Hydraulically centralized 15-slot subsea drilling template
The Statoil Volve field lies in the Norwegian sector of the North Sea and is, unusually, producing oil via a modified jackup rig, the Maersk Inspirer. The choice of the jackup for the drilling and then the subsequent production presented some interesting challenges, not least in relation to the drilling template, a vital component at the outset of field development.
Ann Vicens, project engineer, Claxton, was intimately involved throughout the project to design, fabricate and install the Volve template. She explains some of the issues that she and her team had to deal with:
The decision to produce the field via a modified jackup rig had serious implications for the way the wells were spaced out on the seabed. For practical reasons, the jackup had to be fixed in place before the template. This meant we were then forced to locate the template – crucially, without the help of any physical guides – very accurately on the seabed to ensure that the slots precisely reflected the layout of the well bay. Failure to achieve this would have created serious difficulties when tying back the individual wells and would also have a serious impact on the fatigue life of the risers.
In fact, it was determined that the template had to be installed such that the centres of the outermost slots were within 200 mm of a reference point provided by the location of the corresponding well centres on the rig. Obviously, the template also had to have the same heading as the well bay; it was calculated that a 1° error would reduce the tolerance for the template’s position from 200 to 120 mm.
The installation solution that Claxton proposed involved slinging the template beneath the jackup well bay for its journey out to the field, which meant the template would be submerged and subject to considerable drag. To overcome this loading, a robust seafastening arrangement was devised whereby upstanding beams on the template were bolted to the underside of the well bay. Once at the required field location and after the jackup was raised, the template was run to the seabed on a drillstring secured within the central slot. Two optical gyros, rather than the usual one, were used to monitor the template as it was lowered to the seabed; one was mounted on the drillstring and the other on the template itself. By comparing the data from the two gyros, it was possible to detect any bend in the drillstring and so obtain an accurate indication of the template’s heading, pitch and roll, and also any displacement relative to the well bay throughout the installation process. Calm weather and a slack tide during the installation combined with a remotely operated vehicle (ROV) that was big enough to be able to nudge the structure into place – as well as a good deal of patience – helped to ensure that the template was ultimately set down in exactly the required position and orientation.
“We take great pride in our role in this challenging project,“ says Vicens. “This was one of the biggest templates that UWG, now part of Claxton, had designed, and we supplied it on time and within budget. We also developed the installation procedures, and had two of our engineers on the rig to oversee what turned out to be a highly successful offshore operation. I think the greatest benefit to the client, however, was the ability to tie back the Volve wells to the production jackup without any undue difficulties.”
The Volve template has enabled Maersk to employ the Inspirer jackup in a role for which it was not originally designed. It has also defined a new option for field developers; Claxton has received expressions of interest in the technology from at least one other operator. In conclusion, it could be said that Volve has itself provided a valuable template for further applications of this nature.
The Volve template being installed
Central to the development
The Volve template was designed and built by Acteon company UWG, which is now incorporated into Claxton. It has 15 slots in three banks of five; the central slot was used to run the template as well as for drilling. The template was approved by DNV, whose representatives witnessed all the pre-delivery tests. Weighing in at 50 t, the structure was built in five parts in Great Yarmouth, UK. It was then transported to Haugesund in Norway for assembly before being sea-fastened to the Maersk Inspirer for the journey to the field.
A key feature of the template is the mechanism used to centralize the conductors within the slots during the primary cementing operations. It was a condition of the design that point loading on the production risers through the template during well construction and throughout the life of the field should be eliminated. There was to be no contact between the conductors and the template after the conductors had been cemented in place. This meant having centralizers that could be withdrawn from the slots once they had performed their important role.
Claxton’s solution to this problem was to build three retractable hydraulic cylinders into each slot. Activated by the ROV, these were extended to hold the 30-in. conductors in place during cementing, and were later retracted. The cylinders, each rated at 2100 psi, were grouped in five banks of nine and were powered from the ROV using the same environmentally acceptable hydraulic fluid used to control the ROV’s other mechanical functions. It is believed that this is the first time such a conductor centralizing system has been used.