A large three-leg jack-up drilling unit was chartered to perform a complex drilling programme, in deep water, located alongside a new platform jacket.The owner had constructed a special service module (700 m/tonnes) which was to be carried by the jack-up to site, positioned on the canterlever structure, at the aft end. Once on location the module would be skidded onto the jacket, where skid beams had been installed, and then the drilling derrick would be skidded over the top of the service module, and the drilling programme could begin. The voyage distance was some 500 nm, from a port to the offshore site, in the autumn, through an area prone to storms and heavy traffic.The Constellation offshore marine services company company’s client was the rig owner. Several issues were required to be resolved:
• The unit’s deck load, with the module, would be on the limits of permitted loads for field moving and outside the limits for ocean tow.
• The calculated motions of the unit in the Constellation offshore marine servicescompany design storm for afloat operations would be exceeded if the module was carried.
• The voyage and new location presented navigational and positioning and elevating hazards, due both to the season and the fact that the bottom conditions at the new jacket location were suitable for the unit when preloading without the module, but not with the module. The bottom was also quite hard and the unit had specially constructed high strength steel legs which, although of light weight, in comparison with their length (150 m) were very susceptible to shock load damage if they contacted a firm bottom when thee barge had any significant heave. In fact the permitted motion limits for going on and off location were extremely strict.
• The service module had no dedicated or approved seafastening system.
• The vessel’s class approved operations manual had no approved transit or jacking condition for the carriage of the module.
• If the module could not be carried by the unit, then it would have to be. installed by crane barge, which would interfere with the sub-sea pipeline laying programmes for the new oilfield in way of the jacket.
• The whole field development project was some months behind schedule and the commercial pressures were intense.
The problems were resolved and the operation was carried out using the following methodology – the majority of the work being carried out by the engineers of the Constellation offshore marine services company.
• They designed and produced a set of environmental criteria for jacking the barge up with the module. This was approved by class.
• They designed a seafastening system for the module which was capable of meeting the calculated forces induced by the unit motions in a design storm whose values lay mid-way between ocean move criteria and field move criteria.
• In combination with the seafastening design calculations the stability, both static, dynamic and damage conditions were recalculated for the intermediate storm values and found acceptable.
• The tow route to site from port was designed to take maximum advantage of shelter and jacking sites identified and approved such that the unit was never more than six to eight hours from an approved location.
• The tow procedure documentation and C of A contained expressed conditions and requirements for adherence to actions in the event of forecast or actual weather
conditions reaching set limits. These requirements defined the maximum conditions for transit, jacking up and down, heaving to, proceeding towards next stand-by location and required weather windows for all the operational steps. The procedure for locating and jacking at the platform jacket was additionally modified.
• The tow power and number of tugs required for the operation was closely defined and the requirements for tug assistance when positioning at a stand by location were also stipulated.
.The pre-loading weights and procedures were recalculated and approved to enable the unit to jack-up on the new location, part pre-load, skid off the module then carry out full pre-load.
• In addition to class approval, the Constellation offshore marine services company had to obtain, on behalf of the owner, approval from the regulatory regime of the country in whose waters the jacket was located, for all the operational steps.