Construction of rafts for Nyos-Monoun Degassing Project
Ｙｕｔａｋａ Ｙｏｓｈｉｄａ
Yoshida Consulting Engineer Office (3-46-10, Tsukigaoka, Morioka, 020-0121 JAPAN; E-mail:yoshiday@d2.dion.ne.jp)

1. Introduction

Since gas disasters of Lakes Nyos and Monoun in mid-1980s, many scientific studies of these lakes have been
carried out, and mechanisms of accumulation and sudden release of CO2 gas have been made clear. As CO2 gas
is still accumulating in bottom waters, the lakes have a potential to explode again. In 1999, the degassing
project was funded by USAID Office of Foreign Disaster Assistant (OFDA). Since then, detailed technicalexamination of man-controlled degassing and related scientific monitoring of the lakes have been conducted as
Nyos-Monoun Degassing Project (NMDP). The rafts were built as an essential part of these degassing andmonitoring systems.
2. Types of rafts
From 1999 till 2003, six rafts of three types were made in this project. They were installed at Lakes Nyos andMonoun. The outline of the rafts is given in Table 1.

Table 1.

History of installation and outline of rafts built in OFDA supported degassing project.

InstallationLocation

TypeSize

Frame material

WeightFloats

Buoyancy

Oct. 1999

Lake Nyos

Climate station

2.5m * 2.5m

Wood

250kg

4(large) + 4(small)

900kg

Nov. 1999

Lake Monoun

Climate station

2.5m * 2.5m

Wood

250kg

4(large) + 4(small)

900kg

Jan. 2001

Lake Nyos

Degassing pipe raft

3.55m * 3.0m

Zinc-coated steel

740kg

8 (large urethane filled)

2,240kg

Jan. 2001

Lake Nyos

Instrument raft

3.55m * 3.0m

Zinc-coated steel

650kg

8 (large urethane filled)

2,240kg

Jan. 2003

Lake Monoun

Degassing pipe raft

3.55m * 3.0mZinc-coated steel

740kg

8 (large urethane filled)

2,240kg

Jan. 2003

Lake Monoun

Instrument raft

3.55m * 3.0m

Zinc-coated steel

650kg

8 (large urethane filled)

2,240kg

The raft for the climate stations is relatively small in size. Except for some parts, most of the materials werepurchased in Cameroon. Although the main frame was made of wood, the rafts at Lakes Nyos and Monoun are
still working as of January 2006. A pair of rafts, one for the degassing pipe and another for the control unit, werebuilt. They were assembled in Japan. After inspection, the rafts were transported and reassembled at lakeside.
No electric or engine power was used for reassembly. After launching, the pipe raft was combined with thedegassing pipe prepared by French engineers. Finally, pipe raft and instrument raft were fixed on the deepest
point of each lake.
3. Characteristics of the rafts
The rafts needed to meet the requirements that they should be (1) endurable under tropical climate, (2) lightand compact for easy transportation, and (3) easy to reassemble onsite where no electricity and heavy tools wereavailable. Thus wood was chosen as materials for the rafts used for the climate station, and zinc-coated steel waschosen as frame materials for the pipe- and instrumental rafts. The steel frame was assembled using stainlesssteel bolts and nuts only. Buoyancy was calculated so as to support the weight of the instrument, pipe andcontrol unit. Urethane-filled floats were used for the pipe- and instrumental rafts, since these floats were lightbut hardly collapse.
* Three pictures of rafts are shown in the next page.

photo 1. Climate station on Lake Nyos

photo 2.

photo 3.

Rafts at Nyos lakeside

Recent degassing fountain and pipe raft at Lake Nyos (Jan. 2006)