Modern Offshore Support Vessels Class and Statutory Perspectives

Transcription

1 OSV Singapore 2005 International Conference on Technology & Operation of Offshore Support Vessels At the National University of Singapore, Republic of Singapore Abstract Modern Offshore Support Vessels Class and Statutory Perspectives Ahmad Sarthy Ship/Offshore Structures and Statutes, J. L. Ham Ship/Offshore Engineering Systems American Bureau of Shipping - Pacific Division The world today has about 4000 offshore support vessels of various types. While statistical data are not very precise, there appears to be more than 200 under construction at present. These modern vessels, intended for fleet replacement on the one hand, and to meet the more demanding needs of deeper water operations on the other, are of much improved designs and packed with multifunctional capabilities. This paper performs a simple analysis of existing fleet and order-book databases and highlights some trends. It goes on to outline some basic requirements of offshore support vessels from classification perspectives, followed by provisions for specialized functions and capabilities, such as fire fighting and dynamic positioning. Current statutory requirements applicable to this vessel type are also discussed. Some modern support vessel design characteristics are given with a view to illustrate some design trends. Keywords/Acronyms: ABS AHTS IMO MARPOL OSV SOLAS 1 Introduction : American Bureau of Shipping : Anchor Handling, Towing and Supply Vessel : International Maritime Organization : International Convention for the Prevention of Marine Pollution at Sea : Offshore Support Vessel : International Convention on Safety of Life at Sea The origin of offshore support vessels may be traced to the Gulf of Mexico where oil exploration first moved offshore in the 1950s. Then, surplus World War II vessels, wooden fishing boats, and shrimp trawlers were used to supply offshore rigs with cement, mud, spare parts, crews, fuel and food. In 1955, Alden and John Laborde [17] developed the first purpose-built vessel to supply offshore rigs and platforms, featuring a bow wheelhouse and a long flat afterdeck that became the standard for offshore support vessels. This pioneering design bearing close resemblance to a pickup truck withstood the test of time and has become a generic ship type of its own right. As a ship type it has since been given a variety of names: offshore support vessel is perhaps the more all-encompassing one; others are offshore supply vessels, anchor handling and supply vessel, anchor handling towing and supply vessels, or other combinations, depending largely on the functions the vessel is designed and equipped to perform. Modern Offshore Support Vessels: Class and Statutory Perspectives 281

2 One of more of the following design and operating capabilities can generally be expected of a vessel of this type: - Large and open aft deck and equipment for cargo, anchor handling and towing operations - Highly maneuverable, particularly at low speed or static operations - Storage of consumables for offshore exploration and production activities: such as drilling fluids, bulk mud and cement, potable water, fuel, chemicals, etc. As exploration and production activities move into deeper waters, a certain shift in the basic design in offshore support vessel is evident. Just on drilling operations alone, more cargoes in terms of hardware such as drill pipes on the one hand, to consumables such as drilling fluids on the other, have to be supplied per trip than for shallow water activities. Deepwater also mean further away from shore base, this means the need for higher speed. Deployment and retrieval of anchors of various types in deep waters need more powerful winches than for shallow water. These and other factors dictate, on the whole, the modern offshore vessels, to be larger and more powerful, and in harnessing advances in modern equipment design, more technologically sophisticated than they ever were. Added to these challenges is the demand for these vessels to be equipped for emergency responses. Thus, response capabilities such as fire fighting, standby and rescue, and anti-pollution are also added, in various degrees, on to the already multi-functional role. Modern offshore support vessels are therefore truly action-packed vessels. 2 A Review of Offshore Support Vessel Fleet Demand for OSVs is expected to surge due to at least three fundamental factors: - The aging fleet - The unsuitability of existing fleet to support deepwater activities - The expected continued growth in offshore exploration and production activities fueled by high oil prices. Presently, the average age of the world s fleet of OSVs is more than 20 years. It is dominated by Anchor Handling Towing (AHT) and Anchor Handling Towing Supply (AHTS) vessels representing 33% and 19% of the total existing fleet respectively [1]. The orderbook as of January 2005 indicates domination by Supply Vessels (SV) and AHTS with 50% and 33% respectively [1]. In terms of the age structure of the existing fleet, 45% of the world fleet is aged 25 years and over and 29% is aged between 20 and 25 years old. Assuming the same level of demand and an average vessel life of 25 years, significant renewal of the existing fleet will be required in the years to come to satisfy the demand. This trend is already being observed by the structure of the current orderbook [1]. In terms of vessel types, the largest demand for renewal will come from the AHTS sector as 495 vessels are currently 25 years or older and 960 will be 25 years and over by 2010 assuming no renewal. Similarly, SV newbuilding will increase since 370 vessels are currently 25 years and older. Standby/rescue vessels newbuilding will also increase as 170 vessels are currently 25 years and older [1]. 282 Modern Offshore Support Vessels: Class and Statutory Perspectives

