Lightweight Subsea Manifold Design
Authors: Kirkbride Peter , Brown P.G. , Bloomer R.A. 
Published: October 2008

This paper reviews the design development of the Shell/Enterprise Nelson Subsea Manifold. This facility provides full manifolding for up to eight production wells with gas lift distribution facilities. Water injection facilities are also provided for up to three wells. A lift weight of 147 tonnes was achieved for this design, allowing installation by a number of diving support vessels.

A critical consideration in the development of a light weight manifold is the optimisation of the piping layout. The steps taken to optimise the piping layout are described. Optimisation is considered as minimisation of piping weight and space envelope while maintaining satisfactory diver access.

The structural design considerations involved in developing a light weight structure to support and protect the piping are discussed.

INTRODUCTION

With respect to the current drive for economy in the oil and gas industry, it is becoming more important than ever, to evaluate all available options, for the development of new and existing offshore fields. Increasingly, development of smaller accumulations, particularly via subsea developments is encountered in the mature North Sea environment.

To enhance cost attractiveness of subsea production systems, it is imperative that the designed facility addresses all opportunities to reduce capital and operating expenditure. This entails systematically optimizing fabrication, installation, commissioning and operating of the system.

This paper reviews the design development of the Shell/Enterprise Nelson Subsea Manifold designed by JP KENNY CALEDONIA. The production system is a cluster development with individual production and water injection trees tied-in by jumper spools to the main manifold assembly. The system is illustrated in Figure 1.

DESIGN OBJECTIVES

Detailed design of the Nelson Manifold facility was performed under an EPC contract. The main EPC contractor was FMC, who subcontracted the manifold and protective structure design and fabrication to Consafe, who in turn employed JP Kenny Caledonia as design contractors. The design objectives of the piping systems within the facility are detailed within the flow schematic (fig. 2). These objectives can be summarised as follows:To provide manifolding for up to eight production wells into two production flowlines and one production/test flowline. Each well was to be capable of producing into any of the three flowlines;To provide gas lift manifolding to each of the eight production wells;To provide water injection manifolding to allow injection to three wells;To allow round trip pigging of all three production/test flowlines;To house the umbilical termination assembly (UTA) and the requisite controls for all manifold valving;To distribute the controls for all subsea wells.

The piping systems were to be contained within a support and protection structure to meet the following objectives:To withstand all phases of construction, installation and operation;To withstand a 70 tonne static snagging load on any structural member;To provide protection against dropped objects up to 50 kg mass of 250 mm diameter with terminal impact energy of 2 Kj;To promote overtrawlability and minimise snagging of fishing equipment.