Recommended Practice on Riser Interference

Abstract

Abstract A new recommended practice on riser interference (DNV-RP-F203) will be issued in 2009. The objective of the recommended practice is twofold:to recommend a methodology for engineering analysis, providing a rational design criteria and guidance for assessment of riser interference, andto achieve and document the industry consensus. Assessment of collision between adjacent risers becomes more important with increasing water depth. The space between risers is limited both at the topside onboard the vessel as well as the riser gateway and at the seabed. Riser interference comprises complex physical phenomena, not yet fully understood, and research is still ongoing. When several risers/umbilicals are needed in a limited space, clearance between risers and risers and mooring lines should be documented. The overall focus has changed somewhat since the draft was released for industry hearing in 2005. The 2005 version covered only top tensioned riser systems in steady current. The updated version covers all dynamic riser system including dynamic umbilicals and the focus is to document properly that no collision occurs. The scope is first of all to propose a framework for feasible analysis strategy and practical design procedures with focus on how to assess whether the risers collide or not. This paper presents the methodology for analysis, and the criteria which are the basis for the recommended practice. Introduction A more accurate methodology for assessing riser interference has become increasingly important when oil and gas exploration moves to deeper water. The risk of interference between Top-Tensioned-Risers (TTRs) increases with increasing riser length. The general design practice is that riser collisions are not allowed under normal or even extreme conditions. The recommended practice considers analysis procedures and design criteria for riser interference assessment. The overall objective of the document is to recommend a methodology for engineering analysis, and to provide rational design criteria and guidance for assessment of riser interference. Riser interference comprises of complex physical phenomena, not yet fully understood, and research is ongoing. The aim is to achieve and document the industry consensus as per today. The scope of work is first of all to propose a framework for feasible analysis strategy and practical design procedures with focus on how to assess if the risers collide or not. Whenever the terminology " risers?? is used it generally also applies for umbilicals and flexible risers in addition to TTRs. In order to derive a complete framework for feasibility and practical design procedures, the following topics need to be considered:overall framework and design approach;safety philosophy;environmental conditions and loads;analysis strategies and hydrodynamic interaction models, andacceptance criteria and fundamental requirements.