Improved Accuracy of Laser-Assisted Airgap Surveys Offshore
Authors: Mes M.J. 
Published: October 2008

Summary
Phillips Petroleum Company Norway (PPCoN) regularly monitors the airgap of all Subsiding offshore platforms 1n the Ekofisk field, located in the central North Sea. Subsidence causes the elevation of the platform decks to decrease with respect to the Mean Sea Level. This if of great concern. Expensive platform remedial work may become necessary if the amount of subsidence becomes too large and the platform airgap too small. The airgap is the distance between the crest of the design wave (related to the Mean Sea Level) and the lowest point of the main cellar deck beams.
Short term Laser water level measurements are used in combination with seabed pressure gauge measurements to obtain an accurate platform elevation with respect to the Mean Sea Level, from which the airgap can be computed. Actual data are presented and interpreted to obtain specifications for airgap surveys to obtain dated airgap values of known and optimum accuracy. In particular, recent improvements to the laser measurement system and the interpretation procedures will be discussed, which reduce the R.M.S (Root Mean Square) Error levels of the survey results to approximately +/- 7 cm.
The airgap measurements are also used to confirm reservoir subsidence predictions which predict when airgap closure occurs - and expensive remedial work such as a jackup operation must be completed.
This paper describes the development of the:specifications for laser assisted Airgap surveys,computation of survey uncertainty levels.

INTRODUCTION
Phillips Petroleum Company Norway (PPCoN) is the operator for the Ekofisk Field, located in the center of the North Sea. Production of hydrocarbons has caused compaction in the reservoir, which translated into subsidence at the seabed. This subsidence has reached over 5 meters in the center of the field. A group of platforms near the center of the field, generally called "The complex", were jacked up 6 meters in the summer of 1987, while a concrete barrier wall was placed around a gravity structure, the "tank", in 1989. The Tank is also part of the complex (Figure 1).
There are 2 other Ekofisk field platforms for which subsidence could lead to additional expensive remedial structural work.. The Ekofisk Alpha (2/4 A) and Bravo (2/4 B) platforms also experience subsidence of such magnitude that the airgap must be monitored. We conduct biannual so-called airgaps surveys using procedures described in Reference 1.

Measurement principle
We operate a pressure gauge based survey network (Figure 2) from which we derive subsidence and oceanographic data. The airgap survey measurement principle is illustrated in Figure 3. We have three platforms which can be used as benchmark platforms. These platforms are the Albuskjell (2/4 F), Tor (2/4 E), and Edda (2/7 C) platforms. There is geophysical, geological and foundation information available which suggests that these platforms should not subside or settle. This confirms that these platforms are suitable as benchmark platforms within close measurement tolerances. We have primarily relied on the Albuskjell platform as a benchmark because that platform has historically experienced the least amount of power outages or pressure gauge failures.