Analysis of Risers and Umbilicals for Disconnectable System - Challenges and Techniques
Authors: Dixon Mark A.J. , Al-Sharif Majid M. , Krasin Tim 
Published: May 2011

A Disconnectable Fluid Transfer System for Dynamically Position Ship Shaped provide great means for managing risk as it relates to avoiding harsh environmental events and protecting the subsea facilities in the event of a drive off. A set of alarm systems on the vessel, provide disconnect criteria due to weather conditions, fire, process emergency shut down and vessel offset from nominal position. A buoy, connected to the vessel, detaches to allow the vessel to either relocate or resolve the issue triggering the disconnect. The buoy sinks to a preset level via an attached clump weight. This operation has profound implications for the riser and umbilical system design as it must be designed for two operating modes, connected and disconnected, along with the transient dynamic cases going from one to the other under both planned and emergency scenarios.

The paper presents the challenges faced during the Phoenix field riser and umbilical analysis to define the operational envelope for these components, including the development of the load case matrix from the huge parameter space to satisfy regulatory requirements, and the operational importance of the connection and disconnection procedures. Additionally the paper addresses the implemented passive monitoring system developed as part of the integrity assurance for the disconnectable system.

Introduction

The application of floating ship-shaped vessels as offshore platform solutions, either as tanker conversions or newlybuilds, was first introduced in the late 70s of the last century. Since then, the number of these platforms steadily grew and nowadays there are nearly 200 of these in operation. The increase is down to more flexibility and value they offer compared to other options. Depending on development philosophy, they can have processing facilities onboard (Floating Production Unit - FPU), they can be used for storage only (Floating Storage Offloading- FSO) or both (Floating Production Storage and Offloading - FPSO). Further development of on-board facilities is currently under way and the first Liquefied Natural Gas (LNG) FPSOs will be commissioned in the near future.

Geographically, they can be found in virtually every offshore oil and gas producing corner of the world, West Africa, Middle East, North Sea, Asia-Pacific and South America. Depending on the local environmental conditions, these vessels can be spread moored, turret moored with weathervaning capabilities and in hurricane/typhoon-prone areas they are moored through a disconnectable turret, which allows them to release the turret and sail away until the weather conditions improve and they can resume operation.

Until recently, the only oil and gas producing part of the world without FPSOs in operation has been the Gulf of Mexico (GoM). The reasons for this vary, from available infrastructure to previous lack of regulations for the FPSO operations in the GoM (Regg 1999, [1]). However, the trend is now changing and last year saw the first FPU, Helix Producer 1, in the GoM going into operation in the Helix ESG operated Phoenix field.