3 2.1 OSV Owners/Operators The tables in Fig. 1 below show the dominant players in the OSV sector. Fig. 1 World dominant OSV owners/operators 2.2 The AHT and AHTS Sub-sector The current size of the AHTS and AHT fleet is 1284 and 329, and the orderbook stands at 69 and 7 respectively. New generation deepwater vessels have horsepower greater than 8,000 BHP and winch power greater than 250 tons [1]. Fig. 2 shows an example of vessel size and power evolution in the Gulf of Mexico. Fig. 2 Evolution of AHTS particulars The orderbook shows a trend of an increase in length overall (LOA) that is associated with deepwater service. In terms of LOA, 66% of the total fleet is in the range meters and over 20 years of age. The orderbook shows that 46% is in the meters range, 20% in the meters, and 7% in the over 90 meters range [1]. See Fig. 3. Modern Offshore Support Vessels: Class and Statutory Perspectives 283

4 % 13% <40 4% % % % % % % % >90 1% <40 1% % % AHTS - Existing Fleet LOA AHTS - Orderbook LOA Fig. 3 Distribution of AHTS fleet LOA (meters) - existing vs. orderbook In terms of age structure of the fleet as a function of LOA, Fig. 4 indicates that a significant proportion of the vessels due for renewal are in the meters range. Driven by deep-water service, fleet renewal has focused on the higher LOA ranges. Count of LOA BuildYear LOA < >2000 Grand Total 0.0% 0.0% 0.0% 0.4% 0.6% 1.0% <40 0.0% 0.2% 0.3% 0.2% 0.0% 0.6% % 1.6% 1.9% 0.2% 0.1% 4.5% % 21.0% 20.2% 0.9% 0.3% 44.9% % 10.0% 16.4% 3.0% 3.3% 32.7% % 0.6% 2.1% 3.6% 1.6% 7.9% % 0.0% 3.7% 2.3% 1.5% 7.5% >90 0.0% 0.1% 0.1% 0.2% 0.5% 0.9% Grand Total 3.2% 33.4% 44.8% 10.7% 7.9% 100.0% Fig. 4 AHTS Existing Fleet LOA vs. Age Profile 284 Modern Offshore Support Vessels: Class and Statutory Perspectives

5 >20,000 18,000-20,000 16,000-18,000 14,000-16,000 12,000-14,000 10,000-12,000 8,000-10,000 6,000-8,000 4,000-6,000 2,000-4,000 <2, Existing Fleet BHP Distribution >20,000 2 >18,000-20,000 4 >16,000-18,000 6 >14,000-16, ,000-14, ,000-12,000 8,000-10,000 6,000-8,000 4,000-6, ,000-4,000 1 <2, Orderbook BHP Distribution Fig. 5 Distribution of AHTS fleet BHP Existing vs. Orderbook The propulsion BHP rating also shows a definite shift to higher ranges. In the existing fleet 54% has a BHP rating in the range 2,000 4,000 BHP, 10% in the range 10,000-16,000 BHP, and 7% above 16,000 BHP. In contrast, 36% of the orderbook has a BHP rating greater than 10,000 with a concentration in the 10,000-12,000 BHP range, i.e. 48% of vessels with BHP>10, % of the orderbook is concentrated in the 4,000-8,000 BHP range. A profile of the age distribution versus BHP is given in Fig. 5. It is observed that 30% of current fleet with a BHP in the range 2,000 6,000 BHP has a service life greater than 25 years. The correlation of the BHP vs LOA over the existing fleet and orderbook is ~0.79. A typical correlation plot is given in Fig. 6. Modern Offshore Support Vessels: Class and Statutory Perspectives 285

