Design Guideline for Riser Collision
Authors: Kalleklev Arve Johan , Mørk Kim J. , Sødahl Nils , Nygård Magne K. , Horn Agnes Marie 
Published: October 2008

The paper describes the main topics and findings of the JIP Management of Riser Contact, performed as a co-operation between Aker Kvaerner Technology and Det Norske Veritas, and sponsored by 5 major oil companies. The overall objective is to establish a design practise that properly accounts for contact between top-tensioned risers in operational conditions typical for the major deepwater locations.

The results, findings and recommendations for design and analysis are embedded in a Design Guideline document, reflecting industry consensus on methodology for engineering analysis and design for riser collision. The Design Guideline document constitutes the first steps towards a formal DNV Recommended Practise document, and includes a state-of-theart introduction to the main building stones for riser collision assessment.

So far, design codes have not allowed riser collision under normal operational or even extreme conditions. In the Design Guideline document, riser collisions are allowed provided that the riser array is subjected to state-of-the art analyses and sufficient fatigue (FLS) and ultimate (ULS) capacity are documented.

The paper discusses the main items regarding riser interference comprising hydrodynamic interaction, local response of risers in contact and acceptance criteria for design.

Introduction

A more accurate tool for assessing riser collision has become increasingly more important when oil and gas exploration offshore moves to deeper water. The risk of collision between marine risers increases with increasing riser length. Collisions may be damaging to the risers, both from a high cycle and low cycle fatigue point of view if the collisions lead to excessive plastic displacements.

The current design practice is that riser collisions are not allowed under normal or even extreme conditions. Top-tension and riser spacing are the primary parameters for design of riser arrays against collision. The costs related to increased toptension and/or riser spacing at floater termination may be very high due to possible implications on floater- and tensioner system design. Hence, riser collision has become a critical issue for cost effective design of deep-water explorations utilising top-tensioned riser arrays.

Det Norske Veritas and Aker Kvaerner Technology have prepared a project Design Guideline [1] for the Joint Industry Project "Management of Riser Contact". The document is based upon and being an extension of a former DNV report prepared for the Norwegian Deepwater Program [2]. The Design Guideline includes a state-of-the-art introduction to the main building stones for riser collision assessment, comprising:global analyses with hydrodynamic interaction model;local analyses, andacceptance criteria. The safety philosophy and design principles adopted in DNV-OS-F201 "Dynamic Risers" [3] apply. However, any recognised code considering the set of limit states discussed in the Design Guideline is in principle acceptable. The basic principles are in agreement with most recognised codes and reflect state-of-the-art industry practice and latest research.