internalsubdivisionjackups (1)

ABS TECHNICAL PAPERS 2005

Internal Subdivision of Jack-ups A new standard to resolve an old concern

By J. Andrew Breuer – ABS Americas Robert Bowie – ABS Americas Julian Bowes – LeTourneau Marine Group

Presented at 10th Conference on the Jack-up Platform, London City University, London, UK, September 2005 and re-printed with the kind permission of the City University of London

Abstract

Three Jack-up Drilling Units were lost during transit in heavy weather between 1988 and 1990. These losses motivated the Offshore Industry to investigate the reasons for the accidents and try to find ways to prevent similar recurrences. The main findings of the Industry studies were operational in nature; however the investigations also disclosed weaknesses in the subdivision standards of Jack-ups that need correction.

The American Bureau of Shipping (ABS), the Classification Society that classes the majority of the jack-up fleet, took the lead to review the current damage stability criterion and the internal subdivision Rule requirements for Jack-ups. An ad-hoc committee was formed that included designers, builders, regulators, and drilling contractors to investigate requirements outlined in the current Rules. The research by the committee resulted in a new criterion for damage stability and subdivision for these units.

This paper presents the historical background, the activities of the ad-hoc committee, the procedures leading to the new standard, the approach to resolving the concerns, and the technical support for the new criteria.

Key words: damage, intact, jack-up, regulations, rules, stability

The New Standard

A “Rule Change Notice” (RCN)[1] proposed by the ABS Special Committee on MODU in their annual meeting of 2004 was approved by the ABS Technical Committee in July of 2004. The RCN is the document that precedes the changes to the published Rules. These committees are comprised of Industry representatives under the policies and procedures of the ABS.

The RCN has introduced a number of changes to the 2001 ABS MODU Rules[2] including an additional standard for residual stability after damage for Self-elevating Drilling Units (jack-up). The new Rule applies to all units contracted on or after January 1st, 2005. The new Rule supersedes and eliminates the requirement for maximum size of watertight compartment that was conditionally published a year earlier. The relevant part of Stability and Subdivision of the current ABS Rules are attached as an appendix.

Background

The causes and rationale leading to the development of the new subdivision is presented in The Future of Jack-Up Stability - Learning From Our Past, a paper presented to this Conference in 2003[3].

The many differences between MODUs and conventional vessels, is the reason for the introduction to intact and damage stability standards in the ABS MODU Rules of 1968[4]. Some Jack-up losses while under tow in the 1950’s, confirmed the need for a robust subdivision. At the time of publication of the first Rules for MODU’s, most designs had a certain level of subdivision probably guided more by the functional needs than by damage stability standards. The 1968 Rules established a one compartment flooding standard by requiring the “flooding from the sea of any one main compartment which may reasonably be expected to be flooded.” The expectation after damage was “…sufficient reserve buoyancy and stability to survive considering the adverse effects of wind and sea.”

Further changes to the Rules in 1973, narrowed the meaning of “reasonably be expected to be flooded” to compartments exposed to the collision of an attending vessel and compartments adjacent to the bottom and upper deck. In practice, this

Internal Subdivision of Jack-ups – A new standard to resolve an old concern 107


encompassed any compartment in a jack-up. In the late 70’s, ABS changed the application of these Rules and the flooding through the deck was no longer considered.

Changes to the ABS Rules in 1988, extended the damage stability considerations to compartments containing sea water systems but left many internal compartments with no restrictions to their size. Further, the 50 knot wind criterion is often not restrictive enough when applied to internal compartments

Regardless of the language in the Rules, most designs have a reasonably well subdivided hull. The peripheral compartments are sized to meet the damage stability standard and the internal compartments are subdivided to meet functional needs. Designs with “open architecture” had subdivision of internal compartments, but often the size of main compartments was limited by some self-imposed a damage stability standard.

