Continued frontier development is driving industry's need to refurbish and significantly modify many of the world's 388 active jack-up units for novel projects, improved efficiency and sometimes harsh environments. Frontier development also is spurring the building of new units. Responding to the technical challenges posed by these modifications and the need for new designs, ABS has released a detailed technical Commentary to its Rules for Building and Classing Mobile Offshore Drilling Units (MODUs).
The commentary—a further step in providing industry more specific direction on how to apply ABS Rules to these new projects—particularly focuses on jack-ups, which are the predominant type of drilling unit addressed in the Rules for Building and Classing MODUs.
ABS provides classification and certification services for 75 percent of the world fleet of MODUs.
William J. Sember, ABS vice
president Offshore Development, advises that rapid advancements in technology and accelerated frontier development have challenged designers and generated increasing questions with respect to interpretation and applicability of the ABS Rules.
"This document serves as a supplement to the MODU Rules. The document also addresses industry questions and is intended to facilitate communications regarding future classification requirements for both modified units and new-builds," said Sember.
The commentary addresses some 27 issues, including clarification of load considerations, material selections and structural analysis requirements, and applies to Part 3 of the MODU Rules, which details criteria for the hull and equipment associated with all structures.
The Rules include a total of five parts: general introduction; materials and welding; hull and equipment; machinery; and surveys after construction.
ABS Principal Engineer Jer-Fang Wu served as project leader for the document.
"The commentary addresses a broader range of unit designs and gives owners more detailed guidance on the application of the MODU Rules, particularly those sections applicable to self-elevating jack-up units," said Wu.
Bret Montaruli, ABS manager of Offshore Engineering, explains that because ABS Rules need to be applicable to all designs, they are very broad in scope and are often open to interpretation.
"The commentary, therefore, addresses designers' questions, such as: 'We know what the Rule says; but when applied to our design, what does it mean?'" said Montaruli.
As an example of the clarification provided, Wu says ABS has enhanced the original "Structural Analysis" section to include a discussion on "Analysis of a Primary Structure."
"The commentary," advised Wu, "incorporates the following key elements to clarify all analysis-related issues, giving readers the 'background' or 'why' for formulas comprised within the Rule:"
1. Subsection on general concept of the analysis
2. Description of stepping wave through the structure
3. Description of steep waves
4. Loading directions of the waves
5. How to include the P (force)-Delta effect in the analysis
6. Details of the hydrodynamic leg-modeling procedure
7. Consideration of gravity loads and buoyancy
8. Importance of leg buoyancy
9. Clarification of total-elevated load
10. Emphasis of the effects contributed from Spudcan during the analysis
11. Clarification of the wave load effects on a mat-supported unit during analysis
"We plan to review and update the commentary periodically to ensure its applicability to current technologies and frontier developments," added Wu.