stationed in a pre-installed mooring system. Two 2000t capacity shearlegs would be used to raise the base of the buoy, reducing the draught at the deepest point to allow the semi- submersible cargo vessel to manoeuvre beneath it. The semi- submersible vessel would then deballast, lifting the buoy clear of the water. A sheltered site would be required for this operation because the lifting operations would be very weather sensitive. [FIGURE OMITTED]
5.1.10. The buoy would be transported on the semi-submersible cargo vessel to the final breaking and disposal site on the UK mainland. Access holes would be cut in the sides of the tanks to allow personnel to enter. The inside of the tanks would then be cleaned by high pressure water jetting, and the resulting effluent collected and filtered. The filtration unit would remove solids for authorised disposal, leaving an oily water mixture for further treatment and eventual disposal.
5 1.11. Transfer of the buoy ashore would be complicated because of the uneven weight distribution, caused by the haematite ballast at its base. The buoy would have to be cut into two sections, with the upper section being removed first. exposing the lower regions of the storage tanks. The ballast would be excavated and removed before transfer of the lower hull section to shore.
5.1.12. Mechanical breaking of the structure would be conducted according to strict operational procedures. Components which had been in contact with crude oil when the facility was operational would be assumed to contain scale, contaminated with naturally occurring radioactive materials. These would be decontaminated and the scale disposed of in accordance with current regulations. Once the scale had been removed, the main hull sections would be mechanically cut into manageable pieces and offered for sale as scrap steel. All internal and external anodes would be removed separately. All material, including inert, hazardous and non-hazardous wastes would require disposal via an acceptable route complying with all relevant legislation.
5.2. DESCRIPTION OF DEEP SEA DISPOSAL OPERATIONS
5.2.1. Deep water disposal could be carried out with or without the topsides in place and the advantages and disadvantages of these options have been assessed. It has been concluded that removing the topsides would increase the risks to the workforce, but would not result in any significant reduction in environmental impact. Therefore it is not planned to remove the topsides for the deep water disposal of Brent Spar (AURIS, 1994, 1&2; Global, 1993, 4).
5.2.2. The deep water disposal option would involve less complex engineering works and less offshore activity than the onshore dismantling option.
5.2.3. The Spar would be 're-entered', the topsides made safe and the state of the structure evaluated. In order to minimise the environmental impact of deep sea disposal, as much of the potentially hazardous materials as possible will be removed consistent with safety. feasibility and cost (Metocean, 1993,7). This would involve draining down remaining equipment (eg. lubricating oil, fuel oil etc.,), removing loose equipment and some of the fittings and transporting recovered materials ashore for disposal at an authorised site .
5.2.4. Explosives experts, would then board the buoy and place the charges, eventually used to sink the Spar. Sufficient charges would be placed to ensure that when they were detonated, all the buoyancy tanks would be opened to the sea, even if some of them failed to detonate. Flooding of these tanks would be sufficient to sink the buoy.
5.2.5 Two tugs accompanied by an attendant survey vessel would then tow the buoy to the selected deep sea disposal site. Two of the anchor chains would be used to tow Brent Spar out to the deep sea dump site. The remaining chains would be used as emergency tow lines.
5.2.6. Three general areas have been identified by the Scottish Office Agriculture and Fisheries Department (SOAFD) as potentially suitable for disposal of redundant offshore structures. as follows:
These sites are located within U.K. waters and lie in water depths in excess of 2000m. A detailed survey has been undertaken by Shell Expro and SOAFD to confirm the 'suitability' of the potential disposal sites and to provide baseline environmental data for future monitoring programmes.
5.2.7. Towing would be conducted using conventional maritime practices. All routes from the Brent Field to potential dump sites pass to the north of the Shetland Islands and to the north and west of the Outer Hebrides. Whichever site is selected, the earlier stages of the planned routes would essentially be common. The routes will be determined by the requirement for adequate water depth, sufficient clearance from installations and the need for sufficient sea room for manoeuvre should bad weather or accident result in breaking of the tow. The first part of the tow, from the present location to the 200m contour, would be subject to detailed survey, to ensure that the buoy did not encounter any unexpected obstructions. The route for the latter stage of the tow would depend on the disposal site selected. The tow is planned to take between 15 - 25 days, depending on the weather conditions and final location. [FIGURE OMITTED]
5.2.8. When the Spar arrived at the disposal site, the correct position would be confirmed and the tow lines mechanically released from the tugs. All vessels would then stand off and the explosives would be detonated simultaneously by remote control.