Despite the subdivision, a number of rigs have been lost under tow. The parting of towing lines in heavy seas, and subsequent loss of heading control is generally the initiating event in most losses. Flooding of several compartments after the loss of watertight integrity of the main deck followed. Some of the losses up to 1979 resulted in the loss of life, but in all incidents since then, the crew was evacuated to safety.

The losses of the ROWAN GORILLA I, the INTEROCEAN II, and the WEST GAMMA in 1988, 1989, and 1990 respectively were associated with wet tows in the North Atlantic and the North Sea. Industry and regulatory bodies, recognizing the common factors involving these and other losses, focused their attention to develop standards to prevent the recurrence of these accidents. Organizations such as the International Association of Drilling Contractors (IADC), UK Offshore Operators Association (UKOOA), and the UK Heath and Safety Executive (UK HSE) Jack-up Safety in Transit (JSIT) Working Group engaged in a number of investigations starting in 1991.

The most immediate and significant change was the adoption of operating procedures that resolved the root causes of many incidents. These procedures were published in similar but independent guidance by IADC, UKOOA, and the US Coast Guard. The implementation of the operating procedures resulted in a drastic reduction of incidents.

Independently, the UK HSE published the 4th edition of the Offshore Installations Guidance Notes (GN)[5]. The GN included subdivision standards that applied to any one compartment which appeared to be a substantial raise of the safety standards. Due to the self-imposed standards by designers, most units deemed suitable to operate in the North Sea satisfied the new requirements. However, further inspection by the UK HSE raised concerns about the stability of units

The (JSIT) Working Group continued investigations and a series of model tests were carried out to evaluate the stability of jack-up and their survivability after damage. The conclusions are many and have been published in several reports and papers; some of them presented to this Conference in previous sessions. The model tests confirmed the validity of the long standing “area ratio” intact stability criterion. The GN 4th edition extended the application of the area ratio criteria to damage stability but the tests could not correlate this parameter to survivability after damage. Further, test results failed to identify a stability parameter that reflects adequate subdivision and an acceptable level of survivability after damage.

ABS initiative

Starting with the investigation of the loss of the ROWAN GORILLA I, ABS has had an active participation in most committees and the JSIT working group. The attention by the UK HSE on the apparent lack of subdivision of some units encouraged ABS to take the lead in a renewed effort in the search for adequate stability standards. ABS’ dominating share (87%) of the jack-up classification market gave the organization access to the information and the ability to reconvene a group representative of Industry.

The leading aspect of Jack-up subdivision and the statutory conditions in the year 2000 can be summarized as follows:

• Most regulations only address waterline damages to the peripheral compartments disregarding the scenario found on most jack-up losses. The record shows that typically jack-up floundered after the flooding of several internal compartments.

• The UK GN had been withdrawn and ABS Rules applied to a limited number of internal compartments. Most other regulations have no restriction to the size of internal compartments. Should designers not apply their self-imposed subdivision standards, it was conceivable to design a jack-up that could have a damage stability problem after the flooding of a compartment.

108 Internal Subdivision of Jack-ups – A new standard to resolve an old concern


• All units classed by ABS operating on the UKCS in the period analyzed maintained a subdivision that met the UK GN standards. Few units with ABS class, working outside the North Sea, appear to lack subdivision of the internal compartments.

ABS Ad Hoc Committee for Jack-Up Stability

Having confirmed that all the subdivision of the ABS Classed jack-ups operating in the UK CS met the requirements of the GN 4th edition, , ABS invited a number of industry representatives to form an Ad Hoc Committee (AHC) with the following goals:

• Evaluate the ABS Rules and determine if new or additional subdivision standards were needed • Evaluate the status of the fleet classed by ABS and identify any designs that showed an unusual vulnerability to

damage • Develop new standards to close any loophole allowed by the Rules that could lead to inadequate subdivision

The Ad Hoc committee agreed that the current Rules could allow the design of a jack-up with a well subdivided “collision belt” with a very large internal compartment (open architecture) with its centroid close to or on the centroid of the waterplane. Open architecture, such as the one shown in the illustration below, if applied well, can provide an excellent subdivision. However, the Class or statutory standards in force at the time could not prevent a design with an internal compartment that, if flooded, would place the rig on an extremely vulnerable waterline. Taken to the extreme, if the damaged waterline would be at the main deck elevation, it would comply with the 50 knot wind stability criterion and yet be easily turned over by the effect of waves and green water.

