Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

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1 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area Report No. 338 February 2003

2 P ublications Global experience e International Association of Oil & Gas Producers has access to a wealth of technical knowledge and experience with its members operating around the world in many different terrains. We collate and distil this valuable knowledge for the industry to use as guidelines for good practice by individual members. Consistent high quality database and guidelines Our overall aim is to ensure a consistent approach to training, management and best practice throughout the world. e oil and gas exploration and production industry recognises the need to develop consistent databases and records in certain fields. e OGP s members are encouraged to use the guidelines as a starting point for their operations or to supplement their own policies and regulations which may apply locally. Internationally recognised source of industry information Many of our guidelines have been recognised and used by international authorities and safety and environmental bodies. Requests come from governments and non-government organisations around the world as well as from non-member companies. Disclaimer Whilst every eff ort has been made to ensure the accuracy of the information contained in this publication, neither the OGP nor any of its members past present or future warrants its accuracy or will, regardless of its or their negligence, assume liability for any foreseeable or unforeseeable use made thereof, which liability is hereby excluded. Consequently, such use is at the recipient s own risk on the basis that any use by the recipient constitutes agreement to the terms of this disclaimer. e recipient is obliged to inform any subsequent recipient of such terms. Copyright OGP All rights are reserved. Material may not be copied, reproduced, republished, downloaded, stored in any retrieval system, posted, broadcast or transmitted in any form in any way or by any means except for your own personal non-commercial home use. Any other use requires the prior written permission of the OGP. ese Terms and Conditions shall be governed by and construed in accordance with the laws of England and Wales. Disputes arising here from shall be exclusively subject to the jurisdiction of the courts of England and Wales.

3 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area Report No: 338 February 2003

4 Contributers Erik Hjelde TotalFinaElf Exploration Norge AS Chairman Bob Hemmings Shell Exploration Egil Olsen ExxonMobil International Ove Tobias Gudmestad Statoil ASA Kjell orvald Sørensen Norsk Hydro asa Michael Hall ConocoPhillips

5 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area Summary e objective of this document is to present the experienced gained by the industry in the period in a state-of-art review of the technical challenges and other assessment issues considered in order to identify the best disposal option for disused offshore concrete gravity substructures within the OSPAR Maritime Area. OSPAR Decision 98/3 provides the regulatory framework for decommissioning all offshore structures. In respect of gravity based concrete structures the Decision states that e dumping, and the leaving wholly or partly in place, of disused offshore installations within the maritime area is prohibited, but adds that if the competent authority of the Contracting Party concerned is satisfied that an assessment shows that there are significant reasons why an alternative disposal is preferable to reuse or recycling or final disposal on land, it may issue a permit for a concrete installation...to be dumped or left wholly or partly in place. e part of the concrete platform where such alternative disposal options may be assessed would be the concrete substructure; ie the load bearing structure supporting the topside facilities. No derogation possibility exists for the topside facilities. ere are altogether 27 concrete platforms located within the maritime area of the OSPAR Convention, in Norwegian (12), British (12), Dutch (2) and Danish (1) sectors of the North Sea. Between the adoption of Decision OSPAR 98/3 and July 2002, decommissioning of 4 concrete platforms has been considered. Related studies have been carried out and completed and they represent most of the knowledge gained by the industry since e two North Sea operators who have presented decommissioning proposals on behalf of the their co-ventures, have considered the following main disposal options for four disused offshore concrete platforms: Removal for onshore disposal Removal for deep water disposal Partial removal (cut down the structure down to -55m to respect the IMO Guidelines) Leave in place is report highlights the main findings on the four key elements in the comparative assessment of each disposal option: Technical feasibility Safety for personnel Environmental impact Cost is review identifies several uncertainties associated with the removal of both first and second-generation concrete gravity structures such that a case-by-case evaluation will be required to assess the specific circumstances for each installation. e first generation of offshore concrete gravity platforms installed in the 1970s were not designed or constructed for future removal operations. Although provisions for removal were incorporated into the design of later, second-generation concrete platforms, these may not be fully effective because the obstacles to and hazards associated with removal were not appreciated. An important development over the period of this review has been the introduction of a comprehensive programme of consultation involving a wide range of stakeholders, experts and other users of the sea to view the question of decommissioning from as many angles as possible. is consultation and engagement process has been pivotal in arriving at balanced conclusions in respect of the major decommissioning activity that has taken place between 1998 and

6 International Association of Oil & Gas Producers e need for monitoring of concrete substructures left in place is highlighted. Concrete structures left in the marine environment will degrade slowly and may be expected to remain standing for 500 to 1000 years. Shorter-term contamination of the marine environment due to residual oil in storage chambers and pipe-work is not expected to be significant. Future liability is addressed where the responsibility remains with present owners unless otherwise agreed with the regulators. It is particularly the long-term liability that is of concern for both the industry and the authorities. 2

