2. Overview of Classification Societies Overview and History of Classification Societies

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1 Applying Classification And Regulatory Standards To Novel Design Concepts For Deepwater Installations - Cooperation Between Classification Society And Designers Authors: Luiz P. Feijo ABS Joseph H. Rousseau ABS Jose H. Vazquez, Ph.D. Bennett & Associates, LLC Abstract: As the exploration and production of oil and gas moves into increasingly deeper waters and harsher environments, designers are meeting the challenges associated with such installations by developing novel concepts for which there are no prescriptive regulatory or industry standards. These novel concepts may incorporate technical characteristics to enhance capabilities for the selected site of operation in order to reduce overall costs, simplify construction and installation, or offer greater operational efficiencies. To keep pace with such technological developments, classification societies must find ways to adapt existing requirements to these new designs in order to participate in the innovation process. This paper uses the development of the MinDOC deepwater floating production installation concept to illustrate how the designer and the selected classification society can work together to define and apply appropriate standards. Such a cooperative approach has helped both the designer and the classification society to more fully understand the innovative technology, concepts and functionalities that are being considered and to identify how these approaches can be harmonized with the applicable regulatory and industry standards while maintaining the feasibility of the project. 1

2 1. Introduction Worldwide energy demand has been growing over the last decade at an average rate of 2.3 percent annually (1) and a steady growth is projected until 2030 (2).. As the majority of the energy produced in the world is petroleum based, continued growth is dependent on finding and developing new oil and gas sources. Deepwater fields constitute one of the few remaining untapped sources for new oil & gas production. Over the past 30 years floating production units have therefore evolved into a mature technology that allow for development of deep water oil and gas reservoirs that would be otherwise unreachable and unprofitable. Floating production, storage and offloading (FPSO) vessels, semi-submersible production units, tension leg platforms (TLPs) and spars are proven floating structures that are now commonly used for offshore production operations in deep waters. Classification societies have an important role in the offshore industry. They issue classification approval in accordance with their own Rules and Guides and provide guidance on compliance with increasingly complex regulatory regimes. Their services cover the development, construction, and operation of offshore units and affiliated companies may also provide guidance for the safety, structural and systems integrity aspects of facilities throughout their life cycle. To push the boundaries associated with increasing water depth and environmental loads, technology is constantly evolving to extend operational capabilities and to provide a flexible solution for the development of short-lived or marginal fields. This evolution aims to provide technically advanced solutions suitable for the application while limiting the capital expenditures by simplifying construction and/or offering greater operational efficiency/flexibility. The technological evolution is reflected in the development of novel concepts for which no, empirical regulatory or industry standards exist. To keep pace with technological development, classification societies must find ways to adapt existing requirements to these new designs without stifling the innovation process. The development of the MinDOC concept is an example of the need for the classification society to determine how to fit a novel concept into the existing classification and regulatory frameworks. This challenge is more easily overcome when there is a cooperation between the designers of the concept and the classification society to develop a clear and complete understanding of the features (strengths and weaknesses) and characteristics of the vessel in order to establish appropriate acceptance criteria. 2

3 This paper outlines the cooperation between Bennett & Associates (BASS, the designers of the MinDOC concept) and the ABS (the classification society) throughout the project, with the objective of defining the requirements suitable for the project. Examples are included to illustrate a few practical cases of this cooperative effort. 2. Overview of Classification Societies 2.1. Overview and History of Classification Societies Classification societies provide a mechanism for the self-regulation of the marine industry. As independent organizations they have no commercial interests related to the design, building, ownership, operation, management, or repair of vessels. As a consequence, Classification societies can direct their primary focus to the development and implementation of standards that promote the safety of life, property and the marine environment. Classification societies origins can be traced back to the late 18th century. In the period since, classification societies developed and became recognized as providing a valuable thirdparty verification service to the shipping industry. As a result of their established position, classification societies became involved in the development and implementation of safety standards when the offshore oil and gas industries emerged in the mid-twentieth century. As the offshore industry continues to develop and more sophisticated technologies are being utilized in drilling and production operations, the need for classification societies to provide third party verification and regulatory compliance support remains as essential as it was at the time of their origins in the eighteenth century Role of Classification Societies In their mission statement, the classification society ABS expresses its corporate objectives to serve the public interest as well as the needs of their clients by promoting the security of life, property and the natural environment primarily through the development and verification of standards To fulfill their mission, classification societies: Develop Rules and Guides to establish minimum acceptable standards for the design of marine vessels. 3

