In early 2007 the operator was looking at options to drill additional gas wells. A hydraulic workover unit (HWU) had been used to previously sidetrack a number of wells offshore Dubai and a decision was taken to investigate this option further. The goal was to drill the same well profile as a jack-up did on a previous campaign but at lower costs. The work scope for the HWU campaign would include the retrieval of the existing completion, setting a whipstock, milling through two casing strings, drilling 8–1/2" build-up section into the top of the reservoir, lining off with a 7" liner, horizontally drilling the reservoir section in 6–1/8" hole and finally completing with a 5–1/2" completion. Due to time constraints all planning including contracting was performed in a two months window prior spud. The operator selected three mother wells on a platform connected to the main field facilities, which included accommodation and logistical support. With the reservoir being at around 3,300 feet TVD, a total of 4,285 feet were drilled in the three sidetrack wells. A regional first achieved is the drilling of 1,500 ft of 8–1/2" and running of a 7" liner with a HWU. This paper will describe the planning, challenges, limitations, learnings from the campaign and the resulting innovations for future HWU projects, which includes the concept of 'using self-elevating-workover-platform' or 'lift boat' and purpose built, minimized footprint tank farms for standalone production platforms. Introduction The operator has previously used hydraulic workover units (HWU) on project campaign basis for well repairs, tubing change-outs and simple sidetracks. Until that time sidetracks drilled with HWU's were either 3–3/4" or 4–1/8" bit size with lateral lengths of around 2000 ft. Some of the HWU sidetracks had also used 2–7/8" off-bottom liners with swell packers. The intention from the subsurface team was to have a 6" minimum reservoir section of 1500 to 2000 ft horizontal lengths with a 4–1/2" open hole completion. With the given target, an in-zone sidetrack from a donor well was not possible and with a large pressure differential between the reservoir and upper formations the build-up section needed to be cased off. A further requirement was the necessity for a gas tight re-entry mechanism into the donor well, sealing off any potential leak paths into the annuli of the donor well. Conclusively, after a detailed engineering review of various options a 8–1/2" inch build section lined with a 7" liner tied back into the 9–5/8" donor casing was chosen. The reservoir was to be drilled in 6–1/8" open hole, which was identical to the well design from the previous Rheas well drilled using a jack-up rig. Drilling the 8–1/2" build section and running 7" liners with a hydraulic workover unit (HWU) was highlighted as a major challenge, but detailed technical reviews and 'drilling the well on paper' exercises, indicated the feasibility and the project sanction was awarded. Hydraulic Workover Unit (HWU) and Facilities A standard hydraulic workover unit (HWU) uses four hydraulic driven cylinders with one set of stationary and one set of traveling slips to push or pull tubulars. Depending on the design of the HWU the traveling distance or stroke varies between 9 to 12 feet only, which explains the slow tripping time of a HWU compared to a standard jack-up rig, which can generally handle triple stands and rack them in and out the derrick. If the HWU does not have the racking capability, due to platform load and/or space restrictions, then all tubulars have to be singled-in and out the work basket during each trip. During the Rheas campaign an average of 20 joints per hour while tripping were achieved, which meant that a trip typically took eight hours (excluding BHA handling) at a measured depth of 5000 feet. In regards to the pushing and pulling capabilities there are various designs and sizes available on the market; The Rheas project used a 340k HWU, which means this HWU was able to pull 340000 lbs and push 180000 lbs.