Searching for a reasonably simple standard, that would allow early design stage planning of the subdivision, the AHC proposed a standard limiting the maximum size of any one compartment. ABS made an exhaustive research of all designs representative of the market. Older designs and those built as one-offs were not researched. Standard designs, built with more than one subdivision arrangement, were investigated in each version.

The AHC completed its task and submitted a proposed Rule change to the ABS Special Committee of MODU in the May 2003 meeting. The new subdivision standard applied to all watertight compartments in the unit and limited their maximum size to 33% of the reserve buoyancy of the unit at its deepest waterline.

In the investigation it was established that all the existing designs that showed a rational subdivision met the standard. The very few units that did not, showed one or maybe two compartments that were clearly oversized.

The Special Committee on MODU accepted a modified version allowing the size of the compartment to 40% of the reserve buoyancy. The Rule change was approved by the ABS Technical Committee. The new Rule applied to all units contracted after January 1st, 2004. The Special Committee also directed the AHC to reconvene to develop a different standard that would address several valid objections raised during the meeting:

• The Maximum compartment size did not consider the relative position of the flooded compartment with respect to the center of the rig.

• The new designs seemed to require larger compartment to accommodate the increasing size and number of below deck equipment. Newly designed rigs have rooms with volume very close to the 33% limit and the new Rule lacked the technical robustness to allow such compartments even when well located on the unit.



The AHC reconvened in August 2003 with Bob Bowie as Chairman. More members joined to include representatives the following companies:

• ABS • Keppel AmFELS • Atwood Oceanics • Bennett and Assoc. • Diamond Offshore • ENSCO • Friede & Goldman • GlobalSantaFe • GustoMSC-ODA • Keppel Marine Group • Keppel FELS • LeTourneau • Maersk Contractors • Noble Drilling • Pride International • Transocean

In the fist meeting, held in August 2003, the following tasks were proposed to be completed by February 2004 • Establish an acceptable extent of flooding, including which and how many compartments were to be assumed

flooded concurrently. • Establish new residual damage stability criteria. • Evaluate the current fleet of jack-up against the selected extent of damage and residual stability. • Establish the mode of flooding and the methodology for damage stability analysis for the new criteria.

To provide the needed agility, the AHC appointed a Technical Research Team (TRT) with a small number of members, with strong technical background and experience in MODU Stability. The team, lead by, the following members:

• Julian Bowes – Chairman (LeTourneau) • J. Andrew Breuer (ABS) • Tom Burns (Friede& Goldman) • T. O. Cheung (Keppel Marine) • Anis Hussain (Keppel AmFELS) • Ray Jacques (LeTourneau) • Richard Michel (Bennett and Assoc.) • CJ Mommaas • Chris Morlan (GustoMSC-ODA) • Sean O’Connor (ENSCO) • Joe Rousseau (ABS) • Raquel dos Santos – Secretary (ABS) • Julian Soles (GlobalSantaFe) • Charles Springett (Consultant) • Joost van Santen (GustoMSC)

The TRT met 11 times in a six-month period and it reported or met with the Ad Hoc Committee four times. Between meetings, an exhaustive list of tasks was set to test and validate the ideas presented on the previous meeting. The following sections present some of the ideas, the results of the investigation, and rationale for acceptance and rejections.

Flooding scenarios and extent of damage

The reports of the jack-up rigs lost in transit indicate that the flooding extended to several compartments. The compartments involved were mostly operational compartments and not tanks or voids. The source of flooding water was through the topsides and not as the result of a breach to the side shell or bottom plating.