7 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area Table of contents 1 Introduction Description of concrete gravity platforms Design Construction Installation Population of concrete gravity platforms Concrete gravity platforms in the OSPAR Maritime Area Concrete gravity platforms outside the OSPAR Maritime Area International regulatory requirements for decommissioning Decommissioning alternatives Removal Removal for deep water disposal Partial removal Leave in place Safety Environmental impact Re-float for onshore disposal Deepwater disposal Cutting to -55 metres Leave in place Long-term fate of concrete structures Monitoring Liability Cost Decommissioning experience and future plans Recent work on disposal of concrete platforms Future decommissioning plans Public consultation Conclusions Appendix 1 Concrete gravity platforms within the OSPAR Maritime Area Appendix 2 Concrete gravity platforms outside the OSPAR Maritime Area Reference List

8 International Association of Oil & Gas Producers 1 Introduction In 1996, the International Association of Oil and Gas Producers - OGP, (then the Oil Industry International Exploration and Production Forum - E & P Forum) published a report (E&P Forum report number 240) on decommissioning offshore gravity-based concrete structures, from the perspective of the international regulatory regime in force at that time. At its Ministerial level conference in 1998 Contracting Parties to the OSPAR Convention agreed a new and binding Decision (Decision (98/3) on disposal of disused offshore installations. At the heart of this Decision was the recognition that re-use, recycling or final disposal on land will generally be the preferred option for decommissioning offshore installations. Nonetheless, recognising the particular problems associated with the decommissioning large concrete structures, the decision also set out conditions whereby these structures might be left in place (wholly or partially) or dumped at sea, including a detailed consultation mechanism that would engage all contracting parties. e final decision on decommissioning would still reside with the national competent authority. e objective of this document is to update the earlier 1996 report, taking into account knowledge and experience gained by the industry in the period 1998 to 2002 and in the light of the new regulatory conditions for the North East Atlantic, focusing in particular on the issues and risks associated with the decommissioning options considered. 4

9 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area 2 Description of concrete gravity platforms 2.1 Design A concrete gravity platform is one that is placed on the seabed and by its own weight is capable of withstanding the environmental forces it may be exposed to during its lifetime. Most of the platforms are additionally stabilised by skirts that penetrate into the seabed. ese platforms are huge in size and weight. Some of them are among the most impressive structures ever built. e weights of the concrete substructures range from 3,000 tonnes to 1,200,000 tonnes, and support topsides weighing from between 5,000 to 52,000 tonnes. Some of the concrete substructures have oil storage ranging from 400,000 to 2,000,000 barrels (approximately 50,000 to 270,000 tonnes) (see Appendix 1 and 2 for further details). Main purpose of most concrete gravity platforms was to provide storage facilities for oil at the offshore location at a time when no, or few export pipelines were available for transport of oil from the oil fields to shore. e aim was to provide sufficient storage capacity in the platform base storage cells to enable continued production from the field. e stored oil would then typically be pumped from the platform storage cells via an offloading system to shuttle tankers. Concrete structures were also designed to provide sufficient support for topsides loads of more than tonnes e requirement for new fixed concrete structures with offshore storage capabilities has gradually decreased with the development of offshore pipeline infrastructure and the introduction of new technology including sub sea engineering, flexible risers and based on Floating Production Storage and Offloading installations (FPSOs). One advantage of the concrete gravity based structures compared with conventional piled steel jacket structures, was that they could be floated/towed out to the installation site and installed with the topsides already in place. e installation could thus to a great extent be completed onshore/inshore before tow-out to the field, thereby minimising offshore hook-up and commissioning work. Since the 1970s, several concrete platform designs have been developed. Most of the designs have in common a base caisson (normally for storage of oil) and shafts penetrating the water surface to give support for the topside structures. e shafts normally contain utility systems for offloading, draw down and ballast operations, or they serve as drilling shafts. e most common concrete designs are: Condeep (with one, two, three or four columns) see Figure 2.1 ANDOC (with four columns) see Figure 2.2 Sea Tank (with two or four columns) see Figure 2.3 C G Doris see Figure 2.4 Ove Arup see Figure 2.5 e first concrete gravity platform to be installed in the North Sea was a C G Doris platform, the Ekofisk Tank, in Norwegian waters in June During summer 1975, three other concrete platforms were installed, two Condeeps and another C G Doris platform; all placed in the UK sector of the North Sea. After these first successful installations of concrete gravity platforms, a number of different designs was developed. e last concrete platform was installed in