4 Conduct design review of methodologies, documents and drawings to verify the design of a vessel complies with the applicable classification society Rules and Guides. Carry out surveys during the construction of a vessel to verify it is being fabricated in accordance with the approved drawings and in-line with good marine practice. Survey the installation of the sub-sea foundations, anchoring systems, mooring arrangements, hull structure and topsides deck modules for offshore facilities which have a specified location. Perform periodic surveys over the life of a vessel to verify that it remains in compliance with the applicable Rules and Guides. In addition to verifying compliance with the classification requirements, classification societies also act on behalf of coastal authorities and flag States. When acting in such a capacity, a classification society is working under the authority of the appropriate coastal State or the country s maritime authority to verify compliance with applicable national requirements and international regulations. Currently, over one hundred flag States authorize ABS to undertake inspections and verifications on their behalf and to issue the associated statutory certificates. As represented in Figure 1, the activities of classification societies are based around the principles of personnel safety, integrity of assets and protection of the natural environment. Figure 1: Key classification society activities 2.3. The Classification Rules The classification society Rules and Guides are the technical standards used in the classification process. They present criteria for: the design of structural, mechanical, piping, electrical, components and systems; 4

5 the selection, testing and certification of materials of construction, and the methods of joining materials; surveys and testing during new construction; equipment acceptance; and periodic surveys after construction. These criteria are based on published sources and also developed by ABS itself. Sources include the International Maritime Organization (IMO) and other statutory instruments, the International Association of Classification Societies (IACS) unified requirements, and recognized national and international standards. ABS develops additional criteria or refines existing criteria based on its own research, and on the body of knowledge acquired during years of experience in the offshore and marine industries. The Rules and Guides are developed in cooperation with the marine and offshore industries with the objective of setting the standards to be applied during design and construction projects. If a new technology results in a need to develop a new standard or to align with an existing standard, the new Rule is prepared by and submitted for internal review. The proposed Rules are then referred to an ABS Committee structure for review. The ABS Committees include professionals and members of the industry such as owners, operators, shipbuilders, designers and government representatives. They include geographic Committees in the ABS operating divisions as well as Special Committees for certain Rule sets. The Technical Committee performs a final review based on input from the Divisional and Special Committees, and final approval is carried out by the internal Rules Committee. Once the rules are approved, they are published and become effective based on the date of contract signing for a vessel. As discussed above, the evolution of the industry has created a variety of new technologies. The proposed technology typically involves some level/degree of novelty. Novel Concepts are defined as applications or processes that have no previous experience in the environment being proposed. In working with novel concepts it is important to identify the nature of the concept being proposed; it can be: an existing design or process being used in a novel application, an existing design or process that challenges the present boundaries of current applications, or a novel design being used in either a novel or existing application. 5

6 A good understanding of the novel concept and its characteristics by the classification society is necessary to develop an appropriate set of requirements. 3. The MinDOC Concept 3.1. History The MinDOC was conceived in 1997 by Alden Doc Laborde, chairman and founder of Gulf Island Fabricators, who was seeking a deepwater floater that could be built along the Gulf of Mexico to bolster the sagging markets for fixed platforms. In an effort to bring some of the business that was going to foreign builders for the deepwater, he sought several local designers to form a consortium to design, develop and market a Floating Production Unit that could be built in either fabrication yards or shipyards along the Gulf of Mexico. The original members of the MinDOC LLC consortium contributed their respective expertise to the concept: Gulf Island Fabricators - Fabrication & Construction Bennett & Associates - Naval Architecture & Hull Design W.S. Nelson - Topsides Design and Deepwater Project Management W.H. Linder - Topsides Designs and Deepwater Project Management. This team worked together for 4 years developing the Original MinDOC and MinDOC 2. The Original MinDOC and MinDOC 2 were competitors to the classic spar concepts in that they exhibited very low motions, have very deep draft, and did not rely on permanent (high-density) ballast to enhance their pendulum stability. MinDOC 2 was developed in 2000 for a specific Gulf of Mexico application.. The MinDOC 2 was not easily dry towed, which proved to be a disadvantage in case the hull had to be built overseas. This precipitated a design spiral to develop MinDOC 3, which was capable of being transported on several of the existing heavy lift vessels; and was therefore suitable for worldwide construction. The MinDOC 3 concept obtained Approval In Principle from ABS in 2001, proving that the design was sound and developed in accordance with the classification rules. An important lesson learned by the consortium during the development and pricing of the concept in 2000 was that the cost of the various component pieces of the MinDOC may vary dramatically. The nodes can be as much as double the cost of the column and pontoon construction. Construction in a fabrication yard does not follow the same cost basis found in a shipyard. Their different areas of specialization mean that each has areas where they are more or 6