6 Fig. 6 AHTS existing fleet - BHP vs. LOA 3 Typical OSVs Then and Now In the past, the sizes of OSVs are traditionally in the range of 40m to around 70m. The types of cargos carried on board were typically pipes, casings, stores, chains, machineries and equipment for offshore rigs, etc. In general, the conventional design of offshore supply vessels satisfied with the definition in IMO Resolution A469(XII) [10]. The hull forms vary from straight chines in the early eighties to streamline curves in the nineties and today. Within the main hull, supply vessels are typically provided with deep tanks dedicated for the carriage of drill water, liquid mud and brine. Some supply vessels carry bulk tanks below the deck for the carriage of liquid mud, cement, etc. Forward of the midship, supply vessels are fitted with a forecastle, deckhouses and Navigation Bridge. The aft deck areas are normally fitted with cargo rails for securing of deck cargos and wooden dunnage for protection of deck plating. For supply vessel that perform anchor handling and towing operations, typically a stern roller will be fitted at the aft end and a towing winch, towing gears and anchor handling gears are fitted at suitable locations on deck. In the earlier trends, the propulsion systems consisted of conventional twin-screw diesel engines. In the later stage, fixed propeller nozzles were fitted in order to gain higher power ranges. Static Bollard Pull in the eighties was normally in the range of 30 to around 60 tonnes. Fig. 7 shows a typical design built in the early 80s. 286 Modern Offshore Support Vessels: Class and Statutory Perspectives

7 Fig. 7 A typical OSV built in the early eighties (The above sketches are solely for illustrative purpose) In contrast, new generation OSVs are required to provide support for deepwater drilling operations and deliver more cargoes at greater distance from shore and faster. They are larger in size as compared to their predecessors in order to provide bigger cargo deck areas, and to optimize the underdeck spaces for accommodating increase number of bulk tanks for liquid mud, brine and cement. Due to higher specification for towing and anchor handling, new generation OSVs are now equipped with sophisticated anchor handling equipment, and thrusters for dynamic positioning capabilities. Specification for higher Bollard Pull has increased in trend in the range of 150 to 200 tonnes. A significant number of new generation OSVs is fitted with diesel-electric propulsion systems, particularly those with enhanced dynamic positioning systems. Modern Offshore Support Vessels: Class and Statutory Perspectives 287

8 These vessels have significantly improved dry bulk and liquid cargo capacities through better compartment design and also better space efficiency of electric propulsion system. Some of the new generation supply vessels are equipped with ROV for underwater operations, lifting appliances with high safe working loads for cargo handling, platform access system to facilitate transfer of personnel from the vessel to offshore platform, etc. Though there are not many changes to the position of the accommodation and navigation bridge, comfort and good ergonomics are becoming increasingly important in the design of these spaces. Emphasis on the crew comfort to make working and living conditions on board as safe and comfortable as possible is also now becoming part of the specification of a new generation of offshore supply vessels. Traditionally, OSV s deckhouse front is simply a vertical plane transverse bulkhead. Obviously, such a design has obtuse windage area, high wind resistance, and absorbs full impact from wave and green sea in adverse weather (see Fig. 8). Fig. 8 A heavy weather frontal assault (The above photo is solely for illustrative purpose) Recent advances in design make deckhouse front bulkhead rake aft to reduce wind resistance and green sea loads. In this respect, rounded deckhouse front is being conceived also. Navigating Bridge is now designed in a similar concept to airport control tower in such a manner as to achieve all-round visibility. 288 Modern Offshore Support Vessels: Class and Statutory Perspectives

9 Fig. 9 A typical new generation OSV (The sketches above are solely for illustrative purpose) 4 Classification and Statutory Requirements of OSVs ABS is currently one of the major Classification Societies for OSVs with more than 1000 currently in ABS class. ABS first established Rules specifically for smaller vessels with the publication in 1973 Rules for Building and Classing Steel Vessels under 61 meters in Length. This was evolved in 1997 into Rules for Building and Classing Steel Vessels under 90 meters in Length [2] essentially to extend the Modern Offshore Support Vessels: Class and Statutory Perspectives 289

