Spool-Tree Development for 2500-m Water Depth
Authors: Prado Luiz A.R. , Sanches Eduardo M. , Olijnik Luiz A. 
Published: October 2008

This paper presents the development of the work performed by a petroleum equipment company with a major oil company in developing a subsea spool christmas tree to be installed in 2500m water depth. The paper describes the technological agreement, the tree arrangement and the solution adopted in each significant component; how interfaces points were defined for instalation and operation; and future work plans on prototype fabrication and tests.

Introduction

The history of oil production in Brazil has been marked by continuous efforts in developing technology for always increasing water depths. Nowadays the barrier to be transposed is to reach the fields up to 2500m of water depth. Considering the Brazilian scenario, where there are many wells in deepwater oil fields, Petrobras decided to undertake this technological development.

In the development of deepwater oil fields, where cost reduction is of capital importance, the use of an appropriate tree is vital. The use of a spool tree is a great advantage because it is possible to retrieve the tubing string without removing the tree.

With the concept phase completed, the design has been defined as a result of the united work that joins the operational and technological experiences.

Tree and wellhead system overview

The 2500m spool tree is totally diverless and is installed without guide lines. It has been designed to land on 16-3/4" × 10,000 psi wellhead. Material mechanical properties are controlled to allow service in high concentrations of hydrogen sulfide (H2S). The design provides a vertical connection flowline system to land the flowline modules on the tree. The spool design was adopted due the advantages in time installation and workover that it provides. One of the main characteristic of this dual 5" × 2" horizontal tree is the fact that it will be installed using a dynamically positioned rig with a GLL 16-3/4" BOP system, due to the water depth.

Wellhead description.

The wellhead is a guidelineless system with a 16-3/4" housing external profile.

Tree description.

The subsea christmas tree is composed by 5-1/8" production bore size and 2-1/16" annulus bore size. Vertical wireline access is provided for the production bore. The tree is rated to 5000 psi maximum working pressure. Figure 1 shows the production stack-up.

All hydraulically operated gate valves are fail close and have a hydraulic override to be operated by a ROV which allows mechanical operation in case of problems with hydraulic supply lines.

The production tubing string is suspended by a tubing hangers set inside the spool body of the tree.

Operation for the DHSV is provided by two separate lines during tubing hanger installation and after tree installation by one line from the surface with by-pass for both lines on tubing strings.

There is an internal helix inside the spool to orient the tubing hanger. After the tubing hanger is run, the tree cap will land on the tree spool and seal to it and the tubing hanger, using stab subs.

The external tree cap was selected as it resembled a more traditional connection for the DHSVs and electrical connector. This allow access to the production string if a light intervention is needed using a workover BOP, without the tree cap retrieval. Figure 2 shows the tree under intervention.

Tree / Flowline interfaces.

Design is suitable to allow flowline modules running independently of the tree, connecting vertically on it. The flowlines will be run using three modules (production, annulus and umbilical). This allows increased size of the lines, as well as size reduction of the Flowline Modules, as the pig valve wil