10 International Association of Oil & Gas Producers Figure 2.1: A typical Condeep design Figure 2.2: A typical ANDOC design (Anglo Dutch Offshore Concrete) Figure 2.3 A typical Sea Tank Design Figure 2.4 A typical concrete gravity platform designed by Doris Engineering Figure 2.5 platform where the base is of concrete with storage capacity on which a steel jack-up rig is fixed 2.2 Construction e lower part of the concrete gravity structure including the skirts, is built in a dry dock. When the lower part of the caisson or storage tanks had been fabricated and has reached a certain height, the concrete substructure is floated out of the dry dock and moored at an inshore deep-water site where the pouring of concrete continues. As the construction advances the structure is more or less continuously ballasted down to maintain a workable height for slip-forming activities. e outfitting of the shafts then takes place before the deck structure is installed. e topsides on some concrete substructures are installed inshore, in components, by a heavy lift vessel before being towed offshore. On others, the deck structure and modules are installed as a complete unit onto the concrete substructure in sheltered inshore waters. e concrete substructure is ballasted with water so that only about 5 metres of the columns protrude above water. Barges then position the complete topsides over the concrete columns. e concrete substructure is then de-ballasted and gradually the weight of the topsides is transferred onto the concrete substructure. 6

11 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area A number of incidents has shown the deep ballasting operation to be very critical as extreme water pressure is applied to the concrete substructure. One concrete substructure collapsed during such an operation in e implosion that followed as it sank caused the concrete substructure to be completely broken up. Other structures have shown severe cracking without reaching a catastrophic stage. Such uncertainties in question be an important issue when addressing the technical challenges of potential re-floating during decommissioning. ese factors are discussed later in Section 5.1. A distinct benefit of installing the complete topsides with modules on the concrete substructure in sheltered waters is that most of the hook-up and commissioning work is performed before towing the complete platform to its final location offshore. is has meant that the platform could be operational very shortly after it was safely installed. 2.3 Installation Concrete gravity platforms installed prior to 1979 were equipped with a simplified installation system consisting of a combined water depletion and grout system. is system was used for drainage of water under the platform and in the skirt compartments during platform installation. Following platform installation, the system was used for placing grout under the platform, thereby securing full contact between the platform underside and the seabed. Water and grout return lines were also installed. ese were used for draining out the displaced water, while injecting grout under the platform and enabled the installation team to check that the grout had been distributed evenly under the platform. e grout thus ensured that the contact pressure was equally distributed over the foundation area. ere is, however, uncertainty as to whether the grout would stick to the underside of the platform during a removal attempt, or whether it would fall off when the platform lifts off from the seabed. A sudden loss of the grout may have an adverse effect on the stability of the platform (see also Section 5.1.2). From 1979, platforms installed in the North Sea (so-called second generation installations) were equipped with a more sophisticated installation system involving separate water removal system for use in the installation phase. is system was not filled with grout during the grouting operation but was sealed off. It was intended that this system could be used to inject water under the platform in a controlled way during a possible re-float operation, in order to assist in loosening the platform from the seabed. 7

12 International Association of Oil & Gas Producers 3 Population of concrete gravity platforms 3.1 Concrete gravity platforms in the OSPAR Maritime Area e OSPAR region covers the whole of the North East Atlantic area including the North Sea. All together there are 27 concrete platforms in this maritime area (see Appendix 1 for details). ere are 12 concrete gravity base platforms in Norwegian waters in water depths from 70 to 330 metres. e earliest, the Ekofisk Tank, was installed in e largest concrete platform ever built is the Troll Gas platform installed in e UK sector has 12 concrete platforms, the majority of which were installed in the 1970s. e last concrete structure to be installed was the Harding platform in 1995 (concrete base only). Two concrete platforms are located offshore the Netherlands and one offshore Denmark. e Arne South platform in the Danish Sector was installed in 1999 and is the last concrete platform to be installed in the OSPAR Maritime Area. Of the 27 concrete platforms in the North Sea, 16 have facilities for oil storage within the base of the structure. Figures 3.1, 3.2 and 3.3 show respectively, the type (in terms of first or second generation), location of the concrete gravity platforms within the OSPAR Maritime area as well as the number in different water depths. e notation second generation indicates that removal was addressed as a design condition during design and construction. Figure 3.1: Number of concrete platforms in the OSPAR Maritime Area Figure 3.3 Number of Concrete Platforms per depth interval in the OSPAR Maritime Area 8

13 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area DRAUGEN STATFJORD C DUNLIN A A B C GULLFAKS CORMORANT A D A B C BRENT B NINIAN CENTRAL TROLL-A OSEBERG-A Norway FRIGG-TP1 FRIGG-CDP1 FRIGG-TCP2 HARDING BERYL A MCP-01 SLEIPNER-A EKOFISK 2/4-T SOUTH ARNE A Denmark F3-FB-1P UK RAVENSPURN NORTH CP Germany HALFWEG Netherlands Figure 3.2 Locations of Concrete Platforms in the OSPAR Maritime Area 3.2 Concrete gravity platforms outside the OSPAR Maritime Area Concrete has also been used for platform construction in other parts of the world, albeit to a lesser extent than the North Sea: notably in Australia, where three structures were installed in the mid-1990s; the recently installed Malampaya concrete structure in the Philippines; the massive Hibernia platform offshore Canada, and two small structures in the shallow waters of the Baltic. ese latter two platforms at Schwedeneck See are currently being decommissioned, and removal is expected in the near future. Details of the platforms are provided in Appendix 2. 9