7 less competitive in terms of cost, which tends to make for somewhat competitive overall pricing on either facility. Figure 2: The MinDOC 3 Concept Shortly after the development of the MinDOC 3 began, the two topsides designers withdrew from the consortium and sold their interests to Gulf Island Fabricators and Bennett & Associates since the development costs were primarily for the development of a new hull form. A combination of a soft deepwater market and continued cost of development resulted in Gulf Island selling the design to Bennett & Associates in Presently, Bennett & Associates possess the intellectual property rights to all of the MinDOC designs Novel Features The MinDOC designs are novel in that they are neither spars nor semi-submersibles, but a combination of features of both the spar and the semi-submersible. From its inception the original MinDOC introduced several novel concepts for deep-draft floaters. The subdivision of 7

8 the hull featured a center trunk that permitted access to any one compartment without compromising the watertight integrity of adjacent compartments. The access trunk also housed all the piping for the simplified ballast and bilge systems, providing accessibility to all valves. The ballast system was simplified as compared with typical semi-submersible design and featured an over-the-top pump-in and pump-out system that eliminated the possibility of transferring ballast from one column to another. Additionally, the design was configured to prevent accidental transfer of ballast water from a lower compartment into a higher compartment, should a valve either malfunction or be inadvertently left open. The Titan MinDOC 3, currently under construction for ATP Oil & Gas, features the stability of a spar (pendulum stability) and that of a semi-submersible (waterplane stability), a combination that produces high payload capacity and excellent motion characteristics. The pendulum stability is achieved by the use of high density ballast in the lower raft compartments. Portions of the MinDOC (lower raft, lower columns and upper pontoons) are free-flooding to reduce the scantlings of these members in areas not required for watertight integrity. Buoyancy of the unit is primarily obtained from the upper columns. This configuration allows flexibility for weight growth and design changes that are common during the course of topsides design of a floating production unit. Heave Motions (ft/ft amplitude) Heave Motions for MinDOC 3 -- Titan (with Rig) Angle = 0 Angle = 45 Angle = 90 Angle = 135 Angle = 180 Angle = 225 Angle = 270 Angle = 315 Pitch Motions (deg/ft amplitude) Pitch Motions for MinDOC 3 -- Titan (with Rig) Angle = 0 Angle = 45 Angle = 90 Angle = 135 Angle = 180 Angle = 225 Angle = 270 Angle = Period (s) Period (s) Figure 3: MinDOC Motion Response Amplitude Operators (RAOs) From a dynamic response perspective, the MinDOC 3 also behaves more like a spar than a semi-submersible, with very good heave and pitch motions, as needed to support dry trees, toptensioned risers (TTRs) and steel catenary risers (SCRs) 8

