This paper reviews the present means used to deploy the diver closer to the job site and thereby increasing bottom working time. The paper does not address the situation where the support vessel can locate directly over the work area.
The six basic means adopted at present are as follows:
Cross haul of diving bell towards job site.
Deploy diving bell/basket from a boom.
Guide wire system to guide bell to job site.
Mobile diving bell.
Diver lock out submarine.
Orientate diving from above the job site, i.e.platform based.
The increased use of large semi-submersible vessels has stimulated development in this area simply due to the stand off required and this is particularly apparent when supporting surface diving on an adjacent structure.
The technique of cross hauling is well established and has been used on diving support vessels with moonpool launched bells for a number of years. The method is to attach a second lifting line to the diving bell, via a monkey plate, which passes down through the moonpool and back up to the side of the vessel. This wire passes over a guide sheave or fairlead to an independent winch.
After launch, the weight of the bell is transferred to the cross haul line, which moves the bell closer to the side of the vessel. The amount of movement can be increased with the aid of extension booms.
This technique has also been used on drill rigs and platforms, and in a developed form on the BP Iolair and Shell Stadive. Both vessels, being large semi-submersibles, are at a disadvantage when compared to a smaller monohull DSV for working alongside a platform. Both vessels have long booms to improve outreach and employ motion compensation on the cross haul line.
Cross hauling is a simple and cost effective means of moving the bell but there are inherent disadvantages:
Limited lateral movement - cannot reach cross hauling point due to catenary tension in main lift wire.
Difficult to monitor exact position of the bell.
Cross haul wire is subject to corrosion and wear and must be frequently inspected.
When cross hauling, conventional guide wires are not used which maintain orientation of the bell. This can give problems with the recovery of side mate bells into cursors in particular.
Boom deployment is different to cross hauling in that the bell/basket lifting wire actually passes over the end of the boom. An adaption of this is to ‘push’ the lifting wire further outboard after the bell has been deployed. Such a system is used effectively on the Philips SS, a semi-submersible diving support vessel, when operating alongside a platform, with the saturation bell.