14 International Association of Oil & Gas Producers 4 International regulatory requirements for decommissioning Decommissioning procedures for disused offshore installations are generally set out in national legislation, with accompanying guideline and practice documents. Internationally, there are a number of agreements relating to aspects decommissioning, principally addressing partial removal and disposal at sea. e IMO Guidelines and Standards for the removal of Offshore Installations adopted by IMO Contracting States in 1989, set out conditions for removal of installations with the aim of protecting navigation and the safety of other legitimate users of the sea. In essence the guidelines suggest that where complete removal is not possible, partial removal should leave an unobstructed water column of 55 metres. e London Convention 1972 (formerly known as the London Dumping Convention) is an agreement that regulates dumping material at sea (including offshore installations). e 1996 Protocol to the London Convention 1972 categorises offshore installations as platforms or other man-made structures at sea, and although the Protocol is not in force, the Contracting Parties to the 1972 Convention have adopted Guidelines for assessing disposal options. In addition to being signatories to the London Convention, States littoral to the North East Atlantic are also signatories to the OSPAR Convention. Annex 3 to the agreement contains the provision relating to the prevention and elimination of pollution from offshore installations. Although this Annex sets out general conditions, subsequent measures agreed by Contracting Parties have tightened the regime as regards disposal at sea. In particular, Decision 98/3 contains a virtual prohibition of disposal for all installations with a limited and small number of exceptions including large concrete structures. Any proposal for disposal at sea (including leaving in place is subject to an extensive international consultation exercise, but with the final decision resting with the national competent authority (taking into account the views of other Contracting States). 10

15 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area 5 Decommissioning alternatives In the specific case of the OSPAR region, while the regulation allows for disposal at sea as a decommissioning option, the option only relates to the concrete substructure. Topsides need to be removed to land unless there are exceptional or unforeseen circumstances or where the topside support structure is an integral part of the sub-structure. is is frequently the case for concrete gravity structures. Any recommendation to dispose of a concrete substructure at sea needs to be supported by a detailed comparative assessment of the disposal options. e following sections set out the main issues that need to be considered in determining the best disposal option for a concrete substructure. 5.1 Removal As explained in Section 2.1, the first generation of offshore concrete gravity platforms installed in the seventies were not designed or constructed for a future removal operation. Later concrete platforms were designed with removal in mind, but the extent of the challenges and possible obstacles and hazards that might occur may not always have been fully appreciated in the original design. Hence, the uncertainties identified in the first generation concrete platforms may also be valid for the second-generation concrete substructures Removal method For large concrete gravity platforms, the most likely removal method will, in essence, be to reverse the method of installation. However, there are a number of issues that the installation operation did not need to consider but that would require consideration upon removal. All concrete platforms located in the North Sea today have been installed by controlling the level of water ballast within the concrete substructure. When on location, a careful increase of the water level allowed safe and accurate positioning of the platform. An adjustment of the relative water levels in the cells of the caisson allowed an on-bottom correction to achieve a true vertical position of the platform. On most of the structures significant amounts of cement grout were injected under the base slab of the platforms to ensure a uniform distribution of loads on to the seabed. In principle, a reverse installation could also minimise the offshore work by allowing removal of all the topside facilities to shore. ese can then be removed in a sheltered location where the weather conditions allow a more efficient execution of work. However, studies have shown that weight increases during the operating phase may require a significant amount of the topside loads to be removed before engaging in a re-float operation. is is because limited buoyancy may be available to lift the structure from the seabed. A weight uncertainty also arises due sand produced from the reservoir trapped in the storage cells, the possible adherence of under-base grout and soil, marine growth and the absorption of water in the cement matrix. e exact weight of the topsides may also add to the uncertainties as considerable amount of equipment have been added during an operational life often more than 30 years. To secure an adequate weight tolerance for the re-float operation, a number of offshore lifts may thus be required prior to removal in order to reduce the overall weight. All piping penetrations through the concrete hull below water level have to be closed to ensure a watertight structure. Any excessive leaks will jeopardise the platform s ability to remain afloat in all phases until it is safely located in a dry dock for final deconstruction. is period could last for up to three years after initial removal. Essential equipment required during the re-float phase will be the water ballast systems and pipe connections inside the concrete substructures. Originally, these systems ensured a 11