While the possible distribution of water into multiple compartments is unlimited in number and percentage of fill, a regulatory representation of the extent of flooding would be the assumption of a selected number of compartments, filled to a fraction of their capacity. The alternatives proposed to the AHC were:



1. Four compartments filled to 25% capacity, or 2. Two compartments filled to 50% capacity, or 3. Three compartments filled to 33% capacity, or 4. One compartment filled to 100% capacity

It was also suggested that only combinations of adjacent compartments should be considered but this alternative became moot when the TRT favored the One-Compartment filled to 100% capacity approach. Eventually, the TRT proposed and the AHC accepted this alternative in spite of the difference between the single compartment flooding and the flooding scenario experienced by the lost rigs. Various arguments supported this choice:

• The one compartment option was determined to be the most conservative. The effect of free surface in multiple compartments may or may not add to the conservative but the multiple options on how to apply it, would lead to conflicting results.

• A multiple compartment approach would require the analysis of the many combinations of two, three, or four compartments. This without consideration to the infinite combinations of different levels of flooding that each compartment could have suffered.

• Partial filling of compartments, as discussed below, requires an analytical procedure that is not available in most software packages, and the input and results can be interpreted in conflicting ways.

Mode of Flooding

On a typical incident in transit, the compartments were “flooded” from the topside. The water level in each compartment was possibly, but not necessarily, the same as in the other flooded compartments but different from the level of the sea. The conventional damage stability analysis assumes that the compartment is in direct communication with the sea (DCS) and most naval architecture software only provide that option for analysis.

A great number of damage cases were analyzed for twenty-two different designs. In most cases, each member of the TRT researched their own designs but ABS addressed the cases where the designer was not available or when the team member lacked the resources. One series of tests evaluated the one-compartment filled to capacity scenario. In spite of the apparent simplicity, each member of the TRT had a different approach, and the results were not quite comparable. Eventually, it was agreed that the assumption of DCS was a better analytic choice despite its less realistic approach. The following reasons were presented to support this approach:

• Most software lacks the capability of analyzing the effect of simple filling but all can do the DCS. • Simple filling analysis can be done with several conflicting approaches and thus creating a new issue to

standardize. • If the new criteria also apply to compartments exposed to peripheral compartments (as it was eventually agreed),

each peripheral compartment would have to be analyzed twice; one as simple filling and once in DCS.

An observation against the DCS choice is that the damage compartment is not likely to be filled to capacity. However, in a damage scenario that is significant as determined by its effect on the stability analysis, a compartment damaged with DCS is likely to be filled very close to 100% and the conservatism of the "loss of buoyancy" compensates for the difference between "close to" and "100% fill."

Again, the AHC chose the simpler and conservative approach over the more laborious and physically correct one.

Residual stability Criteria

This part of the research was the most difficult to agree as the alternatives are many. Even after narrowing the options to a few, the evaluation of each one as it was applied to the twenty-two jack-up designs selected became a very arduous effort.

The proprietary nature of each design was discussed in the early stages of these analyses; agreement to maintain the confidentiality of the information was established. Results would be disclosed to all members of the TRT but the results would not be associated with any particular design. This meant that ABS personnel, who are bound by their internal confidentiality procedures and the ABS Code of Ethics, would develop many of the displays and statistics.



Several criteria were suggested as a way to evaluate resistance to capsize and sinkage. From the many suggested the following seven were selected as reasonable and further investigated by the TRT:

1. Maximum size of compartment 2. Minimum GM after damage 3. Reserve of dynamic stability. Fraction of energy to capsize retained after damage 4. Reserve of buoyancy. Fraction of reserve buoyancy retained after damage 5. Final waterline after damage below margin line 6. Green water on deck 7. Range of Stability & Stability Range

The following rationale was developed for each of them:

1. Maximum size of compartment

This criterion was considered because it is part of the requirements in the MODU Rules at the time. The 40% of the reserve of buoyancy maximum compartment size was adopted for its simplicity and ease of application; it has many of the properties expected from practical Rules.