9 The Titan MinDOC 3 is fitted with a drilling unit, making it capable of fully drilling its own wells. This feature provides significant economical advantages by eliminating the need of pre-drilling wells and working over the vertical access wells. The wells can be drilled with a surface blow-out preventer (BOP) stack and are completed with surface trees. Also, this MinDOC features a riser support structure built into the hull rather than the topsides where riser tensioners are typically located. This allows the drill deck elevation to be significantly reduced, providing added protection for the risers at the waterline. Some of the column compartments have been designed for the storage of consumable liquids such as drill water, diesel oil, base oil and drilling mud. One compartment was designed to accept methanol storage while in service. Other compartments house equipment and at one time had a space for an auxiliary generator. Compartments in the upper pontoons and lower columns adjacent to the critical connections were designed to be dewatered to provide access for inspection in these areas as required. This would also allow for repairs in the dry should the requirement ever present itself Surprises Even tough the initial model testing of some of the MinDOC 3 configurations showed negligible to no vortex-induced motions (VIM), the second set of tests done on the Titan MinDOC presented evidence that VIM may be possible on a bare hull. The amount of VIM was not large enough to impact the TTRs or SCRs, since the MinDOC hull has a peak VIM amplitude of about one half of the column diameter (i.e., sway in the order of 25ft), deemed not significant enough to rule out any of the desired riser types. Due to the fact that the field at which the unit will first be located is in one of the highest loop current areas, it was decided to fit the upper columns with strakes. Review of the testing identified that the installation of strakes on the inboard portions of the three upper columns would significantly reduce the amplitude of the VIM and the range of currents over which VIM would occur. Further investigation proved that this would not adversely affect the construction of the unit or interfere with the mooring system. There were a couple of other surprises pertaining to the analytical portion of the approval process. One of these was the level of detail required on appurtenances and their connection points to the hull. This was the case for the strakes and anodes. Another early surprise that was 9

10 addressed through conversations between BASS and ABS was the pronounced effect on fatigue life of large thickness insert plates in critical connections. BASS and ABS spent considerable time discussing the validity and applicability of the various idealized stress concentration factors (SCF) formulae for plate thickness changes, and finally settled on a combination of theoretical calculations based on beam theory, and detailed solid model finite element (FE) analysis. The initial design called for thickness changes with ratios as large as 4 to 1. For areas that needed to be fabricated early on, the final design added transition plates to avoid the large SCFs near the critical connection. For areas such as the stabbing post and the connection to the topsides leg, a series of detailed axisymmetric and solid-element analyses were used to quantify the SCFs, accounting not only for thickness changes but for eccentricity of the tubular members Other MinDOCs The MinDOC is a highly scalable design and can accommodate either small or large topside loads and configurations. Several versions have been developed that range in topside loads of 3,000 ST to 50,000 ST and water depths of 1,500 feet to near 10,000 feet, and having traditional rectangular as well as T-shaped and triangular topsides. MinDOC can be designed for surface trees, a hub to tie back subsea wells or a combination of both as is true for the Titan. A version for topside integration at a quay side has been designed to permit installation in remote areas. Although the three-column version is preferred for several reasons pertaining to efficiency, four-column versions have also been studied. The three-column version offers added simplicity for production crews, and the loads applied to the critical connections are balanced and minimized if the unit is kept on even keel. Multiple columns may produce uneven loading of the connections on different ballast conditions. As the expensive part of the MinDOC construction is at the nodes, the three-column version presents important cost savings. However, since the buoyancy comes primarily from the columns, the four-column designs do offer the potential for higher topsides loads. 4. Application of Rules and Regulatory Requirements to the MinDOC concept Upon initial evaluation of the MinDOC concept it was evident that there were no specific classification rules directly applicable to the configuration. ABS had then to consider the 10

11 MinDOC as a novel concept and develop requirements which would meet the classification objectives. Novel concepts do not necessarily present a higher inherent risk. In fact, in many cases, they may prove in operation to pose less risk than prior technologies. However, when they are initially developed, they often are presumed to be high risk. by parties that share those risks (particularly regulators, classification societies, and investors).. One means to help address the risk perceptions is to include as many of those stakeholders as practical in performing the novel concept review activities. A novel concept review needs to be able to identify hazards and critical issues for the project using the level of information typically available at the FEED (Front End Engineering Design) stage. As such, the level of Class review in support of the concept increases in complexity as more analyses, testing, etc. are performed. Thus, we see aspects of the design increasing in certainty. The novel concept approach was used in evaluating the MinDOC designs and provided a methodology for both the designer and ABS to move together in the development of improvements in the marine market. Understanding the concept, identifying the hazards involved with the new technology. Finding the departure from existing codes, defining equivalencies and evaluating safeguards are some of the steps generally taken when evaluating novel concepts or technologies. In following these basic steps, it was noted that the novel design had features and characteristics which were similar to other existing concepts that could be used in the definition of requirements to be applied on the project. As the concept was in developmental phase, several uncertainties were identified that would not allow ABS to proceed with full definition of the requirements to be applied. However, the designers had to proceed with the development work, so ABS had to act efficiently in identifying any departures from codes and defining appropriate equivalencies. Where no applicable code existed, new standards had to be developed. 5. Cooperation between Classification Society and Designers To meet the challenge of having to determine the applicable requirements in time to enable the designers to proceed with the development of the project and not delay the project construction schedule, ABS and BASS worked in close cooperation so that the concept was 11