10 envelope of length. These Rules specify more functional, equipment and arrangement requirements as conditions of classification as well as introducing optional classification notations for specific functions and services of the vessel. 4.1 The Past Practice In the earlier days, classification of OSVs concerned itself with hull structural and essential propulsion and auxiliary machinery requirements only. Special functions such as towing are not a part of classification requirements. In addition to basic classification requirements, where delegated and authorized by Flag Administrations, ABS also carried out plan review and surveys, during new construction and in service, to verify compliance with statutory requirements. In the 1960s and 1970s, The International Convention of Loadline (ICLL) 1966 and Safety of Life at Sea (SOLAS) 1960 were the main international statutory requirements. Tonnage was then a Flag Administration s domain and various national tonnage rules were used. The U. S. tonnage rules at the time permitted exemption of certain spaces (such as bulk mud room) from tonnage measurement. This resulted in many of the earlier OSVs falling under 500 GT and thus did not have to comply with SOLAS. It was not until 1981 that IMO adopted the Resolution A.469(XII) Guidelines for the Design and Construction of Offshore Supply Vessels [8] recognizing that both the unique design features and service characteristics of OSVs differ from those of conventional cargo ships, and introduced some specific safety requirements. In it, Offshore Supply Vessel is defined as a vessel - which is primarily engaged in the transport of stores, materials and equipment to offshore installations, and - which is designed with accommodation and bridge erections in the forward part of the vessel and an exposed cargo deck in the aft part for the handling of cargo at sea. IMO Res. A.469(XII) prescribes a standard of safety intended to be equivalent to the International Convention for the Safety of Life at Sea, 1974 (SOLAS 1974), but modifies certain provisions to suit the nature of OSVs, in particular in respect of stability criteria: - Intact Stability: The IMO resolution prescribes alternative stability criteria as OSV s characteristics render compliance with the IMO Res. A.167 impracticable. In addition, due to the open ended and low profile stern, it requires OSVs to maintain a minimum freeboard at the stern of at least 0.005L. - Subdivision and Damage Stability: To safeguard against collision damage, the IMO resolution superimposes damage stability requirements with assumed damage extents of 760mm measured transversely inboard from side shell and vertically for the full depth of the vessel. However, IMO Res. A.469 (XII) was not a mandatory document and had no major impact on most of the OSVs built in the earlier period. 4.2 The Present Practice Perhaps the most significant change in the regulatory regime is in recognizing the multi-role nature of OSV operations. Thus in developing its Rules for Building and Classing Steel Vessels Under 90 meters (295 ft) in Length in 1997, ABS incorporates pertinent provisions of IMO Resolution A.469(XII), IMO Resolution A.673 (16) and SOLAS. 290 Modern Offshore Support Vessels: Class and Statutory Perspectives

11 Vessel functions such as towing, fire fighting, oil recovery, and safety standby were introduced and offered as optional class notations. Vessel performance capabilities such as dynamic positioning are also introduced, along with human factor related requirements such as habitability and navigating bridge ergonomics. IMO statutory requirements are now becoming universal and most, if not all, OSVs need to be issued with certificates under the conventions of SOLAS, MARPOL, Loadline and International Tonnage. Some details are introduced in the following sections. 4.3 Certification of Vessel s Functions Vessels with towing capability Requirements for Towing Vessel are primarily for structures and stability, which includes [4]: - Structures in way of towing hook, towing winch, towing bollards, towing guide rollers and fairleads and their structural attachment designs - Braking power of winch - Static bollard pull along with guideline for Bollard Pull Test Procedure - The minimum specified breaking strength of towline. - Stability criteria for towing operations - Provision of towing hook or towing winch quick release Vessels with external fire fighting capabilities In addition to specific requirements for fire pumps and monitors and their control for fighting fires external to the vessel, the requirements include the vessel s stability (as may be affected by the monitors), the ability to maintain station while the fire monitors are in operation, and the protection of the vessel itself against external radiant heat [4]. There are three Fire Fighting Vessel class notations: Class 1, Class 2, and Class 3. Fire fighting vessel Class 1 will be assigned to vessel with water spray protection for cooling the vessel s surfaces to enable close operation at early stages of fire fighting and rescue operations, whereas vessels fitted with equipment for extended operations of fire fighting and cooling structures on fire will be assigned Class 2 or Class 3. In general, Classes 2 and 3 have higher monitor range and capacity than Class 1. Class 3 vessels have greater capacity monitors and pumps than Class Vessels for offshore support Offshore Support Vessel notation is assigned to vessels designed for support service to offshore installations such as in the transport of stores, materials, hazardous and noxious liquid substances, and equipment to offshore installations. Assignment of this notation requires compliance to specific requirements for liquid cargo systems, dry bulk systems, side shell, frames, stability (intact and damage) and cargo decks [4]. Other services, and corresponding class notations, such as anchor handling services (AH) and well stimulation (WS) are also available Vessels with oil recovery capability Two Oil Recovery Vessel notation are offered [4]: Class 1 to vessels intended to recover oil of unknown flash point and Class 2 to vessels intended to recover oil with a flash point exceeding 60 C. Essentially, the requirements are intended to prevent the ignition of oil vapors while recovering and transporting recovered oil as well as to protect the vessel and crew. These Rules also features Modern Offshore Support Vessels: Class and Statutory Perspectives 291

