Subsea remote intervention systems generally become more complex as time passes, in order to gain technical and commercial advantage
In 1996 BP Exploration Operating Company UK Ltd and Stolt Comex Seaway embarked on an innovative programme of subsea remote intervention that tackles one on of the most inhospitable subsea locations - ‘The Splash Zone’ This is a very turbulent place to work affecting the productivity of divers and ROVs similarly no matter how sophisticated the support vessel or platform based system A suite of 5 tools were developed to inspect and intervene in the splash zone down to, but not limited to, 13m depth These tools were not deployed by divers or ROVs but by abseilers working in the dry, mainly from the cellar deck
This paper describes the unique solutions derived from stepping back from complex technology and applying simplification that negates the need for high levels of technical support skill This approach led, as intended, to the realisation of tools exhibiting productivity gains and improved technical results This development paves the way for significant cost savings in future years
A glossary of terms is included at the end of the paper
The majority of annual IRM work on platforms is in the splash zone from the sea surface to the first horizontal structural frame and conductor guide frame These structural members and supporting legs receive the majority of the sea's wave slam The top 5 metres of the water column are turbulent even when the sea is ‘calm ’ Divers and ROVs move with the swell, which is rarely less than 15m, as they are neutrally buoyant The only way to maintain station is to ‘hold on’ and ‘clamp on’ respectively The diver is uncomfortable because of his compliance and there are considerable forces transmitted through a rigidly clamped ROV to riser or member Dynamic station keeping through forces transmitted through thrusters or fins, to react against the disturbance, are too slow for fine control near the sea surface Divers and ROVs can perform ‘acceptable ’ video from depths deeper than 2–3m below the surface as they oscillate in the surge Trying to maintain position is impossible without clamps for vehicles and the ‘illegal’ dangerous practice of ‘hogging in’ for divers The wave loading in a small swell is still considerable on an anchored body which is approximately neutrally buoyant in the water
A negatively buoyant vehicle is less affected by wave loading but would have to generate thrust in an upward direction to maintain depth This problem is overcome with a suspended space frame of weight very much greater than the lift from the vertical component of the wave motion Therefore the supporting structure of the vehicle would have to exhibit minimal surface area to reduce wave slam Reduction of horizontal surface area also minimises drag, potentially negating the need for reaction thrusters The resulting space frame is an ‘ROV without thrusters’ A ‘backward’ step?