6.1. INTRODUCTION
6.1.1. Both options are significantly different in terms of their engineering complexity and uncertainties inherent in their execution, but both are considered technically feasible (AURIS, 1994, 1&2; Global, 1993, 4; McDermott 1993, 11).
6.2 OPTION 1 - HORIZONTAL DISMANTLING
6.2.1. The main objective of this operation is to transport the Spar ashore for onshore breaking, to recycle the steel and to properly dispose of any waste materials. Transportation requires a reduction in the draught of the buoy, sufficient to permit entry to a suitable port and transfer to an onshore quay. To do this requires an intermediate stage of transfer to a semi- submersible cargo vessel. This option involves nineteen principle operations and is technically complex. [FIGURE OMITTED]
6.2.2. The operations, whilst they have been used in the offshore oil and gas industry, involve technically demanding marine engineering techniques. Although all stages are considered technically feasible, the damage to the storage tanks would have to be repaired underwater to ensure that they were water-tight and could provide buoyancy during the upending operation. A detailed survey will be required to examine the structure before operations commence to confirm integrity.
6.2.3. The upending operation would have to take place in deep water due to the draught of the buoy (currently 109m), which would be reduced to 102m by removing the topsides. This operation would be subject to offshore weather and sea conditions. The original up-ending operation was subsequently demonstrated to have caused local yielding of the submerged part of the structure. It is believed that a similar condition would occur during reverse up-ending, potentially leading to stability and control problems, particularly if the yielding led to a breach and accidental flooding of the storage chambers. This coupled with the difficulties of assuring integrity of the field repairs to the tanks introduces technical risks. To mitigate this, the up-ending operation would need to be carried out slowly (over a period of 5-10 days) which make this a highly vulnerable to poor weather
6.2.4. Following up-ending, the draught of the Spar would be reduced from 102m to 15m, enabling it to be towed in a horizontal aspect to a suitable inshore location (e.g. Scapa Flow) for transfer to a semi-submersible cargo vessel. The horizontal tow from the up-ending site to a sheltered site involves a technical risk, since pumping equipment installed to facilitate the reverse up-ending would have to be removed prior to the tow. As a result of this, there would be no capability to remove any water that subsequently leaked into the buoy. This makes the Spar vulnerable to grounding or sinking in the event of damage occurring during the tow.
6.2.5. The 15m draft of the Spar is too deep for direct transfer onto a semi-submersible cargo vessel and would require two sheerlegs to lift it into position. The lift is also critically dependent upon weather and sea state, hence the preference to complete the operation in a sheltered area such as Scapa Flow. Once on the vessel, the Spar would be transported to a suitable onshore site for decontamination and breaking on the UK mainland.
6.3. OPTION 4 - DEEP WATER DISPOSAL [FIGURE OMITTED]
6.3.1. This option involves towing the Spar to a licensed deep water disposal site and sinking it. Seven principle engineering and marine operations have been identified. In relative terms, the option is considered technically simple and again, all aspects of the operation are within normal offshore industry practices. Proper disposal would require that the buoy does not implode or break up during sinking. The preferred method would be to use linear explosive devices to breach the ballast tanks and allow flooding of all the buoyancy compartments of the Spar.
6.3.2. Preparatory works would be required prior to the tow to remove as much of the 'regulated materials' as is reasonably practicable and to install explosive devices with dual fail-safe detonation system.
6.3.3. The tow and sinking operation would be carried out in the summer months. Its exact timing would be subject to the availability of a good weather window. No repairs would be required to the damaged tanks as the buoy has sufficient intrinsic buoyancy for the tow. The buoy would be towed in the vertical aspect from the Brent Field. Detailed route planning would be conducted to ensure the requirement for adequate water depth, sufficient clearance from installations and the need for sufficient sea room to manoeuvre should bad weather or accident result in loss of the tow. The first part of the tow, from the Spar's present location to the 200m depth contour, would be subject to a route survey, ahead of the tow.