16 International Association of Oil & Gas Producers gradual filling of water ballast to ensure a controlled touchdown on the seabed. On the first generation of platforms, these ballast systems where typically only designed as installation aids and not maintained or grouted up after the structure was in place. On some structures it will be necessary to inject water under the base slab to mobilise additional upward force to be able to pull the base skirts out of the subsoil. is water injection has to be carefully monitored in parallel with the water de-ballasting during the re-float operations. For safety reasons it is preferred that the re-float operations are performed with no personnel on the platform. e towing route to shore will have to be carefully evaluated to ensure sufficient draught during the towing operation. Some structures may have such deep draughts that the inshore sheltered areas that they can enter may be limited Technical uncertainties Each of the platform designs described in Section 2 has its own features depending on the service for which they were intended. e feasibility of a removal operation will depend on clarification of a number of uncertainties that will exist, even if the concrete platform was initially designed with future removal in mind. Studies recently undertaken have identified the following main common uncertainties and difficulties related to the removal of concrete gravity base structures. ese are: Sealing and testing of penetrations Structural integrity in re-float phase Under-base grout Sudden uncontrolled release Under base injection Mechanical systems Sealing and testing of penetrations Sealing of penetrations and cracks in the concrete substructures are seen as major concerns. e problems include limited or no access to penetrations and cracks, inability to test a sealed penetration, difficulties in detecting and sealing cracks etc. Conductor penetrations in drill shafts may be particularly difficult to address. Although cracks may have been sealed during the operational phase of the installation, these may re-open and cause leaks during re-floatation and towing operation as the loads change. Structural integrity in re-float phase During the re-float operations the concrete platform may need to be de-ballasted to a greater extent than during the installation. Additional uplift forces to overcome friction and suction in the seabed may be required. It may also be difficult to empty one cell or buoyancy compartment during de-ballasting. is will require additional de-ballasting in the remaining compartments to compensate for the non-emptied cell(s). is, in turn, may give high differential pressures in the compartments, that may lead to total collapse if the structural strength is exceeded. A preventive measure would be to introduce compressed air into the cells. is would assist in maintaining the overall structural integrity and mitigate the stresses in certain structural elements. However, this must be carefully evaluated as it may introduce a risk of overstressing vital structural parts. An excessive pop-up to a level where the air pressure exceeds the 12

17 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area ambient water pressure could introduce severe structural consequences, as well as being a hazard to personnel and vessels involved in the re-float operation. An excessive differential loading between the cells may cause collapse of internal walls on some types of structures. is concern is also applicable if the platform experiences excessive tilt during the re-float phase. As each individual concrete structure has its own characteristics, a thorough structural analysis checking all applicable load cases will be required to eliminate these uncertainties. e current applicability of the codes used in the original design and any experience gained, have to be duly considered. Over the last 30 years, the design codes have introduced more stringent structural strength requirements. All structural analysis for removal operations should therefore be based on conservative assumptions reflecting any deterioration and any uncertainties that affect the design. e safety factor should not be lower than specified in current design codes for construction, installations and operations. is structural check will also be necessary for second-generation concrete platforms having re-float as a load condition in the original design. Allowance must be made for designs that did not fully recognise the challenges and possible obstacles that might occur during a refloat operations; often taking place over 30 years after installation. Under base grout On some platforms, grout was injected under the slab to ensure a uniform soil pressure after installation. Also, during completion of the production wells, grout was injected and is expected to have been spread underneath and become attached to the slab. Prior to a refloatation there is no method available to assess the amount of grout under the base slab, or whether or not the grout will remain attached to the base. If a re-floatation is carried out and a large amount of grout is attached to the underside, inshore deconstruction is not advisable, since there is no method to remove the grouting from the underside within an acceptable risk. Both mechanical equipment and explosives have been evaluated for use in detaching the grout. However, it should be noted that use of such methods might cause a sudden release of a large amount of grout and cause instability of the substructure causing it to sink. Sudden uncontrolled release After release from the seabed, the concrete platform could have unbalanced buoyancy that could cause an uncontrolled release from the seabed. Uncertainties in platform weight and centre of gravity, soil resistance, under base grout lost before, during or after re-float, and possible soil suction may contribute to unbalanced buoyancy. Some platforms have an accumulation of drill cuttings inside the concrete shafts. Deposits of produced sand in the storage compartments also add to the uncertainty in knowing the exact weight of the structure. is could lead to an unpredicted instability and pitching of the structure after being released from the seabed. Under base injection Injection of water under the base slab will require certain strength in the upper soil layers under the platform. Exceeding this threshold will result in failure of the soil, causing channelling or piping thus allowing water to escape preventing a pressure build-up under the base. Placing gravel around the base of the substructure could in some instances reduce the risk for developing channelling in the soil. 13

