Hydrographic
Survey
I'm
often asked what it is I do for a living so here's a little about
working as a hydrographic surveyor.....
Hydrographic (or offshore) surveyors work on all kinds of projects all
over the world, the
vast majority of it connected with engineering for the offshore oil and
gas
industry. Cable surveys, seismic exploration and a whole host of other
activities - anything that involves subsea positioning or
data
collection - likely to involve surveyors. Various
types of surface and acoustic positioning systems are used to determine
the
position of towed and remotely operated vehicles during these
operations. Surveyors are generally part of a team which may include
ROV crew, geologists, geophysicists, draughtsmen, engineers and others
- all in
addition to the marine crew required to operate the vessel itself.
For more information about cable and pipeline route surveys see here.
In addition to surveys and various types of inspection and
investigation offshore survey vessels, at least those equipped with
ROVs (Remotely
Operated Vehicles),
are often involved with a variety of construction and engineering
projects. For examples and pictures of such click on the following
link: Offshore construction
and engineering . For
pictures of offshore platforms
and other structures
see here.
Most survey and construction vessels are in operation 24 hours a day, with the majority
of the crew working 12 hour shifts. The amount of time spent onboard
varies greatly but in general varies from two to six weeks with a
corresponding length of time on leave.
Outside of the seismic industry there are two broad types of survey
vessels, one is vessels equipped with ROVs which perform
surveys using sensors such as high resolution MBE
(MultiBeam Echosounders), Pipetrackers, video and more. They often
assist in a variety of contstruction tasks using manipulators to
operate valves on subsea templates, install things on the seabed and
more. The second type of survey vessel may be equipped with hull
mounted MBE systems and an assortment of towed sensors such as Side
Scan sonars , Sub-bottom profilers, caesium magnetometers, sparkers and
boomers.
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3D View of a
wreck - made from MBE data gridded at 0.5x0.5m
The same shipwreck as above as "seen" by the 100 kHz Side Scan Sonar
After an acoustic survey geotechnical sampling is often conducted using
vibrocorers, CPT rigs, drop cores or other sediment sampling equipment.
This information is used to "ground truth" the interpreation of
geophysical data and to find out how the various sediments will affect
a pipelay (or other engineering activity)..
A Sub Bottom
Profiler trace showing two wrecks and various sediment layers..
 |
 |
Pipelay
vessel S7000 with survey support vessel
|
SV
Geobay, ROV survey / construction support vessel |
Before an oil or gas field is developed detailed maps of the seabed are
made in
order to design pipeline routes and the like. All kinds of
information are gathered regarding the shape of the seabed
and the sub-surface sediments. These data are commonly presented as
composite charts which present all the necessary data for engineering
design. Digital Terrain Models are often also viewd in 3D in order to
improve the visualisation of the seabed.
Prior
to pipelay there is often a requirement seabed intervention work such
as rock dumping,
dredging and trenching, all of which needs to be documented. Surveyors
work on rock dumpers, pipelay barges and pipelay support vessels during
these operations. Once a pipeline has been laid detailed surveys are
conducted to document its as-laid status.
 |
 |
 |
| Allsea's
pipelay vessel "Solitaire" |
Saipem's
S7000, the largest pipelay vessel in the world. |
CSO
Apache laying an umbilical |
Piper
Acergy, formerly LB200, near Kristin platform, May 2007
Frequently
ROVs are used, either as a platform for survey sensors or in order to
investigate targets located during other types of survey.
Photo: Andy Edwards
Sometimes ROVs catch fish during operations - not normally as spectacular as this one though!
Cod
as seen by ROV in almost 300m of water
Rabbit fish - not an unusal sight in deep water....
It is not unusual to find wrecks during seabed surveys, the excitement
of unexpectedly finding the remains of a Greek wreck in 2000 metres of
water is indescribable. However, I have only occasionally been invloved
in projects where we have specifically looked for wrecks.
The ultimate
survey experience - finding an uncharted Greek wreck in 2000m of water
HUGIN
AUV
An exciting development in recent years is the introduction of AUVs -
these Autonomous Underwater Vehicles are not physically connected to
the support vessel but are pre-programmed to a specified line plan and
then launched. Data are downloaded on recovery. This type of vehicle
can produce high resolution data with outstanding accuracy in water
depths down to at least 3000m.
The images of shipwrecks
above are
taken from 100 kHz Side scan sonar records operating at 200m range
Below
are some pictures of some other ROV survey / construction
vessels, pictures taken during September 2006 at work on the Ormen
Lange field.