6.4. SUMMARY OF ENGINEERING COMPLEXITY
6.4.1. While both options are technically feasible, deep sea disposal involves fewer operations (7 versus 19) and these are less complex and involve less technical risk than those required for horizontal dismantling. On this basis, the deep sea disposal option is preferred.
7.1. INTRODUCTION
7.1.1. Both options have a common safety and risk element regarding the initial entry and preparatory works aboard the Spar. This involves providing safe access to the buoy and will require ventilation of spaces, testing and monitoring of the atmosphere and placement of temporary life support, safety and fire systems. The safety and risk exposure during these works are comparable to those experienced in normal offshore construction activities.
7.2. OPTION 1 - HORIZONTAL DISMANTLING
7.2.1. The large number of activities required for this option means that it is highly labour intensive and involves some complex and potentially hazardous operations. The safety and risk exposures are consequently high (AURIS, 1994,2).
7.2.2. Risk analysis indicates that the probabilities of fatal injuries are between 0.030 - 0.088 for this option. This is due, in part, to the hazardous nature of some of the more complex operations, but mainly to the high exposures (large number of man hours) to the hazards of the onshore breaking operations.
7.2.3. The onshore breaking and disposal of the structure introduces significant potential for occupational health risks. The workforce would be exposed in varying degrees to low specific activity scales, asbestos, heavy metals etc., during the breaking and subsequent disposal of waste. Strict controls would be necessary to protect them from unnecessary exposures.
7.3. OPTION 4 - DEEP WATER DISPOSAL
7.3.1. This option involves a much smaller workforce and relatively straight forward marine operations. Consequently the exposures are low (AURIS, 1994,2).
7.3.2. Risk analysis indicates that the probabilities of a fatal injury occurring are between 0.005 - 0.014 for this option. This is due in part to the comparatively low hazard potential of the deep sea disposal operations, but is mainly the result of the low exposures (low number of man-hours) involved.
7.3.3. Occupational health risks will be minimal in this option, as onshore dismantling is not required.
7.4. SUMMARY OF SAFETY AND RISK IMPLICATIONS
7.4.1. The complex and labour intensive requirements of horizontal dismantling make the health and safety risks higher for this option than deep sea disposal. On this basis, the deep sea disposal option is preferred.
8.1. INTRODUCTION
8.1.1. Each option has the potential to affect different environments. Horizontal dismantling operations take place in the North Sea, the shallow coastal environment, an estuarine environment and the onshore dismantling and disposal locations. Deep sea disposal operation takes place in the North Sea and the NE Atlantic.
8.1.2. Brent Spar contains small, residual quantities of materials and substances which have the potential to cause impact if released to the environment. Three groups of materials have been subjected to particular consideration, these are heavy metals and quantities of other regulated materials, petrogenic hydrocarbons and naturally occurring radioactive materials.
8.2. OPTION 1 - HORIZONTAL DISMANTLING
8.2.1. Successful execution of the horizontal, onshore dismantling of the Spar would result in negligible impacts to the marine environment of the North Sea (AURIS, 1994,1).
8.2.2. Impacts to onshore, terrestrial environments associated with dismantling the buoy and the removal of its contents are considered mostly short lived and geographically localised, similar to the everyday impacts that might be experienced at any coastal, industrial facility.
8.2.3. Material retrieved from the buoy during onshore dismantling would either be sold for scrap, recycled or disposed of at authorised sites. These sites are properly designed, constructed and managed for disposal of materials and no impacts would be anticipated from these activities.
8.2.4. The complex operations involved create a potential for unplanned events to occur, and subsequent impact to North Sea or coastal environments through the accidental release of materials, or at worst, premature loss of the structure. Impacts from accidental releases would be confined to the immediate vicinity of the structure. Depending on the location, premature of loss of the structure could cause significant local impacts on fishing, shipping and other users of the sea, particularly if it occurred in a shallow water coastal area.
