Equipment and Material Selection to Cope With High Pressure/High Temperature Surface Conditions
Authors: Walton D. 
Published: October 2008

Abstract
The Elgin and Franklin project is the largest development on the UK Continental Shelf in recent years, and the largest High Pressure and High Temperature (HP/HT) field developed to date. The field layout, which was selected following the conclusion of the Front End Engineering Development studies, required that the two separate Wellhead Platforms had to be capable of handling the high surface pressures and temperatures from the reservoir, prior to transporting the products to the main central processing platform. On this Processing, Utilities and Quarters (PUQ) platform, processing is performed at more "conventional" pressures and temperatures. This paper describes the general layout of the Wellhead Platforms, identifies the HP/HT issues that were considered critical to the successful design of the surface facilities, and describes some of the particular challenges encountered during the design and manufacturing stages of the project. The paper demonstrates the importance of a carefully considered programme for the development and pre-qualification testing of critical equipment.

Introduction

Field Layout.
The Elgin and Franklin fields are located in Blocks 22/30c and 29/5b of the UKCS, in water depth of approximately 92m. The overall field layout is shown in Fig. 1, which shows that the two Wellhead Platforms are independently connected to the main Elgin PUQ platform. The wellhead jackets are conventional four legged steel jackets, with grouted piles, the typical arrangement of which is shown in Fig. 2. One notable feature of their design is the incorporation of the "twisted base" concept which eliminates the requirement for battered legs and therefore improves the access from the attendant jack-up drilling rigs which are required to operate in cantilevered mode over the Wellhead Platforms. Neither of the Wellhead Platforms have permanent drilling facilities, as all wells are intended to be drilled from an attendant mobile jack-up drilling rig, which can gain access to the wells through removable hatch covers contained within the upper weather deck of the Wellhead Platforms. Similarly, all future workover operations shall be performed using temporary facilities which will be installed on the Wellhead Decks when required.
The Elgin Wellhead Platform is directly adjacent to the PUQ, to which it is connected by a steel bridge. In addition to providing personnel access, the bridge also carries the various pipework and cable systems to provide utilities, power and instrumentation that are necessary to support operations on, and receive hydrocarbons exported from, the Wellhead Platform. As shown in Fig. 3 and Fig. 4 the Elgin Deck, which weighs under 2000 Te, comprises two main levels plus a weather deck. The lower deck contains the wellheads, anti-scaling and washwater pumps. The mezzanine deck houses the Christmas trees, the associated flowlines and manifolds (test and production), the choke and shut down valves as well as methanol injection pumps. The structure also contains electrical equipment rooms and all the safety equipment necessary to support the anticipated personnel levels, including a freefall lifeboat and liferafts. The weather deck has removable hatches to allow access to the wellheads from the attendant drilling or workover rig.