12 clearly understood and that proper considerations were given in the establishment of the classification requirements. A few items were immediately identified as being novel in nature: the triangular configuration, the columns with different diameters along their length, and the upper pontoon arrangement. Other items were familiar in principle but applied differently, such as the deep draft configuration and the internal arrangement of the hull. A key decision to be made was how to qualify the MinDOC concept within the existing general classes of vessels: as a column stabilized unit such as a semi-submersible, or as a spar. This definition would greatly impact the development of the main aspects of the design such as the hull structure, mooring system and the vessel systems such as ballast and bilge. Figure 4: The MinDOC 3 under construction BASS provided ABS with a comprehensive explanation of the hull configuration and supplied general drawings with the basic details of the proposed hull. Based on outline drawings, hull subdivision and hydrostatic characteristics, ABS stability engineers developed a preliminary model of the hull and carried out an independent stability analysis. The results proved that the hull presented stability characteristics very much comparable to the spars, with the center of buoyancy located above the center of gravity and the stability curve sloping upwards indefinitely. This single finding was sufficient to qualify the hull as having the basic characteristics of a spar, 12

13 therefore a large amount of the requirements applicable to a spar (3) would be valid for application on the MinDOC design. Throughout the design and fabrication phase, ABS and BASS worked closely together in several aspects to determine the regulatory basis applicable to each individual novel detail of the project. This cooperation allowed a thorough understanding of the features and how to better apply existing requirements, adapting them as needed to obtain the direction for safe application of the new concept. A few practical examples can be found on items below Definition of Ballast System The ballast system design philosophy is directly related to the conclusion that the MinDOC has a stability behavior similar to a spar. Semi-submersibles heavily depend on the ballast system to maintain stability in damaged conditions, while the MinDOC does not present such a dependence on the ballast system configuration. Due to the internal subdivision and stability characteristics, the application of ballast system requirements of the spar (3) was appropriate for the MinDOC. This conclusion came after a series of discussions and interactions with BASS, who supported ABS in the decision making process by providing additional information, calculations and analysis. In the end, MinDOC proved to have a passive ballast system Definition on the Internal Subdivision of the Hull Upper Columns The hull upper columns were originally designed to accommodate a machinery room with a diesel generator, switchgear, control panels and other equipment. This imposed a series of restrictions to the systems design, primarily due to the concerns with downflooding points on the hull deck, where green waters are expected on the harsh environment of the Gulf of Mexico mainly during hurricane season corresponding to the 100-year return period. It is noted that model testing showed that the water these areas may see is due to wave/column run up, which doesn t have the impact of true green-water. Openings for ventilation, engine exhaust, multiple access points, etc, are typically not existent on spars. ABS and BASS reviewed plans and details of the openings to determine the most suitable means of closure as well as the design basis of the main access hatches so that the concerns were properly addressed. 13

14 5.3. Hazardous Areas Inside the Columns Compartments As part of the design flexibility of the MinDOC, the subdivision on the upper levels are such that there may be tanks containing hazardous fluids such as methanol and diesel fuel on the same level of machinery spaces and service spaces. To optimize the hazardous areas definition, BASS and ABS evaluated the design and created enough subdivisions on those spaces that created spaces segregating the hazardous areas from safe areas. By having pumps and other electrical devices on safe areas, the overall level of safety of the facility has increased, meeting the purposes of classification. Another related example is the hawse pipe location. Due to design constraints, the hawse pipe had to penetrate hazardous areas and tanks containing hazardous liquids, imposing a major regulatory difficulty. Due to the open communication and collaboration between ABS and BASS, void spaces were created surrounding the hawse pipes and the locations imposed by the mooring system and the tank arrangement dictated by the topsides design could be implemented Upending Procedure Due to the stability requirements during the upending process, tank arrangements in the upper pontoon and lower columns had to be carefully designed. Some compartments were required to be flooded for upending, but during normal operation the same compartments provided important access for inspection of the hull internal structures. BASS worked diligently in explaining to ABS the upending procedures and laying out the limitations of the operational condition. ABS was then able to define the basic requirements to be met regarding drainage of these compartments. Additionally, there were initial concerns about the upending process. To this effect, BASS developed its own simulation of the free-flooding process that takes the hull from its horizontal wet tow position to an upright position using SIMO and in-house programming as well as contracting an independent engineering company to confirm the results using their own program based on the MOSES software. Finally, scale model testing was also conducted, confirming the numerical simulations of the upending process. 14