12 requirements for locations of recovered oil tanks, structural fire protection of exterior bulkheads, spill coamings, pump rooms and machinery spaces, associated machinery and systems Vessels for safety standby service Requirements include special features for evacuation and reception of personnel from an offshore installation and the rescue and care of persons from another vessel or from the sea. Three Safety Standby Service are offered [4]: Group A, Group B and Group C, depending on the number of survivors the vessel is capable of accommodating. Other requirements include speed and maneuverability, navigation bridge design, rescue zone, accommodations for survivors, rescue and safety equipment and navigation and communication equipment Vessels for escort service Escort service vessel is intended for accompanying another vessel in transit and where needed to provide assistance to disabled vessels involving maneuverability due to loss of propulsion or steering or both. Special requirements [4] include intact stability criteria for an escort vessel, static and dynamic bollard pull, towing gears, and verification of steering capability Dynamic Positioning Systems ABS offers four levels of notations for DP Systems [3, 5] with ascending degree of redundancy: DPS-0, -1, -2 and -3. DPS-0 has no redundancy and is essentially for manual position control with automated heading control function only. DPS-1 has fully automated position control and heading control functions, but no redundancy. DPS-2 is most common; it has fully automated position control and heading control functions, and will do so with any single system component failure. DPS-3 requires added consideration of flooding or fire in any single compartment. Certain flag or coastal state administrations may require compliance with IMO s MSC/Circ. 645 Guidelines for Vessels with Dynamic Positioning Systems [13] Propulsion redundancy Propulsion redundancy notations [3, 5] are available for vessels fitted with redundant propulsion and steering systems such that with any single failure, the vessel can continue to navigate at half its speed or 7 knots, whichever is less, for at least 36 hours. R-1 and R-2 are notations intended for vessels where failure does not include lost of any single compartment due to fire or flooding, while R-1S and R-2S are notations that do, see Fig Modern Offshore Support Vessels: Class and Statutory Perspectives

13 Fig. 10 Propulsion redundancy notations (R1, R2, R1-S, R2-S) Navigating Bridge ABS provides notations for integrated bridge based on ABS Guide for Bridge Design and Navigation Equipment/Systems [6]. In recognition of the importance of navigating bridge equipment layout and ergonomic design of the bridge itself, ABS published two other documents to help designers to achieve better bridge designs : - ABS Guidelines for Bridge Design and Navigation Equipment/Systems [7] - ABS Guidance Notes on Ergonomic Design of Navigation Bridges [8] Crew Habitability ABS Guide for Crew Habitability on Ships [9] introduces a comprehensive set of habitability criteria. "Habitability" is defined as the acceptability of conditions on-board a ship in terms of vibration, noise, indoor climate and lighting as well as physical and spatial characteristics, according to prevailing research and standards for human efficiency and comfort. Two notations are offered. HAB for basic habitability and HAB+ for more stringent accommodation, vibration and indoor climate criteria. Meeting the criteria of this Guide will also fulfill the physical design requirements of International Labor Organization (ILO) Conventions 92 and Some Rule Requirements Hull Structures Vessels having Length of 65 meters and above are required to comply with stillwater bending moment and shear forces calculations of the Rules [2]. Double bottom as required by SOLAS is incorporated in the Rules. In principle, double bottom shall be fitted fore and aft between the peaks or as near thereto as practicable for vessels of ordinary design of 500GT or over. Double bottom need not be fitted in way of deep tanks provided the safety of the vessel in the event of bottom damage is not thereby impaired. Modern Offshore Support Vessels: Class and Statutory Perspectives 293