8.3. OPTION 4- DEEP WATER DISPOSAL
8.3.1. Successful execution of the deep water disposal option would result in small localised impacts at an authorised dumping site in the NE Atlantic. It would have little or no risk to the North Sea or coastal environments (AURIS, 1994,1 ;Metocean, 1993,8&9).
8.3.2. Regulated materials contained within the topsides of the Spar would be removed, as far as is reasonably practicable, in order to minimise any potential for environmental impact. These materials would be disposed of at authorised land-fill sites and no impacts are envisaged from these activities.
8.3.3. The deep ocean environment supports low densities of animals and a small range of species, and is essentially isolated from the surface and upper ocean. From the results of available deep ocean survey data, the environmental impacts of deep water disposal would be small and confined to a small area of the deep sea bed at the NE Atlantic disposal site, and to that part of the water column immediately adjacent to the sea bed.
8.3.4. Exploitation of resources of the deep ocean (> 1 ,500m) is currently limited to laying of submarine cables and military activities. The sites have been selected to avoid interactions with these interests. This option provides no potential for interaction with other resource users at the present time or, indeed, for the foreseeable future.
8.3.5. The potential for unplanned events exist during the tow. There is less risk of premature sinking of the Spar as its floatation in the vertical does not rely on integrity of the storage tanks. As the majority of the towing route is in deep water, away from the coast, the impact of premature sinking would be less significant than a similar event occurring in shallow coastal waters.
8.4. SUMMARY OF ENVIRONMENTAL CONSIDERATIONS
8.4.1. - The relatively small amount of contaminants on and in the Brent Spar means that the impacts arising from their discharge or release into any environment would be small and localised.
8.4.2. In the event of premature sinking of the Spar, the physical presence of the structure on the sea bed would be of greater significance to other users of the sea, in the shallow North Sea or coastal environment than in the deep waters of the North East Atlantic. Although the areas of the North Sea along the tow routes would be exposed to this risk in both options, only the horizontal dismantling option would expose the coastal environment to risks of this nature .
8.4.3. Deep sea disposal will have a small localised impact at the deep water disposal site but no effect on the coastal or onshore environment. As planned, horizontal dismantling will have negligible impacts on the marine environment, and any effects onshore will be extremely localised. However, there is greater potential for an unplanned event during horizontal dismantling, and if this were to occur in shallow inshore waters there could be a significant impact on other users of the sea. The environmental impacts of each option are therefore evenly balanced.
9.1. INTRODUCTION
9.1.1. The cost estimates in this section have been compiled to reflect the latest figures derived during option development.
9.2. OPTION 1 - HORIZONTAL DISMANTLING
9.2.1. The Cost estimates for horizontal dismantling total Ѓ46 million. The cost breakdown is summarised below: _
CONTINGENCY (specific items) Ѓ6 594.2
Sub-Total (specific Items) Ѓ39 565.5
3. FEASIBILITY STUDIES Ѓ864.0 _
4 MANIFOLD COVER Ѓ1 317.8 _
5 DEBRIS CLEARANCE Ѓ468.5
6. PROJECT MANAGEMENT Ѓ2 671.6
CONTINGENCY (common items) Ѓ1 064. 4
Sub-Total (common items) Ѓ6 386.2
TOTAL Ѓ45 951.7
9.3. OPTION 4 - DEEP WATER DISPOSAL
9.3.1. The cost estimates for deep water disposal total ,11.8 million. The cost breakdown is summarised below:
2. TOW & DISPOSE
CONTINGENCY (specific items) Ѓ1 238.9 Sub-Total (specific Items) Ѓ7 433.2
TOTAL Ѓ11,766.8
9.4. SUMMARY OF COST CONSIDERATIONS
9.4.1.- Horizontal dismantling represents a significantly higher cost than deep water disposal. The more attractive option economically is deep sea disposal.
