Erha and Erha North Development: Steel Catenary Risers and Offloading System
Authors: Nolop N.C. , Wang H.H. , Kan W.C. , Sutherland J.B. , Elholm E.S. , Hoyt D.S. , Montbarbon S. , Quintin H. 
Published: March 2009

The Erha and Erha North field development in deepwater Nigeria consists of multiple subsea wells connected to a floating production, storage, and offloading vessel (FPSO) via flowlines and steel catenary risers (SCRs). Processed crude oil is exported through an oil offloading system (OLS), consisting of a catenary anchor leg mooring (CALM) buoy and dynamic offloading lines (OLLs).

The SCRs were identified as a key technical issue due to high-fatigue performance requirements, especially under potential sour service conditions. In addition, the associated flexible joint performance in a relatively high-temperature environment also extended the design envelope for this critical component. Innovative clad overlay and clad welding technologies were developed within the Project timeframe and efficiently executed both onshore and offshore. A rigorous design and qualification effort, including the development of novel sour environment fatigue testing and ultrasonic inspection technologies, ensured system integrity.

The OLS was also identified as a key technical challenge early in the Project. The innovative, U-shaped OLL design is a first in the industry. The OLLs presented performance challenges similar to those of the SCRs, but in addition, the complicated coupled buoy/mooring/OLL motion behavior posed a unique challenge for OLL fatigue design. A dedicated qualification and validation program was devised to confirm system integrity. The design and execution teams utilized new technology and lessons learned from past projects to ensure that the installed system would meet challenging acceptance criteria. This paper discusses challenging issues and resolutions for the SCR and OLL designs. The extensive qualification programs, including CALM buoy model tests, SCR cladding and welding qualification tests, and the flexible joint qualification, are also discussed.

Introduction

The Erha and Erha North development, located in 1,200 m of water approximately 97 km offshore Nigeria, consists of three subsea drill centers tied back to an FPSO. Drill Centers East and West (DCE and DCW) are located approximately 1.5 km from the FPSO, and Drill Center North (DCN) is an 8-km subsea tieback to DCE. The subsea production system includes trees and manifolds connected to 6-in. and 10-in. insulated steel flowlines that tie back to the FPSO via SCRs. Injection fluids are transferred from the FPSO via the SCRs and flowlines for subsea injection at the drill centers. The FPSO is a new-build, spread-moored vessel with installed capacity for 210,000 barrels per day (bpd) oil processing, 150,000 bpd water injection, and 340 million standard cubic feet per day (Mscfd) of gas injection. Processed crude is stored in the FPSO and regularly offloaded to tankers through two 22-in. offloading lines and CALM offloading buoy. Figure 1 shows the field layout.

Figure 1. Erha and Erha North Field Layout (available in full paper)

The Erha field contains ten flowlines/SCRs, five for DCE and five for DCW. A stab and hinge-over (SHO) arrangement connected to a suction pile initiates each line, which terminates at the FPSO via a flexible joint.

The OLS includes two 22-in. outside diameter steel OLLs. The OLLs are suspended between the FPSO and the CALM buoy in a U-shaped arrangement.