18 International Association of Oil & Gas Producers Mechanical systems e de-ballasting operations as well as any under base water injection will require mechanical systems that are proven to be fully reliable in all functions and operations. e original systems are very likely to have deteriorated after many years in seawater unless they have been properly maintained and tested during the in-service life of the platform. Demanding requirements on the durability and reliability of the system were not fully accounted for during the design, as they would stay idle for decades prior to use, without the opportunity to test the system. e original carbon steel piping may, therefore, have to be changed before the system can be used. Part of the piping embedded in concrete may have to be flushed and smaller diameter, flexible or expanding piping inserted into the old and deteriorated pipelines. Prior to the operations, any parts used for removal must be thoroughly inspected, tested and commissioned. However, it may often be difficult to inspect or even impossible to replace these systems. e only alternative is then to install an external ballast piping system linking each buoyancy compartment together that would be located outside the concrete substructure. is will involve additional risks with extensive use of divers. A new buoyancy system would require penetrations to be made in the storage tanks that would introduce potential new points of leakage. An external system would also be exposed to dropped objects and impact from collision with support vessels. Such operations have not been executed before and could add a considerable cost to the project. Methods and procedures need to be developed and tested inshore before a conclusion can be drawn on their feasibility. It is also questionable if such solutions will give the required reliability needed to launch a re-float operation within the acceptance criteria. Case-by-case evaluation Finally, it is important to note that each platform will have its own and unique problems (for example weight increases, stability, cracks, structural strength, high probability of leakage etc), and that each platform therefore should be considered on a case by case basis. Only indepth studies for each installation can conclude whether its re-floatation is possible or not. Appropriate risk analysis is a tool that can be used to establish the risk level compared to the acceptance criteria set for similar offshore operations Towing A towing operation to a sheltered inshore location needs to be considered before a full removal is considered acceptable. e major differences between an installation tow and a removal tow are related to the risk of: Grout attached to the underside of the base slab can fall of and hit a live pipeline; Grout falling resulting in instability of the platform and causing it to sink; Major leakage may occur in sealed penetrations and cracks, causing the platform to sink during an offshore or an inshore phase of the towing route (it could hit an offshore live pipeline, block the entrance to a harbour etc). Towing points on the concrete platform also need to be thoroughly inspected and tested and, if necessary, replaced before a re-floatation and towing operation is attempted. 14

19 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area Inshore/onshore deconstruction e inshore and onshore deconstruction phase for a typical concrete substructure is estimated to take two to four years. erefore, the concrete substructure needs to be kept floating for at least two to three years. e concerns for the inshore deconstruction phase are basically the same as for the re-floatation/ towing operation, however, there are differences, as described below. Detachment of grout from the underside of the base slab while the floating substructure is being cut into small pieces represents an unsafe working site for personnel. Sudden loss of grout is likely to cause instability of the substructure resulting in a tilt, and in a worst case scenario, the sinking of the substructure. If uncontrollable leaks arise due to failure of previously sealed penetrations, in-service deterioration of the piping system and structure, or unpredictable loss of grout and soil from the underside of the base slab, it could have catastrophic consequences resulting in loss of life. If the structure sinks at an inshore location the environmental consequences may be more severe than if it occurs at an offshore location. e increased consequences include the assumption that more fuel will be required onboard the structure to keep the temporary buoyancy system and other temporary systems running required for the deconstruction work, and that the distance from the installation to shore will be only a few hundred metres. On the other hand, it is assumed that any inshore releases can be managed more effectively by use of pumps etc. Concrete substructures that have been used for oil storage would be require cleaning to remove any free oil that could be released prior to its onshore disposal and possibly before any re-float operation is carried out. Of particular concern are the storage cells of the platforms where no access is possible except via a piping system. Concrete is a semi permeable material and it should be assumed that oil has penetrated into the pores of the concrete walls. e extent of oil contamination of the concrete walls inside the storage cells is, however, considered to be relative small as the concrete material is normally very dense. Furthermore, a layer of wax is likely to be deposited on the concrete walls, limiting the oil penetration into the wall. It may be very difficult to remove the oil contained in the concrete pores by water flushing, steam cleaning or other cleaning methods. us the reuse potential of this concrete material may be limited to for example for use as road hardcore or landfill Reuse at another location If a concrete platform can be safely removed from its present location within the acceptance criteria set, a reuse at another location would then be evaluated. However, a number of criteria have to be fulfilled at its new location such as: satisfactory soil condition, water depth, environmental conditions, fulfilling current design codes and level of safety. Reuse of the concrete substructure as, for example bridge foundation or quay support, could be a practical solution compared to an expensive deconstruction work. Each platform would have to be assessed for the particular re-use opportunities that may present themselves. 15

