This paper describes a design technique for the lower portion of the jacket including mudmats and mudline horizontal framing. This technique considers the net weight of the jacket, the weight of any piles "hung" on the jacket, and any storm wave loads which may place an overturning load on the temporary foundation. Consideration is given to the support offered by the jacket leg extensions, mudmats, and jacket horizontal framing at the mudline, and the amount of load taken by each. The primary consideration is the structural integrity of the jacket structure at the mudline when subjected to a combination of loads that could cause the jacket to partially lift off the mud and/or partially sink into the mud. A redistribution of the load will occur which could as much as double the load transferred to some structural braces. The proposed technique insures the failure of the soil before the failure of critical jacket members by assignment of structural design loads which are usually much higher than the allowable soil bearing value used in mudmat sizing.
Mudmat geometry, location, and materials are discussed as well as the structural design loads for different types of mudmats and jacket members. Two design examples are presented with a basic plan for a solution.
Offshore jacket structures must be supported by mudline soils during a short period of time between the setting of the jacket in. position and the driving of piling to permanently secure the jacket to the seafloor. During this time, the jacket is exposed to waves generally up to five feet in height, but sometimes a sudden storm will bring about waves up to twenty feet in height. Such waves can tip a jacket over, causing it to partially sink in the seafloor mud and/or cause failure of jacket horizontal braces at the mudline (see Figure 1). Sometimes the damage does not occur until an uprighting operation is attempted by a derrick barge to free the jacket. Once the lower bracing is damaged, the jacket is generally too heavy to lift from the water for repairs, often causing the necessity to cut up and scrap the jacket and fabricate a new one, causing a great loss of time and production revenues as well as the cost of the jacket.
A fairly simple design technique can be used to minimize the chances of damage to a jacket during the time that the jacket is temporarily supported by the seafloor. The primary purpose of this paper is to present the design technique that a structural engineer may follow to prevent structural failure of a jacket as well as providing adequate support on the sea bottom.
The structural engineer should refer to a paper presented to OTC in 1980, "Temporary Seafloor Support of Jacket Structures," by Helfrich, Young, and Ehlers, for information on geotechnical considerations for a jacket design (1).
