Design for Impact of Dropped Objects
Authors: Wenger Arne , Edvardsen Geir , Olafsson S. , Alvestad Terje 
Published: October 2008

The piperack and wellhead areas of the platforms in the Statfjord field have been designed for protection against accidentally dropped objects.

This paper deals specifically with the design effort for the "B" and "c" platform decks. Results from three series of full scale tests are reported. Drops of 9 ! inch drill collars from heights ranging from 8m to 42m were performed on plate thicknesses ranging from 12mm to 47mm. Stiffened, continuously welded plates were selected for the piperack areas and areas surrounding the wellheads, while simply supported loose hatch covers were used directly above each well head.

Based on information from the tests, an experimentally correlated design approach has been developed for determining necessary plate thickness for steel decks. In addition some practical recommendations for design of impact resistant deck structures are presented.

INTRODUCTION

It was recognized early in the development phase of the Statfjord field that simultaneous drilling and production operations increase the risk of damage to completed wellheads from accidentally dropped drilling objects. To protect against this risk, the operator decided to take steps to protect the wellheads and other critical equipment. On Statfjord "A" platform deck, this protection consisted of heavy wooden mats encased in steel plate. The critical load case used for the design was represented by a vertically. falling 9½ inch drill collar.

The Statfjord "B" and "C" platform deck designs resulted in a maximum possible fall height for drilling equipment considerably greater than for the "A" platform. Consequently, full scale testing was conducted to verify design calculations and optimize on thicknesses.

LOADS

Selection of design loads was ba8ed on a survey of equipment to be handled. Idealized load cases were listed for the typical objects. Since the loading is random, a realistic evaluation of maximum object weights with corresponding maximum drop heights was made.

Heavy objects, having sufficient contact area during impact to be caught by the grid system, will represent the most severe cases for the stringers and girders. In most cases restrictions will be laid on the crane operators concerning allowable lifting heights for such objects since the lifting frequency is low. This paper is, however, limited to objects with a small contact area during impact which will represent the governing load case for unstiffened plates.

PARAMETERS

General

Each impact case on even seemingly identical structures are very often "one of a kind cases" since minor parameter changes for the separate structural elements may change the response of the structure significantly. The following parameters will influence the energy absorbtion for dropped objects:Impact speedContact surface during impactLocation of impact on structure (stiffness)Object mass/weightMass of impacted structureBoundary conditionsObject stiffnessOrientation of velocity vector during impactType of materialMaterial thickness

Impact Speed.

The impact energy to be absorbed is proportional to the squared velocity and can be expressed by the well known equation:(Available In Full Paper)