10.1. CONSULTATIONS
10.1.1. Discussions on the proposed Abandonment Plan for the Brent Spar were initiated between Shell Expro and the Department of Trade and industry in 1992. Under the Petroleum Act 1987, consultation on the Abandonment Plan is required with the following organisations:
10.1.2. During the first quarter of 1994, Shell Expro notified these organisations of the proposed Abandonment Plan, appraised them of the options considered, and their preferred choice. Consultation meetings with the fishermens' organisations have also been held through the Scottish Fishermen's Federation. Detailed discussions have been held with the Scottish Office Agriculture and Fisheries Department, Her Majesty's Industrial Pollution Inspectorate and the Health and Safety Executive. Other interested government bodies were also notified of the Abandonment Plan, including the Ministry of Defence, Hydrographer of the Navy, Crown Estates Commissioners for Scotland and the Department of Transport.
10.2. ACCEPTABILITY
10.2.1. No objections have been raised to Shell Expro's conclusion that deep sea disposal is the preferred option for abandoning the Brent Spar.
11.1. BEST PRACTICABLE ENVIRONMENTAL OPTION
11.1.1. From a technical perspective the deep sea disposal option is demonstrated to be simpler than the horizontal dismantling option. The latter involves more complex engineering and marine operations and higher risk of unplanned events and technical uncertainties.
11.1.2. The safety and risk implications demonstrate that the complex and labour intensive activities involved in horizontal dismantling constitutes a higher safety and risk exposure to the workforce, compared to deep sea disposal.
11.1.3. In either option, it can be demonstrated that there is a small or negligible environmental impact. The operations associated with the deep water disposal would have small localised effects at the deep water site. The planned operations associated with onshore dismantling and disposal would have negligible impacts.
11.1.4. Although horizontal dismantling would lead to negligible environmental effects, provided no-mishaps occur, the more complicated activities involved lead to an increased potential for unplanned events. The areas that might be affected have higher sensitivity than those affected by deep sea disposal, particularly in the event of premature loss of the buoy where the physical presence of the wreck could present an obstruction to shipping or fishing activities.
11.1.5. From a cost perspective the deep water option is significantly less expensive compared with horizontal onshore disposal.
11.1.6. Consultations have been held with responsible government departments and other interested parties. No objections to the deep water disposal option have been raised.
11.2. CONCLUSION
The Best Practicable Environmental Option assessment significantly favours Deep Sea Disposal of the Brent Spar on the basis that:
Alternative methods are technically complex
It greatly reduces the risks to personnel engaged in the abandonment
It offers negligible environmental disadvantages and reduces the risk to other assets and resources at sea and on the coast
It is the lowest cost option It is acceptable to the authorities and their consultees
The Deep Sea Disposal option has therefore been selected for the abandonment of the Brent Spar Buoy in the 2nd and 3rd quarters of 1995.
1. The Environmental Impacts of Two Possible Disposal Options for the Brent Spar Buoy. The University of Aberdeen, January 1994.
2. A Risk Analysis of Two Possible Disposal Options for the Brent Spar Buoy. The University of Aberdeen, January 1994.
3. Statement of International and UK Law and Practice with Regard to the Disposal of Radioactive Wastes at Sea Arising from the Abandonment of the Brent Spar.
Environment and Resource Technology Ltd, July 1993.
4. Brent Spar Removal and Dumping. Global Maritime, April 1993.
5. Likelihood And Extent of Contamination of Low Specific Activity Scale, Sand and Sludges within the Oil Storage Tanks and Associated Systems on the Brent Spar Loading Facility.
ICI Tracerco, May 1993.
6. Radiological Impact Assessment of the Disposal of. The Brent Spar in Accordance with the Principle of Best Practicable Environmental Option (BPEO).
ICI Tracerco, May 1993
7. Brent Spar Abandonment: Assessment of Items and Materials for Retention. Metocean, March 1993
8. Evaluation of Environmental Aspects of the Deep Water Disposal Option. Metocean, April 1993.
9. Best Practicable Environmental Option Evaluation Report. Metocean, June 1993
10. Decommissioning and Abandonment Screening Study for the Brent Spar Facility.
McDermott Engineering (Europe) Ltd, March 1993
11. Feasibility Study for the Onshore Disposal of the
Brent Spar Facility.
McDermott Engineering (Europe) Ltd, April 1993.
Page 31
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