Development of an IS0 Standard for Floating Structures
Authors: Wolfram W. Robert 
Published: October 2008

An internationally constituted IS0 work is currently working on the development of Part 4, Floating Structures, of IS0 Standard 13819, Standard for Offshore Structures for the Petroleum and Natural Gas Industries. Part 4 deals with all types of floating structures used for production and/or storage of oil and gas, including monohulls, semisubmersibles, tension leg platforms, and spar-shaped vessels. Part 4 will set performance criteria and present design methods for the hull, riser system and moorings, and will list site-specific requirements for a permanently located facility. The work group intends to make reference to other appropriate codes and standards, particularly relevant classification society design practices, whenever possible. The new IS0 standard, when completed, is intended to replace corresponding American Petroleum Institute Recommended Practices (API RPs) for FPS, spar, and TLP system design.

Introduction

The increasing use of floating structures for development of oil and gas fields offshore has demonstrated the lack of any set of standards with broad industry acceptance for the design of such structures. Floating Production, Storage and Offloading (FPSO) vessels, which have been used for a number of years, primarily in mild environments, have often been designed as conversions from existing tankers using design practices taken directly from classification society rules for ships. The recent increase in the application of FPSOs for full field development in rough weather areas, such as the North Sea, has highlighted the need for sitespecific design requirements, particularly in the areas of fatigue resistance and inspectability offshore, to facilitate long-term operation without the' need for drydocking.

Semisubmersible-based Floating Production Systems (FPSs) have been designed primarily using classification society rules for mobile offshore drilling units (MODUs). Spar shaped vessels, for which no operating experience exists, have no single set of standards applicable to their design. Only tension leg platforms (TLPs) are covered by a specfic RP (API RPZT). This RP is closely related to typical offshore platform design practices contained in API RP2A. As a result the requirements for hull design can vary significantly from the similar shaped semisubmersible FPS, whose design is based on MODU practices.

The area of mooring design is covered by two different standards, API RPZSK, and "North Sea" practice embodied in codes such the Nonvegian Maritime Directorate (NMD) Regulations for Mobile Offshore Units. North Sea practices have many similarities to API's, but also some marked differences, particularly in the specification of design criteria. Finally, riser design is just now being standardized with the development of the soon-to-be-issued API RP2R.D.

In the well-developed offshore operating arenas, such as the Gulf of Mesico and the North Sea, the choice of design practice for the various structures was often influenced by strong national regulations. However some nations are moving toward less specific goal setting regulations and the industry is also expanding into less developed areas where national regulations may be weak or nonexistent. Furthermore, field developments are increasingly being undertaken by joint ventures of several operators, with project teams made up of representatives of the various partners and or "alliances" with contractors.