The January 1998 edition of the Rules for Classification of Mobile Offshore Units contains a revision of the following rule chapters, entering into force on 1st of July 1998:


Pt.0 Ch.1              User Information, Amendments and Indexes

Pt.1 Ch.1              General Regulations

Pt.3 Ch.1              Structural Design, General

Pt.3 Ch.2              Special Designs, Equipment and Stability

Pt.6 Ch.1              Miscellaneous Notations


Some of the Rule chapters have been subject to minor amendments, which have been inserted in Sec.3 of Pt.0 Ch.1.


Important Rule Changes:

The most important rule changes covered by the revised chapters or by Sec.3 of Pt.0 Ch.1 are as follows:



Pt.1 Ch.1 Sec.3      General regulations - Classification Procedure


The rules in this section stipulate general conditions for suspension and deletion of class.


However, the wording relates only to automatic suspension in case of overdue annual, intermediate and renewal surveys and 'force majeure' cases, and could incorrectly and unintentionally be interpreted to mean that class suspension could not be used in other cases.


To clarify this, a new paragraph has been included, stating that immediate class suspension may be effected also in cases where a deficiency or a survey related to a class notation is not dealt with by the time stipulated by the surveyor.


Furthermore, a clarification on conditions for reclassification in cases when outstanding debt to the Society is the reason for class withdrawal, has been included.


Pt.1 Ch.1 Sec.4      General regulations - Legal Provisions


The regulations concerning liability and jurisdiction have been rewritten.


Pt.3 Ch.1                 Special Designs, General


This Rule chapter has been subject to a general updating:


- The Rule requirements are now compatible with DNV Classification Notes 31.4, 'Strength of Main Structures of Column Stabilised Units' and 31.5, 'Strength Analysis of Main Structures of Self-elevating units' (e.g. in respect to the definition of 'Special areas' and to jack-up installation forces, respectively).

- The environmental criteria previously stated in the Rules are generally replaced by references to DNV Classification Note 30.5, 'Environmental Conditions and Environmental Loads'.

- The requirements for local pressure loads have been updated and clarified.

- All specific items concerning semi-submersibles and jack-ups have been relocated to Pt.3 Ch.2 Sec.2 and to Pt.3 Ch.2 Sec.3, respectively.


Pt.3 Ch.2 Sec.2      Special Designs, Equipment and Stability - Column-Stabilised Units


Requirements for semi-submersible units have been added concerning:


-                                                               explicit criteria for tanks where filling to the top of the air pipe is not a relevant design load condition

-                                                               explicit criteria as to how to consider internal tank pressure, head loading and environmentally induced load components.


Pt.3 Ch.2 Sec.3      Special Designs, Equipment and Stability - Self-Elevating Units


Jack-up foundation criteria are to be found in DNV Classification Note 30.4, 'Foundations'. The Rules have been updated by referring to this Classification Note with respect to foundation design for jack-ups. Furthermore, the jack-up-specific requirements for local pressure have been updated and clarified.


Pt.4 Ch.1 Sec.4      Machinery and Systems Design, General - Platform Piping Systems


The requirements for ballast system design have been rewritten to be in line with the requirements given in Pt.3 Ch.2.


Pt.4 Ch.2 Sec.6      Propulsion and Auxiliary Machinery - Shafting and Vibrations


Since the end of the seventies many ships were equipped with generators driven from a main gearbox power take-off (PTO). Throughout the eighties frequent bearing failures have been reported. Focusing mainly on the lateral vibrations of the generators, stringent instructions for approval were applied, and the rules concerning lateral vibrations measurements were revised in 1995.


These amendments did not give the expected results in practice, and it has later appeared that this is due to not giving a sufficiently specific acceptance limit. The rules have now been amended to account for this and the fact that no failures have been reported over the last years.


Pt.4 Ch.2 Sec.9      Propulsion and Auxiliary Machinery - Turbochargers


The Rules include a requirement for overspeed testing of turbocharger wheels. At the time the Rules were developed, all such wheels were designed with a bore. An overspeed test was introduced in order to secure adequate level of safety in relation to wheel burst.


Lately, wheels without a bore have been designed. Documentation shows that the stress level is significantly lower than of a comparable wheel with a bore. Also burst tests of such designs have been performed that verify that safety against burst is far higher than that of the designs with a bore.


To reflect this, the requirement for overspeed testing of turbochargers has been amended, saying overspeed testing of turbocharger wheels may be waived if the wheel is without bore.


Pt.5 Ch.2 Sec.2      Accommodation Units and Other Offshore Support Units - Accommodation Units


The Rules regarding accommodation units have been updated so that the requirements for structural strength reflect the current Rule requirements for stability, including reserve stability.


Pt.6 Ch.1 Sec.2      Miscellaneous Notations - Helicopter Decks


While the requirements for helicopter decks in the Rules for Classification of Ships have been updated in the past, the Rules for Classification of MOU have not be similarly updated. This chapter has been generally updated in order to make them more compatible with the helicopter deck rules contained in the Rules for Classification of Ships.


Pt.7 Ch.5                 Management of Safety and Environmental Protection (SEP)


The SEP Rules for MOU are similar to the SEP Rules for ships, and have been issued in a common booklet. For description of the amendments, see Rules for Classification of Ships, Pt.7 Ch.5.