20 International Association of Oil & Gas Producers 5.2 Removal for deep water disposal Removal method e activities in this alternative are essentially the same as those discussed in Section 5.1 Removal. e main difference is that for this alternative the complete topsides (including the main support frame) would need to be removed before the re-float of the concrete substructure takes place. Alternatively, the topsides could be removed when the structure is afloat at an inshore sheltered area, and then the concrete substructure could be towed to an approved deep-water site for disposal. As much of the internal and external steelwork as practicable is likely to be removed for reuse or recycling onshore. Following the re-float operation, the concrete substructure will be towed to an approved deep-water location. By taking water out of the cells and then submerging the substructure by pumping water into the columns an implosion could occur, which would effectively demolish the concrete. For some concrete substructures this method would not be possible due to the design features. In those cases it is likely that the complete substructure would hit the seabed and be severely deformed and disintegrate Technical uncertainties e technical uncertainties in the re-float stage are essentially those valid for the removal and onshore disposal alternative described in Section Towing operation e risks associated with the towing operation are the similar to those for towing to an inshore location. However, the towing route to a deepwater location for disposal may be substantially longer than for removal to land and the weather conditions encountered might be more severe. us the length of good weather periods may be critical. 5.3 Partial removal Partial removal of a concrete substructure represents a removal of parts of the substructure to such an extent that it fulfils the Guidelines given by the International Maritime Organization (IMO), namely to leave a free water column of 55 metres above the remaining structure for safety of navigation (see also Section 4) Removal method Mechanical Means is option presupposes that all the topsides and the external/internal steel works are removed and taken to shore for recycling or deconstruction before the deconstruction of the concrete part commences. Offshore deconstruction alternative entails cutting the concrete substructure into pieces at the offshore location. e concrete pieces are likely to be left next to the remaining substructure. Alternatively they may be lifted on to a vessel and transported to shore for recycling or deconstruction. e internal steel outfitting in the shafts would be removed in reverse installation order to the greatest extent possible. However, it may not be possible to remove some of the outfitting before the concrete structure has been deconstructed down to the level of the actual outfitting. e only controlled method of cutting reinforced concrete is by using cutting tools such as diamond wire or saws controlled by divers. Use of explosives has been evaluated, but studies concluded that it could not be the preferred option as it is not possible to guarantee that present methods will successfully cut the heavily reinforced, pre-stressed structure at the first 16

21 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area attempt. e environmental consequences (noise and possible disturbance of fish and marine mammals) may also be reasons for not using explosives. Mechanical cutting the concrete shafts could either be done from inside a dry shaft or from the outside. An external cofferdam would be required if making the cuts from the inside to prevent ingress of water. Personnel would be required to operate the cutting machinery inside the shaft. For the concrete substructures of column and caisson types, the shafts can be cut to obtain the required depth. However, if the top of the caisson reaches into the >55-metre zone, parts of the caisson would have to be removed. is would represent extensive additional underwater work. For concrete substructures with no shafts, the preferred cutting method would be to cut the substructure down to -55 metres, piece by piece, either lifting away each piece or toppling them outwards. e actual cutting operations would require extensive underwater works that ideally should be performed by remotely operated means. However, extensive use of divers in various operations would almost certainly be required. Initiating structural collapse is option pre-supposes the use of explosives to initiate structural collapse of the concrete structure. e explosives may be placed on the outer surface and/or the inner surface of the structure. e platform is expected to remain as a pulverised heap of concrete and reinforcement on the seabed, and may represent a hazard for bottom trawls. To make the site over-trawlable, the remains of the structure may be re-distributed on the seabed and/or rock may be dumped to cover the remnant structure. Rock dumping may also reduce minor leaching of hydrocarbons to the water column (from residuals attached to the structure and any accumulated drill cuttings). All possible precautions would have to be taken to limit the effect that the explosives would have on fish and other sea mammals present in the area. e time of the year selected for the operation, the type of explosives and the position of the explosives on the structure etc, will be important to limit the effect on the marine environment. However, despite all precautions taken, it is inevitable that some fish would be killed within a few hundred metres of the explosion Technical uncertainties e various methods proposed for cutting the concrete substructures down to -55 metres are considered to be theoretically feasible although there are a number of critical operations that would need to be proven. No experience exists today of cutting such heavily reinforced prestressed concrete structural members under water. e traditional tools used on land such as diamond wire or saw have not been exposed to underwater conditions such as the North Sea. Studies have revealed that prior to launching any offshore works, extensive development and testing of equipment will be required to prove its practical feasibility and efficiency. Diamond wire tool e most likely cutting technique is a diamond wire tool. Different contractors have advanced this as a feasible method. However, the tool will need to be fabricated and tested before a clear conclusion can be drawn on the capability of such a tool to cut reinforced concrete under compression. In the past there have been difficulties with the diamond wire tool, especially if the material to be cut is a composite material and under compression. Most of the load-bearing sections 17

