Abstract

Detailed seabed mapping by use of multibeam echosounder (MBE), sidescan sonar (SSS) and subbottom profiler (SBP) mounted on ROV's has been common practice for several years in the North Sea region. The details from this type of survey compared to vessel based surveys have added valuable information for planning and detailed engineering for field development. The details revealed by ROV seabed mapping is even more necessary as field development progresses towards deep water (> 1000 metres).

As survey ROV's and survey sensors typically are rated to 1000 metres or less, it requires significant planning and preparation to prepare a spread capable for deep-water operations. Several challenges, known and unexpected, experienced during a mapping campaign off the coast of West Africa in water depths down to 1450 metres are discussed below. The paper describes the pre-studies and analyses carried out prior to mobilisation, problems encountered during mobilisation and survey, and the corresponding problem solving.

The modifications made to the ROV prior to the survey proved to be very profitable as survey speeds in excess of 1.5 knots were obtained in water depths of 1400 metres. The data quality appeared to be good, although some of the survey data partly suffered from background noise.

Background

Even with significant experience in ROV seabed mapping it is a comprehensive task to build a ROV survey spread suitable for deep-water. The task appears simple at first sight. We just want to do in 1500 metres water depth what we have done for years in 300-600 metres water depth.

But after the most obvious thoughts of "what is the depth rating of the ROV" and "how long is the umbilical" more complex questions arose. As the answers turned out to be negative for our standard survey spread for both the above questions, new ideas had to be developed. But first a target objective was established. The work task included more than 800 line km of seabed mapping and therefore survey speed became an important issue. We aimed for a speed of 1.5 knots (0.77 m/s). This was approximately 10 times faster than the only known deep-water ROV MBE survey at that time (but less than the speed we operate with in the North Sea).

In addition to the survey speed, the following main tasks were identified as the key to successful mobilisation and performance:

  • Upgrade of deep rated WorkROV to Survey ROV.

  • Identification and integration of deep rated survey sensors.

  • Fast and reliable data transmission from ROV to surface.

Survey spread

The survey spread should include a vessel with good DP capabilities and high accuracy sub-surface positioning system, e.g. HiPAP (High Precision Acoustic Positioning) from Simrad, and DGPS based heading and attitude sensor (Seapath from Seatex). Low noise environment around the vessel is an advantage for the quality of the acoustic link to the ROV.

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