Overcoming the challenges of installing the disconnectable transfer system (DTS) of The Helix Producer 1 in Phoenix field
The Helix Producer 1 is currently among a fleet of vessels being used by BP to help deal with the aftermath of the blowout of its Macondo well in the Gulf of Mexico.
Before its unexpected call up by BP, the Helix Producer 1 had been about to begin producing oil from the Phoenix field, where it had replaced Chevron’s Typhoon mini-tension-leg platform, which had overturned during Hurricane Rita in 2005. As reported here, the installation in the Phoenix field of the vessel’s DTS posed some interesting challenges.
The arrival of the Helix Producer 1 would probably not have caused much of a stir in many of the oil-producing regions of the world. But it certainly did in the Gulf of Mexico, simply because it is the first ship-shaped floating production unit to enter service there.
Another notable feature of the Helix Producer 1 is that it is designed to operate without moorings: it is dynamically positioned. This enables the vessel and its crew to quickly escape from the path of any serious hurricane that passes through the Gulf, especially one directly threatening the field over which the vessel is eventually due to sit: the Phoenix field in Green Canyon Block 237.
Hydrocarbons are produced from Phoenix via a DTS, the most obvious component of which is a riser buoy held in a cage-like structure roughly midway down the Helix Producer 1’s port side. As there will be periods when the buoy is not connected to the ship, it needs its own mooring system to prevent movement causing damage to the risers.
Engineering and installing the mooring system, and installing the DTS buoy itself, have constituted a significant project for InterMoor over the past couple of years, as staff engineer Dusty Mathus explains. “The DTS buoy, which weighs around 300 t, has a six-point chain-and-wire mooring system that was designed to be preset. In addition, there is a 68-t clump weight in the form of a spreader bar with chains attached to it suspended beneath the structure on a wire tether nearly 600 m long. The approximate water depth at the DTS’s centre is 650 m.
“The magnitude of the clump weight and the length of the tether were calculated so that, when disconnected from the vessel, the DTS buoy would be drawn down to a depth of just over 50 m, so avoiding the worst of any wave forces generated by a passing hurricane. The buoyancy calculations assumed the buoy would also support the weight of its mooring system, four risers and two umbilicals. In the event, a relatively minor change to the field development schedule, certainly minor when compared with the vessel’s move to Macondo, meant that there was a period when this assumption was invalid and things became more complicated than we had expected.”
InterMoor’s original workscope revolved around the installation of the preset mooring system and its subsequent hook-up to the DTS buoy. This was when the plan was to bring the Helix Producer 1 and the buoy in at the same time, the latter already occupying its position in the port-side cage. However, InterMoor’s scope expanded significantly when it was decided, based on the unexpected availability of specialist support vessels, to install the DTS buoy ahead of the Helix Producer 1’s arrival, an undertaking described as wet-parking the buoy.
“Wet-parking the DTS buoy provided a series of interesting challenges,” says Mathus. “Although we did not anticipate too many problems installing the buoy together with its clump weight, it was clear that without the moorings and risers we had insufficient ballast to set it down at its safe depth. Working with Helix Energy Solutions’ engineers, we came up with the idea of a temporary ballast system in the form of four chains weighing a total of 110 t that could be attached directly to the underside of the structure.”
Mathus worked out the details for wet-parking the buoy with its temporary ballast system. These involved using the Q4000 semisubmersible to lift the buoy into the sea and an anchor-handling vessel, the Kevin Gros, to handle the clump weight and temporary ballast system.
Mathus is proud to report that the novel operation was performed without incident in just two days. This was during November 2009. The moorings were preset at about the same time. In January 2010, the four risers were laid down on the sea floor. During March and April, the risers and two umbilicals were pulled into the DTS buoy and then the moorings were hooked up, with ballast being progressively removed throughout the two processes. Finally, in late April, the Helix Producer 1 took up position in the field and claimed its entry in the industry’s history books as the first ship-shaped production unit to enter service in the Gulf of Mexico.
Events since then have rather overshadowed the Phoenix story. However, the DTS buoy remains moored at Phoenix awaiting the vessel’s return. (A second temporary DTS was quickly fabricated for use on the Macondo well; InterMoor was also involved in this mooring exercise.) At the time of writing, a date for the return of the Helix Producer 1 to the Phoenix field has not been set.