15 Heave 3,000 2,500 Shear Force Along the Hull During Upending 20ft above keel 88ft above keel 132ft above keel Heave at BL (ft) Time (s) Shear(Kips) 2,000 1,500 1, Hull Angle (deg) 20 Pitch 450,000 Bending Moment Along the Hull During Upending 0 400,000 Pitch (deg) Time (s) Moment(Kips-ft) 350, , , , ,000 20ft above keel 100,000 88ft above keel 50, ft above keel Hull Angle (deg) Figure 5: Upending Sequence and Response 15

16 During the upending, peak stresses occur at the connection of the lower column to the upper column. This was obvious from BASS s analyses, but due the fact the geometry was not a simple cylinder, traditional shear/moment diagrams were not easily produced, as initial requested by ABS. After discussions, it was agreed that shear/moment values throughout the upending process at three key locations (base, middle and top of lower column) would suffice to confirm that the critical area was, indeed the top of the lower column. The discussions between BASS and ABS proved to be quite beneficial in reaching a consensus that safety was not compromised In-Service Inspection Requirements and Definition of Fatigue Safety Factors For being a new concept, no inspection experience existed in order to define the critical points and therefore the application of the safety factors on the fatigue calculations. During the initial phases of design review, ABS worked in conjunction with BASS to define the in-service inspection requirements of the critical elements and joints. By acquiring a better knowledge of the configuration of the hull structures and the details of connections, brackets, and stiffeners, ABS was able to develop a better understanding of the structural characteristics and therefore be better prepared to assign criticality factors and to define adequate inspection requirements. This allowed BASS to progress with the design using parameters that are acceptable to classification Balancing Cyclic Loads in a Static Finite Element Model BASS conducted a global spectral fatigue analysis of the entire MinDOC hull, with nominal element sizes for fatigue screening, thereby identifying critical areas for more detailed analyses. Once the critical areas were identified, detailed shell-fe models were built and finely meshed (t by t) to conduct more elaborate fatigue assessments of these regions. Discussions with ABS early on focused on the way in which the model was constrained to balance the hydrodynamic pressures. As it turned out, ABS had a preferred approach whereby the structural model is constrained by springs representing the mooring lines. BASS, on the other hand, postulated that the mooring lines do not really balance much of the wave-induced load, and it was best to simply use inertia relief, thereby not forcing any unbalanced loads to end up at the fairleads. Since the FE model needs to be stable, the choice of boundary conditions became a concern to ABS. BASS was able to prove that as long as the constraints make the model stable 16

17 while keeping it statically determinate, inertia relief ensures that the sum of the forces (and moments) are equal to zero by selecting the proper accelerations to counter the excitation loads. Therefore, each of the reactions at the selected constraints must also be equal to zero. B.C. AT TOP B.C. AT BOTTOM Figure 6: Deflected Shape and Stress Contours of Inertia Relief Model with different BCs Through a series of examples, ABS became convinced that, indeed, the choice of statically determinate constraints only changes that rigid part of the deflections, not the relative deflections, and therefore the strains and stresses were unaffected by the boundary conditions (provided they formed a statically determinate system). Another issue of early discussion was the specific approach to use for the detailed fatigue analyses of the areas deemed to be critical for fatigue. Upon discussion of the specifics of applying the Weibull approach, BASS became concerned that different Weibull parameters might be used even within a single connection. For this reason, BASS chose to forgo the Weibull approach and performed a full spectral analyses on detailed (t x t mesh) models for all critical areas. In cooperation with ABS, and after proving that the chosen approach yielded conservative estimates of fatigue life, BASS used a modification of the hot-spot approach to estimate fatigue life in all critical areas Hull vs Topsides Requirements and Results The deck leg design requires the solution to transferring static and dynamic loads from the topsides at six discrete points into the hull structure. A highly detailed finite element model of the hull and the topsides was used to examine load cases on the deck legs. Worst case vertical 17

