Maersk Oil’s original multiplatform Dan Field complex in the Danish sector is one of the grand old dames of Europe—it was the first permanent production facility anywhere in the North Sea region.

Containing an estimated 2 Bbbl of oil in place, it is the country’s oldest producing field after coming onstream in July 1972 following its ground-breaking discovery early the previous year. The Dan A wellhead and Dan B production platforms were installed 200 km (120 miles) offshore and have performed admirably since first oil, and so far Maersk estimates that close to 28% of the country’s total oil production has come from the Dan Field alone.

The operator also has added several other satellite installations in and around Dan in the southwestern part of the Danish North Sea sector over the decades to maintain production levels from the field’s main central area and flanks as well as employing innovative drilling and subsurface EOR techniques throughout the field’s life to improve the reservoir’s initially low recovery factor (originally under 10% but well into the 30% range by the mid-1990s, thanks largely to horizontal wells, fracturing and water injection solutions).

Makeover
However, after four decades of sterling service—already well beyond the initially envisaged 25-year operational lifespan—the operator decided it was not yet time to retire the facility and instead opted to give platforms A (the wellhead platform) and platform B (the production center) a comprehensive makeover.

This led to a lifetime extension program called the Dan Bravo Rationalization project, which saw Maersk select Boskalis Subsea Services carry out work on the two platforms between 2013 and 2015 and achieve a number of offshore industry firsts along the way.

According to Bert van der Velden, commercial director at Boskalis Subsea Services, the major subsea structural reinforcement campaign on the wellhead platform (Dan A) “has extended the platform’s life by another 40 years at least.”

Largest 3-D photogrammetry
Speaking at the Subsea Expo event in Aberdeen, Scotland, earlier this year, van der Velden highlighted in particular the project team’s undertaking of the world’s largest-ever 3-D subsea photogrammetry on the structure.

First the platform jacket’s members and nodes had to be completely cleaned back to bare metal using grit blasting and flush grinding by air and saturation divers. It was then marked out with about 2,900 magnetic coded markers, with about 20,000 overlapping high-definition images subsequently being taken by a work class ROV between July and September 2013.

These photographs were converted into an exact geometric 3-D model using the information from the markers, which was used for the onshore fabrication of subsea reinforcement structures (K-node clamps). High accuracy was required for these steel-to-steel clamps, van der Velden said, which reinforce the jacket structure where a horizontal member is intersected by cross members. These clamps were fixed to the jacket with the help of two diving support vessels (DSVs), the Constructor and Protea DSVs.

Complex installations
A new conductor guide level also was installed on the Dan A platform by Boskalis as well as two old conductor guide levels removed from it.

These installations required the careful maneuvering of various pieces of fabricated steel weighing up to 5 tonnes through slots in the facility’s structure guided by divers and rope access teams. Two temporary knuckle boom cranes also were installed on vertical platform members 12 m (39 ft) above sea level to help with the maneuvering of the new steel sections as the platform’s single existing crane had only a 2.3-ton capacity.

Van der Velden said 136 metric tonnes of new steel was installed, involving 88 connections, with 220,000 man hours spent on the job, a project personnel total of 320 people and more than 200 vessel days required. Operations were performed in water depths varying from 10 m to 41 m (33 ft to 135 ft). The last part of the offshore campaign was to remove the temporary cranes before the winter season began.

Industry first
The reinforcing of the Dan A platform in this way and to this scale had never been done before elsewhere, van de Velden added, who said the majority of its work on Dan was focused on the wellhead facility to preserve its structural integrity until at least 2042.

He also pointed out that this complex and challenging work program was all carried out subsea on a mature platform, sometimes in unfavorable and challenging weather and visibility conditions, while it continued to produce hydrocarbons.

A further key aim was to carry out as much of the work as possible onshore to help avoid more costly work offshore, with a close working relationship established between the operator and the contractor. The latter was contracted for the installation, fabrication, procurement, equipment testing and structural examinations.

There were no lost time incidents reported over the course of the 200-day offshore work program, he added, which also included fitting a new boat landing to Dan B.