22 International Association of Oil & Gas Producers in any concrete substructure, including the concrete shafts, consist of high strength concrete with an inner and outer dense layer of steel reinforcement and pre-stressing tendons in steel ducts. e pre-stressing tendons ensure that the concrete section remains in compression at extreme wave loads to avoid cracking in the concrete. e pre-stressing tendons were installed in purpose-built ducts in the shafts, tensioned and bonded to the structure by injection of grout in the annulus between the tendons and the duct walls. If the bonding between the cable and the grout is not properly performed, an enormous amount of energy could be released when the pre-stressing tendons are cut. e effect on the concrete of such a release of energy is not fully understood. Another problem, which has been experienced in the past, is controlling the tension in the diamond wire. Any over-tensioning will cause the diamond wire to break. Excess transverse feed velocity of the wire or the presence of vibrations in the tool/ wire could result in over-tensioning the wire. If the wire breaks during the final cuts, the wire has to be cut and abandoned, since the gap created by the wire will close due to shear leg effects or effect of the tension wires. us, a new cut has to start above or below the previous cut. Weaknesses have also been revealed in some of the diamond wire types making them unsuitable for cutting steel material. Diamond saw tool A diamond cutting saw is more likely to be used when access is restricted to only one face of the concrete section to be cut. Studies have shown that the diameter of a diamond saw could reach 3.5 metres to be able to cut structural elements with thickness 80 to 100 cm. is cutting tool would require heavy support to be fixed to the concrete surface to guide the cutting tool in a controlled manner. Jamming of the diamond saw is also very likely for the same reason as described for the diamond wire tools. Explosives e ability of explosives to cut thick (up to one metre) concrete walls effectively underwater with substantial amounts of pre-stressing and reinforcing steel is not well proven and involves many uncertainties. e firing of explosive charges to topple the structures is a point of no return and is likely to result in an unplanned situation from which it may be impossible or extremely difficult and dangerous to recover. Explosives may, however, be used to make the final cut to enable the toppling or bending of a cut section outwards to reach the -55 metre requirement. Structural stability For the non-shaft concrete substructures, the cutting operation of structural members will weaken the structural integrity gradually. By removing structural members the ability to withstand wave forces will be reduced. If it is not possible to complete the work in one summer season, it is very likely that the winter storms will deteriorate the structural strength further; to such an extent that it will be hazardous to send divers back to resume the work the following summer. e storage tanks will also be problematic to deconstruct, since there are no practical methods to divide the structure into smaller parts underwater. e other concern with this disposal option is the stability of the section for the period after the final cuts are made until a heavy lift vessel lifts off the section. e cuts have to be planned and performed in such a way, as to maintain the stability of the section as long as possible. us, three or four sections of the circumference of the legs have to remain intact until a sufficient weather window is forecasted. Holes therefore have to be 18

23 Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area pre-drilled into the concrete walls by divers or remote operated vehicle (ROV) to be able to insert the diamond wire cutting tool and perform the cuts of the sections. As mentioned above, the critical period will be when making the final cuts. If the cutting tool fails during these final cuts the cut section may be lost if the weather worsens. 5.4 Leave in place e leave-in-place alternative presupposes that the topsides are removed and taken to shore for disposal and if considered a hazard, external steelwork would also be removed to shore Work to be done e modules and support frames, forming the topsides, would be removed first. Before removing the deck, the accessible steelwork inside the platform would be removed as far as practicable. On some concrete substructures the support frame consists of concrete beams, often forming part of the main structure. In such cases it is likely that these structural parts would remain with the concrete substructure. Flushing and cleaning of any oil storage tanks would be performed to reduce the content of hydrocarbon and other residuals to a minimum. e internal walls in the storage tanks would not be exposed to the sea outside, but would remain protected inside the storage tanks for natural degradation to take place as the concrete structure slowly deteriorates. Environmental impact assessments are required to demonstrate that any impacts arising are within acceptable limits. e necessary navigation aids would be installed on the substructure in accordance with applicable national and international requirements. e navigation system would be designed in an easily maintained package with back-up systems (for example by means of a helicopter but not dependent on a helicopter deck). A programme for maintaining a reliable navigation system would be designed, agreed with the competent authorities and introduced. Debris around the concrete substructure would be recovered, where practicable and brought to shore Technical uncertainties Removal of topsides would include known technical operations, but could still be very challenging requiring detailed planning and control to prevent major unforeseen events. e hydrocarbon and other residues left in the storage cells of the concrete substructures present an additional challenge. e design allows the cells to be flushed through a complex pipe work system, but on some platforms, this system provides the only access into the cells. Rigorous inspection and conventional cleaning methods by scraping or through use of solvents are either not feasible or environmentally unattractive. Both alternative access and cleaning techniques would need to be developed or a thorough assessment performed to demonstrate the acceptability of any potential impacts to the environment by the gradual release and natural degradation process as the structure slowly deteriorates. See also Section 7. 19