Anchor Handling and Rig Move for Short Weather Windows During Exploration Drilling

Abstract

The paper describes field experience with a method for efficient anchor handling and rig move during North Sea exploration drilling campaigns with a semi submersible drilling rig requiring anchors for positional fix. A conventional anchor handling operation including a rig move operation is presented, followed by a detailed presentation of an innovative field operation including a method for combining anchor pick up, rig move and pre-setting. Four anchor handling boats were used to pick up anchors at an abandoned well. Thereafter the anchors and ground chains were sent off to next drilling location with three of the boats. These boats pre-set the anchors. The anchor pre-setting sequence was selected such that one boat was released after only a short period at the new location. The last anchor handling vessel, the smallest one, was used in the rig move operation as a tug. When the rig arrived at the new drilling location, the three remaining boats were involved in connecting the anchors. As a result of the pre-set anchors, the mooring operation could be performed in a weather window, being roughly one half of the weather window needed for normal mooring operations. The anchor handling boats were released sequentially as early as possible reducing the cost of the entire operation significantly. As will be described a traditional anchor handling operation would have resulted in two days waiting on weather in addition to the extra time spent on anchor handling because the weather conditions deteriorated. The paper also describes the challenges of retrieving pre-laid anchors tensioned to the anticipated maximum 100 years load.

Different Categories of Working Units

“…Anchor handling operations may contribute 10-20% of the total well costs of offshore exploration drilling [8]. In a conventional anchor handling operation, the rig's winches are used to tension the anchors.…”

Positioning capability of anchor handling vessels in deep water during anchor deployment

“…Meanwhile, the AHOs have a significant economical influence on the offshore projects. According to Saasen et al [1], AHOs may carry 10-20 % of the total well exploitation cost because hiring the anchor handling vessel (AHV) is expensive. Considering daily hire rates on the spot market, costs may be as high as 900,000 Norwegian krones.…”