14 Local strengthening of the OSVs side structures in the region subjected to impact due to contact with offshore structures is incorporated in the Rules. Where heavy cargo is carried on deck, effective means such as steel cradle, steel or wooden dunnage are to be provided so that cargo weight is uniformly distributed in the deck structures. Adequacy of the deck structures in way of heavy cargoes shall be evaluated and the stresses in deck members are not to exceed the allowable values specified in the Rules. Structures in way of towing gears, winch foundations, stern rollers, A-Frames, cargo rails supports, bulk tanks, mud tanks and cargo deck structures are to be evaluated using first-principle approach demonstrating compliance with Rule-specified allowable stresses. Carriage of limited amounts of hazardous and noxious liquid substances, where applicable, is to meet IMO Resolution A.673(16) Guidelines for the Transport and Handling of Limited Amounts of Hazardous and Noxious Liquid Substances in Bulk in Offshore Support Vessels [11] Subdivisions and Stability The stability criteria in the Rules are in line with IMO Res. A.469 (XII) Fire Safety Fire safety measures as specified in SOLAS are incorporated in the Rules for OSVs of 500 GT and above. 4.5 Other Statutory Issues SOLAS (as amended) Chapter VI Carriage of Cargoes As required under this section of SOLAS, cargo and cargo units carried on or under deck shall be so loaded; stowed and secured as to prevent as far as is practicable, throughout the voyage, damage or hazard to the ship and the persons on boar, and loss of cargo overboard. Cargoes such as pipes, pipe casing, containers, machinery and equipment, bulk tanks, anchors and chains, etc. shall be stowed on board and secured by means of suitable lashing/securing systems. Cargo deck of OSVs is fitted with arrangement of fixed securing devices such as lashing eyes/rings, container sockets, etc. An approved Cargo Securing Manual prepared in accordance with the IMO MSC/Cir.745 (as amended with MSC/Circ. 1026) Guidelines for the Preparation of Cargo Securing Manual and in conjunction with the IMO Code of Safe Practice for Cargo Stowage and Securing is required on all cargo vessels where this regulation applies, and this includes all OSVs Carriage of Personnel other than Crew Where the vessel is intended to accommodate more than twelve industrial personnel (persons other than the crew), the Flag Administration should be consulted for its special requirements IMO Res. A.863(20) Code of Safe Practice for the Carriage of Cargoes and Persons by Offshore Supply Vessels (OSV Code) This Code [14] was issued in 1997 and, as stated in the document itself, the purpose of this Code is to provide, for both operator of the offshore installation and OSV contractor, an international standard to avoid or reduce to a minimum the hazards which affect offshore supply vessels in their daily operation of carrying cargoes and persons to, from and between offshore installations. The Code emphasizes issues such as: 294 Modern Offshore Support Vessels: Class and Statutory Perspectives

15 - Establishment of procedures between operators of offshore installation and the OSV contractors, including emergency procedures, mooring, cargo handling and personnel transfers - Cargo safety issues in terms of complete knowledge of the cargo (weight, requirement for stowage, hazardous nature, etc.); proper containment, stowage and securing, as applicable; the effect of green water on exposed cargoes and the ensuing vessel s stability; and the safety of cargo handling personnel - Safe handling of bulk cargoes including color-coding of transfer hoses - Other personnel safety issues such as protective equipment While the OSV Code is not mandatory at present, its voluntary compliance should be encouraged IMO MSC 80 th Session (11-20 May 2005), Amendment to 1974 SOLAS (as amended) Reg. II-1/3-8 Mooring and Towing To take into account the increased capacity of today s large capacity tugs and the lack of awareness of the safe working load for the towing fittings which has been considered to be the prime cause of recent and numerous failures of towing equipment and fittings on board ships, the Committee adopted new regulation II-1/3-8 which addresses strength requirements for shipboard fittings of new ships > 500 gt and constructed on/after 1 January As per new resolution MSC.194(80), the regulation requires each towing and mooring fitting to be clearly marked with any restrictions associated with its safe operation, taking into account the strength of its attachment to the ship s structure. The regulation refers to recommended standards contained in new MSC/Circ The design load applied through the tow line onto fittings used for normal towing (harbor/maneuvering) is 1.25 times the intended maximum towing load where as the design load for escort towing and normal mooring is 1.25 times the IACS Recommendations for towing and mooring line breaking strength. Figure 11 Application of Design Load In all cases, the applied load need not be taken to be more than two times the design load as shown in Figure 11. Suitable safety factors (100% yield stress in bending and 60% yield stress in shear) are assigned so that the safe working load does not exceed the design load applied to the fitting. Modern Offshore Support Vessels: Class and Statutory Perspectives 295