While the criterion ensures that the unit will not sink after the flooding of any one compartment, it provides no certainty against capsizing or progressive flooding.

The TRT analyzed the effect of compartment location on damage. For that purpose the damage stability analysis was done with the center of the compartment on regularly separated positions from the end and centerline. The results demonstrated that the location of the compartment is as critical to survivability as the volume. The graph below shows the relative size of a compartment (when compared to the reserve buoyancy before flooding) that would cause the loss of a jack-up. Compartments with the centroid at or near the center of flotation of the hull only produce a parallel sikange. Loss of stability would follow if the compartment was the same size of the reserve buoyancy. Compartments bounded by the side of the hull can capsize the unit even when the volume is relatively low. The 40% ratio adopted previously was proved not to be restrictive enough for compartments away from the center of flotation.

2. Minimum GM after damage

This criterion was considered by its simplicity. However, one concern leading to this investigation is the flooding of central compartments. A large compartment causing a parallel sinkage would result in a very high metacentric height but the unit is left extremely vulnerable to environmental loads. The unit after damage with even keel and the main deck awash, is when this weakness is at its worst.

3. Reserve of dynamic stability. Fraction of energy to capsize retained after damage

This criterion was considered because it would reflect the unit’s resistance to capsize in a dynamic mode. This is analogous to the area ratio criterion so successful in evaluating intact stability. However, model tests demonstrated that this latter criterion does not correlate well with the resistance of the damaged rig to capsize. Furthermore, the model test films show the capsizing to be a quasi-static event where energy to capsize is not a leading parameter.



The capsizing can be seen to occur after an ever increasing heel is created by the pounding from a series of waves crashing on deck.

4. Reserve of Buoyancy. Fraction of reserve buoyancy retained after damage

In a closer analysis, this criterion is very similar to that of the compartment size restriction. It was dismissed for the same reasons used for disregarding the Maximum Size of Compartment Criterion.

5. Final waterline after damage below margin line

This criterion has the simplicity expected from a Rule and it can be evaluated with hand calculations. Also, it is a recognized criterion with a great acceptance for many types of vessels. Conventionally, the Margin Line lies on the shell plating, 3 inches below the watertight deck edge.

The analysis on a few of the rigs in the set of 22 selected designs showed that, even when the effect of wind is ignored, this criteria is more conservative than that currently applied to the peripheral compartments. The adoption of such criterion would impose a severe increase of the damage stability requirements, even for designs that have been deemed to be satisfactorily subdivided.

6. Green water

The stability effect of green water on deck was carefully considered. The models tests have shown how the effect of water on deck is the dominating force that caused capsizing. ABS Corporate Technology Department (R&D) hydrodynamicists were consulted on this subject. The conclusions from this discussion were that, while the computational tools are available, they are complex and must be validated. Further, the effect of green water would eventually have to be reduced to simple tools. Such a research would be very long and of uncertain results.

The criterion was not investigated any further because the schedule set for the development of the new Rule had no room for a lengthy research.

7. Range of Stability & Stability Range

The selected criterion seems now an obvious choice, but it became so only after long discussions and many analysis.

Range of stability (RoS) and Stability Range (SR) were determined to be an excellent complement to the existing criterion to evaluate the residual stability after damage. Realizing that the existing criteria for residual stability (no downflooding occurrence under a 50-knot wind) provided an acceptable subdivision standard for peripheral compartments the research focused on the flooding of central compartments; specifically, damage that caused parallel sinkage with little or no list or trim.

The chart below shows the detrimental effect of an increasingly severe extent of flooding on the righting arm curve. The severity is measured in terms of the ratio of the volume flooded over the volume of the reserve buoyancy. It can be easily explained as a “loss of freeboard after damage”.

The RoS is defined as the angular difference between the first and second intercept of the righting arm curve and the zero-arm axis. The SR is the angular difference between the first and second intercept of the same curve and the wind heeling arm curve.