18 and lateral accelerations for the operating and survival conditions were calculated by hydrodynamic analyses and utilized as design conditions to evaluate the strength of the deck legs. Loads resulting in the highest vertical and lateral acceleration of the topsides for the operational and storm conditions were applied to a MinDOC global structural FE model, and solved using inertial relief. Conservative design dictates assuming the maximum topsides weight for each condition. To that end, the future weight of the topsides, which includes equipment that may or may not be added in the future, was used. The center of gravity of the topsides was also considered variable, according to the position of heavy drilling equipment or "slot" positions, which can change the overall center of gravity of the topsides enough to affect the load distribution on the deck legs. BASS, in collaboration with the deck installation contractor, proposed an alternative design approach to the unique configuration of the connection. Detailed solid-element models were created to establish stress concentration factors for the deck leg to stub connection, accounting for the different thicknesses of the deck leg and the stub, and for possible misalignment due to fabrication/installation tolerances. These analyses were carefully discussed with ABS to achieve sufficient level of confidence that the proposed detail will perform as needed. The methodology was presented to ABS and considered suitable to demonstrate the strength of the connection Booby Hatch Slamming vs Wave Run Up The design of secondary structure on the tops of the columns became a topic of discussion due to the possible effects of wave slamming and the pressures that they may experience. Of particular concern were the booby hatches that provided access into the column spaces. Although damage stability had shown the MinDOC capable of withstanding the accidental flooding of the upper access trunk, it contained valve actuators and other components that preferably would be protected. BASS held numerous discussions with ABS to illustrate that these components on top of the column were not subjected to wave impact during the model testing, but the water to which they were exposed was from wave run up that came over the column tops. Additionally 3D PDMS models were shown to illustrate the booby hatches and in particular the water tight doors were shielded by large machinery, structural elements and ancillary devices making it improbable that any direct wave slam could impact these structures. 18

19 After allowing ABS to study the tank tests and the 3D models ABS determined the structure was adequate for the intended purposes. Figure 7: Key Model Test showing wave run up Figure 8: 3D PDMS model of column top equipment 6. Conclusions While there may be a debate as to whether or not the MinDOC platform is a novel concept, there is no question that the rules existing at the time of its design needed to be carefully evaluated to ensure that innovation is not hampered, while ensuring that the classing process remains true to its mission. Through early and constant cooperation between designer and 19

20 classification society, a thorough understanding of the features, strengths and weaknesses of the MinDOC and of the rules and recommendations was shared. Such a cooperative approach helped both the designer and the classification society more fully understand the innovative technology, concepts, and functionalities that are being considered and to identify how these approaches could be harmonized with the applicable regulatory and industry standards while maintaining the feasibility of the project. An encouraging result of this cooperation was that although different methodologies were often employed by the designer and the classification society, the results were similar enough to illustrate that workable solutions can be produced using distinct procedures. The fact that multiple analytical approaches did show favorable results is a strong argument in demonstrating that the risks of a properly designed and reviewed novel design may be well within acceptable limits. MinDOC will take its place among the other, more traditional deepwater units, and show that novel ideas can gain the acceptance they need to succeed while remaining in compliance with the classification standards. 7. Acknowledgements The authors would like to acknowledge the following professionals for their invaluable contribution and support in the preparation of this paper: William T. Bennett and Kenneth A. Ullrich. Bennett & Associates LLC Kenneth Richardson ABS Additionally, the authors would like to thank the clients: Bluewater Industries and ATP Oil & Gas for their permission and encouragement to compile and publish the specifics of the Classing of the first MinDOC. 8. References (1) - International Energy Annual report 2006, U.S. Energy Information Administration (2) International Energy Outlook report 2009, U.S. Energy Information Administration (3) Guide for Building and Classing Floating Production Installation, ABS publication 20