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INS aided subsurface positioning for ROV pipeline surveys

Michiel van de Munt, Robert van der Velden, Karel Epke

 

ROV positions are established using a combination of surface and

subsurface navigation systems aided by heading and motion sensors that

provide global coordinates for the subsea sensor platform in near real

time. In past decades, the performance of these systems has gradually

improved but not sufficiently to keep pace with the rapid move into

(ultra) deep water and the related optimisation of pipeline design,

especially for gas pipelines with hot mixtures. The current designs

for ‘hot’ subsea pipelines require very tight measuring tolerances

generally beyond the unaided capabilities of the acoustic positioning

systems.

 

A subsea pipeline which operates at temperatures and pressures above

seabed ambient conditions will tend to expand. If this expansion is

restrained, for example by axial friction between the pipeline and the

seabed, then an axial force will be developed in the ‘hot’

pipeline that could lead to buckling .Accurate positioning of such a

newly installed pipeline in X, Y, Z, is important to relatively check

for any significant small pipeline deflections that could lead to

potential buckling during its lifetime. These accurate ROV pipeline

surveys are referred to as Out-Of-Straightness (OOS) surveys.

 

Since the early 1990’s, Doppler Velocity Logs (DVLs) are installed

on ROVs to enhance the USBL position by integrating the relative short

term accuracy of the DVL with the absolute long term accuracy of the

USBL systems. Integration was based on a variety of mathematical

routines of which Kalman filtering is the most commonly used. In the

past decade, with the launch of commercial AUVs, the combination of

INS and DVL became a primary positioning solution for these platforms,

whilst acquiring high precision geophysical data in deeper water.

Absolute positioning of these platforms is achieved through adding

sparse DGNSS or acoustics during post processing. Migration of the

same technology onto the ROV platform is a challenge and is not as

straightforward as it may seem.

 

The presentation demonstrates the outcome of various trials conducted

by Allseas with INS aided ROV positioning along offshore pipelines,

including comparisons between various combinations of ROV positioning

survey sensors.