Rolls-Royce Subsea Construction Vessel Delivered to Island Offshore
Island Crown is the latest vessel to come from the relationship between Island Offshore and Rolls-Royce. Once again the UT 776 CD has been selected as the basis for a vessel to meet some special offshore requirements. Over the past few years, the Norwegian shipowner has added several batches of the UT 776 platform supply vessel to its fleet, including two fuelled with LNG.
This time around, the design brief was a vessel that will usually spend its time connected to an offshore platform, acting as a hotel for people working on the platform. Island Crown can also carry out a separate set of functions – subsea construction and ROV operation – and is additionally equipped to transport all the liquid and bulk supplies needed by rigs and platforms.
It was logical to choose the latest and longest version (99.8m) of the Rolls-Royce UT 776 CD which has proven low motion in a seaway. Also specified were other Rolls-Royce ship systems similar to these on other Island Offshore vessels, such as a four-generator set diesel electric propulsion system, Azipull main thrusters and twin tunnel thrusters with a swing up azimuth thruster at the bow. This arrangement provides the high level of redundancy required for the DP2 class Dynamic Positioning System.
STX Offshore, now the Vard Brevik shipyard, was chosen to build the vessel. Island Crown has the UT 776 CD superstructure layout favoured by Island Offshore. This provides comfortable accommodation for a marine crew of about 40 including supernumeraries. The hotel block is placed aft of the main superstructure, with a hospital and services at main deck level and two decks of 2-person cabins, day and meeting rooms above. The main galley supplies the food, and there are also pantries in the hotel section.
Platform workers living on the ship have to transfer between ship and platform at the change of shift. To do this, they use a transfer gangway positioned at the port aft corner of the deck. Island Crown is flexible enough to link to many different types of platform, with a variety of heights above water level of the platform entry port. To cater for these variations, and also the relative motion between vessel and platform in a seaway, the access system is made up of two main components: one is a telescopic tower and staircase which takes care of the initial height differences; the other is the gangway itself, linking the top of the tower and the docking point at the entry port. The gangway is hydraulically positioned but free to pivot, allowing for relative motion. Its length can also be varied. Normally, the vessel will maintain a fixed distance from the platform under DP, at a heading that as far as possible minimizes the energy required by the thrusters to keep station. Uptime supplied the gangway and Ulmatec provided the telescopic tower.
Passengers transfer first up a gangway from the main deck to the tower staircase, then up to the transfer gangway. If the system has to match a very low platform height, the tower telescopes down and the deck gangway then leads downwards to a mezzanine deck at the foot of the staircase.
Passengers and crew can be transported to and from land by helicopter. The helicopter platform is above the hotel near the point of minimum ship motions with access to the reception area.
Island Crown is designed to the very demanding SPS (special purpose ship) rules which, among other things, set strict limits on stability and angle of heel under various collision or damage scenarios. With a hundred people on board, there needs to be extensive evacuation capability, in this case two enclosed lifeboats and two Man Over Board boats.
For its platform service and subsea construction roles, Island Crown is equipped with a long boom crane mounted on the starboard side with capacity ranges from 10t at 40m to 100t at 10m. Also on this side is the ROV handling area with cursor just aft of the hotel block. Other subsea operations can be carried out from the port side.
Island Offshore’s new vessel is built and equipped to a very high standard, and provides offshore operators with a ship of good pedigree and capable of efficiently fulfilling several distinct roles.