Taking Advantage of Benign Metocean Conditions to Provide Surface Trees on Barges as Alternate to TLPs or Deep Draft Floaters
Authors: Valenchon C. , Gassert M. , Midgley S. 
Published: October 2008

The paper will present different versions of barge concepts with surface trees from the smallest unit "The Wellhead Barge - WHB® " supporting only surface trees and completion rig, to the largest unit "The Multifunction Barge - MFB® ", an FPSO with surface trees and drilling facilities.

A spread moored barge is already used as a production unit on the NKOSSA field offshore Congo and a spread moored FPSO will be installed offshore Angola in 1350 m water depth on the GIRASSOL field in early 2001. Although none of these facilities has surface trees on board, their motion behavior is compatible with both drilling and surface trees.

West African unique metocean conditions allow deepwater field operators to consider spread-moored barge-type floaters to support surface wellheads and perform drilling operations. In other deep offshore areas, such as Gulf of Mexico and Offshore Brazil, Tension Leg Platforms (TLP's) and deep draft vessels are presently the only floaters able to support surface trees.

The paper will present results of computational work and their correlation with tank tests on the MFB®, and WHB® performed on 1/60 model including hull, mooring and risers under wave and current loadings.

Emphasis will be placed on risers, drilling arrangement, and hydrodynamic behavior within the well bay.

Economical advantage, which results from the use of the "Surface Tree Barges", will conclude the paper.

Barges with surface trees take advantage of benign metocean conditions of Gulf of Guinea to offer a single vessel as drilling and production support for field development. Surface trees and drilling facility are integrated on a low cost, high carrying capacity floater able to support any kind of processing plant with or without crude storage.

Introduction

BOUYGUES OFFSHORE associated with SEDCO FOREX has been developing barge-type wellhead floaters since 1995 starting with the Multifunction Barge (MFB®) (Ref. 1), an FPSO with surface trees and drilling and deriving towards the Wellhead Barge (WHB®) (Ref. 2) which is a MFB® without storage (Fig. 1). In 1995 Institut Français du Pétrole initiated the riser design and today 2H OFFSHORE ENGINEERING is providing assistance in riser design and analysis.

Generic development work for the WHB® and the MFB® has been technically and financially supported by BPAMOCO, CONOCO, ELF, EXXON, SHELL and STATOIL and site specific studies have been performed.

Barge type wellhead platforms have been developed in order to present a cost-effective support for deepwater discoveries of the Gulf of Guinea.

In essence, the reasons that WHB®/MFB® are cost effective, are linked to the fact that spread moored barges are tailored to mild environmental conditions where they are able to combine surface trees with any kind of processing plant on the same low cost, high carrying capacity, floater.

In mild environments such as the Gulf of Guinea and some areas of Indonesia, the overall motion behavior of a barge is compatible with high-pressure top tensioned risers.