RIGHTING ARM FOR DIFFERENT RoB (Added Weight)

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0 5 10 15 20

Heel (deg)

Rig

htin

g A

rm (f

t)

0% of RoB 15% of RoB 30% of RoB 45% of RoB 60% of RoB



The choice was made for the first one because model testing demonstrated that the wind overturning moment had little effect on the capsizing event. The testing showed the unit consistently capsizing toward the direction of the incoming waves. The disregard of the wind also simplifies the analysis; an additional advantage that cannot be ignored.

It must be noted that while the RoS is established independent of the wind curve, the wind was retained as the environmental effect in the waterline damage analysis.

Validation of the selected criteria

Damage stability analysis was carried out on the selected group of twenty-two jack-up designs; including the design used for the model testing. This series was carried several times with varying parameters. As understanding of the results grew and trends were identified, the studies focused on the results for the jack-up loaded with the allowable VCG in ABS record and “as loaded” VCG obtained from the operations manual for each unit.

Results were further identified by the location of the damage compartment into three groups:

• Peripheral compartments, • Internal-lateral compartments, • Internal central compartments

Results were displayed in various chart formats. In the analysis of results, it was determined that while the RoS best correlated with the resistance to capsize if the detrimental effect of the static angle of heel after damage was also considered.

The chart below presented the result in the most eloquent way and the studies focused in this way. These are some notable aspects of the data.

• The double circular markings correspond to the model test results when the rig did not capsize. It showed the trend and assisted in determining the minimum acceptable RoS.

• Each marker represents a damage stability analysis performed. The designs are not identified in the chart to maintain the confidentiality of each design. However, members could identify the results of their own designs and compare how they fit on the scatter.

• A few markers appear to show extremely low results. The ABS members of the TRT separately evaluated the cases and sometimes discussed with the designer of the specific jack-up. When analyzed individually, these cases would often be clearly the result of a less than ideal subdivision. Some of the cases showing very low RoS were on large compartments resulting from changes of subdivision after construction. Some were for units that operated at drafts deeper than conceived at design.



Static Angle w/o Wind vs Range of Stability (RoS)

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Model Test (NOT Capsize) New Damage Stability Criteria

Peripheral Compartments Peripheral Compartments (VCG as Loaded)

Internal-Lateral Compartments Internal-Lateral Compartments (VCG as Loaded)

Internal-Central Internal-Central (VCG as Loaded)

Criteria:RoS ≥ 7+(1.5 * Static Angle w/o wind)

RoS min = 10 deg

Selection of the required RoS

Determination of the actual RoS was subject of a lively discussion. A cautious approach would suggest a very high value but a realistic approach prevailed and accepted that most designs are reasonably subdivided. Two prevailing factors dictated the selected level:

1. The current criteria that govern the size of peripheral compartments established the acceptable level of subdivision. Therefore, the boundary of minimum RoS should not be more severe than the RoS available after damage of those compartments.

2. The trend dictated by the model tests is a strong identifier and the minimum to be established. It would be reasonable to expect that the capsizing conditions in the model test should be located on the left side of the boundary line. The selection of a “median” line is based on the recognition that the environmental condition to which the model was exposed is far more severe than what is intended in the Rules.

On the above basis the RoS ≥ 7º + (1.5 Θs) was established based on a boundary line that separated the reasonably sized compartments from the oversized ones. Further, the discussion focused on the minimum. It was argued that while the trend of the limiting line was acceptable, the minimum that applied for centrally located compartments was too low. A minimum RoS of 10 degrees was established and unanimously approved.

A very important decision followed when the TRT recommended that downflooding should not be considered as a factor in the evaluation of the RoS. There are several reasons for this determination:

• Downflooding through weathertight protected openings (tank vents, doors, hatches, etc) is already controlled by the existing 50-knot wind criteria.

• Downflooding through openings such as ventilation intake and exhaust is taken into account in the intact stability studies and the downflooding angles are larger than the RoS limits. The difference is large enough to account for the effect of damage.

