Assessment of Collision Risks Associated with Jack-Ups Moving on to Location Alongside Jackets

ABSTRACT The paper describes a study to investigate the likelihood of impacts occurring between a jack-up spudcan and a jacketmember as the rig manoeuvres close to the jacket. The study involves assessment of likely impact locations and velocities and detailed finite element impact analyses to investigate damage to both the jack-up and jacket. Statistical analyses to determine the probability of damage occurring are described upon which basis the main conclusions are drawn. INTRODUCTION A Joint Industry Project (JIP) [1] has been carried out to investigate the problems associated with jack-up leg impacts during touch-down. These impacts cause impulsive loads at the spudcans which occur whenever the rig moves on and off location. To avoid damage to the legs, limiting weather criteria are normally applied. For example, the information in operations manuals on board rigs is limited either to the maximum wave height that can be tolerated or to limiting combinations of roll (pitch) amplitude and period, usually given for just one or two water depths. In the former, water depth is ignored and both neglect the effects of seabed soil characteristics. The results of the JIP [2],[3],[4] demonstrated that rig motion characteristics on touchdown are a function of rig type, water depth (leg length), vessel characteristics and wave frequency. The impact forces increase with increasing seabed stiffness and also vary sufficiently rapidly that leg dynamic response is appreciable. Similar uncertainties regarding motions exist in the case of a jack-up moving into a drilling or tender assist mode next to a jacket. Thus the possibility of contact occurring between the two has to be taken into account even though the seastate in which such an operation might proceed may be significantly less than that which applies when the jack-up is attempting to touch-down in a clear area. Tug breakdown or the failure of a mooring line may also give rise to impact speeds approaching those induced by wave action. A study, funded by the Health and Safety Executive, was carried out to investigate the likelihood and level of damage occurring during such manoeuvring operations. OBJECTIVE & PROCEDURE The overall objective of the study was to determine the environmental and operational parameters which apply when jack-ups move on and off location adjacent to a jacket, and to identify those which control the frequency and extent of possible damage to the installation or the jack-up. The present study applies to a Marathon Le Tourneau 116-C enabling the use of motions analyses, leg failure surface data and the FE model for this rig previously developed for the JIP. The analysis procedure was as follows:determine likely impact locations and velocities based on typical jack-up manocuvring proceduresdetermine the probability and frequency of contact between the spudcan and the jacketperform detailed time domain non-linear finite element (FE) impact analysesdetermine the level of damage to the jacket and jack-up from FE analysis resultsassess the maximum impact velocities for an acceptable level of damagedetermine the probability of damage occurring given that an impact has occurred