Abstract

Eni Petroleum will be developing the Subsea Production Facilities for the K2 Project on behalf of its partners Anadarko, Conoco Phillips and Unocal. The development is located in 3900 ft of water depth, to tie-back three to five subsea oil wells in Green Canyon Block 562 (GC562) in the Gulf of Mexico to the Marco Polo TLP facility at GC608. This paper broadly discusses several challenging design considerations particular to the wet insulated tie-back Steel Catenary Risers (SCR). These SCRs are made up of heavy wall pipe with wet insulation to guarantee flow assurance requirements and are also designed to provide buoyancy necessary to reduce the top tension loads on the host facility. It also focuses on the interdependent aspects of the interference and top load requirements, which imposes significant optimization design effort. It also addresses the fatigue design phenomena that are very challenging to quantify but are essential to assure feasibility and deliver an optimized design solution, together with the material property and dimensional tolerances and weld acceptance criteria required for the construction and installation phases of the project.

Introduction

The use of sub-sea tie-back as a production scheme for the GOM deepwater developments is becoming a preferred solution for many operators. These developments may be associated with reservoirs producing at high pressure and temperature, and may involve the tie-back to existing production floating facilities located at significant distances from the wells. These production facilites may be Spars, TLPs or Semi-submersible structures, each with its own peculiarities in terms of motions, maximum allowed payload, configuration and distribution of existing production and export lines and process facilities. All these factors have to be considered by designers when dealing with the SCR production alternative, and are commonly associated with significant challenges in the design effort.

In the K2 Project, the design of the production SCRs involved several iterations to achieve an optimized solution, where the effects of hang-off load limitation, hang-off location and attachment method, wave-loading fatigue, vortex induced vibrations (VIV) fatigue, extreme dynamics and interference analysis between risers and between risers and tendons, had to be carefully considered, in a short time frame, in order to accommodate the client's production schedule. The paper will present a broad view of the main design aspects associated with each of the design cycles developed for the project.

General Information

The Marco Polo platform consists of a mini-TLP in 4285 ft water depth, in Block 608 in the Green Canyon area of the Gulf of Mexico. The riser system for Marco Polo consists of an array of top-tensioned production risers combined with 12-inch and 18-inch SCRs for oil and gas export that are connected to the Marco Polo 14-inch oil and 18inch/20-inch gas export pipelines [1]. The platform hull consists of a four column TLP (Figure 1) with provision for several future tieback risers hanging off the platform base tank (wing tanks), located between the tendons support structure (TSS).

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