16 Concluding Remarks This paper reviews the existing fleet and the order book of offshore support vessels and examines some of their design characteristics. It deduces that the highest demand is in the anchor handling/towing/supply sector. Vessel lengths have increased with majority in the 60-70m ranges as opposed to 50-60m ranges in existing fleet. As can be expected, dominant BHP is now over 10,000 as opposed to ranges in existing fleet. Obviously, the current orderbook is more than fleet replacement, it aims to meet the deepwater support market as well. The new vessels are larger (longer and deeper), more powerful, more cargo space, more space efficient, and technologically a great deal more sophisticated. They are also more likely to be multirole than their predecessors. The paper reviewed evolution of classification rules and international regulations. It is seen that, not unlike the OSVs themselves, classification and regulatory requirements have increased their degree of sophistication, both to keep up with the technology employed in these vessels as well as to address a variety of their operational functions. They now provide for certification of various functional capabilities from carriage of cargoes of various forms to emergency responses such as fire fighting and safety standby. They also address the vessel s performance capabilities such as dynamic positioning and bridge ergonomics. With the maturing of these hardware-focus requirements, it is foreseeable that regulatory regime will in the future focus on safe operation of these vessels. As, no doubt, the operations of these increasingly more sophisticated vessels in harsher and more demanding environments will indeed be an interesting challenge for the industry. Acknowledgement The authors would like to thank their colleagues in ABS, in particular Mr. Franck Violette who performed the statistical analyses of fleet databases. The authors also like to express their appreciation to Mr. David Neo, Dr. Dev Dutta and Mr. Andrew Mak for reviewing the paper and to Mr. A K Seah for editorial assistance. References [1] LR-Fairplay and Clarkson s databases [2] American Bureau of Shipping, Rules for Building and Classing Steel Vessels Under 90 meters in Length 2001, Part 3 Hull Construction and Equipment [3] American Bureau of Shipping, Rules for Building and Classing Steel Vessels Under 90 meters in Length 2001, Part 4 Vessel System and Machinery [4] American Bureau of Shipping, Rules for Building and Classing Steel Vessels Under 90 meters in Length 2001, Part 5 Specialized Vessels and Services [5] American Bureau of Shipping, Rules for Building and Classing Steel Vessels, Part 4 Vessel System and Machinery, 2005 [6] American Bureau of Shipping, Guide for Bridge Design and Navigation Equipment/System, 2001 [7] American Bureau of Shipping, Guidelines for Bridge Design and Navigation Equipment/System, January 2000 updated June 2002 [8] American Bureau of Shipping, Guidance Notes on Ergonomic Design of Navigation Bridges, October Modern Offshore Support Vessels: Class and Statutory Perspectives

17 [9] American Bureau of Shipping, Guide for Crew Habitability on Ships, December 2001 [10] International Maritime Organization, Resolution A.469(XII) Guidelines for the Design and Construction of Offshore Supply Vessels, IMO London, 1981 [11] International Maritime Organization, Resolution A.673(16) Guidelines for the Transport and Handling of Limited Amounts of Hazardous and Noxious Liquid Substances in Bulk in Offshore Support Vessels, IMO London, October 1989 [12] International Maritime Organization, Resolution A.534(13) Code of Safety for Special Purpose Ship, IMO London, November 1983 [13] International Maritime Organization, MSC/Circ.645 Guidelines for Vessels with Dynamic Positioning Systems, IMO London, June 1994 [14] International Maritime Organization, Resolution A.863(20) Code of Safe Practice for the Carriage of Cargoes and Persons by Offshore Supply Vessels (OSV) Code, IMO London, 1997 [15] International Maritime Organization, MSC/Cir.745 (as amended by MSC/Circ. 1026) Guidelines for the Preparation of Cargo Securing Manual, IMO London, June 1996 [16] International Maritime Organization, Resolution A.714(17) Code of Safe Practice for Cargo Stowage and Securing, IMO London, November 1991 [17] Offshore Magazine, August 2004, Anniversary Special: 50 key events, technologies shaped the offshore industry Modern Offshore Support Vessels: Class and Statutory Perspectives 297

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