• The purpose of the new Rule is mainly to restrict any compartment size in association with the stability properties of the hull

It was further agreed that the RoS standard should apply to all compartments. This may be objectionable because the existing standard that restricted the size of peripheral compartments was already acceptable. However, the wider



relevance simplified the application of the Rule. Also, by extending the application, it closed a possible loophole created when an effectively internal compartment is extended to the shell plating to be only exposed to waterline collision.

RoS – a higher standard

It has been argued that the new Rules impose a higher standard by “raising the bar”. This is true when looking at it from a Rule point of view. However, the TRT made a conscientious decision to adopt a standard that would follow the better practice by the experienced designers. During the evaluation of the designs it was confirmed that most jack-up rigs operate with a subdivision that exceed the old standards. The graph below compares the righting arms restrictions before and after the new Rule. The illustration below, shows the difference between the earlier criteria the new criteria

Practical application

The new standard has been applied to several, new and existing, designs since last January. Despite its apparent simplicity, designers and ABS have found technical difficulties when analyzing the RoS after damage. This difficulty is not a new finding; in fact it was reported by J. A. van Santen in 1986[6]. The difficulty is mostly found when “variable trim” righting arm calculation can result in more than one balanced trim. Further difficulty is created by the conventional approach to develop righting arms from different “fixed” directions. In actuality, the rolling of the hull does not necessarily follow this restricted pattern and leads the conventional analysis to unexpected results. The rationale to resolve this challenge is currently investigated by ABS on the basis of a presentation by J. A. van Santen and others. A solution is expected soon.

Conclusion

The new requirement for residual stability after damage closes a gap left open by the ABS Rules since first published. The Rule provides guidance to limit the size of the hull compartments but is not an assurance against rig loss after flooding.

The Rule is neither a reflection of the incident that the rig is expected to suffer, or the residual stability that will prevent the loss of the unit. Further, this Rule consolidates the many flooding scenarios that may be expected into a simple one. The RoS was selected as the residual stability parameter that appears to best reflect the many affected by damage. As in many other Rules, this is a simplification that provides a level of safety comparable to or higher than one obtained by far more complex methods. It also reduces the risk for errors in interpretation and application.

Despite the possibility of unusually large internal compartments allowed by the previous Rules, the research found that most built designs are reasonably subdivided. Even rigs that have one or two compartments that fall short of the new subdivision standard are not necessarily at unreasonably high risk.

Open architecture designs (i.e. a collision belt surrounding one very large compartment) are in a special group that must be considered. Open architecture has been found to be a very viable solution to resolve the challenges of flooding. While a few of such rigs show vulnerability to flooding the internal compartment, the great majority were carefully located and sized to offer especially high residual buoyancy and stability. In addition to the residual stability qualities, open architecture resolves the uncertainty of the extent of flooding, the concern on the adequacy of watertight closures, and provides a residual freeboard round the deck.



References [1] American Bureau of Shipping – Rules for Building and Classing Mobile Offshore Drilling Units, 2001 - Notice No.

4 – December 2004 [2] American Bureau of Shipping – Rules for Building and Classing Mobile Offshore Drilling Units, 2001 [3] The Future of Jack-Up Stability - Learning From Our Past. Ninth International Conference-The Jack-Up Platform,

Design, Construction & Operation, 2003,London, England [4] American Bureau of Shipping – Rules for Building and Classing Offshore Mobile Drilling Units, 1968 [5] UK Health and Safety Executive, Offshore Installations: Guidance on design construction and certification, Fourth

Edition, 1990 [6] Stability Calculations for Jack-up and Semisubmersibles – J. A. van Santen – Conference in Computer Aided

Design, Manufacture and Operation in the Marine and Offshore Industry


internalsubdivisionjackups (1)

Documents

new subdivision

subdivision standards

internal subdivision

new standard

current abs rules

new rule

damage stability standards

abs technical committee